Fab ica ion and cha ac e isa ion
o MgLi hin ilms o
neu ological implan s
Disse a ion
zu E langung des akademischen G ades eines
Dok o s de Ingenieu swissenscha en
(D .-Ing.)
de Ch is ian-Alb ech s-Uni e si ¨a zu Kiel
o geleg on
Lisa Hanke
aus
Hambu g
Kiel
2024
E s gu ach e : P o . D . Eckha d Quand
Zwei gu ach e : P o . D . Regine Willumei -R¨ome
D i gu ach e : P o . D . Ma kus Val ine
Da um de Dispu a ion: 25.10.2024
Con en s
Con en s
Abs ac 1
Zusammen assung 2
1 Mo i a ion 3
2 Fundamen als 5
2.1 Magnesium and i s alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1.1 Applica ions in medicine . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1.2 Mg alloys o medical applica ions . . . . . . . . . . . . . . . . . . . 7
2.1.3 MgLi................................... 7
2.2 Spu e ed hin ilms............................... 9
2.2.1 Mic os uc u e o hin ilms . . . . . . . . . . . . . . . . . . . . . . 9
2.2.2 Mgalloy hin ilms ........................... 11
2.3 Co osion .................................... 12
2.3.1 Mgco osion .............................. 13
2.3.2 In luencing ac o s ........................... 14
2.3.3 Mg hin ilm co osion . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.3.4 MgLico osion ............................. 17
2.3.5 Li2CO3 o ma ion............................ 19
2.4 Co osion measu emen echniques and easibili y o MgLi hin ilm mea-
su emen s .................................... 20
2.4.1 Weigh -loss measu emen s . . . . . . . . . . . . . . . . . . . . . . . 20
2.4.2 Hyd ogen e olu ion . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
2.4.3 Po en iodynamic pola isa ion . . . . . . . . . . . . . . . . . . . . . 22
2.4.4 In-lineICP-MS ............................. 24
2.4.5 Fu he measu emen s . . . . . . . . . . . . . . . . . . . . . . . . . 27
3 Resul s 28
i

Con en s
3.1 Publica ion: S uc u al cha ac e isa ion and deg ada ion o Mg-Li hin
ilms o biodeg adable implan s . . . . . . . . . . . . . . . . . . . . . . . . 28
3.2 In luence o spu e ing pa ame e s on he co osion a e o Mg-3Li . . . . . 41
3.3 Analysis o long- e m co osion in di e en solu ions . . . . . . . . . . . . . 44
3.4 Publica ion: In es iga ion o in-si u ion elease and su ace ilm o ma ion
o hcpMg-Li hin ilms............................. 48
3.5 Publica ion: Tailo ing o Mg and MgLi hin- ilm co osion a es wi h di-
elec ic ba ie discha ge plasma ea men . . . . . . . . . . . . . . . . . . 59
3.6 Addi ion o an alloying elemen - MgAgLi . . . . . . . . . . . . . . . . . . 69
4 Conclusion and Ou look 70
Appendix lxxii
A.1 Supplemen S uc u al cha ac e isa ion and deg ada ion o Mg-Li hin ilms
o biodeg adable implan s . . . . . . . . . . . . . . . . . . . . . . . . . . . lxxii
A.2 Supplemen In es iga ion o in-si u ion elease and su ace ilm o ma ion
o hcpMg-Li hin ilms.............................lxx i
A.3 Supplemen Tailo ing o Mg and MgLi hin- ilm co osion a e wi h dielec-
ic ba ie discha ge plasma ea men . . . . . . . . . . . . . . . . . . . . lxxx
Full Lis o Publica ions lxxxi
Con e ence P esen a ions lxxx
Symbols and Abb e ia ions lxxx i
Lis o Figu es lxxx iii
Lis o Tables lxxxix
Bibliog aphy xci
Acknowledgemen s c i
ii
Abs ac
Abs ac
Due o hei mechanical p ope ies, biocompa ibili y and deg adabili y, magnesium alloys
a e p omising ma e ials o a ious medical implan s, such as bone ixa ions o s en s.
Compa ed o o he ma e ials, nega i e long- e m e ec s o he implan s o a second ope -
a ion o emo e he implan can be a oided. Ano he ad an age o he deg ada ion o he
ma e ial is he po en ial he apeu ic e ec o he alloying elemen s i sel . In his wo k,
magnesium-li hium alloys a e conside ed as biodeg adable ese oi s o li hium which
can be used o neu ological ea men s. Fo his pu pose, small implan s wi h de ined
s uc u es need o be ab ica ed. This manusc ip desc ibes he equi emen s and p op-
e ies o he hin ilms p oduced by magne on spu e ing. Pa icula a en ion is paid
o he co osion p ope ies, as hese de e mine no only he li espan o he implan s bu
also he concen a ion o eleased ions. Di e en possibili ies o ailo he p ope ies o
he ilms a e also discussed.
In a i s s udy, alloys wi h ou di e en li hium ac ions we e compa ed in e ms o hei
s uc u al and mechanical p ope ies and co osion a e. A subdi ision o he p ope ies
acco ding o he phases p esen in he ilm was ound. While he single-phase alloys ex-
hibi ed a co osion a e simila o ha o unalloyed magnesium ilms, which only sligh ly
inc eased wi h li hium con en , seconda y phases some imes esul ed in s ongly inc eased
co osion a es due o gal anic co osion in he absence o passi a ing su ace ilms. How-
e e , he co osion a e could also be con olled by o he ac o s, such as o ien a ion o
g ain bounda y modi ica ion by changing he spu e ing pa ame e s du ing he manu ac-
u ing p ocess.
The elease o li hium du ing deg ada ion de e mines he e ec i eness o he implan s as
a ese oi o neu ological ea men s. I has, he e o e, been s udied o e longe pe iods
as well as a he onse o co osion. The onse o con ac wi h he solu ion is o pa icula
in e es as he su ace ilms and su ace p ocesses signi ican ly in luence co osion, espe-
cially o hin ilms. A li hium ca bona e ilm, which o ms in ai and dissol es du ing
imme sion, was iden i ied on he s udied hin ilms. Subsequen selec i e dissolu ion o
he li hium leads o a change in concen a ion h oughou he measu emen .
Since he bulk s uc u e o he ilm and he su ace a ec he co osion esis ance, bo h
can be al e ed o adjus he p ope ies. A plasma ea men wi h a dielec ic ba ie
discha ge plasma esul ed in a change in su ace chemis y and mo phology. The modi-
ied su ace showed imp o ed co osion esis ance a e only a ew seconds o ea men .
The addi ion o sil e o he alloy inc eased he ensile s eng h while no signi ican ly
a ec ing co osion a e.
This s udy, hus, p o ides a i s insigh in o magnesium-li hium hin ilms as neu ological
implan s and di e en p ocesses and op ions o conside o op imising he ilms o he
applica ion.
1
Fundamen als
(compa ison: a= 3.2076 ˚
A, c/a = 1.6227 o 0.65 w % s. a= 3.193 ˚
A and c/a = 1.6068
o 5.24 w % Li) [42].
The change o hcp o a mo e cubic-like s uc u e o he p esence o bcc leads o he a ail-
abili y o mo e slip sys ems o mechanical de o ma ion. Wi h a c/a a io close o 1,
he p isma ic and py amidal slip a e mo e p onounced, leading o a highe duc ili y and
maximum elonga ion [77, 78]. The elonga ion a ies depending on he Li con en . Zhou
e al. ound, e.g., an inc ease om a ound 15 % o 41 % om Mg3.5Li o Mg8.5Li [79].
On he o he hand, while low amoun s o Li can inc ease he s ain ha dening and yield
s eng h, u he Li addi ion and addi ional slip lowe s he possibili y o s ain ha dening
and educes he ensile s eng h [78]. Fu he mo e, he plas ic aniso opy is educed, and
in compa ison o pu e Mg, he g ain g ow h du ing ec ys allisa ion is inc eased [77]. β-
MgLi shows an especially high o mabili y, duc ili y and sligh ly educed Young’s modulus
bu also lowe s eng h and low he mal and c eep esis ance [72, 80].
Figu e 2.2: Phase diag am o Mg-Li. Rep oduced and adap ed wi h pe mission om Sp inge
Na u e [76].
While Li could be equally dis ibu ed in he single phase ma e ial o g ains om each
phase in he mixed ma e ial, a seg ega ion a he g ain bounda ies is ound e en o low
Li ac ions [81]. This can ei he lead o an enhanced emb i lemen o imp o ed g ain
bounda y sliding. Fo mixed-phase ma e ials a e hea ea men and mechanical ea -
men , g ain bounda y sliding was ound o lead o supe plas ici y [72]. The Li seg ega ion
a g ain bounda ies possibly in luences he di usion o Li, which is in gene al lowe o
α-phase han β-phase [82, 83], leading o an easie anspo o Li o he su ace o e g ain
bounda ies also o he hcp phase. The accumula ion o Li could also a ec he co osion
p ope ies o he ma e ial which will be desc ibed la e .
In addi ion o he phases, he p ope ies o MgLi a e also in luenced by mic os uc u e.
Di e en pos - ea men s such as hea ea men s o mechanical ea men s, e.g. olling,
change g ain sizes and ex u e [40, 72, 84]. This can lead o ha dening o he ma e ial.
Especially o nanos uc u ed Li- ich ma e ials, an imp o ed s eng h is ound [40]. O he
elemen s such as Zn, Al o REE a e o en added o imp o ed p ope ies [40, 65, 66].
Wo king wi h MgLi also has addi ional challenges due o he high eac i i y o Li. This
leads o high oxida ion [72] and complica es he p ocessing. Especially o β-MgLi, ad-
8

Fundamen als
di ional ageing e ec s esul in he o ma ion o p ecipi a es and ec ys allisa ion o a
pa ial α-phase e en a oom empe a u e [85, 86]. This can lead o age so ening and
de ec o ma ion a in e aces be ween bo h phases. The e o e, he p ope ies also depend
on he ime be ween sample p epa a ion and expe imen .
2.2 Spu e ed hin ilms
The de ini ion o hin ilms di e s in he li e a u e. While some desc ibe ha a hin ilm
is hinne han 1 µm [87, 88], i can also include s uc u es in he µm- ange o e en 100s
o µm i he ma e ial is s ill small in compa ison o he subs a e [89]. Thin ilms can be
ab ica ed by di e en echniques using solid, liquid o gaseous deposi ion [90]. The mos
commonly used echniques a e chemical (CVD) o physical (PVD) apou deposi ion. In
PVD, he ma e ial o choice is b ough in o he gas phase by physical means such as
hea o ion bomba dmen . The a oms will hen deposi and o m a laye on he chosen
subs a e wi hou any addi ional chemical eac ions aking place.
Spu e ing as one o he PVD echniques is ca ied ou in a acuum chambe which is
looded wi h an ine gas such as a gon o non- eac i e spu e ing [91]. An applied ol age
s a s a plasma be ween he a ge ma e ial and he subs a e. Due o he bomba dmen
o he a ge by compounds om he plasma, a oms o he a ge will be ejec ed and
deposi ed on he subs a e. Fo a highe spu e ing a e and lowe necessa y ol age
and p essu e, pe manen magne s can be placed behind he a ge s o he so-called
magne on spu e ing [90, 92]. This leads o a longe , spi al pa h in he plasma and, hus,
u he ionising by in e ac ion o gas a oms. The e o e, he amoun o ions hi ing he
a ge is inc eased, leading o a highe spu e ing a e. A side e ec is he o ma ion o
speci ic spu e ing enches due o he small a ea o a ack. The composi ion o ma e ials
including mul iple elemen s can di e om a ge o deposi ed ma e ial since, depending
on he a omic numbe o he elemen s, he a oms ha e a di e en angula dis ibu ion.
2.2.1 Mic os uc u e o hin ilms
Du ing spu e ing, pa ame e s o he p ocess, such as he applied powe , p essu e o gas
and gas low, can be adjus ed. This can hen inc ease o dec ease he ene gy o he a oms,
he e o e in luencing he mic os uc u e o med du ing deposi ion.
The de elopmen o mic os uc u e dependen on he empe a u e o he subs a e (Ts)
and mel ing empe a u e (Tm) as Ts/Tmwas desc ibed by Mo chan and Demchishin in
1969 [93], indica ing di e en zones 1-3 depending on he subs a e empe a u e since i
in luences he ene gy a ailable o ac i a ion and, hus, di usion.
Tho n on hen added he dependence on he p essu e and de eloped he s uc u e-zone
model shown in igu e 2.3 [94, 95]. The model is mainly desc ibed by zone 1-3 and a
ansi ion zone T be ween 1 and 2 [94, 95, 97].
•In zone 1 wi h he lowes Ts/Tm, he low di usion leads o a high de ec densi y,
shadowing and open g ain bounda ies. The diame e o he g ains inc eases wi h
9
Fundamen als
Figu e 2.3: S uc u e-zone model de eloped by Tho n on showing he mic os uc u e o spu -
e ed ilms dependen on he empe a u e (subs a e empe a u e Tsand mel ing empe a u e
Tm) and he ine gas p essu e. Rep oduced wi h pe mission om Else ie [96].
empe a u e.
•In he ansi ion zone T, su ace di usion is al eady possible, esul ing in a dense
s uc u e wi h he o ma ion o less de ined g ains o ib ous s uc u es.
•Fo zone 2, a smoo h su ace and dense g ain bounda ies be ween columna g ains
a e eached, leading o be e mechanical p ope ies.
•In zone 3, olume di usion and ec ys allisa ion can occu , o ming ound g ains
and mic os uc u es simila o bulk ma e ial.
•On he o he axis, he inc ease in p essu e shi s he ans o ma ion empe a u es
o highe alues due o a loss in kine ic ene gy.
The e ised s uc u e-zone model by Messie e al. exchanged he p essu e wi h a sub-
s a e loa ing po en ial and claims he T zone as a subzone o zone 1 [98]. Ba na and
Adamik also added he impo ance o impu i ies on he inal s uc u e [99], and Ande s
adjus ed he model by swi ching o a gene alised empe a u e and no malised ene gy lux
o accoun o o he deposi ion echniques [100]. Thus, he di e en s uc u e-zone mod-
els can gi e an insigh in o he app oxima e s uc u e o hin ilms bu need o be de ined
and adap ed o he sys em o in e es [97].
In compa ison o bulk ma e ials, ewe p ecipi a es a e ound in spu e ed ilms. This is
due o he as cooling a e du ing deposi ion, leading o non-equilib ium supe sa u a ed
solu ions o ma e ials which can possess e y di e en mel ing poin s [32].
In addi ion o he mic os uc u e desc ibed in he s uc u e-zone model, changing he
spu e ing pa ame e s also in luences he ilm s ess, speci ically, he in insic ilm s ess
de ined by he s uc u al diso de [96]. Fo he ab ica ion o s able, possibly ees and-
ing hin ilms, an op imisa ion o low ilm s ess is necessa y o p e en such ilms om
de o ma ion a e li -o om he subs a e.
10
Fundamen als
2.2.2 Mg alloy hin ilms
Fo medical applica ions, Mg alloy hin ilms a e o in e es in di e en o ms such as
coa ings o ees anding ilms o , e.g., s en s o sca olds [26, 61]. Fab ica ion by spu -
e ing leads o he ad an age o an easy change in s uc u e (amo phous o c ys alline),
adjus men o hickness, p oduc ion o supe sa u a ed single-phase ma e ials and s uc-
u ing by o he hin- ilm echniques [27, 61].
Spu e ed Mg alloy hin ilms show a s ong p e e ed o ien a ion and, hus, highe ex-
u e han bulk ma e ial. This is in luenced by a p e e ed g ow h in [0001] di ec ion due
o he lowes su ace ene gy o he (0001) planes [28], leading o MgAg and WE alloys
o a columna g ow h wi h g ain sizes in µm ange and a g ain leng h equal o he ilm
hickness [28, 29].
The in luence o di e en alloying elemen s such as Ag, Zn, Ca and REE on he ilm
p ope ies ha e been es ed. While o MgAg ypically only 2 w % Ag a e soluble in Mg,
spu e ing can o m me as able solid solu ions up o 6 w % be o e p ecipi a es occu [28].
By adjus ing spu e ing pa ame e s, e en me allic glasses such as Mg50Zn5Ca, which hus
ha e a high solubili y and no p ecipi a es, can be ab ica ed [30]. The pa ame e s need
o be chosen as low powe and high p essu e o educe di usion.
Fo many alloying elemen s such as Ag, Y and Gd, an inc ease in la ice pa ame e wi h
addi ion o he alloying elemen is ound [27, 31]. The su ace oughness is also educed
o spu e ed AZ alloys in compa ison o spu e ed Mg [101].
The ex u e and addi ion o elemen s in luence he mechanical p ope ies due o solid
solu ion s eng hening, high de ec densi ies and small g ain diame e s [28, 31]. A highe
yield s eng h is de e mined o MgAg, MgY and MgGd when including mo e o he alloy-
ing elemen , while he Young’s modulus s ays simila [28, 31]. The maximum elonga ion
is o en dec eased a he same ime [28]. The in luence o spu e ing pa ame e s o he
di e en alloys was analysed. S udies claim an in luence o he spu e p essu e on he
maximum elonga ion due o lowe di usion and inc easing ine gas a oms addi ion o
he ilms o highe p essu e [29]. A change o p essu e and powe can change be ween
b i le and duc ile ilms, e en leading o amo phous ma e ials [29, 30, 102].
While he adap ing o pa ame e s o changing he mic os uc u e is dependen on he
speci ic alloys, a gene al ou e o he s uc u ing and ab ica ion o ees anding hin ilms
in a hickness ange o 10-250 µm was de eloped by Ha ne e al. [26] and is desc ibed
as pe o med in he s udies o his wo k. The ou line is shown in igu e 2.4. I includes
UV-li hog aphy, magne on spu e ing and e ching s eps.
In he i s s ep, a silicon wa e is coa ed by spin coa ing wi h an image e e sal esis
(a). A e ans e ing a mask s uc u e on o he wa e by illumina ion wi h ul a iole
ligh and emo ing he non-illumina ed pho o esis (b), a laye o Al wi h a hickness o
app oxima ely 150 nm is added by magne on spu e ing (c). The esis is hen dissol ed
in ace one so ha only he s uc u e emains as an Al ha d mask (d).
