coa ings
A icle
The E ec o Hea T ea men on P ope ies o Ni–P
Coa ings Deposi ed on a AZ91 Magnesium Alloy
Ma in Buch ík * , Michaela K ys ýno á, Jiˇ íMásilko and Ja omí Wasse baue
Ma e ials Resea ch Cen e, Facul y o Chemis y, B no Uni e si y o Technology, Pu kyˇno a 464/118, 61200 B no,
Czech Republic
*Co espondence: [email p o ec ed]; Tel.: +420-736-445-019
Recei ed: 10 June 2019; Accep ed: 19 July 2019; Published: 23 July 2019


Abs ac :
The p esen s udy epo s he e ec o phospho us con en in deposi ed elec oless nickel
(Ni–P) coa ings, he hea ea men on he mic oha dness and i s mic os uc u al cha ac e is ics,
and he in luence o he empe a u e on he mic os uc u e o he Mg alloy subs a e du ing he hea
ea men . The deposi ion o Ni–P coa ings was ca ied ou in he elec oless nickel ba h, and he
esul ing P con en anged om 5.2 o 10.8 w .%. P epa ed samples we e hea - ea ed in he mu le
u nace a 400
◦
C o 1 h a e he coa ing deposi ion. The cooling o he samples o oom empe a u e
was p oceeded in he ai . Fo as-deposi ed and hea - ea ed samples, i was de e mined ha wi h he
inc easing P con en , he mic oha dness was dec easing. This may be caused by he changes in he
s uc u e o he Ni–P coa ing. The X- ay di ac ion pa e ns o he as-deposi ed Ni–P coa ings showed
ha he mic os uc u e changed hei na u e om c ys alline o amo phous wi h he inc easing P
con en . The hea ea men o p epa ed samples led o he signi ican inc ease o mic oha dness
o Ni–P coa ings. All he hea - ea ed samples showed he c ys alline cha ac e , ega dless o he
P con en and he p esence o ha d Ni
3
P phase, which can ha e a posi i e e ec on he inc ease o
mic oha dness. The me allog aphic analysis showed changes o subs a e mic os uc u e a e he
hea ea men . The p epa ed coa ings we e uni o m and wi h no isible de ec s.
Keywo ds:
Ni–Pcoa ings; Ni
3
Pphase; Mgalloys; AZ91; hea ea men ; mic oha dness; c ys alli esize
1. In oduc ion
Magnesium alloys a e he ligh es s uc u al me allic ma e ials [
1
,
2
]. Due o hei excep ional
p ope ies, such as a low densi y, s i ness, speci ic s eng h, good cas abili y, and machinabili y, hey a e
desi able in a ious indus ies [
3
–
5
]. One o he bigges limi s o he widesp ead use o magnesium
alloys is hei poo co osion, wea esis ance, and low ha dness [
1
,
5
–
7
]. These p oblems a e o en
esol ed by means o su ace coa ings. Elec oless nickel (Ni–P) deposi ion seems o be an app op ia e
a ian o p o ec magnesium alloy subs a es [
8
]. Elec oless Ni–P coa ings a e mainly used due
o hei excellen co osion esis ance, high ha dness, and wea esis ance. Howe e , p ope ies o
Ni–P coa ings a e s ongly dependen on hei chemical composi ion, i.e., he phospho us (P) con en
in he coa ing [
9
]. In e ms o he chemical composi ion, elec oless Ni–P coa ings can be di ided
in o h ee g oups: Low phospho us (1–5 w .% o P), medium phospho us (6–9 w .% o P), and high
phospho us (10–13 w .% o P) [
9
–
11
]. Low phospho us Ni–P coa ings a e p edominan ly c ys alline
and less co osion esis an compa ed o he medium and high P coa ings. They a e cha ac e is ic wi h
a high ha dness and good mechanical and ibological p ope ies. The c ys alline cha ac e o he low
phospho us coa ings indica es ha he numbe o phospho us a oms in in e s i ial posi ions is no
su icien o he dis o ion o he nickel la ice [11,12].
High phospho us Ni–P coa ings a e known o excellen co osion esis ance due o hei
amo phous mic os uc u e [11–13].