A Bosch p ocess is hen ca ied ou by ICP-RIE (induc i ely coupled plasma - eac i e
ion e ching) o e ch up o a dep h o a leas 20 µm o he ilms p oduced in he ollowing
s udies, o en u he o up o 60 µm (e). A e cleaning in ace one and isop opanol, AlN
is added wi h a hickness o 500 nm by eac i e magne on spu e ing using an Al a ge
and ni ogen gas ( ) be o e he inal spu e ing o Mg alloy (g). Du ing he li -o in
20 w % KOH, he sac i icial AlN laye is dissol ed (h). The ees anding hin ilms can
hen be cleaned in dis illed wa e and isop opanol be o e d ying hem wi h ni ogen.
11
Fundamen als
Figu e 2.4: Fab ica ion s eps o ees anding Mg alloy hin ilms using UV-li hog aphy, mag-
ne on spu e ing and sac i icial laye s as pe o med o his wo k, based on he p ocess o
Ha ne e al. [26].
2.3 Co osion
Co osion can occu by chemical o elec ochemical means o wi h he addi ional in luence
o biological ma e [103 a, 46 b]. I is, in gene al, de ined as he deg ading o des uc ion
o a ma e ial due o eac ions wi h he en i onmen . While co osion was de ined in he
pas mainly o me als, i can now include he deg ada ion o all ma e ials [46 b]. The
ype o co osion desc ibed in he s udies o his hesis is elec ochemical, mo e speci i-
cally, me al co osion and oxida ion in an aqueous solu ion. The o e all eac ion includes
an anode, a ca hode, an elec oly e and a pa h o he elec ons o ans e be ween he
elec odes, o ming a co osion cell. O he ac o s, such as empe a u e, pH, and sal in
he solu ion, can u he in luence he eac ions ha ake place. [104 a]
While he anodic eac ion includes he co osion and dissolu ion o he sample by oxi-
da ion, he ca hodic educ ion eac ions can include di e en eac ions depending on he
eac ion medium and elec ode po en ial. The p obabili y o s eng h o co osion is de-
sc ibed by he elec ode po en ial, which is always de ined as he po en ial compa ed o
ano he elec ode. Me als wi h a lowe elec ode po en ial a e mo e p one o co osion
and will ac as anodes i hey a e in con ac wi h me als wi h a highe po en ial. [103 b]
While co osion in aqueous solu ion, in gene al, is de ined by he ion elease, his does
no ha e o be a con inuous elease o e he su ace. The di e en ypes o co osion a e
spli in o homogeneous and local co osion, which includes, bu is no limi ed o, gal anic,
dealloying, in e g anula , ili o m, pi ing and c e ice co osion [103 c, 46 c, 104 b]:
•Fo homogeneous o uni o m co osion, he eac ion akes place on he o e all
su ace. I o en occu s i no s able passi a ing laye is o med. While measu emen s
and calcula ions o co osion a es only assume uni o m co osion, o he co osion
ypes also ake place and need o be conside ed.
•In gal anic o mic ogal anic co osion, wo me als o me al phases a e in con ac
12
Fundamen als
wi h each o he , leading o as e co osion o he compound wi h he lowe elec ode
po en ial.
•Simila o gal anic co osion, dealloying co osion, also called selec i e co o-
sion, desc ibes he dissolu ion p ocess o one speci ic alloying elemen due o he
lowe po en ial.
•I a di e ence in po en ial occu s a g ain bounda ies due o, e.g., p e e ed accumu-
la ion o one compound a bounda ies, his could lead o in e g anula co osion.
•When co osion p oduc s a e o med du ing co osion, hey can o m ilamen s on
he su ace, which s a om su ace de ec s and lead o an ac i e anodic head o he
ilamen and a g owing ca hodic a ea o co osion p oduc s. This p ocess is called
ili o m co osion. I o en occu s i he co oding ma e ial is coa ed.
•Pi ing co osion occu s i he ma e ial i sel is no homogeneous o he passi a ion
laye o ms c acks o o he de ec s, leading o a p e e ed a ack and o ma ion o
pi s a hose places. I can also be in luenced by highly eac i e anions in solu ion.
•C e ice co osion akes place i ano he objec limi s he olume abo e an a ea
o he sample and less exchange o solu ion occu s, leading o localised co osion.
2.3.1 Mg co osion
Mg has a e y low elec ode po en ial o -2.37 V s SHE [46 a]. The edox eac ion consis s
o he anodic Mg ion elease and ca hodic hyd ogen e olu ion. Mg ions and hyd ogen can
hen u he eac o Mg hyd oxide. [105]
Oxida ion: Mg −−→ Mg2+ + 2 e–
Reduc ion: 2 H2O + 2 e–−−→ 2 H2+ 2 OH–
Redox: Mg + 2 H2O−−→ 2 H2+ Mg(OH)2
Since Mg o en eac s o MgO and o ms an oxide laye in ai , he eac ion o Mg(OH)2
can also be ew i en as
2 Mg + O2−−→ 2 MgO
MgO + H2O−−→ Mg(OH)2
The change and eac ion o he su ace laye a e in luenced by he s abili y o such laye s
on he su ace and solubili y in wa e . The s abili y is ypically desc ibed by he Pilling-
Bedwo h a io (PBR) [106, 107]. The a io de ines he s ess o a su ace laye on he
subs a e by calcula ing he a io o he co osion p oduc ’s mola olume o he o iginal
me al’s mola olume. A alue o 1, he e o e, de ines a s ess- ee ilm, and any alue
o PBR=1-2 is de ined as ela i ely s able. While <1 means ha ensile s ess occu s, a
13

Fundamen als
PBR>1 leads o comp essi e s ess. Fo MgO, he PBR is <1 wi h 0.8, bu o Mg(OH)2,
a alue o 1.8 and, he e o e, a mo e s able ilm is eached [108].
The s abili y o su ace laye s is, howe e , also dependen on an applied po en ial and he
pH o he aqueous solu ion. Those pa ame e s and he esponding p oduc s a e gi en in
Pou baix diag ams, showing di e en a eas o eac ions [109]. The eac ion and o ma ion
o Mg(OH)2is jus s able o a highe , alkaline pH [109] and, hus, he pH in luences he
co osion by educing he co osion a e o a highe pH [110, 111].
Figu e 2.5: Co osion a e dependen on he pH in Hank’s balanced sal solu ion
(bu e ed) o h ee exempla y Mg ma e ials (high pu i y Mg HP Mg, AZ91, ZE41 Zn+REE).
Rep oduced and adap ed wi h pe mission om Else ie [111].
A a pH o 7.4, which co esponds o he pH in a human body, nei he Mg(OH)2no
MgO a e s ably o med and he e o e do no hinde u he eac ion and dissolu ion [112].
Howe e , Mg(OH)2can s ill o m a lowe pH due o an inc ease o pH close o he sample
by he eac ions aking place [32]. Since he co osion a e educ ion due o he hyd oxide
laye is highe o a highe pH, he open ci cui po en ial (po en ial o he sample s.
e e ence in a ce ain sys em i no ol age is applied, EOCV) can also shi pH dependen ly
[113, 114].
An addi ional poin o conside when desc ibing he co osion o Mg is ha he ca hodic
and anodic eac ions canno be easily sepa a ed. Fo a s anda d me al, he ca hodic
eac ion - in his case, hyd ogen e olu ion - akes place i a nega i e po en ial is applied,
while a posi i e po en ial leads o mo e anodic eac ion, hus, me al dissolu ion and a
dec ease in he ca hodic eac ion a e. Howe e , o Mg alloys, an enhanced ol age
also leads, in many cases, o an inc ease in hyd ogen e olu ion and a as Mg dissolu ion,
which al eady s a s in he ca hodic egion. This is called nega i e di e ence e ec (NDE)
[105, 115, 116]. Ce ain heo ies, such as he p esence o Mg+ions o he in luence
o he p o ec i e ilm as ca hodic egions o hyd ogen e olu ion, ha e been p oposed.
Bo h hypo heses we e con adic ed by o he s udies [34, 115, 117–119]; hus, no inal
explana ion has been de e mined.
2.3.2 In luencing ac o s
The co osion p ocess and a e o Mg a e e y complex and in luenced by se e al ac o s,
such as he s uc u e, including mic os uc u e and ex u e, he solu ion in which co osion
14
Fundamen als
akes place and addi ional elemen s. The impo ance o hese ac o s will be discussed in
he ollowing sec ions.
In luence o s uc u e
The s uc u e o he Mg samples can highly in luence he co osion a e. This is o en used
o lowe he a e by he mal o mechanical ea men s [120]. He eby, wo main ac o s a e
changed: g ain size and o ien a ion. The in luence o g ain size is widely s udied. G ain
bounda ies, in gene al, coun as de ec s, inc ease he a ea a ailable o co osion s a
and, he e o e, could inc ease he co osion a e, especially i a change o composi ion
occu s be ween g ain olume and bounda y [121, 122]. Howe e , i is o en ound ha
small g ains lead o imp o ed co osion esis ance due o he o ma ion o a mo e uni o m
passi a ing ilm [121–123], smalle p ecipi a es [32] o a change in po en ial a he g ain
bounda ies [124]. The o ma ion o a be e passi a ing ilm also explains why sho - e m
measu emen s migh show highe co osion a es o small g ains since he ilm has no
been s ably o med.
The o ien a ion o g ains and, hus, he ex u e also changes he co osion a e. E en
hough all g ains consis o he same phase, each plane can show a sligh ly di e en co -
osion a e, mainly in luenced by he s eng h o bonds be ween he a oms. The e o e,
heo e ically, he denses plane should possess he bes co osion esis ance, which is he
(0001) plane in he case o Mg [125, 126]. Howe e , planes wi h he lowes ac i i y o
eac ions also ha e he lowes ac i i y owa ds, e.g., oxida ion, he e o e o ming hinne
oxide o p o ec i e laye s [127–129]. A uni o m ex u e migh also be c i ical o low co -
osion addi ional o he speci ic plane since i p e en s gal anic coupling [130]. The e o e,
he p e e ence o a s uc u e o good co osion esis ance has o be de e mined o each
pa icula sys em o sample alloy and su ounding.
In luence o solu ion
As al eady desc ibed o he pH in sec ion 2.3.1, he solu ion in which co osion akes
place s ongly a ec s he co osion a e. This is e en mo e ue i addi ional ions a e
in ol ed [131]. Kwon e al. showed a s ong di e ence in he co osion beha iou o a Mg
alloy i di e en ions we e p esen in he solu ion e en hough he pH was held cons an
[132]. Sil a e al. ound ha he oxygen con en can also in luence he co osion [133].
When wo king unde physiological condi ions, ei he jus in a sal solu ion o in i o
o in i o, he sys em is complex. While added o ganic componen s such as molecules
and cells can also in luence [134], he expe imen s in hese s udies a e ca ied ou in sal
solu ion; hus, his e ec is mainly desc ibed in he ollowing.
In sal solu ions such as Hank’s balanced sal solu ion (HBSS), hyd oca bona es, ca bon-
a es, and ca bon dioxide a e o en p esen as bu e ing sys ems o om ai , leading o he
o ma ion o MgCO3o mo e complex ca bona es [135]. The o ma ion o ca bona es,
in gene al, was ound by San ucci e al. [112] o be mo e s able wi h inc easing pH and
inc easing concen a ion, while MgCO3speci ically is s able in a b oad pH ange. Addi-
ional sal s, including elemen s such as Ca, can lead o he o ma ion o u he ca bona es
o phospha es. Ca phospha e is especially known o o m a s able p oduc which educes
he co osion a e [131, 136] ( able 2.1). I also has a PBR o 1.27 and, hus, o ms a
15
Fundamen als
su ace ilm wi h low ilm s ess [108].
A complex sys em including cells o in i o measu emen s can in luence he co osion
a e [134, 137], bu he in luence and p ocess o co osion p oduc o ma ion a e s ill
impo an . Zhao e al. ound, e.g., Mg ca bona es and phospha es on Mg alloys a e
implan a ion in o a mouse model, indica ing ha he e ec o sal s in solu ion is also o
in e es o he inal applica ion [138].
In luence o pu i y
When discussing he in luence o addi ional elemen s on co osion, wo possibili ies ha e
o be conside ed - impu i ies and alloying elemen s.
Impu i ies a e, in gene al, unwan ed addi ions o elemen s. When compa ing ul a-pu e
Mg wi h high-pu i y Mg, he co osion a e o ul a-pu e Mg is signi ican ly lowe [139].
The impu i ies a e p oblema ic i hey ha e low solubili y and, hus, o m addi ional
phases wi h highe elec ode po en ial which can lead o mic ogal anic co osion. Since
nea ly all elemen s ha e a highe elec ode po en ial han Mg [46 a], his esul s in as e
Mg dissolu ion. One impu i y which is o en included is i on. I he concen a ion exceeds
0.005-0.017 % [140, 141], i can inc ease he co osion a e. Techniques such as spu e ing
migh p e en he o ma ion o addi ional phases and, he e o e, educe such e ec s [27].
Elemen s can also be added on pu pose as alloying elemen s o in luence he p ope ies.
This can include changing he mic os uc u e o su ace p ope ies o educe he co osion
a e, a leas in compa ison o Mg wi h low pu i y [120]. REE ha e a simila elec ode
po en ial o Mg, and he po en ial o Ca is e en lowe , leading o a lowe o e all po en ial
o he in e me allic phase [142]. A dec ease in size o he second phase o con inuous
dis ibu ion p e en ing he p opaga ion o pi s can, e.g., educe he co osion a e [108,
143–146]. Since Mg alloys can o m passi a ing su ace laye s, spu e ed Mg alloys ha e
also been discussed as sel -healing laye s, which can p e en gal anic co osion [120].
2.3.3 Mg hin ilm co osion
The s uc u e o hin ilms p oduced by spu e ing esul s in wo main ac o s which can
educe he co osion, namely a s ong ex u e wi h basal (0001) planes on he su ace and
a supe sa u a ed solid solu ion p e en ing he o ma ion o addi ional phases up o highe
ac ions o he alloying elemen [27, 31, 126, 147].
The e o e, less pi ing co osion and o he local co osion is ound o spu e ed samples
since less gal anic coupling can occu wi h p ecipi a es [32]. Blawe e al. also discussed
he inco po a ion o alloying elemen s o hin ilms in o he passi a ing laye s, leading o a
be e co osion esis ance i dis ibu ed homogeneously, which can u he be in luenced
by changes in mic os uc u e [101].
Fo WE alloys, no e ec o he alloying elemen s on he co osion a e occu s since no p e-
cipi a es a e ound [102]. E en an inc ease o he alloying elemen does no lead o changes
as s udied o up o a ound 23 a % o Y o 3.6 a % o Gd [31]. Fo MgAg, he addi ion o
he elemen e en dec eases he co osion a e by inc easing he s anda d po en ial when
o ming a solid solu ion [27]. A educ ion is also measu ed o amo phous MgZnCa, o
which he co osion a e is possibly educed by educing he hyd ogen e olu ion ia Zn
16
Fundamen als
PBR [108] Ksp[149]
Li2CO31.35 8.15·10−4
Mg(OH)21.80 5.61·10−12
MgO 0.80 -
LiOH 1.26 -
MgCO32.04 6.82·10−6
Mg3(PO4)22.29 1.04·10−24
Li3(PO4)21.20 2.37·10−11
Ca3(PO4)21.27 2.07·10−33
Table 2.1: Pilling-Bedwo h a ios (PBR) and solubili y p oduc cons an s (Ksp a 25 ◦C o
possible co osion p oduc s o MgLi in sal solu ion.
inco po a ion [30]. Fo his ma e ial, an in luence o he spu e ing pa ame e s on he
co osion by changing om polyc ys alline o amo phous ma e ial is desc ibed.
2.3.4 MgLi co osion
The elec ode po en ial o Li is e en lowe han o Mg wi h -3.05 V s SHE [46 a]. Thus,
Li enhances he eac i i y and he co osion a e. I is ound o inc ease he hyd ogen
e olu ion by inc easing he ca hodic kine ics [35, 148]. While Li is s udied o educe
he co osion esis ance sho e m, s udies also show enhanced esis ance o e longe
measu emen imes [33]. This migh be in luenced by a s onge inc ease in pH du ing
he beginning, which s abilises Mg(OH)2on he su ace a e wa ds [73].
Since he in luence o co osion p oduc s on he co osion esis ance is in luenced by he
s abili y and solubili y, PBR and solubili y p oduc cons an s o possible componen s
o med by MgLi in sal solu ions a e lis ed in able 2.1. The e ec o Li on he co osion
is highly in luenced by he exac amoun o Li. The main change depends on he MgLi
phase. In gene al, he co osion esis ance is o en desc ibed o be he highes o β-MgLi,
ollowed by α-MgLi and hen α+β-MgLi [150].
The co osion a es and in luences o each phase a e desc ibed in he ollowing sec ions.
α-phase
Fo MgLi wi h a hcp s uc u e, he co osion is gene ally assumed o be simila o he
co osion o pu e Mg [151]. Thus, he o ma ion o MgO and Mg(OH)2will occu [152].
Since Li has a lowe elec ode po en ial han Mg, he co osion po en ial o he alloy
dec eases wi h inc easing Li con en [153]. Du ing he co osion, anodic and ca hodic e-
gions de elop on he su ace. Hyd ogen and co osion p oduc s a e o med in he ca hodic
egions and he co osion p og esses as ili o m co osion [150, 154]. While Li can lowe
he o e all elec ode po en ial, Li e al. ound o α-MgLi wi h di e en Li ac ions he
bes co osion esis ance o he highes Li ac ion due o he in luence on mic os uc u e
and su ace ilms [153].
In addi ion o he co osion p oduc s on Mg, Li-con aining componen s a e ound. Fo
MgLi-based alloys such as, e.g., LA51 (Mg-5Li-1Al in w %), Li2CO3and LiOH a e o med
in ai on he su ace [155].
17
Fundamen als
co osion. The measu emen gene ally does no show co osion in a non-dis u bed sys em
and a ies signi ican ly om he co osion a e o weigh loss o hyd ogen e olu ion [180].
One p oblem is also he assump ions o he Ta el ex apola ion, which only desc ibes
one eac ion occu ing o each b anch. In p axis, his does no hold ue, especially
conside ing he NDE, which, oge he wi h he o ma ion o oxides and hyd oxides, o en
p e en s a c ossing o bo h linea ex apola ions a EOCV [118, 180–182]. The hyd ogen
e olu ion and o ma ion o bubbles on he sample su ace can u he in luence he esul s
[183], especially conside ing small measu emen a eas as o en used o he hin ilm
samples.
Howe e , an ex apola ion om he ca hodic b anch can be used o es ima e a co osion
a e and compa e di e en samples placed in he same se -up [180, 184].
2.4.4 In-line ICP-MS
Du ing he co osion in solu ion, he sample i sel ge s dissol ed. The e o e, addi ional
in o ma ion ega ding he co osion a e and, e en mo e impo an ly, abou he speci ic
eleased elemen can be de e mined by analysing he dissolu ion p oduc in he elec-
oly e.
Samples o solu ion wi h dissol ed species can be analysed by echniques such as in-
duc i ely coupled plasma - mass spec ome y (ICP-MS) o induc i ely coupled plasma-
a omic emission spec ome y (ICP-AES, also called ICP-OES, OES = op ical emission
spec ome y) o de e mine, e.g., ca alys loading, s abili y o pa ially me allic ca alys s
o he concen a ion in solu ion a e a de ined du a ion o co osion [66, 79, 185–187].
Howe e , his only gi es he in eg al in o ma ion o e ime be ween measu emen s. Based
on p e ious measu emen echniques [188], an in-line echnique was p esen ed by Ogle and
Webe in 2000 [189]. The AESEC (a omic emission spec oelec ochemis y) combines
ICP-AES wi h an elec ochemical low cell ups eam in which he sample is added as he
wo king elec ode. Thus, he dissol ed elemen s can be di ec ly analysed downs eam and
du ing elec ochemical measu emen s. Simila se -ups wi h ICP-MS ha e been de eloped
[185, 190, 191], and bo h ypes a e used o he s udy o ime- esol ed co osion o ca al-
ysis [185, 188, 192]. Bo h will be discussed; howe e , he main ocus is on he echnique
used o s udies in his hesis, hus, a combina ion o he low cell wi h ICP-MS.
In bo h cases, he sys em consis s o he low cell - a cell wi h de ined sample su ace in
con ac wi h solu ion and de ined olume o e he sample h ough which he elec oly e
is pumped wi h a de ined speed -, he spec ome e and he da a acquisi ion sys em.
Mul iple impo an pa ame e s ha e o be aken in o accoun du ing he measu emen .
While, e.g., he low a e can be de e mined o compa e he ion elease o di e en samples
measu ed wi h sligh ly di e ing a es, his a e will also di ec ly in luence he co osion
p ocess by, e.g., hinde ing p ecipi a ion, changing passi a ion laye s o pH changes [188,
193–195]. The low cell includes a h ee-elec ode se -up wi h a coun e elec ode and
e e ence elec ode. In igu e 2.9, he se -up o he low cell [196, 197] desc ibed in his
wo k is shown. In he ICP-MS sys em, he elec oly e is con e ed in o an ae osol in
he nebulise by A low and hen anspo ed in o he sp ay chambe , om which only
a ew d ople s will be ans e ed in o an A plasma o b ing he elemen s in o he gas
phase and ionise hem. A e wa ds, he ions a e passing in o a acuum chambe o mass
de e mina ion. He e, a i s , any emaining neu al pa icles a e sepa a ed om he ions,
and he beam is ocused be o e i eaches a collision/ eac ion cell. In his cell, any ions
24

Fundamen als
Figu e 2.9: Schema ic o low cell as desc ibed in [196, 197] and inline ICP-MS measu emen .
a) Flow cell as used o he measu emen o Mg alloy samples, including a P coun e elec ode
(CE), e e ence elec ode (RE) and he sample as a wo king elec ode (WE), connec ed by
coppe ape. b) C oss sec ion o low cell. c) O e all se -up wi h solu ion low h ough he low
cell o he ICP-MS.
including mul iple a oms a e spli , in he case o he machine used in his wo k (Agilen
7900 ICP-MS) by collision wi h He a oms. The quan i ica ion dependen on he mass
akes place using a quad upole mass spec ome e in high acuum o a oid ecombina-
ion. While ICP-MS allows he quan i ica ion o iso opes and de ec ion down o a limi o
pp (pa s pe illion), he elec oly e needs o be much mo e dilu ed han o ICP-AES.
[185, 188]
The dissolu ion a e jmis hen de ined by he concen a ion c( ) a e calib a ion, he
low a e o he elec oly e and he a ea o sample Ain con ac wi h said elec oly e
[185, 188]:
jm( ) = ·c( )/A (2.6)
To calcula e he dissolu ion cu en densi y i, he Fa aday cons an F, elec ons eleased
pe a om nand he mola mass Ma e aken in o accoun
i( ) = jm( )·n·F/M (2.7)
While his cu en densi y (i i is added up o all elemen s) can, in heo y, be compa ed
o he esul s o , e.g., po en iodynamic pola isa ion i a ol age sweep is pe o med in he
low cell (e.g. [182]), he in luence o side eac ions, signal b oadening and ans e ime
om low cell o ICP-MS need o be aken in o accoun [182, 188, 189].
By applying a po en ial o cu en , a change in composi ion du ing and a e passi a ion
egions can also be de e mined. The combina ion wi h he low cell and iden i ica ion
o each elemen indi idually, hus, makes he iden i ica ion o he s oichiome y o dis-
sol ed elemen s possible. The e o e, selec i e dissolu ion due o dealloying, p e e en ial
co osion, ilm o ma ion du ing co osion, o ca aly ic eac ions can be measu ed ime
esol ed. The combina ion wi h elec ochemis y also helps o ecei e in o ma ion no di-
ec ly a ailable om ICP-MS, such as he oxida ion s a e o he elemen s a e co osion
[185, 188].
Se e al s udies by bo h in-si u ICP-MS and ICP-AES ha e been ca ied ou o u he
unde s anding he co osion p ocess and p o ec i e laye s du ing co osion. The sys em
also allows o analyse he ion dissolu ion du ing o he elec ochemical measu emen s [198]
o eac ions unde addi ional in luences such as, e.g. ligh o sc a ching [185, 191, 196,
25
Fundamen als
199, 200].
The echniques we e used o se e al s udies on Mg and MgLi. Fo Mg co osion, s udies
by Thomas e al. in es iga ed he in luence o impu i ies o Fe, p o ing an inc ease in
ac i i y [201]. The in luence o a pH change and applied anodic po en ial was es ed by
Ross ucke e al. [110, 182], showing a lowe NDE o low pola isa ion and high pH.
Compa ed o ex-si u measu emen s, he dependence on ime and change a e applying
anodic pola isa ion can be di ec ly obse ed. Fo Mg co osion, ea lie s udies sugges he
possibili y o he o ma ion o Mg+addi ional o Mg2+ as a eason o he NDE occu ing
o Mg, howe e , analysis by ´
Swia owska e al. [117] and Lebouil e al. [115] showed only
Mg2+ and a he sugges ed he g ow h o ca hodes by co osion p oduc o ma ion i an
anodic po en ial is applied. This also leads o an imp o ed s abili y o he dissolu ion
a e anodic pola isa ion when he sample is held a EOCV. The o ma ion o hyd ogen
gas can be iden i ied as noise in he measu emen s [117].
Fo MgLi, speci ically wi h high Li con en (β-phase), he composi ion o a passi a ion
laye o med du ing co osion was analysed by de e mining he change in s oichiome y
when applying an inc easing ol age and sc a ching he su ace [162, 166, 200], showing,
e.g., a p e e ed Li dissolu ion a e he sc a ch in he s udy by Yan e al. [200]. Hou e
al. ound a g ea e Mg:Li a io in he dissol ed ions han in he ma e ial i sel and de-
e mined he p esence o Li2CO3by a change o a a io simila o bulk when he ol age
is inc eased o e a ce ain h eshold [166]. Howe e , Yan e al. [162] did no ind he
same di e ence in a io bu a he ound a Mg- ich laye de e mined by Mg- ich pa icles
eleased a e sc a ching.
Fo he s udies o hin ilms, he possibili y o gain insigh in o he s a o he co osion
in-si u is especially impo an since he su ace a ec s he o e all sample p ope ies sig-
ni ican ly due o he high su ace- o- olume a io. Addi ionally o hin ilms on subs a e,
ee-s anding hin ilms can also easily be placed in o he low cell, howe e , special ca e
has o be aken o p e en co osion mainly as c e ice co osion when he hin ilm is ben
a ound he O- ing [197].
Since he dissol ed ions a e di ec ly de ec ed, no e ec s such as he NDE can in luence
he assumed co osion a e compa ed o he ac ual ion elease. Since he de ec ion limi
is e y low, he e is also no e ec o concen a ions ha a e oo low due o he hin ilms,
especially since, in any case, he echnique analyses in sho ime ames, and hus, no
long- e m accumula ion o co osion p oduc s is necessa y. I also allows o he iden i-
ica ion o ligh elemen s such as Li, he e o e allowing he s udying o nea ly any Mg
alloys.
While he echnique can be used o longe in-si u expe imen s, he measu emen imes
o hin ilms a e limi ed o p e en c e ice o pi ing co osion h ough he whole ilm
o a oid con ac be ween he elec oly e and he subs a e ma e ial o sample holde
addi ional o he sample. Thus, no long- e m co osion p ope ies a e analysed. Since
he machine is e y sensi i e, he elec oly e needs o be adjus ed. While he co osion
a es we e mainly de e mined in HBSS (155 mmol) a a pH o 7.4±0.2 and a empe a u e
o a ound 37±1◦C (see 2.4.1, 2.4.3) o simula e physiological condi ions, o he in-si u
measu emen s, he concen a ion was adjus ed o 15 mmol HBSS du ing he expe imen s
o his wo k. The pH was close o 7.4 bu no empe a u e con ol was applied. This is,
howe e , possible by, e.g., a wa e ba h hea ing [188].
In-line ICP-MS migh no be necessa y o de e mine he co osion a e, which can be
done wi h less e o wi h o he echniques, bu i p o ides a u he unde s anding o
co osion p ocesses and elemen -dependen dissolu ion o MgLi hin ilms.
26
Fundamen als
2.4.5 Fu he measu emen s
While se e al echniques ha e been discussed o de e mine he co osion a e and analyse
he co osion p ocess, hose o en need o be combined wi h addi ional measu emen s
[105]. Those can be used o s udying he su ace p e- and pos -co osion o in es-
iga e a change in su ace composi ion by, e.g., o ma ion o co osion p oduc s (e.g.
SEM/EDX (scanning elec on mic oscopy/ene gy dispe si e X- ay spec oscopy), XRD
(X- ay di ac ion), XPS (X- ay pho oelec on spec oscopy)), o change in su ace mo -
phology (e.g. AFM (a omic o ce mic oscopy), SEM) o de ining he o e all change
du ing co osion. Addi ional measu emen s such as EIS (elec ochemical impedance spec-
oscopy) can gi e u he insigh in o he elec ochemical p ocesses aking place a di -
e en ime scales and help o iden i y he o ma ion o co osion laye s. Thus, since
he co osion o Mg and Mg alloys in aqueous solu ion and especially in mo e complex
elec oly es is a complex sys em, no single echnique can gi e all esul s necessa y o
unde s and he ongoing p ocess.
27
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To analyse he iabili y o MgLi hin ilms as a local d ug ese oi o he ea men o
neu ological diso de s, in he ollowing s udies and chap e s wi h addi ional expe imen al
esul s, he ab ica ion, s uc u al p ope ies and co osion a e o MgLi hin ilms wi h
a ying Li con en will be discussed. In addi ion o he gene al es o he p ope ies o
such alloys, he s udies op o u he unde s and he co osion, he in luence o pa ici-
pa ing pa s such as phases, Li con en , mic os uc u e, o ien a ions o su ace ilms on
he co osion a e and a mo e de ailed co osion p ocess and ion elease will be desc ibed.
This no only p o ides a mo e ho ough unde s anding o he unde lying mechanisms oc-
cu ing bu also allows he unde s anding o adjus men s o he co osion a e by changing
he spu e ing pa ame e s o applying addi ional pos - ea men s.
3.1 Publica ion: S uc u al cha ac e isa ion and deg a-
da ion o Mg-Li hin ilms o biodeg adable implan s
To de elop MgLi hin ilms o medical ea men s, ees anding ilms o he alloys ha e
o be p epa ed and s udied. In his wo k, hin ilms in αand α+β-phase we e ab ica ed
by magne on spu e ing wi h low ilm s ess. The mic os uc u e and phases we e anal-
ysed, showing no only β-phase o highe Li ac ions bu also addi ional Li2CO3and a
change in o ien a ion. In mechanical es s, a lowe ensile s eng h was ound o all MgLi
composi ions compa ed o pu e Mg, and he maximum elonga ion was educed excep o
samples wi h he lowes Li ac ion. While he co osion a e inc eased o an inc ease in
Li ac ion in he α-phase due o highe eac i i y and he in luence o o ien a ion changes,
a lowe co osion a e was ound a high Li ac ion in he mixed phase, indica ing a pos-
sible e ec o p o ec i e su ace ilms.
Own con ibu ions o he ollowing a icle
(concep - 50 %, planning - 75 %, expe imen s - 75 %, analysis - 90 %, w i ing - 90 %):
•Sample p epa a ion and p epa a ion op imisa ion
•XRD, SEM and EDX in es iga ion
•Tensile es ing, weigh loss and po en iodynamic pola isa ion measu emen s
•In e p e a ion and discussion o he esul s
•W i ing o he manusc ip
28
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The pape Hanke, L., Jessen, L.K., Weishei , F., Bha , K., Wes e ns ¨oe , U., Ga be-
Sch¨onbe g, D., Willumei -R¨ome , R., Quand , E. S uc u al cha ac e isa ion and deg a-
da ion o Mg–Li hin ilms o biodeg adable implan s. Scien i ic Repo s 13, 12572(2023)
[202] published by Sp inge Na u e is open access and he use is pe mi ed by he C e-
a i e Commons A ibu ion 4.0 In e na ional License (h p://c ea i ecommons.o g/
licenses/by/4.0/).
29

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S uc u al cha ac e isa ion
and deg ada ion o Mg–Li hin ilms
o biodeg adable implan s
Lisa Hanke
1, Lea K. Jessen
1, Felix Weishei
1, K a hika Bha
2, Ul ike Wes e ns öe
3,
Die e Ga be‑Schönbe g
3, Regine Willumei ‑Röme
2 & Eckha d Quand
1*
F ees anding hin ilms o Mg–Li (magnesium–li hium) alloys wi h a Li mass ac ion be ween 1.6%
(m/m) and 9.5% (m/m) we e p epa ed and s udied wi h espec o hei s uc u e and deg ada ion
p ope ies. Wi h inc easing Li con en , he mic os uc u e de ia es om hexagonal Mg–Li wi h
s ic columna g ow h and p e e ed o ien a ion, and addi ional cubic Mg–Li and Li2CO3 occu . The
co osion a e was measu ed in Hanks’ balanced sal solu ion by po en iodynamic pola isa ion and
weigh loss measu emen s o in es iga e biodeg ada ion. In luences o he o ien a ion, phase and
p o ec i e laye o ma ion lead o an inc ease in co osion om 1.6 o 5.5% (m/m) om 0.13 ± 0.03 o
0.67 ± 0.29 mm/yea when measu ed by po en iodynamic pola isa ion bu a simila co osion a e o
9.5% (m/m) and 3% (m/m) o Li o 0.27 ± 0.07 mm/yea and 0.26 ± 0.05 mm/yea .
Magnesium and i s alloys a e widely s udied as ma e ials o applica ions in he medical ield due o hei bio-
deg adabili y. Di e en elemen s such as, e.g., Ca, Zn o a e ea h elemen s (REE) a e included o imp o e
mechanical p ope ies o ailo he deg ada ion a e o adjus hem o applica ions as, e.g., s en s o bone
implan s1–4. Addi ional o he ad an age o ha ing an implan which deg ades a e i is no longe equi ed,
he possible he apeu ic e ec s o he implan s a e explo ed by, e.g., loading s en s wi h d ug-elu ing laye s5 o
using he co osion p ocess and changes in he en i onmen such as pH and hyd ogen e olu ion di ec ly o i s
an ibac e ial p ope ies6.
In line wi h he idea o use he implan i sel as ea men , an alloy including he he apeu ically ac i e elemen
li hium will be analysed in his s udy. Li hium is used in ea men s o mood diso de s, in pa icula bipola
diso de , and is also s udied o ha e e ec s on Alzheime ’s and Pa kinson’s disease7–10. Magnesium i sel shows
also neu ological e ec s11. The e o e, he deg ada ion and, hus, con inuous elease o bo h he magnesium
and addi ional elemen s would allow a local ea men in he b ain. I a con olled and local elease is achie ed
by unde s anding he deg ada ion o he ma e ial, side e ec s which can occu du ing he ea men wi h Li12
could be educed.
Fo Mg–Li, he s uc u e in bulk ma e ials di e s om pu e Mg by a educ ion o he dis ance in c-di ec ion
and a phase change o a body cen ed cubic (bcc) phase (β phase) o highe Li ac ions (Mg–Li phase diag am,
Fig.1 13). This change leads o addi ional non-basal slip on he p isma ic planes, winning, and mo e duc ile
p ope ies e en in hcp Mg–Li alloys. Fo highe Li ac ions, he addi ion o he second phase can signi ican ly
change he mechanical p ope ies. Li e al. showed ha c acks a e p e e ably o med a he phase bounda ies
which is acili a ed by he di e ence in he numbe o gliding sys ems p esen in bo h phases and, hus, a di e -
ence in s ess accumula ion14. Addi ionally, he ageing o he second phase and change om bcc o hcp phase
e en a oom empe a u e in luences he p ope ies o e ime14,15.
The co osion a e o Mg–Li alloys is in luenced by se e al ac o s such as he high ac i i y o Li, change o
mic os uc u e and su ace ilms. The low elec ochemical po en ial o Li leads o an inc ease in he ca hodic
kine ics and a mo e signi ican shi o he pH. Fili o m co osion is ound o be one o he main co osion p o-
cesses occu ing o Mg–Li alloys in he α o α + β ma e ials16–18. Fo ilms wi h mixed phases, mic o-gal anic
coupling is ound as a main ac o o an inc ease in co osion a e wi h p e e ed co osion and pi ing a he
phase bounda ies19,20. Howe e , he di e si y o mic os uc u e and p o ec i e laye s o med du ing co osion
makes a clea indica ion o he in luence o he di e en ac o s on he co osion a e di icul . Li e al. showed
ha he co osion a e dec eases om α + β > α > β18. The lowe co osion a e o he bcc phase is assumed o be
OPEN
1Ino ganic Func ional Ma e ials, Ins i u e o Ma e ials Science, Facul y o Enginee ing, Kiel Uni e si y, Kiel,
Ge many. 2Ins i u e o Me allic Bioma e ials, Helmhol z Cen e He eon, Gees hach , Ge many. 3Ma ine
Clima e Resea ch, Ins i u e o Geosciences, Facul y o Ma hema ics and Na u al Sciences, Kiel Uni e si y, Kiel,
Ge many. *email: eq@ .uni-kiel.de
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due o he high densi y and s abili y o a o med p o ec i e laye . While o he Mg ich phase, mainly a po ous
Mg(OH)2 laye is expec ed o o m du ing co osion21,22, he laye s uc u es o su ace ilms o med on Mg–Li
including he bcc phase a e complex. Xu e al., e.g., analysed he s uc u e o he na u al ilm o med in ai as a
Li2CO3 ilm on he su ace, Mg oxide and Li oxide ilm unde nea h and Mg- ich ilm be o e he bulk ma e ial23.