Coa ings 2019,9, 461; doi:10.3390/coa ings9070461 www.mdpi.com/jou nal/coa ings
Coa ings 2019,9, 461 2 o 9
Duncan [
11
] s a ed ha Ni–P coa ing is in non-equilib ium s a e a e deposi ion. Ni–P coa ing
is o med by a c ys alline solid solu ion o P in Ni, called he
β
phase (low phospho us), he o al
amo phous
γ
phase, which exis s be ween 11–15 w .% P (high phospho us), o he mix u e o
β+γ
phase (medium phospho us). These me as able phases a e cha ac e ized by decomposi ion
eac ions du ing he hea ea men o o m he equilib ium
α
phase (solid solu ion o P in Ni) and
Ni3P phase.
C ys alline nickel ( he
α
phase) and he Ni
3
P phase a e only s able p oduc s a e he hea
ea men . These s able phases begin o o m om 300
◦
C. The op imal empe a u e ange o he hea
ea men o he Ni–P coa ings is wi hin he empe a u e ange o 300 o 400
◦
C. Riedel [
10
] s a ed
ha i is ad isable o pe o m hea ea men a 400
◦
C o 1 h o achie e he maximum ha dness o
Ni–P coa ings. The inc ease in p ecipi a e size and coa ing g ain coa sening was obse ed a applied
empe a u es highe han 400
◦
C and longe ea ing imes du ing he hea ea men p ocess, ega dless
o he P con en [14,15].
A sui able hea ea men p ocess can esul in an inc ease in he coa ing ha dness, up o 1300 HV.
This is because o he ec ys alliza ion o a non-equilib ium
β
phase (low phospho us), an amo phous
γ
phase (high phospho us), o hei mix u e (medium phospho us) in o he equilib ium c ys alline
αphase, wi h a simul aneous p ecipi a ion o he ha d in e media e Ni3P phase [10,14].
Kuma [
16
] epo ed ha he c ys alli e size o Ni changes wi h he inc easing hea ea men
empe a u e. F om he oom empe a u e o 100
◦
C, he e was only a negligible change in he Ni
c ys alli e size. Be ween 100 and 300
◦
C, he inc ease in c ys alli e size o Ni was e iden due o he
a angemen o Ni a oms in he la ice. Howe e , no o ma ion o any in e media e p ecipi a e pa icles
was de ec ed. Au ho s also lis ed ha he disappea ance o he amo phous phase was obse ed a
330
◦
C, wha indica es he comple e c ys alliza ion o he mic os uc u e. A a empe a u e abo e
300
◦
C, a signi ican inc ease in c ys alli e size was obse ed, p obably due o he o ma ion o Ni
3
P
phase pa icles.
Mos o he published s udies a e ocused on he in luence o hea ea men and P con en in
Ni–P coa ings deposi ed on he s eels. Howe e , he hea ea men o Ni–P coa ings o achie e he
maximum ha dness is pe o med in he empe a u e o 400
◦
C o 1 h. This empe a u e does no
in luence he mic os uc u e o s eels bu may ha e signi ican e ec on magnesium alloys. The e o e,
his s udy deals wi h he e ec s o he hea ea men o Ni–P coa ings wi h he a ious P con en
deposi ed on a AZ91 magnesium alloy. The mic os uc u e o a AZ91 alloy and he cha ac e iza ion o
Ni–P coa ings, such as mic oha dness, phase composi ion, and c ys alli e size, we e e alua ed be o e
and a e he hea ea men .
2. Ma e ials and Me hods
Samples o a cas AZ91 magnesium alloy wi h dimensions o 30
×
30
×
7 mm
3
we e chosen as
subs a es o he elec oless deposi ion o Ni–P coa ings. The elemen al composi ion o he AZ91 alloy,
analyzed using he glow-discha ge op ical emission spec oscopy (GDOES) Spec uma GDS 750
(Spec uma Analy ik GmbH, Ho , Ge many), is lis ed in Table 1. To ob ain an app op ia e su ace,
he samples o he Mg alloy we e g ound using no. 1200 SiC pape be o e he p e- ea men p ocess.
Du ing he p e- ea men p ocess, g ound samples we e deg eased in an alkali ba h and hen pickled in
an acid-pickling ba h o ac i a e he su ace. A e each s ep o he p e- ea men , samples we e insed
in dis illed wa e and isop opyl alcohol and hen d ied in ho ai . The deposi ion o Ni–P coa ings
was ca ied ou in he elec oless nickel ba h wi h di e en Ni
2+
/H
2
PO
2−
a ios. Indi idual a ios o
Ni
2+
/H
2
PO
2−
we e se a 0.1, 0.2, 0.3, 0.45, and 0.75. The chemical composi ion was cha ac e ized
using a Zeiss EVO LS-10 (Ca l Zeiss L d., Camb idge, UK) scanning elec on mic oscope (SEM)
wi h ene gy-dispe si e spec oscopy (EDS) Ox o d Ins umen s Xmax 80 mm
2
de ec o (Ox o d
Ins umen s plc, Abingdon, UK) and he AZ ec so wa e ( e sion 2.4).