O he s udies claim o ilms o med in ai o du ing co osion se e al compounds including ca bona es, oxides
and hyd oxides o bo h Li and Mg, o en sepa a ed in a laye s uc u e16,24,25. P e ious s udies assumed o sus-
pec ed ha he o med Li2CO3 has he main in luence on he highe co osion esis ance o he bcc phase23,26,27.
The Pilling–Bedwo h a io (PBR), which is a measu e o ilm s ess and, hus, iden i ies a s able ilm o
1 < PBR < 2, is > 1 o all Mg:Li a ios o Li2CO3. Thus, i could al eady be o med o lowe Li mass ac ions in
he hcp phase18. Yan e al. sugges ano he possible in luence as Li doping and he e o e s eng hening he MgO
and hinde ing he o ma ion o he mo e po ous and less p o ec i e magnesium hyd oxide. Since he c i ical
Li ac ion o o ming a s able laye o MgO is calcula ed o be a ound 15–18a .% (4.8–5.9%(m/m)), his is in
ag eemen wi h he o ma ion o he laye only on Mg–Li wi h β o α + β28. The e o e, wi h he assump ion o
he o ma ion o a s able MgO laye by Li doping, he highe Li ac ions leads o a dec ease in co osion a e
by changing he ilm s ess.
Howe e , i has o be no ed ha he co osion o Mg–Li wi h di e en phases is a complex sys em, leading o
o he s udies showing he lowes co osion a e o he hcp phase19. The main in luences on he ac ual co osion
a e a e he e o e no jus Li con en and phases bu also he mic os uc u e o he ma e ial.
Fo neu ological implan s, small ees anding ilms and s uc u es in he sizes o a ew µm–mm a e needed.
P e ious o his pape , no ex ensi e s udies o Mg–Li hin ilms ega ding he g ow h and p ope ies a e a ailable
o ou knowledge. As ound o hin ilms o o he Mg alloys, signi ican in luences on he p ope ies in com-
pa ison o bulk ma e ials a e expec ed. Fo di e en Mg alloys such as Mg–Ag o Mg–REE, he s uc u e o hin
ilms was s udied29–33. Fo spu e -deposi ed ilms, he hexagonal close packed (hcp) magnesium phase (α phase)
is s ongly ex u ed wi h a p e e ed g ow h di ec ion o [001] and a columna g ain s uc u e is appa en 31,33–35.
The s ong ex u e in luences he de o ma ion by, e.g., educing wo k ha dening and in luences he e o e he
mechanical p ope ies33. The co osion o hin ilms in compa ison o bulk is shown o be mo e homogeneous
wi h less e ec o pi ing35,36. Addi ionally, he co osion esis ance and oxida ion o di e en planes a e di e en
due o he packing and binding ene gies, hus, he co osion a e is in luenced by he ex u ing o he ilms37.
The (001) plane is ound o show he lowes co osion a e due o he denses packing, howe e , since he as e
oxida ion o o he planes could lead o a p o ec ed su ace, he ac ual in luence on he co osion a e canno be
di ec ly p edic ed38,39. Since he p ope ies a e highly dependen on s uc u e and mic os uc u e, hey can be
in luenced o he same alloy by changing he spu e ing pa ame e s, leading o highe densi ies o di e ences
in he ilm g ow h depending on he ene gy a ailable o di usion o he a oms29,32,40–42.
In his s udy, Mg–Li hin ilms wi h a pu e hcp o an α + β s uc u e (Mg–Li phase diag am, Fig.1) a e p e-
pa ed ia magne on spu e ing. To gain insigh in o he co osion p ocess dependen on he speci ic s uc u e
p esen in he hin ilms, s udies wi h espec o hei g ow h and mic os uc u e a e ca ied ou o allow a co -
ela ion wi h in luences on he co osion a e. Addi ionally, he possibili ies o in luence and une hose o mee
speci ic equi emen s gi en o applica ions a e discussed.
Figu e1. Mg–Li phase diag am, adap ed om13. The egion wi h a Li ac ion om 0 o 50% (n/n) is
depic ed and he concen a ion o sample ilms (Li mass ac ion o 1.6% (m/m), 3% (m/m), 5.5% (m/m) and
9.5% (m/m)) a e ma ked. Low Li concen a ions lead o a Mg ich α-phase wi h a hcp s uc u e and high Li
concen a ions o a β-phase wi h a bcc s uc u e. In he mixed phase egions, α-phase and β-phase a e o med.
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Composi ion and mic os uc u e. The composi ions o he p epa ed Mg–Li hin ilms a e ma ked wi h
ed dashed lines a mass ac ions o 1.6% (m/m), 3% (m/m), 5.5% (m/m) and 9.5% (m/m) in he phase diag am
in Fig.1 ( ac ions gi en in Supplemen a y Table1). As shown, wo sample ypes a e p epa ed wi h Li mass ac-
ions leading o pu e hexagonal phase, while he o he wo heo e ically consis o α and β phase wi h app oxi-
ma ely 89% (Mg–5.5Li) o 22% (Mg–9.5Li) α phase.
Ve y low i on con amina ions a e measu ed o all ilm composi ions. Rep esen a i e XRD di ac og ams o
all composi ions a e displayed in Fig.2a in compa ison o pu e Mg p epa ed by he same p ocedu e. While pu e
Mg exhibi s a s ong ex u e wi h a main peak o (002) a 34.3°, including 1.6% (m/m) Li leads o less p e e ed
g ow h and o Mg–3Li, a andom o ien a ion is iden i ied (Supplemen a y Fig.1). Fo bo h ma e ials in he α + β
phase, he hexagonal phase shows a p e e ed o ien a ion o (110). Addi ional bcc can be iden i ied as small peaks
in he di ac og am, and addi ional s udies o he ecip ocal space allow he iden i ica ion o s ong (110) peaks
o Mg–9.5Li a an angle o χ = 32°–40°, indica ing a s ongly ex u ed β-phase. While he la ice pa ame e a is
only sligh ly dec eased om Mg–1.6Li o highe Li mass ac ions, c is educed, especially o he inc ease o Li
om 1.6% (m/m) o 3% (m/m) (peak shi in Fig.2a, calcula ed la ice pa ame e s in Supplemen a y Table2).
When he second phase is p esen , he pa ame e s do no dec ease u he since he added Li is included in he
addi ional phase. O in e es a e as well he peaks a , e.g., 21.3°, 23.3°, 29.4° and 34.1° o Mg–5.5Li and Mg–9.5Li
which indica e he exis ence o Li2CO3 (Supplemen a y Table3). Because o he o ma ion o his addi ional
phase, he amoun o β-phase is educed.
In es iga ions ia EDX show a laye including ca bon and oxygen on he su ace o Mg–9.5Li, hus, he
Li2CO3 iden i ied by XRD can mainly be assigned o a ilm o med on he su ace o he samples when hose a e
s o ed in ai . The o ma ion o Li2CO3 in humid ai o β-phase Li is also shown in p e ious s udies23,28. A laye
o Li2O is o med on he su ace o alloys wi h high Li con en and can eac u he o Li2CO3 i CO2 is p esen
in he su ounding a mosphe e23. The e is no signi ican composi ional change o e he laye hickness o , e.g.,
Mg–1.6Li (Fig.2b). C oss-sec ional images o he di e en Mg–Li alloy ees anding hin ilms wi h a hickness
o 10µm a e gi en in Fig.3a o analysis o he mic os uc u e. Mg–1.6Li exhibi s a columna g ow h wi h a
cons an diame e o app oxima ely 500nm o 1µm o e he whole ilm hickness. This s uc u e is also iden i ied
o pu e Mg ilms wi h he s ong (001) ex u e p epa ed ia magne on spu e ing30. Fo Mg–3Li, smalle g ains
a e o med close o he subs a e while columns s a a e a ew 100nm wi h inc easing diame e up o 1.5µm o
2µm wi h a ew columns exhibi ing a diame e o a ound 4µm. Less columna g ow h is isible o Mg–5.5Li and
canno be iden i ied o he highes Li mass ac ion. The su ace, howe e , s ill exhibi s a s uc u e which leads
o he iden i ica ion o g ain sizes o app oxima ely 1.5–2µm. The di e ence in he c oss-sec ional images can
no only be assigned o a change o columns o a di e en mic os uc u e because o addi ional phases bu also
o less p e e ed ac u e a he g ain bounda ies du ing bending. This is in luenced by, e.g., oids o med due o
he sel -shadowing o he columns. Fo Mg–9.5Li, oxida ion o he samples also plays a majo ole in he isible
s uc u e since he samples a e highly a ec ed. While a hin oxide ilm is o med o all ilms and is appa en in
he su ace images in Fig.3a, only o ilms wi h highe Li con en (Mg–9.5Li) he oxide g ows signi ican ly un il
he ilm is comple ely oxidized (Fig.2c).
To classi y he mic os uc u es u he , hey can be compa ed wi h he s uc u e-zone model41,43. The subs a e
empe a u e du ing he deposi ion was (49–54)°C o Mg–1.6Li, (54–66)°C o Mg–3Li and (60–66)°C o
Figu e2. S uc u e and composi ion o ees anding hin ilms (a) XRD di ac og ams o Mg, Mg–1.6Li,
Mg–3Li, Mg–5.5Li, Mg–9.5Li hin ilms. The posi ions o he hcp and bcc Mg–Li phases and Li2CO3 a e
ma ked. Addi ionally, he o ien a ion o he co esponding planes o he hcp phase a e indica ed, (b) EDX line
scans o c oss sec ions o Mg–1.6Li and Mg–9.5Li hin ilms as used o co osion measu emen s, (c) EDX line
scans o c oss sec ions o Mg–9.5Li ees anding hin ilms a e 1day and 8–9 mon hs.
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Mg–5.5Li and Mg–9.5Li samples. This leads o T/Tm = 0.36 o Mg–1.6Li and T/Tm = 0.39 o Mg–9.5Li as he
highes and lowes possible alues, leading in heo y o s uc u es in he T-zone, changing in o zone 2. Fo highe
zones, he g ain bounda ies ge dense , and he de ec densi y dec eases, he e o e ewe oids occu which could
in luence he ac u e a g ain bounda ies. Addi ionally, a shi om s aigh e ib es o mo e complex s uc u es
a lowe laye s o Mg–3Li is in good ag eemen wi h he mic os uc u al c oss-sec ions.
Film g ow h. To iden i y and unde s and he di e ences in mic os uc u e, he g ow h o Mg–1.6Li, Mg–
3Li and Mg–9.5Li ilms is addi ionally analysed. The spu e imes we e chosen acco ding o he spu e ing a es
o 10µm o esul in app oxima ely 10nm, 100nm, 1µm and 20µm. The esul s a e displayed in Fig.3b.
The g ow h o he lowes Li mass ac ion s a s wi h island g ow h, simila o pu e Mg44. The ilm co e s
he whole su ace o a hickness > 10nm, esul ing in columna g ow h. The ene gy o he pa icles om he
spu e ing p ocess i sel and he low empe a u e o he subs a e do no allow su icien di usion o a mo e
homogeneous g ow h45. The columna g ow h is addi ionally in good acco dance wi h he s uc u e o med wi h
he p e e ed o ien a ion o (001) because he as es g ow h o hexagonal aces is in di ec ion o he c-axis45.
The ilm g ow h p ocess is s udied o be highly in luenced by including alloying elemen s44. Fo Mg–3Li, he
laye o ma ion s a s wi h laye g ow h, including only ew de ec s in a 10 nm hick laye . A e app oxima ely
(400–500)nm, he g ow h changes o columna g ow h. This change can be a ibu ed o ilm s ess which accu-
mula es o e he laye and changes he ene ge ically a ou able g ow h. Po es and oids can al eady be iden i ied
a e he column g ow h o a laye hickness o 1µm. E en hough columna g ow h is isible, he homogenei y
o he signal on he ecip ocal space mapping om XRD shows ha a andom o ien a ion is o med h oughou
he whole ilm (Supplemen a y Fig.1). Since he g ow h is no as s ic ly o ien a ed as o Mg–1.6Li, i leads o
an inc ease in column diame e and oid o ma ion.
Fo Mg–9.5Li, he ilm g ow h a he beginning canno be di ec ly iden i ied as island g ow h, howe e , a
a ilm hickness o app oxima ely 100nm, a g ain-like su ace wi h a high oughness is isible. E en hough
hese samples we e measu ed di ec ly a e p epa a ion and he oxida ion o he samples is hus minimal, a
colou change o he samples showed a sligh oxida ion e en o he as es possible measu emen and he e o e
Figu e3. SEM images (a) Su ace and c oss-sec ion o ees anding Mg–Li hin ilms. (b) Side iew o c oss-
sec ion o Mg–Li ilms (Li: 1.6% (m/m), 3% (m/m), 9.5% (m/m)) on Si subs a e wi h hicknesses o 10 nm, 100
nm, 1 µm and 20 µm.
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58. Ha ne , D., Zamponi, C., de Mi anda, R. L. & Quand , E. Mic opa e ned ees anding magne on spu e ed Mg-alloy sca olds.
BioNanoMa e ials 16, (2015).
Acknowledgemen s
The au ho s hank D . Heike Helmholz om he Ins i u e o Me allic Bioma e ials o he Helmhol z Cen e
he eon o he suppo wi h he AAS measu emen s. This wo k was suppo ed by he DFG in he amewo k o
he esea ch aining g oup 2154—Ma e ials o B ain (p ojec 270394294).
Au ho con ibu ions
L.H. and L.K.J. concep ion and design o he s udy, L.H. w i ing-o iginal d a , analysis o esul s; L.H., F.W.,
K.B. and U.W. pe o ming expe imen s, E.Q, R.W.-R. concep ion o gene al p ojec , E.Q., R.W.-R. and D.G.-S.
supe ision o he wo k. All au ho s e iewed he manusc ip .
Funding
Open Access unding enabled and o ganized by P ojek DEAL.
Compe ing in e es s
The au ho s decla e no compe ing in e es s.
Addi ional in o ma ion
Supplemen a y In o ma ion The online e sion con ains supplemen a y ma e ial a ailable a h ps:// doi. o g/
10. 1038/ s41598- 023- 39493-9.
Co espondence and eques s o ma e ials should be add essed o E.Q.
Rep in s and pe missions in o ma ion is a ailable a www.na u e.com/ ep in s.
Publishe ’s no e Sp inge Na u e emains neu al wi h ega d o ju isdic ional claims in published maps and
ins i u ional a ilia ions.
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Resul s
40

Resul s
3.2 In luence o spu e ing pa ame e s on he co osion
a e o Mg-3Li
As discussed in he publica ion in chap e 3.1, he co osion a e can no only be in luenced
by he Li con en i sel bu also by he s uc u e, e.g., he o ien a ion. To in es iga e his
in luence u he , he impac o a s uc u e change by adjus men o spu e ing pa ame e s
and, hus, he di ec uning o he co osion a e du ing he spu e ing p ocess, is discussed
in he ollowing s udy.
Powe [W] P essu e [10−3mba ] Spu e ing a e [nm/s]
50 2.3 1.78
100
1.5
3.19
2.3
4
6
150 2.3 4.70
Table 3.1: Se o spu e ing pa ame e s o magne on spu e ing o Mg-3Li o analyse he
in luence o powe and p essu e change on he s uc u e and co osion a e o he hin ilms.
The p ope ies o hin ilms p epa ed by magne on spu e ing can be in luenced by chang-
ing he pa ame e s such as p essu e, powe o gas low. In his s udy, a a ia ion o powe
(50-150 W) and p essu e (1.5-6·10−3mba ) o Mg-3Li is exempla ily in es iga ed o
iden i y possible in luences on he s uc u e o he ilm and he esul ing co osion a e
(spu e ing pa ame e s able 3.1). Figu e 3.1 shows he co osion a es o all es ed
pa ame e combina ions.
50 100 150
0.15
0.20
0.25
Co osion a e [mm/y ]
Powe [W]
Cons an p essu e 2.3*10
-3
mba
(a)
2 4 6
0.10
0.15
0.20
0.25
Co osion a e [mm/y ]
P essu e [10
-3
mba ]
Cons an powe 100 W
(b)
Figu e 3.1: Compa ison o co osion a es o Mg-3Li hin ilms on subs a e spu e ed wi h
di e en spu e ing pa ame e s ( o powe change: a) cons an p essu e o 2.3·10−3mba , o
p essu e change: b) cons an powe o 100 W).
Since he ilm s ess a ies, all co osion s udies we e pe o med on subs a e. Di e en
co osion po en ials be ween -1.70 V and -1.79 V a e measu ed o all pa ame e se s, bu
41
Resul s
no di ec dependence on powe o p essu e can be assigned. The lowes co osion a e
is de e mined o samples p epa ed wi h a powe o 100 W and p essu e o 4·10−3mba .
Especially o he highes p essu e o 6·10−3mba , he po en iodynamic pola isa ion mea-
su emen is mo e uns able wi h high luc ua ions, including s eps om possible pi ing and
depassi a ion, he e o e leading o highe e o s. Since no all pa ame e combina ions
a e es ed, his s udy does no aim o de e mine he lowes o highes possible co osion
a e bu ins ead o iden i y possible co ela ions be ween pa ame e s, mic os uc u e and
co osion a e, he e o e showing uning possibili ies.
Film s ess analysis by p oducing ees anding hin ilms and compa ing he olling a ios
shows ha changing he pa ame e s leads o signi ican ilm s ess ( igu e 3.2).
Figu e 3.2: Compa ison o ees anding Mg-3Li spu e ed a di e en p essu e and powe
( o powe change: cons an p essu e o 2.3·10−3mba , o p essu e change: cons an powe o
100 W). The ilm s ess can be iden i ied by he olling o he ilms.
Fo a p essu e o 2.3·10−3mba , he lowes ilm s ess is exhibi ed o 50 W samples;
highe powe leads o comp essi e s ess. A 150 W, bo h comp essi e and ensile s ess
samples can be ound, depending on he posi ion on he wa e du ing spu e ing. The
su ace o all samples, bu especially o 100 W, a e shiny and, he e o e, smoo h. A
100 W, a change o p essu e leads o la and s able ilms o a p essu e o 4·10−3mba
wi h comp essi e s ess o he o he ilms, hus no allowing a di ec end. Fo all p es-
su es spu e ed a 100 W, he su ace is smoo h. Howe e , a colou change o he lowes
p essu e indica es a change in he sample su ace ( igu e 3.2).
Fac o s such as g ain size, oughness, and g ain bounda ies could also in luence he co o-
sion a e. As discussed by Qu e al., smalle g ains can dec ease he biodeg ada ion a e
o pu e Mg [124]. The imp o ed co osion esis ance could be in luenced by an imp o ed
oxide laye o g ain bounda ies ac ing as co osion ba ie s [124, 203]. Howe e , i he
su oundings do no allow o su icien passi a ion due o high co osion a es [122], he
co osion a e can also be inc eased. G ain bounda ies o e possible a ack poin s o
co osion i he s uc u e is no dense due o inc eased su ace a ea. He e, he g ain size
dec eases wi h inc easing spu e ing powe , and he s uc u e is dense , wi h smalle g ain
s uc u es appa en on he su ace. This educes he a ea o easy a ack. Due o he ad-
di ional ene gy a a highe powe , mo e di usion o a oms is possible, hinde ing a s ong
shadowing. A much highe ene gies, his can addi ionally lead o a shi om he T zone
o zone 2 wi h s onge columna g ow h [204]. Howe e , in he samples discussed he e,
mic os uc u al analysis o c oss-sec ions o all sample ypes does no show a signi ican
change in he mic os uc u al ype ( igu e 3.3).
Inc easing he p essu e can lead o lowe ene gy o he inciden spu e ed a oms and, hus,
mo e la ice de ec s [29]. Howe e , o he s udies desc ibe ha a p essu e change does no
42
Resul s
Figu e 3.3: Mic os uc u e o Mg-3Li hin ilms spu e ed a di e en p essu e and powe
( o powe change: cons an p essu e o 2.3·10−3mba , o p essu e change: cons an powe o
100 W).
in luence he co osion a e [102] o can lowe he co osion a e by leading o ine g an-
ula s uc u es [205]. Fo samples p epa ed a he lowes p essu e o 1.5·10−3mba , he
SEM images con i m he change in he su ace p ope ies, which was al eady appa en due
o he colou change. The spu e ing a such p essu es is less s able, and smalle pa icles
o d ople s can be ound on he su ace and in he ilm. Howe e , since no signi ican
change o he co osion a e o he p essu e o 2.3·10−3mba is measu ed, his canno be
he main in luence o a co osion a e change. Fo samples spu e ed a he p essu e o
4·10−3mba , hin columna s uc u es a e o med. E en hough he su ace is no la ,
no signi ican gaps and no ough sub-s uc u e on he g ains a e o med.
Since he Mg-3Li samples o a p essu e o 4·10−3mba and a powe o 100 W showed
he lowes co osion a e and low ilm s ess, ees anding hin ilms we e p epa ed o
co osion measu emen s, leading o co osion a es o 0.14±0.02 mm/y in compa ison o
0.27±0.07 mm/y o he s anda d spu e ing pa ame e s a 50 W desc ibed in he chap-
e 3.1. Fo bo h samples, a simila Li ac ion (3.5±0.5 w % s 3.9±0.1 w %, measu ed
by AAS) is ound. The e o e, his canno be a main in luence on he change in co o-
sion a e. The main di e ence shown in c oss-sec ional images o he mic os uc u e is
analysed o be a dense s uc u e and smalle columns, indica ed by ewe oids a g ain
bounda ies, and a la e su ace s uc u e wi h less p onounced g ain bounda ies ( igu e
3.4 a). The compa ison o exac XRD peak heigh s o ees anding hin ilms is di icul
since hey canno be ixed comple ely la . Howe e , no signi ican change in o ien a ion
can be obse ed and s onge signals and, hus, mo e low-indexed planes pa allel o he
su ace can be ound o he 100 W sample ( igu e 3.4 b). Thus, by changing he spu e -
ing pa ame e s, he same co osion a e as o Mg-1.6Li in ees anding hin ilms can be
achie ed o Mg-3Li.
As a esul , no di ec end, such as a dec ease in co osion a e wi h inc easing powe
o dec easing p essu e, can be iden i ied. S ill, he possibili y o in luencing he co o-
sion a e is con i med. Mul iple ac o s, including a p e e ed o ien a ion, g ain size, oid
o ma ion, su ace oughness and low ilm s ess, a ec he co osion a e. Thus, he pa-
ame e s ha e o be chosen acco dingly o each alloy ype. To allow a con olled uning o
he p ope ies, he in luence o he pa ame e s on he plasma and, he e o e, ilm g ow h
needs o be u he analysed.
43
Resul s
Figu e 3.4: Compa ison o SEM and XRD analysis o Mg-3Li spu e ed wi h wo se s o
spu e ing pa ame e s (50 W, 2.3·10−3mba and 100 W, 4·10−3mba ) p oducing s ess- ee
ilms.
3.3 Analysis o long- e m co osion in di e en solu ions
The co osion expe imen s o he s udies a e ca ied ou mainly in HBSS o e sho e
pe iods o ime due o he measu emen se -up (po en iodynamic pola isa ion o weigh
loss in sample holde ). To iden i y u he in luencing ac o s and see he co osion long
e m, ees anding samples o Mg, Mg-1.6Li and Mg-3Li a e placed in HBSS wi hou
hea ing (18-23 ◦C) o pH con ol (pH ange o 7-8.5). The pic u es o Mg-1.6Li a e shown
in igu e 3.5. A e a colou change due o he o ma ion o co osion p oduc on he
su ace, holes occu due o inhomogeneous co osion.
Figu e 3.5: Exempla y pho os o co osion o Mg-1.6Li o e 182 days in HBSS.
As can be seen in igu e 3.6, o all h ee sample ypes, he co osion does no ake place
homogeneously o e he whole su ace bu holes a e o med due o de ec s and pi ing. Due
o he olling o mainly Mg-3Li and Mg, he co osion a e canno be di ec ly iden i ied
bu a e a ound 55-60 days, Mg-1.6Li has he la ges emaining a ea o hin ilm (da k
44
Resul s
ilm a ea), indica ing a lowe co osion a e as al eady desc ibed in he pape in 3.1. Fo
Figu e 3.6: Co oded samples o a) Mg (55 days), b) Mg-1.6Li (62 days), c) Mg-3Li (57 days)
in HBSS.
all samples, he o ma ion o a whi e co osion p oduc pa ially e aining he shape o
he hin ilm is ound. To u he iden i y change o he ilm du ing he co osion, XRD
and SEM analysis on he co oded samples we e pe o med. In igu e 3.7, he o ma ion
o a co osion p oduc laye can be easily iden i ied by a change in mic os uc u e. This
su ace laye , howe e , is no dense and is c acked h oughou he whole ilm. In he a ea
o he whi e co osion p oduc , he mic os uc u e is comple ely changed, only lea ing a
po ous ma e ial.
Figu e 3.7: SEM o Mg-1.6Li samples a e 63 days in HBSS. a) C oss sec ion o whi e, co oded
a ea, b) c oss sec ion o da k a ea, c) su ace.
To iden i y he change o he ilm, XRD analysis was ca ied ou on bo h he da k (possibly
emaining me al alloy) and whi e (pu e co osion p oduc ) a ea o Mg, Mg-1.6Li and Mg-
3Li samples. In igu e 3.8, bo h a e di ec ly compa ed o he spec a o unco oded hin
ilms. In he g ey egions, he Mg o MgLi can s ill be iden i ied o all samples bu
is much lowe o no p esen o he whi e egions due o he co osion. Ins ead, MgO
and ca bona e con aining compounds (MgCO3o MgxCa(CO3)x+1) can be ound on bo h
MgLi alloys wi h addi ional in luences o sal s such as NaCl since he samples could no
be comple ely cleaned due o ins abili y a e co osion. I can be assumed ha he whi e
a ea is no only a sal accumula ion o NaCl singe i mimics he shape o he me al ilm
be o e (see igu e 3.6), hus, co osion p oduc s o he hin ilms a e included. The signal
in gene al is lowe due o he educed amoun o ma e ial. In addi ion, XRD only de ec s
c ys alline ma e ial, hus, any amo phous componen s canno be de e mined. A possible
inclusion o Ca in o he co osion p oduc hin s o he impo ance o he solu ion on he
co osion p ocess as discussed in sec ion 2.3.2. HBSS is chosen since i includes he many
elemen s which a e p esen in he human body such as Na, Ca, Mg, K, Cl, S o P and
glucose (H1387, Sigma-Ald ich wi h added sodium bica bona e). O he s udies o Mg
co osion a e ca ied ou in simple NaCl sal solu ions (e.g. [156, 165]) o o he simula ing
body luids (e.g. [163, 187]). To see he e ec o he sal s on he co osion o MgLi, samples
45

Resul s
(a) (b)
(c)
Figu e 3.8: XRD o a) Mg a e 56 days, b) Mg-1.6Li samples a e 66 days, c) Mg-3Li a e
74 days co osion in HBSS on whi e and g ey sample a ea. Addi ionally, XRD di ac og ams
o he samples be o e co osion a e shown.
o Mg-1.6Li we e placed in HBSS, DMEM (Dulbecco’s modi ied Eagle’s medium, DMEM,
31966047, The mo Fishe , wi h 10 % e al bo ine se um, S0615, Me ck), aCSF (a i icial
ce eb ospinal luid) and dis illed wa e as a compa ison wi hou empe a u e and pH
con ol.
In igu e 3.9, images o he samples a e sho imme sion and a e longe imme sion a e
shown. Fo samples in bo h dis illed wa e and DMEM, he sample is nea ly comple ely
co oded be o e 10 days in solu ion while samples in bo h aCSF and HBSS a e mo e s able
wi h la ge a eas in ac o mo e homogeneously co oded a e o e 50 days. An es ima ion
he e o e leads o a co osion a e <0.6 mm/y in DMEM and wa e , <0.1 mm/y in aCSF
and <0.05 mm/y in HBSS. Since he ilm is, howe e , no co oding homogeneously, his
only gi es an es ima ion o he in luence o co osion and no di ec co osion a es.
The composi ion o aCSF is simila o HBSS, con aining mainly sal s and glucose including
ca bona es, phospha es and Na, Ca, Mg, Cl. The DMEM used in his s udies o e all has
a highe concen a ion o sal s han in HBSS and addi ionally includes amino acids and
i amins. The concen a ion o Ca and ca bona es and phospha e con aining componen s
46
Resul s
is highe , hus, an easie o ma ion o such componen s on he o ma ion o a passi a ing
laye canno be he deciding ac o . The o ganic componen s can in heo y also dec ease
he co osion a e [135]. One possible explana ion is a change in pH. DMEM is ound o
bu e a change in pH be e han HBSS [135], hus, due o he lack o pH con ol, an
inc ease in pH can be mo e p onounced in HBSS, leading o lowe co osion a es.
Figu e 3.9: Mg-1.6Li samples in di e en solu ions a e sho (1-2 days) and longe co osion
( o as co oding samples 6 days, o slow co oding samples a ound 60 days).
47
Resul s
3.4 Publica ion: In es iga ion o in-si u ion elease and
su ace ilm o ma ion o hcp Mg-Li hin ilms
In addi ion o he in luences o he mic os uc u e on he co osion a e o he hcp α-MgLi
alloys (Mg-1.6Li and Mg-3Li), which a e discussed in 3.1, he ion elease o he samples
can gi e u he insigh in o changes o he co osion wi h a change in Li con en . The e-
o e, bo h long- e m (3 days) and sho - e m s udies (in-si u s udy du ing he i s 30
min-1 h o imme sion) we e ca ied ou o analyse he elease o bo h Mg and Li o e
ime.
Fo compa ison, hin ilms wi h 5.5 w % o Li we e added wi h a composi ion e y close
o he bounda y be ween hcp and mixed phase.
The di e ence in co osion p ocess p e iously suspec ed by he change in co osion a e
o an inc eased co osion a e wi h Li ac ion and mic ogal anic coupling o Mg-5.5Li
we e con i med, and a p ocess o p e e ed ion elease o e ime was p oposed.
Fu he , a Li- ich laye on he su ace o he α-phase samples s o ed in ai was ound,
which is eleased di ec ly a e con ac in solu ion and, hus, inc eases he Li concen-
a ion du ing he s a o he ea men i no p e- ea ed. The e o e, he o ma ion o
Li2CO3on he su ace o α-MgLi con i ms ha he co osion a e educ ion o β-MgLi
migh no only be in luenced by Li2CO3as discussed in sec ion 2.3.5.
Own con ibu ions o he ollowing a icle
(concep - 75 %, planning - 80 %, expe imen s - 75 %, analysis - 90 %, w i ing - 90 %):
•Sample p epa a ion
•Se -up and co osion o p epa a ion o long- e m s udy
•Online ICP-MS measu emen s
•In e p e a ion and discussion o he esul s
•W i ing o he manusc ip
The pape Hanke, L., Kalchg ube , L., Wes e ns ¨oe , U., Ga be-Sch¨onbe g, D., Quand ,
E., Val ine , M. In es iga ion o in-si u ion elease and su ace ilm o ma ion o hcp
Mg-Li hin ilms. Co osion Science 238, 112361(2024) [206] published by Else ie is
open access and he use is pe mi ed by he C ea i e Commons CC-BY License (h ps:
//c ea i ecommons.o g/licenses/by/4.0/).
48
In es iga ion o in-si u ion elease and su ace ilm o ma ion o hcp Mg-Li
hin ilms
Lisa Hanke
a
, Lukas Kalchg ube
b
, Ul ike Wes e ns ¨
oe
c
, Die e Ga be-Sch¨
onbe g
c
,
Eckha d Quand
a
, Ma kus Val ine
b
,
*
a
Ino ganic Func ional Ma e ials, Ins i u e o Ma e ials Science, Kiel Uni e si y, Kiel, Ge many
b
Applied In e ace Physics, Ins i u e o Applied Physics, Vienna Uni e si y o Technology, Vienna, Aus ia
c
Ma ine Clima e Resea ch, Ins i u e o Geosciences, Kiel Uni e si y, Kiel, Ge many
ARTICLE INFO
Keywo ds:
A Magnesium
A Spu e ed ilms
B ICP- MS
B XPS
C Su ace ilms
ABSTRACT
In his wo k, he dissolu ion p ocess o magne on spu e ed Mg-Li hin ilms was in es iga ed by in-si u low
cell/ICP-MS measu emen s and ex-si u ICP-MS measu emen s a e longe imme sion and addi ional XPS mea-
su emen s. High Li concen a ions a e eleased due o a Li ich ca bona e laye o med in ai . The deple ion o Li
leads o p e e ed Mg elease be o e p e e ed Li elease occu s due o he highe ac i i y o Li and inco po a ion
o Mg in co osion p oduc s. This da a p o ides a baseline o de eloping elease p o iles o medical applica ion,
mo e gene ally, i un a els de ails o he co osion mechanism o ligh weigh MgLi alloys.
1. In oduc ion
Biodeg adable ma e ials such as magnesium (Mg) a e o in e es o
se e al medical applica ions o educe pe manen implan s’side e ec s,
imp o e bone healing o elease bioac i e componen s [1–4]. A key
poin o in e es is he unde s anding and con ol o he deg ada ion a e
o such implan ma e ials o e i y he li e ime o he implan s, a oid
ad e se e ec s om as co oding ma e ials and allow o he adjus -
men o he ma e ial p ope ies o acili a e a be e ea men [5–7].
Unde s anding he deg ada ion p ocess becomes e en mo e c ucial i he
deg ada ion p oduc s, e.g., co osion p oduc s and pH change [4] o ion
elease [8], a e used as he apeu ically ac i e species. A ma e ial o in-
e es o ion elease s udies is magnesium-li hium. While he elease o
li hium (Li) om di e en ma e ials is al eady s udied [9–12], i was
ecen ly p oposed o use Mg-Li hin ilms as a ese oi implan eleasing
Li o he local ea men o neu ological applica ions [8,13] such as he
ea men o bipola diso de , Alzheime ’s o Pa kinson’s disease
[14–17]. The he apy wi h Li is associa ed wi h se e al side e ec s and
has a small he apeu ic window [15,18]. Thus, he con ol o he con-
cen a ion is o high impo ance. Mg can be used as he base ma e ial o
implan s no only because i is al eady widely s udied as a biodeg adable
and biocompa ible ma e ial and used in clinical ials and medical ap-
plica ions [2,19] bu also because i shows possible posi i e he apeu ic
e ec s [19,20]. The ea men wi h Mg-based ma e ials is al eady
es ablished o pa icles loaded wi h addi ional d ugs o using he
deg ada ion p oduc s, such as hyd ogen. He e, bo h he bene i o a
biodeg adable ca ie and he e ec o Mg i sel a e aken ad an age o
[21–24].
Mg-Li alloys can be di e en ia ed in
α
-Mg-Li,
α
+β-Mg-Li and β-Mg-Li
wi h
α
as he hexagonal closed packed (hcp) Mg- ich phase and βas he
body cen ed cubic (bcc) Li- ich phase. Gene ally, he highes co osion
a e is ound in he mixed phase
α
+βdue o he mic ogal anic coupling
o hose phases, while single phases show a lowe co osion a e [25,26].
Fo hcp
α
-phase Mg-Li wi h low Li con en , he co osion p ocess is
discussed o be simila o pu e Mg wi h he o ma ion o oxide and hy-
d oxide laye s [27,28] and ili o m co osion occu ing [26,29]. Li
ca bona e con aining p o ec i e laye s we e also ound o Mg-5Li-1Al
samples in he hcp phase [30]. Fo neu ological applica ions, implan s
need o be p oduced in small sizes. The p ope ies o such hin ilms a e
highly a ec ed by su ace e ec s and can di e om bulk ma e ial [31,
32]. As p e iously shown [8], simila o bulk ma e ial, Mg-Li hin ilms
wi h low Li ac ion show a co osion a e simila o Mg hin ilms in a
medium chosen o simula e physiological condi ions, while he co o-
sion a e inc eases o highe Li ac ions.
The ion elease o Mg-Li-Zn alloys [33] and Mg-Li-(Al)-(RE) [34] was
es ed o iden i y he co osion a e and in luence on cells a e longe
imme sion ime o applica ions as s en ma e ial. Zhou e al. ound an
inc ease in bo h Li and Mg elease o Mg-8.5 w % Li in compa ison o
* Co esponding au ho .
E-mail add ess: [email p o ec ed] (M. Val ine ).
Con en s lis s a ailable a ScienceDi ec
Co osion Science
jou nal homepage: www.else ie .com/loca e/co sci
h ps://doi.o g/10.1016/j.co sci.2024.112361
Recei ed 13 May 2024; Recei ed in e ised o m 29 July 2024; Accep ed 8 Augus 2024
Co osion Science 238 (2024) 112361
A ailable online 13 Augus 2024
0010-938X/© 2024 The Au ho s. Published by Else ie L d. This is an open access a icle unde he CC BY license ( h p://c ea i ecommons.o g/licenses/by/4.0/ ).
Resul s
49
dec eases and e en hough he passi a ion is no s ong, addi ional
compounds such as Li
2
CO
3
o LiOH ha e been ound on he su ace o
Mg-Li based samples [29,30].
In gene al, he Li elease is expec ed o inc ease wi h he Li con en
a ailable in he ilm, which was ound o bo h sho - e m and long- e m
measu emen s. Howe e , he elease is in luenced by no only he Li
con en bu also he deg ada ion p ocess, in he simples case, he
co osion a e. In a p e ious s udy, he co osion a e o Mg-1.6Li was
de e mined o be he lowes , while he co osion a e inc eased due o
he change in o ien a ion, highe ac i i y o Li and gal anic coupling
because o he o ma ion o a second phase o Mg-5.5Li [8]. A highe Li
con en leads o a highe co osion a e and, hus, o a highe elease in
bo h Mg and Li ions. This can be e en mo e p onounced i he inc ease in
Li con en also changes he o ien a ion o he ma e ial and he mic o-
s uc u e [8]. This is in ag eemen wi h he highe o e all ion elease
and, hus, co osion a e measu ed he ein.
Bo h
α
-Mg-Li alloys show a p e e ed Mg elease a e he i s peak
o Li elease in he i s hou , and he e is a nea ly di ec s a o Mg
elease a e imme sion. Thus, a e he dissolu ion o Li- ich com-
pounds, Li is no longe eleased p e e en ially, and a la ge amoun o
Mg is a ailable, possibly due o he deple ion a e Li eac ing on he
su ace. In he 3-day measu emen s, bo h
α
-phase ma e ials show a
simila end o elease wi h a highe elease o Mg a he beginning, as
al eady men ioned o in-si u measu emen s. Howe e , he esul s show
a p e e ed dissolu ion o Li o e longe imme sion imes, possibly due
o he lowe elec ochemical po en ial [50]. The p e e ed dissolu ion o
Li is also obse ed in he ion elease s udies a cons an ol age. While
he Mg elease is nea ly comple ely supp essed o bo h ma e ials a
ca hodic pola isa ion, Li elease s ill occu s, especially o Mg-3Li. Thus,
he elease is no a con inuous elease o bo h ma e ials p esen in he
phase bu p e e ed o one ma e ial e en hough no wo di e en phases
can be ound. Since he Li was ound o be accumula ing a he g ain
bounda ies [53], i is possible ha hese Li- ich a eas a e co oding
p e e en ially; howe e , u he s udies would be needed o p o e he
e ec on he ion elease di ec ly.
The co osion o
α
-Mg-Li, hus, changes om su ace o bulk o he
hin ilms and can be desc ibed by h ee phases (Fig. 8): 1. Li elease
om Li con aining su ace ilm o med in ai , 2. P e e ed elease o Mg
om Mg- ich a ea unde he su ace, 3. Con inuous elease om he bulk
o he ilm wi h p e e ed Li dissolu ion. The elease o Mg is educed
o e ime while he Li elease is less educed; hus, in addi ion o he
highe ac i i y o Li, he inco po a ion o Mg in co osion p oduc s
migh addi ionally educe he Mg elease and lead o a highe concen-
a ion o Li in compa ison o Mg. The anodic pola isa ion by sweeping
o alues abo e passi a ion and he s a o pi ing shows an inc ease o
Mg elease, hus indica ing he p esence o a Mg- ich co osion p oduc .
The o ma ion o he co osion p oduc s can hen change dec ease
co osion o e ime [54] oge he wi h addi ional componen s such as
phospha es (Ca, P) o med due o he addi ional sal s in he solu ion.
Li-con aining componen s ha e no been ound in he ou e laye a e
co osion wi hou s o age in ai , and a e 1 h o imme sion, only a hin
laye o oxides and hyd oxides is o med. Thus, he Mg me al unde -
nea h is s ill de ec able. Fu he measu emen s would be necessa y o
iden i y he composi ion o he co osion p oduc s in de ail which is no
Fig. 8. Schema ics o possible p ocesses o he su ace laye o ma ion and deg ada ion o hcp Mg-Li hin ilms a)-c) in ai and d)- ) in solu ion. a) Fo ma ion o a
Li
2
CO
3
and MgO con aining su ace laye in ai on as spu e ed samples, b) o ma ion o Mg- ich su ace laye s in ai a e sho s o age ime and low Li con en a e
co osion and c) de elopmen o Li
2
CO
3
on co oded samples a e longe s o age ime in ai o ilms wi h highe Li con en . d) The s a o he con ac wi h solu ion
leads o a dissolu ion o he Li- ich ca bona e laye , ollowed by e) p e e ed, s ong Mg elease in sho - e m co osion, be o e ) he elease swi ches o p e e ed Li
elease, possibly due o p e e ed Li elease and o ma ion o co osion p oduc s.
L. Hanke e al. Co osion Science 238 (2024) 112361
8
Resul s
56

pa o his s udy.
Fo he applica ion o such ilms, he Li elease needs o be de e -
mined o e ime. While he he apeu ic ange is discussed o be a ound
0.4–1.2 mM [55], his canno be di ec ly compa ed o he concen a-
ions eached in he 3-day s udies due o he in luence o cells o
placemen in i o on he co osion a e [56], and he ac ual olume o
solu ion and he low on he inal concen a ion. Howe e , his s udy
and he p e e ed elease o Li iden i y ha he Li concen a ion a ail-
able canno be di ec ly de i ed om he deg ada ion a e o he ilms
and will change, especially du ing he beginning o placemen and a e
long imme sion imes when Li is deple ed. Especially he i s Li- ich
laye migh lead o highe Li concen a ions, and emo al o such
laye by, e.g., p e ious imme sion in solu ion migh be necessa y.
5. Conclusions
Mg-Li hin ilms in he hcp phase we e s udied wi h espec o he
su ace chemis y and deg ada ion by a combina ion o in-si u and ex-
si u echniques o de e mining he ion elease and addi ional analysis
o he su ace composi ion by XPS. A p ocess o he o ma ion o su ace
laye s in ai and du ing co osion and p e e ed ion elease is desc ibed:
•I was ound ha bo h spu e ed hcp alloys (Mg-1.6Li and Mg-3Li)
o med Li ca bona e con aining compounds on he su ace a e
long- e m exposu e o ai . This laye is also o med i co oded
samples a e exposed o ai . Howe e , he de elopmen is mainly
isible o Mg-3Li ilms a e 7 days due o he highe Li con en and,
hus, a ailabili y o he elemen .
•Du ing deg ada ion in Hanks’balanced sal solu ion, his su ace
laye is dissol ed, moni o ed by a high elease in Li and educ ion in
bo h ca bona e and Li signal in XPS. The laye does, he e o e, no
con ibu e o he passi i y and in luence on he co osion esis ance
o he hcp hin ilms.
•While a highe Mg concen a ion can be ound du ing he i s hou
o imme sion, a highe Li concen a ion han p esen in he ilm is
de e mined o e a longe ime due o he p e e ed elease o Li and
he o ma ion o Mg- ich co osion p oduc s (con aining oxides and
hyd oxides), which o m a passi a ing laye and dec ease he
co osion a e.
The co osion p ocess and ion elease a e ime-dependen and
change om di ec con ac in solu ion o long- e m co osion. The
combina ion o in-si u sho - e m and s udies wi h longe imme sion
imes, as desc ibed in his wo k, is he e o e bene icial o gain a mo e
comp ehensi e pic u e o he complex deg ada ion beha iou o he hin
ilms.
Unde s anding he deg ada ion p ocess and eac ions in ai is also o
in e es o o he applica ions o MgLi in ields such as ligh weigh
cons uc ion. Fo he medical ield speci ically, he ion elease esul s
also highligh he impo ance o ca e ully analysing he ion elease when
discussing such hin ilms o applica ion as biodeg adable ese oi s o
ea men s wi h dissol ing elemen s. Fo an op imisa ion o he ea -
men , i is no su icien o de e mine he Li elease by deg ada ion a e
since a change o e ime occu s. The change in he a io o bo h ions
mus also be conside ed i bo h can in luence he he apeu ic e ec o
he o he elemen in ol ed. Fu he mo e, a ea men o solu ion dip-
ping be o e es s in i o o in i o migh be bene icial o a oid he impac
o he high Li elease a he s a .
CRediT au ho ship con ibu ion s a emen
Ul ike Wes e ns ¨
oe : In es iga ion. Die e Ga be-Sch¨
onbe g:
Supe ision, Resou ces. Lisa Hanke: W i ing – e iew &edi ing,
W i ing –o iginal d a , In es iga ion, Fo mal analysis, Concep ualiza-
ion. Lukas Kalchg ube : W i ing – e iew &edi ing, Me hodology,
In es iga ion. Eckha d Quand : W i ing – e iew &edi ing,
Supe ision, Funding acquisi ion. Ma kus Val ine : W i ing – e iew &
edi ing, Supe ision, Resou ces, Me hodology.
Decla a ion o Compe ing In e es
The au ho s decla e ha hey ha e no known compe ing inancial
in e es s o pe sonal ela ionships ha could ha e appea ed o in luence
he wo k epo ed in his pape .
Da a A ailabili y
Da a will be made a ailable on eques .
Acknowledgemen s
This wo k was suppo ed by he DFG in he amewo k o he
esea ch aining g oup 2154 –Ma e ials o B ain (p ojec 270394294).
We also acknowledge he CzechNanoLab p ojec LM2018110 unded by
MEYS CR o he inancial suppo o he XPS measu emen s a CEITEC
Nano Resea ch In as uc u e.
Appendix A. Suppo ing in o ma ion
Supplemen a y da a associa ed wi h his a icle can be ound in he
online e sion a doi:10.1016/j.co sci.2024.112361.
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doi.o g/10.1016/j.jma.2015.02.004.
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L. Hanke e al. Co osion Science 238 (2024) 112361
10
Resul s
58
Resul s
3.5 Publica ion: Tailo ing o Mg and MgLi hin- ilm co -
osion a es wi h dielec ic ba ie discha ge plasma
ea men
To con ol he ion elease o applica ions wi h di e en elease p o iles o implan shapes,
he possibili y o uning he co osion a e is ad an ageous. While chap e 3.2 al eady
discusses he possibili y o changing he co osion a e du ing he spu e ing i sel , his
only dec eases he co osion a e i s ill a mic os uc u e wi h a lowe co osion a e is
possible and can be o med.
In gene al, uning he co osion a e o Mg-based alloys means dec easing i since he
s uc u es need o be p esen in he body o a longe ime so ha he implan can se e
i s job. To u he educe he co osion a e o Mg and MgLi hin ilms, an addi ional
ea men by dielec ic ba ie discha ge plasma was es ed. The se -up o he ea men
in ambien ai is simple, cheap and could be easily upscaled. The su ace change by
educing oughness and o ma ion o a ca bona e-con aining laye educed he co osion
a e by a ound 50 % wi hou signi ican ly changing he ilm’s hickness.
Own con ibu ions o he ollowing a icle
(concep - 50 %, planning - 70 %, expe imen s - 40 %, analysis - 70 %, w i ing - 90 %):
•Sample p epa a ion
•XRD, SEM/EDX in es iga ion
•Pa ially po en iodynamic pola isa ion measu emen s
•In e p e a ion and discussion o he esul s
•W i ing o he manusc ip
The pape Hanke, L., Ha ig, T., Weishei , F., Tja d s, T., Pogoda, T., Faupel, F.,
Quand , E. Tailo ing o Mg and MgLi hin- ilm co osion a es wi h dielec ic ba ie dis-
cha ge plasma ea men . Jou nal o Vacuum Science and Technology A 41, 053109(2023)
[207] is ep oduced wi h pe mission om he Ame ican Vacuum Socie y (AVS) and li-
censed unde a C ea i e Commons A ibu ion (CC BY) license (h ps://c ea i ecommons.
o g/licenses/by/4.0/).
59
Tailo ing o Mg and MgLi hin- ilm co osion a es
wi h dielec ic ba ie discha ge plasma ea men
Ci e as: J. Vac. Sci. Technol. A 41, 053109 (2023); doi: 10.1116/6.0002783
View Online Expo Ci a ion C ossMa
k
Submi ed: 22 Ap il 2023 · Accep ed: 28 July 2023 ·
Published Online: 18 Augus 2023
Lisa Hanke,
1
To ge Ha ig,
2
Felix Weishei ,
1
Tim Tja d s,
2
Tim Pogoda,
2
F anz Faupel,
2
and Eckha d Quand
1,a)
AFFILIATIONS
1
Chai o Ino ganic Func ional Ma e ials, Ins i u e o Ma e ials Science, Facul y o Enginee ing, Kiel Uni e si y, Kiel, Ge many
2
Chai o Mul icomponen Ma e ials, Ins i u e o Ma e ials Science, Facul y o Enginee ing, Kiel Uni e si y, Kiel, Ge many
No e: This pape is pa o he Special Topic Collec ion including pape s om he Paci ic Rim Symposium on Su aces,
Coa ings and In e aces (PacSu 2022).
a)
Au ho o whom co espondence should be add essed: [email p o ec ed]
ABSTRACT
Magnesium and magnesium alloys such as magnesium-li hium a e o g ea in e es o he applica ion as biodeg adable implan s. To
con ol he deg ada ion, a ailo ing o he co osion a e is needed. In his s udy, he e ec o a sho (5–20 s) dielec ic ba ie discha ge
plasma ea men in ambien ai on he co osion a e o magne on spu e ed Mg and MgLi hin ilms is p esen ed. The ea men wi h
a mosphe ic plasma o as spu e ed samples leads o a dec ease o he co osion a e o 45%−50% in Hanks’balanced sal solu ion. The
highe co osion esis ance is in luenced by a change in su ace s uc u e and a o ma ion o an MgCO
3
con aining ilm.
© 2023 Au ho (s). All a icle con en , excep whe e o he wise no ed, is licensed unde a C ea i e Commons A ibu ion (CC BY) license
(h p://c ea i ecommons.o g/licenses/by/4.0/). h ps://doi.o g/10.1116/6.0002783
I. INTRODUCTION
Magnesium and magnesium alloys a e widely s udied biode-
g adable ma e ials as candida es o medical applica ions.
Applica ions o in e es each om biodeg adable o hopedic
implan s and s en s up o he possible ield o he apeu ical
ea men .
1–5
To ensu e he du a ion o he in eg i y o he implan
equi ed by he applica ion o he he apeu ic ac i i y, he deg ada-
ion a e needs o be adjus able. Since he co osion a e o Mg and
Mg alloys is o en oo high o he applica ions,
6–9
se e al s a egies
o educe he a e ha e been de eloped: These include alloying o
he bulk ma e ial,
10–12
in luencing he mic os uc u e,
12–14
su ace
ea men s, and coa ings.
15–17
The su ace ea men s ange om
deposi ion o ino ganic
18–20
and o ganic
21–23
deposi ion coa ings
o e chemical con e sion by imme sion in solu ion
24,25
o ion
implan a ion
26,27
o plasma elec oly ic oxida ion.
28–30
In his s udy, he ocus o he applica ion is on minia u ized
implan s ab ica ed by MEMS (mic o-elec o-mechanical sys ems)
echnology which can se e as, e.g., biodeg adable ma e ials o
b ain implan s, ei he as s uc u al implan s, subs a es o addi-
ional ma e ials o ese oi s o he apeu ically ac i e ions. As an
example o deg adable ilms ha elease ions which can ac as ea -
men s, MgLi hin ilms a e s udied since Li is used in ea men s
o mood diso de s such as bipola diso de .
31,32
The possibili y o
include addi ional ions in a coa ing o educe he co osion a e
speci ically o he applica ion as ese oi s is limi ed since hey
could in luence he he apeu ic e ec i hey a e eleased du ing he
deg ada ion. While small Mg s uc u es wi h a hickness o 10–
100 μm can be p epa ed by hin- ilm deposi ion echniques such as
spu e ing,
33–35
he addi ion o a hick coa ing would addi ionally
coun e ac he e o o educe he implan size. Thus, a ea men
ha leads o a hin laye only consis ing o he alloy elemen s i sel ,
possibly also in he o m o , e.g., oxides and ca bona es, is bene i-
cial. Possible echniques o o m hose laye s include, e.g., chemical
ea men in solu ion o o m MgO o Mg(OH)
2,36,37
ion implan a-
ion o ni ogen,
38
o plasma ea men s.
39–42
Kocijan e al. showed
ha plasma ea men in O
2
and H
2
leads o pin–hole ee oxide
laye s and, hus, lowe ing he co osion a e.
39
The o ma ion o an
oxide ilm is also s udied o plasma ea men wi h A /O
2
by
Tiyyagu a
40
while Nakazawa e al. examined he implemen a ion o
ni ogen and oxide a e he ea men o a Mg su ace wi h an
a mosphe ic plasma je .
41
Addi ionally, ca bon con amina ions on
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60
he samples a e emo ed by plasma p ocesses.
39
Li e al. show ha
a dielec ic ba ie discha ge (DBD) plasma ea men can educe
o e en p e en he co osion o MgLi alloys wi h a Li con en o
14.2% (m/m) signi ican ly by he o ma ion o Li
2
CO
3
and oxide-
con aining laye s.
42
A mosphe ic p essu e plasmas a e highly in e -
es ing o in luence su ace p ope ies in an economic way. In com-
pa ison o o he echniques, DBDs can easily be scaled up o
su ace ea men s in indus ial scales.
43,44
By DBD ea men
oxygen g oups o compounds can o m on he su ace by he in e -
ac ion wi h ele a ed oxygen species om he plasma.
45
In he li e -
a u e, DBDs a e also discussed o clean he su ace o Mg om
emaining con amina ions
46
o o lead o he eac ion o ca bon-
con aining componen s o ca bona es addi ional o he o ma ion
o an oxide laye .
47
Since o hin ilms no p io g inding and pol-
ishing can be easily pe o med, he p ocess can, hus, p o ide
ad an ages addi ionally o he educ ion o he co osion a e due
o he o ma ion o p o ec i e laye s including oxides and
ca bona es.
This s udy aims o e alua e he e ec o a DBD plasma ea -
men o ailo he co osion esis ance o hin ilms o Mg and
MgLi. Since he MgLi samples in his s udy ha e a Li con en o
only 1.6% (m/m), i does no lead o addi ional Li ich phases
which could lead o high Li concen a ed p o ec i e su ace ilms as
seen in p e ious s udies,
42
bu migh in luence he ac i i y and
mic os uc u e. Addi ionally, he na u e o hin ilms wi h μm
hickness does no allow he implemen a ion o hick p o ec i e
laye s. Thus, he e ec o he ea men o ming laye s below 1 μm
hickness on he co osion a e is s udied in Hanks’balanced sal
solu ion o simula e he en i onmen in medical applica ions.
Addi ionally, he laye o med du ing he ea men is analyzed
ega ding he s uc u e and chemical composi ion. By including
Mg and MgLi [1.6%(m/m)], he in luence o alloying and a ying
mic os uc u e can p o ide a de ailed unde s a ing o he o e all
p ocess.
II. EXPERIMENT
A. Sample p epa a ion
Magnesium and magnesium-li hium ilms we e p epa ed by
magne on spu e ing (Von A denne CS730S), using a ge s o
pu e Mg and MgLi [2.5%(m/m) Li] om FHR. As a subs a e, 4 in.
silicon wa e s we e cu in o 15 × 15 mm
2
samples and coa ed wi h
aluminium (Al) and aluminium ni ide (AlN) o compa abili y o
ees anding hin ilms. The spu e ing was ca ied ou wi h a base
p essu e o < 5 × 10
−7
mba and an A p essu e o 2.3
−2.6 × 10
−3
mba wi h a gas low o 25 SCCM A . A inal sample
hickness o 10–20 μm o Mg and MgLi hin ilms wi h a Li mass
ac ion o 1.6% (m/m) was eached. Fo one measu emen se , all
samples we e p epa ed in he same spu e ing p ocess, hus, wi h
he same hickness o un ea ed and ea ed samples. Addi ionally,
ees anding hin ilms we e p epa ed wi h he same spu e ing
pa ame e s, ollowing he p ocess desc ibed by Ha ne e al.
34
A e s uc u ing on he wa e by UV-li hog aphy and e ching, a
sac i icial AlN laye was added be o e he inal laye o Mg o MgLi
was deposi ed. Fo he e ching o he sac i icial laye , he samples
we e a e wa ds imme sed in 20 w . % KOH solu ion.
B. DBD ea men
The hin- ilm samples we e ea ed in he gap o a sel -buil
symme ic olume dielec ic ba ie discha ge se up. All ea men s
we e pe o med in s agnan ambien ai . The ees anding hin
ilms we e placed on a Si chip (15 × 15 mm
2
) and ixed on he
edges o ensu e a la e su ace du ing he ea men . A labo a o y
powe supply (SM7020-D, Del a Elek onika) and a unc ion gene -
a o (DDS unc ion gene a o 4025, Peak Tech) we e connec ed o
he high equency high ol age powe supply (Minipuls 4, GBS
Elek onik). The esul ing sinusoidal ol age signal o he high e-
quency high ol age powe supply wi h an ou pu o 1:2000 was
moni o ed ia an oscilloscope (UTD2025CL, UNIT). The ea men
ime o he plasma was con olled by an inhibi ing signal by a
mic ocon olle boa d (A duino nano e e y) connec ed o he high
equency high ol age powe supply. The DBD se up connec ed o
he powe supply can be seen in Fig. 1.
The pa ame e s o he plasma powe supply we e chosen o
allow he o ma ion o ilamen s all o e he sample su ace du ing
he comple e ea men ime, gi ing a s a e o sa u a ion (Table I).
An o e lay o he plasma ilamen s a all imes o he ea men can
be seen in Fig. 1(b). The pa ame e s we e adjus ed i he se up had
o be adap ed in be ween measu emen s o di e en sample se s
due o unc ioning easons o ensu e a homogeneous su ace ea -
men . Fo each sample and measu emen ype, ea men s wi h di -
e en ea men imes o 5–25 s we e ca ied ou wi h he same
pa ame e s o ensu e compa abili y.
C. Sample analysis
The su ace o he hin ilms was imaged and analysed using a
Zeiss Ul a 55 Plus scanning elec on mic oscope (SEM) and an
Ox o d Ins umen s ULTIM MAX 65 ene gy-dispe si e x- ay spec-
oscope (EDX). An accele a ing ol age o 3 kV o imaging and
10 kV o EDX measu emen s was used. Addi ionally, he s uc u e
o he hin ilms was analysed by x- ay di ac ion (Sma Lab
9 kW, Rigaku) wi h a pa allel beam and monoch oma ic Cu Kα
adia ion on a θ/2 θ-scan wi h a ange o 20°−90° wi h a speed o
5−10°/min and a s ep size o 0.03°. The chemical composi ion o
he sample su ace was cha ac e ized by x- ay pho oelec on spec-
oscopy (XPS). Fo his pu pose, an XPS UHV sys em om
Omic on Elec on Spec oscopy L d. wi h a 240 W Al anode was
used. Su ey scans o sc een o he elemen s p esen a he su ace
we e conduc ed a a pass ene gy o 100 eV, a s ep-size o 0.5 eV,
and a e aged o e h ee sweeps. High- esolu ion scans o he cha -
ac e is ic co e hole-le el spec a used o he chemical analysis we e
conduc ed a a pass ene gy o 30 eV, 15 sweeps, and a s ep-size o
0.05 eV. Fo da a analysis, he so wa e CASA XPS (Ve sion
2.3.23PR1.0) was u ilized and cha ge co ec ion was done by shi -
ing he C 1 s main peak o 284.8 eV and adjus ing all he co e-
sponding spec a acco dingly. The co osion a e was de e mined
by po en iodynamic pola isa ion measu emen s in a 155 mmol
Hanks’balanced sal solu ion (H1387, Sigma-Ald ich wi h added
sodium bica bona e) a a pHo 7.4 ± 0.2 (CO
2
egula ion) and a
empe a u e o 37 ± 1 °C. A Ve saSTAT 3–300 po en ios a
(AMETEKSI) and a h ee-elec ode se up wi h an Ag/AgCl e e -
ence elec ode, a P mesh coun e elec ode, and he sample
included in o a sample holde wi h an exposed a ea o 0.916 cm
2
ARTICLE pubs.aip.o g/a s/j a
J. Vac. Sci. Technol. A 41(5) Sep/Oc 2023; doi: 10.1116/6.0002783 41, 053109-2
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Resul s
61

we e used. A e 5 min o measu ing he open ci cui po en ial
(E
OCV
), a linea ol age sweep om −0.3 V s E
OCV
o +0.3 V s
E
OCV
was pe o med. Fu he in o ma ion can be ound in Re . 48.
Addi ionally, he same measu emen was ca ied ou a e 1h o
imme sion ime.
III. RESULTS AND DISCUSSION
A. Thin- ilm cha ac e iza ion
Rep esen a i e c oss sec ions and su ace images o he spu -
e ed hin ilms o Mg and MgLi a e shown in Figs. 2(a)–2(d).
FIG. 1. (a) Scheme o he sel -buil
dielec ic ba ie discha ge sys em. The
elec odes a e connec ed o he high
equency high ol age powe supply.
The op elec ode is a anspa en
FTO-coa ing on glass (Sigma Ald ich,
100 × 100 × 2.2 mm
3
,13Ω/sq) ac ing
as he op dielec ic, making he obse -
a ion o he plasma ea men om he
op possible. The bo om elec ode is
made om aluminum wi h he bo om
dielec ic being made om 6 mm hick
Al
2
O
3
. (b) Pho o o he DBD ea men
o an Mg hin ilm seen h ough he op
anspa en elec ode. Fo he inse ,
pho os ( ideo ames om a ideo wi h
30 ps) o e a ea men ime o 15 s
we e combined o show all ilamen s
du ing a ea men , ensu ing a ea
sa u a ion.
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While o bo h ma e ials a columna g ow h is isible, he su ace
exhibi s di e en s uc u es o bo h sample ypes wi h a mo e
s uc u ed su ace o Mg and la ge g ainlike a eas o MgLi. EDX
analysis o he su ace o he hin ilm iden i y an oxide and ca -
bona e signal o bo h sample ypes wi h no signi ican di e ence
be ween Mg and MgLi [2%–3%(m/m) C and 0.5%–1%(m/m) O].
No u he quan i ica ion o oxide o ca bona e componen s is
ca ied ou due o he possibili y o he in luence o con amina ions
on he exac in ensi y. In Fig. 2(e), XRD di ac og ams show
signals o he hin ilm i sel and he subs a e. Mg and MgLi ha e
a hexagonal closed packed (hcp) s uc u e wi h a s ong p e e ed
o ien a ion o pu e Mg.
Samples o Mg and MgLi on he subs a e and ees anding
ilms o MgLi we e ea ed wi h an a mosphe ic p essu e dielec ic
ba ie discha ge plasma in ai wi h a sa u a ion o ilamen a y dis-
cha ges, as seen in Fig. 1(b).
B. Co osion measu emen s
Exempla y po en iodynamic pola iza ion cu es o an
un ea ed and 15 s ea ed sample a e 5 min o imme sion a e
shown o Mg and MgLi in Fig. 3(a). The co osion cu en densi-
ies and co osion a es can be de e mined ia Ta el ex apola-
ion.
48,49
The ca hodic b anch was used o he es ima ion o he
co osion a e o he ollowing s udies due o he la ge linea a ea.
The cu en densi y o he anodic b anch is in luenced by addi-
ional hyd ogen e olu ion, ilm o ma ion, and passi a ion
egions,
50–52
hus, a co osion a e de e mined on he anodic
b anch may di e om he a e de e mined om he ca hodic
b anch. MgLi samples we e ea ed o 5, 10, 15, and 20 s. To
exclude he e ec o Li on he in luence o he plasma ea men ,
Mg samples we e ea ed o 5 and 15 s o compa ison. The co o-
sion a es o he measu emen s a e shown in Fig. 3(b). A signi i-
can dec ease in he co osion a e o bo h ma e ial ype is
appa en , lowe ing he co osion a e (CR) du ing a ea men o
15 s om CR
un ea ed
= 2.19 mm/y o CR
ea ed
= 0.45 mm/y o
MgLi and om CR
un ea ed
= 1.69 mm/y o CR
ea ed
= 0.38 mm/y
o Mg. Since he e ec o bo h sample ypes is simila and Li o
he di e en su ace s uc u e does no seem o in luence he e ec
o he ea men signi ican ly, u he s udies we e only ca ied ou
wi h MgLi samples.
MgLi hin ilms wi h a lowe co osion a e we e measu ed
wi h he co esponding co osion a es gi en in Fig. 3(c). The main
dec ease in co osion a e al eady occu s a e a ea men ime o
5 s wi h a educ ion o he co osion a e o abou 46%
(0.23 ± 0.03 mm/y o 0.13 ± 0.01 mm/y ). Since he co osion esis-
ance di e s only sligh ly o longe ea men s, he imes we e se
o 5 and 15 s. Since he po en iodynamic pola iza ion measu e-
men s show he deg ada ion o samples only o he sho e m,
TABLE I. The pa ame e s o DBD ea men . Lis ed a e he dis ance dbe ween
bo h elec odes, he equency , and he discha ge ol age U
discha ge.
d
(mm)
(kHz
@5Vpp)
U
discha ge
(kVpp)
Mg, MgLi [Fig. 3(b)] 3.9 20 24
MgLi [Fig. 3(c)] 1.9 17 25.3
MgLi [Fig. 3(c),1h
imme sion] 1.9 19 22.8
FIG. 2. Thin ilms as spu e ed on he subs a e (Si wa e wi h he added
Al + AlN laye ) (a) and (b) SEM c oss sec ion and image o he su ace o MgLi,
(c) and (d) SEM c oss sec ion and image o he su ace o Mg, (e) XRD di ac-
og am, signals o Mg, Al, AlN, and Si a e ma ked. Peaks wi hou symbol a e
kβ, WLa, and edge e ec signals o he Si subs a e.
ARTICLE pubs.aip.o g/a s/j a
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63
addi ional measu emen s a e necessa y o con i m he imp o e-
men o co osion esis ance o e longe ime pe iods. Weigh -loss
measu emen s o e longe e ms a e di icul due o he low sample
weigh o hin ilms and he cleaning esul ing in he emo al o
co osion p oduc s and possible p oduc s o med du ing he ea -
men , hus, no allowing he inal iden i ica ion o he co oded
mass. The e o e, addi ional po en iodynamic pola iza ion measu e-
men s we e ca ied ou a e 1h o imme sion o allow he p io
o ma ion o co osion p oduc s on he su ace o he ilm which
can p o ec he ilm om u he co osion.
53,54
As shown in Fig. 3(c), he co osion a e s ill dec eases a e
he DBD ea men signi ican ly o app oxima ely hal o he co o-
sion a e. Thus, he ea men no only leads o a passi a ion
dec easing he i s co osion be o e a p o ec i e co osion laye is
o med bu also leads o a s onge p o ec i e laye h oughou he
deg ada ion.
No signi ican change in he co osion po en ial E
co
can be
iden i ied o MgLi hin ilms a e he ea men o 15 s in compa -
ison o he un ea ed samples. While an E
co
o −1.85 ± 0.08 V is
measu ed o he un ea ed samples di ec ly a e imme sion, E
co
o samples wi h a 15 s ea men measu ed is −1.85 ± 0.01 V, hus,
he po en ial a ies o un ea ed samples, while i is mo e s able o
he ea ed samples. A e 1h o imme sion, he po en ial is sligh ly
inc eased o −1.78 ± 0.02 V o un ea ed samples and
−1.79 ± 0.03 V o samples a e 15 s o ea men . The inc ease a e
longe imme sion ime can be assigned o he lowe ac i i y o he
ma e ial, possibly due o he deple ion o Li on he su ace
55
and he
o ma ion o o he p oduc s du ing co osion.
53,54
Thus, he DBD
ea men i sel does no in luence he po en ial o he ma e ial.
Fo possible applica ions o Mg o Mg-based alloys as
implan s o small-size applica ions and no coa ings on o he
ma e ials, he hin ilms need o ha e a hickness in he μm ange
wi hou an addi ional subs a e. The p ocess o p oduce he hin
ilms used in his s udy is desc ibed by Ha ne e al.
34
Since he
sac i icial laye o Al and AlN is dissol ed in KOH, he ilms a e
also exposed o KOH o he du a ion o he li -o , esul ing in a
changed su ace. A ea men wi h KOH is epo ed o lowe he
co osion a e e en wi hou addi ional ea men i he sample is
anodized due o he o ma ion o mo e s able MgO and Mg
(OH)
2;36
howe e , his e ec is no obse ed he e, possibly due o
he insu icien hickness and densi y o he laye o med du ing
he simple imme sion. The e ec o he plasma ea men is lowe
on he ees anding samples; howe e , a dec ease in he co osion
a e is s ill isible [Fig. 3(c)]. Addi ional o he change o su ace
s uc u e, he samples on subs a e also had a la e su ace han
ees anding hin ilms which we e only a ached o Si chips, hus,
ensu ing a mo e homogeneous ea men . Thus, an op imiza ion o
he ea men and sample ixa ion o ees anding hin ilms could
imp o e he deg ada ion a e dec ease u he .
The co osion s udies p o e ha only e y sho DBD plasma
ea men s o Mg and MgLi a e equi ed o signi ican ly in luence
FIG. 3. Po en iodynamic pola iza ion measu emen s in Hanks’balanced sal solu ion (pH= 7.4 ± 0.2 and T = 37 ± 1 °C). (a) Exempla y Ta el plo s o Mg and Mg-1.6Li as
spu e ed on he subs a e and a e 15 s DBD plasma ea men . (b) Co osion a e (CR) o as-spu e ed Mg and Mg-1.6Li on he subs a e. (c) Co osion a es (CR) o
Mg-1.6Li hin ilms as spu e ed, a e 1 h imme sion and ees anding hin ilms (wi hou p eimme sion).
FIG. 4. SEM image o MgLi hin ilms, (a) c oss sec ion (inse shows he
su ace ilm wi h highe magni ica ion) and (b) su ace image o he su ace ilm
o med a e 15 s o DBD ea men .
ARTICLE pubs.aip.o g/a s/j a
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15 Ap il 2024 10:22:05
Resul s
64
FIG. 5. XPS spec a o un ea ed and ea ed MgLi hin ilms (a) ull spec um, (b) Mg 2p and Li 1 s, (c) C 1 s (posi ions o ca bona es, C–C and ca boxyla es a e ma ked,
see he supplemen a y ma e ial a o spec a wi h a ull desc ip ion o C compounds, i ed acco ding o Fo ea e al. (Re . 62, Fig. S1) and (d) O 1 s egions. See he sup-
plemen a y ma e ial Tables SII and SIII o he co esponding peak posi ions and ull-wid h-a -hal -maximum.
ARTICLE pubs.aip.o g/a s/j a
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15 Ap il 2024 10:22:05
Resul s
65
Appendix
Appendix
A.1 Supplemen S uc u al cha ac e isa ion and deg a-
da ion o Mg-Li hin ilms o biodeg adable implan s
lxxii

S uc u al cha ac e isa ion and deg ada ion o Mg-Li hin ilms o
biodeg adable implan s - Supplemen a y in o ma ion
Lisa Hanke1, Lea K. Jessen1, Felix Weishei 1, K a hika Bha 2, Ul ike Wes e ns öe 3, Die e
Ga be-Schönbe g3, Regine Willumei -Röme 2, Eckha d Quand 1*
1Ino ganic Func ional Ma e ials, Ins i u e o Ma e ials Science, Facul y o Enginee ing, Kiel Uni e si y, Kiel,
Ge many
2Ins i u e o Me allic Bioma e ials, Helmhol z Cen e he eon, Gees hach , Ge many
3Ma ine Clima e Resea ch, Ins i u e o Geosciences, Facul y o Ma hema ics and Na u al Sciences, Kiel
Uni e si y, Kiel, Ge many
Mg-Li alloy
Li (%(m/m))
Fe (%(m/m))
Ta ge
Film, ICP-MS
Film, AAS
Film, ICP-MS
Mg-1.6Li
2.5
1.60±0.06
1.55±0.03
0.0029±0.0008
Mg-3Li
5
3.07±0.12
3.13±0.08
0.0027±0.0003
Mg-5.5Li
9
5.15±0.69
6.12±0.32
0.0028±0.0004
Mg-9.5Li
14
9.31±0.84
9.88±0.07
0.0033±0.0006
Table 1 Mass ac ions o Li in a ge s (nominal) and p epa ed ees anding hin ilms analysed by
ICP-MS and AAS. Addi ionally, he Fe con amina ion measu ed by ICP-MS is gi en. The measu emen
unce ain ies gi en a e s a is ical de ia ions om minimum 3 samples.
Figu e 1 Recip ocal space maps measu ed wi h 2D- de ec o and sample il ing o χ=0°,15°,30°,45° o
a) Mg-1.6Li b) Mg-3Li c) Mg-5.5Li d) Mg-9.5Li and zoomed in a ea a ound he (002) and (101) hcp and
(110) bcc peak o e) Mg-5.5Li and ) Mg-9.5Li.
Appendix
lxxiii
Mg-1.6Li
Mg-3Li
Mg-5.5Li
Mg-9.5Li
a [Å]
3.208±0.002
3.202±0.002
3.196±0.004
3.200±0.002
c [Å]
5.197±0.005
5.175±0.007
5.156±0.010
5.161±0.008
Table 2 A e age la ice cons an s a und c and s anda d de ia ions calcula ed om XRD esul s o
Mg-Li alloys (Li: 1.6 %(m/m), 3 %(m/m), 5.5 %(m/m), 9.5 %(m/m)).
Mg-1.6Li
Mg-3Li
Mg-5.5Li
Mg-9.5Li
hcp
100
32.16±0.03
32.25±0.03
32.35±0.08
32.27±0.06
002
34.48±0.03
34.67±0.06
34.78±0.07
34.75±0.07
101
36.63±0.03
36.75±0.03
36.87±0.08
36.78±0.05
102
47.90±0.02
48.09±0.03
48.28±0.10
48.21±0.04
110
57.44±0.01
57.55±0.03
57.69±0.09
57.62±0.05
103
63.26±0.02
63.51±0.04
63.79±0.09
63.75±0.05
200
67.40±0.02
67.56±0.03
67.71±0.07
67.57±0.01
112
68.79±0.02
69.00±0.02
69.16±0.09
69.11±0.06
201
70.11±0.02
70.28±0.04
70.42±0.10
70.33±0.03
004
72.85±0.07
73.21±0.09
73.45
73.48±0.01
202
78.24±0.05
78.33±0.01
104
81.90±0.04
82.23±0.06
82.64±0.12
82.65±0.10
bcc
110
36.10±0.07
36.03±0.06
200
51.66±0.10
51.48
211
64.89±0.06
64.84±0.07
Li2CO3
110
21.39±0.09
21.29±0.06
200
23.38±0.06
23.32±0.07
111
23.49
111
29.48±0.04
29.38±0.03
202
30.67±0.09
30.52±0.06
002
31.77±0.06
31.65±0.05
112
34.11±0.01
34.02±0.06
020
36.13±0.03
36.06±0.03
021
39.65±0.01
39.64±0.10
310
39.93±0.02
39.91
221
42.60
112
42.60
220
43.55±0.02
43.45
130
56.77±0.04
331
65.41
Table 3 A e age 2θ angles (°) om XRD di ac og ams o Mg-1.6Li, Mg-3Li, Mg-5.5Li and Mg-9.5Li
wi h co esponding mille indices o hcp and bcc Mg-Li and Li2CO3 wi h. A minimum o h ee samples
was measu ed and s anda d de ia ions a e gi en i he peak was p esen in mul iple di ac og ams.
Appendix
lxxi
Figu e 2 XRD di ac og ams o Mg-1.6Li hin ilms wi h o ien a ions o (002) and (110). The main
o ien a ions a e indica ed.
Figu e 3 EDX line scans o c oss sec ions o Mg-1.6Li and Mg-9.5Li hin ilms a e co osion in HBSS
o e 3-5 days. The inse shows he a ea o he co osion laye .
MgLi alloy
Powe (W)
P essu e (10-3 mba )
Spu e ing a e (nm/s)
Mg-1.6Li
50
2.3
1.45
Mg-3Li
50
3.3
1.75
Mg-5.5Li
50
2.5
1.17
Mg-9.5Li
50
2.3
0.88
Table 4 Spu e pa ame e s o Mg-Li spu e ing a s anda d pa ame e s o s ess- ee ees anding
hin ilms and o Mg-3Li a di e en p essu e and powe .
Appendix
lxx
Appendix
A.2 Supplemen In es iga ion o in-si u ion elease and
su ace ilm o ma ion o hcp Mg-Li hin ilms
lxx i
Supplemen a y - In es iga ion o in-si u ion elease and su ace
ilm o ma ion o hcp Mg-Li hin ilms
Lisa Hanke1, Lukas Kalchg ube 2, Ul ike Wes e ns öe 3, Die e Ga be-Schönbe g3, Eckha d Quand 1,
Ma kus Val ine 2
1Ino ganic Func ional Ma e ials, Ins i u e o Ma e ials Science, Kiel Uni e si y, Kiel, Ge many
2Applied In e ace Physics, Ins i u e o Applied Physics, Vienna Uni e si y o Technology, Vienna, Aus ia
3Ma ine Clima e Resea ch, Ins i u e o Geosciences, Kiel Uni e si y, Kiel, Ge many
1. Calcula ion o dissol ed mass du ing 3-day co osion, measu ed by ICP-MS
Time
cMg [µg/L]
cLi [µg/L]
Blank
17910
<1
Mg-1.6Li,
sample 1
1 h
19139
12
4 h
19539
35
1 day
24593
142
3 days
36108
433
Mg-1.6Li,
sample 2
1 h
22117
16
4 h
19786
43
1 day
27067
226
3 days
43626
661
Mg-3Li,
sample 1
1 h
18132
32
4 h
20292
67
1 day
25677
360
3 days
29598
812
Mg-3Li,
sample 2
1 h
20210
47
4 h
19141
98
1 day
27323
379
3 days
39602
888
Mg-5.5Li,
sample 1
1 h
17134
111
4 h
20852
213
1 day
21810
847
3 days
37944
2120
Mg-5.5Li,
sample 2
1 h
19140
98
4 h
20840
224
1 day
26640
1598
3 days
41950
4714
Table 1. Measu ed Mg and Li concen a ions by ICP-MS in a blank HBSS solu ion (155 mmol, Hanks´
balanced sal s H1387, Sigma-Ald ich wi h added sodium bica bona e (0.35 g/L)), and a e 1 h, 4 h, 1
day and 3 days o co osion o Mg-1.6Li, Mg-3Li and Mg-5.5Li hin ilms in he solu ion a 37±1 °C and
pH 7.4±0.2.
The mass o he elemen (Mg o Li) in he solu ion a a ce ain measu emen s ep mi can be di ec ly
de e mined om he concen a ion ci de e mined (lis ed in able 1) and he olume adjus ed by he
Appendix
lxx ii

e apo a ion loss Vi. Howe e , o calcula e he o e all mass eleased in o he solu ion mg,i, he amoun
o ex ac ed solu ion (in he expe imen s o his s udy 15 ml) needs o be aken in o accoun . Thus,
mg,i can be desc ibed by
𝑚𝑔,𝑖 = 𝑚𝑔,𝑖−1 + 𝑚𝑖− 𝑚𝑙,𝑖−1
Wi h ml,i-1 as he emaining mass o he elemen in he o e all solu ion a e sample ex ac ion o he
s ep be o e. c0 is he concen a ion o elemen in he blank solu ion which is added in he same
amoun as solu ion is ex ac ed.
ml,i =mi
Vi
⋅(Vi−15ml)+ c0⋅15mL
2. Addi ional esul s
Figu e 1. Cu en I measu ed by he po en ios a in he low cell a cons an ol ages o -0.5 V s EOCV,
0.2 V s EOCV and -0.9 V s Ag/AgCl du ing 15 min a e 10 min a EOCV o wo samples pe alloy (Mg-
1.6Li and Mg-3Li).
iMg,CV/iLi,CV
(EOCV)
iMg,CV/iLi,CV
(-0.9 V s Ag/AgCl)
iMg,CV/iLi,CV
(+0.2 V s EOCV)
Mg-1.6Li
44.3±2.0
64.4±5.2
55.3±1.5
Mg-3Li
20.7±2.6
211.1±45.5
29.1±3.7
Table 2. Ra io o a e aged Mg and Li cu en densi y du ing 15 min a a cons an po en ial o EOCV,
anodic pola iza ion o -0.9 V s Ag/AgCl o +0.2 V s EOCV a e 10 min a EOCV o Mg-1.6Li and Mg-3Li
hin ilms.
Appendix
lxx iii
Figu e 2. Dissolu ion a e o wo samples pe alloy (Mg-1.6Li and Mg-3Li) o e LSV om EOCV un il -
0.5 V s Ag/AgCl, measu ed in si u by ICP-MS. The linea sweep is pe o med a e 10 min OCV, 15
min condi ioning and u he 5 min OCV.
Figu e 3. XPS spec a o a) Ca 2p and b) P 2p o Mg-1.6Li and Mg-3Li hin ilms a e 1 h co osion
and s o age in e hanol.
Appendix
lxxix
Appendix
A.3 Supplemen Tailo ing o Mg and MgLi hin- ilm co -
osion a e wi h dielec ic ba ie discha ge plasma
ea men
lxxx
Tailo ing o Mg and MgLi hin- ilm co osion a e
wi h dielec ic ba ie discha ge plasma ea men –
Supplemen a y In o ma ion
Lisa Hanke 1, To ge Ha ig 2, Felix Weishei 1, Tim Tja d s 2, Tim Pogoda 2,
F anz Faupel 2 and Eckha d Quand 1
1 Chai o Ino ganic Func ional Ma e ials, Ins i u e o Ma e ials Science, Facul y o Enginee ing, Kiel
Uni e si y, Kiel, Ge many
2 Chai o Mul icomponen Ma e ials, Ins i u e o Ma e ials Science, Facul y o Enginee ing, Kiel
Uni e si y, Kiel, Ge many
TABLE SI. A omic concen a ions o he su ace o un ea ed MgLi hin ilms and hin ilms
a e 15 s o DBD de e mined by XPS.
Un ea ed Sample
T ea ed Sample
Elemen
Peak used
o Analysis
Rela i e Amoun
(a %)
Elemen
Peak used
o Analysis
Rela i e Amoun
(a %)
C
C 1s
31.16
C
C 1s
13.00
O
O 1s
43.94
O
O 1s
61.74
Mg
Mg 2p
0.45
Mg
Mg 2p
19.59
Li
Li 1s
24.45
Li
Li 1s
4.12
Appendix
lxxxi
Lis o Figu es
Lis o Figu es
2.1 Slip sys ems in a hexagonal uni cell o Mg. . . . . . . . . . . . . . . . . . 5
2.2 Phase diag am o Mg-Li. Rep oduced and adap ed wi h pe mission om
Sp inge Na u e[76]. .............................. 8
2.3 S uc u e-zone model de eloped by Tho n on showing he mic os uc u e
o spu e ed ilms dependen on he empe a u e (subs a e empe a u e
Tsand mel ing empe a u e Tm) and he ine gas p essu e. Rep oduced
wi h pe mission om Else ie [96]. . . . . . . . . . . . . . . . . . . . . . . 10
2.4 Fab ica ion s eps o ees anding Mg alloy hin ilms using UV-li hog aphy,
magne on spu e ing and sac i icial laye s as pe o med o his wo k,
based on he p ocess o Ha ne e al. [26]. . . . . . . . . . . . . . . . . . . 12
2.5 Co osion a e dependen on he pH in Hank’s balanced sal solu ion
(bu e ed) o h ee exempla y Mg ma e ials (high pu i y Mg HP Mg, AZ91,
ZE41 Zn+REE). Rep oduced and adap ed wi h pe mission om Else ie
[111]. ...................................... 14
2.6 SEM images o Mg-3(w %)Li hin ilms be o e and a e ea men wi h
ch omicacid. .................................. 21
2.7 Schema ic se -up o po en iodynamic pola isa ion measu emen s wi h a
h ee-elec ode se -up including he sample in he sample holde as he
wo king elec ode (WE), a e e ence Ag/AgCl elec ode (RE) and a P
mesh coun e elec ode (CE). . . . . . . . . . . . . . . . . . . . . . . . . . 22
2.8 Schema ic o a po en iodynamic pola isa ion measu emen , depic ed as a
Ta elplo ..................................... 23
2.9 Schema ic o low cell as desc ibed in [196, 197] and inline ICP-MS mea-
su emen . a) Flow cell as used o he measu emen o Mg alloy samples,
including a P coun e elec ode (CE), e e ence elec ode (RE) and he
sample as a wo king elec ode (WE), connec ed by coppe ape. b) C oss
sec ion o low cell. c) O e all se -up wi h solu ion low h ough he low
cell o heICP-MS................................ 25
3.1 Compa ison o co osion a es o Mg-3Li hin ilms on subs a e spu e ed
wi h di e en spu e ing pa ame e s ( o powe change: a) cons an p es-
su e o 2.3·10−3mba , o p essu e change: b) cons an powe o 100 W). . 41
lxxx iii

Lis o Figu es
3.2 Compa ison o ees anding Mg-3Li spu e ed a di e en p essu e and
powe ( o powe change: cons an p essu e o 2.3·10−3mba , o p es-
su e change: cons an powe o 100 W). The ilm s ess can be iden i ied
by he olling o he ilms. . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
3.3 Mic os uc u e o Mg-3Li hin ilms spu e ed a di e en p essu e and
powe ( o powe change: cons an p essu e o 2.3·10−3mba , o p essu e
change: cons an powe o 100 W). . . . . . . . . . . . . . . . . . . . . . . 43
3.4 Compa ison o SEM and XRD analysis o Mg-3Li spu e ed wi h wo se s
o spu e ing pa ame e s (50 W, 2.3·10−3mba and 100 W, 4·10−3mba )
p oducing s ess- ee ilms. . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
3.5 Exempla y pho os o co osion o Mg-1.6Li o e 182 days in HBSS. . . . . 44
3.6 Co oded samples o a) Mg (55 days), b) Mg-1.6Li (62 days), c) Mg-3Li
(57days)inHBSS................................ 45
3.7 SEM o Mg-1.6Li samples a e 63 days in HBSS. a) C oss sec ion o whi e,
co oded a ea, b) c oss sec ion o da k a ea, c) su ace. . . . . . . . . . . . 45
3.8 XRD o a) Mg a e 56 days, b) Mg-1.6Li samples a e 66 days, c) Mg-3Li
a e 74 days co osion in HBSS on whi e and g ey sample a ea. Addi ion-
ally, XRD di ac og ams o he samples be o e co osion a e shown. . . . . 46
3.9 Mg-1.6Li samples in di e en solu ions a e sho (1-2 days) and longe
co osion ( o as co oding samples 6 days, o slow co oding samples
a ound60days). ................................ 47
3.10 Compa ison o Mg-1.6Li and Mg-6Ag-1.6Li. a) Di ac og ams o hin ilms.
In ed, he posi ions o a MgAgLi phase (cubic, based on [208]) a e ma ked.
b) Exempla y s ess-s ain cu es o Mg-1.6Li, Mg-3Li and Mg-6Ag-1.6Li. . 69
lxxxix
Lis o Tables
Lis o Tables
2.1 Pilling-Bedwo h a ios (PBR) and solubili y p oduc cons an s (Ksp a
25 ◦C o possible co osion p oduc s o MgLi in sal solu ion. . . . . . . . . 17
3.1 Se o spu e ing pa ame e s o magne on spu e ing o Mg-3Li o analyse
he in luence o powe and p essu e change on he s uc u e and co osion
a eo he hin ilms............................... 41
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Acknowledgemen s
Acknowledgemen s
This wo k would no ha e been possible wi hou all he g ea suppo I ecei ed. The e-
o e, I would like o hank:
My supe iso P o . D . Eckha d Quand o being able o wo k in his g oup on his
in e es ing, in e disciplina y opic and he suppo and help ul sugges ions du ing he las
ou yea s.
P o . D . Regine Willumei -R¨ome o he suppo du ing my PhD ega ding magne-
sium and co osion speci ic ques ions as my second supe iso .
My colleagues o he g oup ino ganic unc ional ma e ials o helping me o lea n he
necessa y machines and o he gene al suppo bo h scien i ic and pe sonal, especially
Lea Jessen o eaching me e e y hing abou Mg hin ilms and measu emen echniques
a ailable, bu also helping me o gene ally eeling welcomed in he g oup when I s a ed.
Thanks o Hanna Lewi z, La s Tho m¨alen, D . Jus in Je e , Felix Weishei and Duygu
Dengiz o being no only a g ea help by p ac ical suppo , answe ing ques ions and
discussing p oblems bu also by mo al suppo . A special hanks goes o La s o keeping
he machines ali e so ha expe imen al wo k was possible.
My colleagues om he esea ch aining g oup ”Ma e ials o B ain”, especially K a hika
Bha , o con inuous discussions o e he ime o he PhD and a lo o help o unde s and
he equi emen s om a biological poin o iew bu also he willingness o lea n he
ma e ial science pa so ha ac ual exchange was possible.
P o . D . Ma kus Val ine o no only hos ing me du ing my esea ch a he TU Vienna,
bu also being a ailable o any ques ions and discussions du ing ha s ay and mo i a ing
me o de elop a deepe unde s anding o my scien i ic p oblems. I would like o ex end
my g a i ude o he g oup membe s in Vienna, Lukas Kalchg ube , D . Lau a Mea s
and Ma eo Olgia i o helping me lea n he machines, answe ing all my ques ions and
helping me wi h u he measu emen s and e e yone else o c ea ing a pleasan wo king
a mosphe e.
Ul ike Wes e ns ¨oe and by ex ension D . Die e -Ga be Sch¨onbe g o ag eeing o mea-
su ing my samples by ICP-MS wi hou which many pa s o my s udies would no ha e
been possible o ele an .
Las bu no leas I would like o hank my amily, especially my pa en s and my sis-
e , and my iends o con inuously suppo ing and mo i a ing me.
c i