Coa ings 2019,9, 461 3 o 9
Table 1.
Elemen al composi ion o he AZ91 Mg alloy, glow-discha ge op ical emission
spec oscopy (GDOES).
Elemen Al Zn Cu Mn Si Fe Ni Z Mg
Con en [w .%] 8.80 0.81 0.00 0.32 0.01
0.004
0.00 0.01 Bal.
P epa ed samples we e hea - ea ed in he mu le u nace LAC LM07 (LAC, s. .o., Židlocho ice,
Czech Republic) a 400
◦
C o 1 h a e he coa ing deposi ion. The cooling o he samples o oom
empe a u e was p oceeded in he ai .
The mic os uc u e o he Ni–P coa ings and AZ91 magnesium alloy was cha ac e ized using an
Axio Obse e Z1m (ZEISS) ligh mic oscope and a Zeiss EVO LS-10 scanning elec on mic oscope.
The mic oha dness o he deposi ed Ni–P coa ings was measu ed using a LECO AMH55
mic oha dness es e (Sain Joseph, MO, USA). The mic oha dness was pe o med and e alua ed
acco ding o he ASTM E384 s anda d. The mic oha dness was measu ed om he pe pendicula cu .
The samples we e g ound and polished using a Teg amin-25 (S ue s) au oma ic g inde wi h a special
holde o p epa a ion o plana specimens. The inal s ep was polishing, using diamond pas e wi h
0.25
µ
m pa icle size. Iso-p opanol was used as a lub ican . The Vicke s me hod was used wi h he
applied load o 25 g o 10 s. The mic oha dness alue was de e mined om 10 alues.
Fo he de e mina ion and cha ac e iza ion o he Ni–P coa ings phase composi ion, he coa ings
we e mechanically sepa a ed om he subs a e, milled, and analyzed in he powde o m using
he Sche e me hod. The analysis was pe o med on an Empy ean (Panaly ical) X-Ray di ac ion
spec ome e wi h Cu-anode (
λ
K
α1
=0.15406 nm,
λ
K
α2
=0.15444 nm) a oom empe a u e. The scan
s ep size was se up a 0.013
◦
. The ob ained da a we e p ocessed using High Sco e Plus so wa e.
The c ys alli e size o Ni and Ni
3
P was calcula ed om he ull wid h hal maximum (FWHM)
acco ding o he Sche e equa ion [17] (Equa ion (1)):
τ=
K·λ
β1/2·cosθ(1)
whe e
τ
is he c ys alli e size,
λ
is he X- ay wa eleng h,
β1/2
is he peak ex ension a hal o he
maximum in ensi y (FWHM),
θ
is he di ac ion B agg’s angle, and Kis he pa icles shape ac o
(Sche e cons an ) depending on he shape o he c ys alli es. K, anging om 0.62 o 2.08, is usually
close o 1. Fo pe ec ly ounded c ys als, Kis equal o 0.89.
3. Resul s and Discussion
3.1. Mic os uc u e and Chemical Composi ion
Table 2shows he esul s o he chemical composi ion o he as-deposi ed and hea - ea ed Ni–P
coa ings, deposi ed on he AZ91 alloy wi h di e en Ni
2+
/H
2
PO
2−
a ios in he elec oless nickel ba h.
The a e age phospho us con en in as-deposi ed and hea - ea ed coa ings was simila o he same
Ni
2+
/H
2
PO
2−
a ios, he e o e he e isonly one alue o eachNi
2+
/H
2
PO
2−
a io. Thea e agePcon en
anges om app oxima ely 5 w .% o 11 w .% o P, bo h o as-deposi ed and hea - ea ed coa ings.
Table 2.
The phospho us con en o elec oless nickel (Ni–P) as-deposi ed and hea - ea ed coa ings in
dependence on he Ni2+/H2PO2− a ios, ene gy-dispe si e spec oscopy (EDS).
Ni2+/H2PO2−Ra io P Con en [w .%]
0.75 5.2 ±0.2
0.45 5.5 ±0.1
0.3 7.4 ±0.1
0.2 10.1 ±0.2
0.1 10.8 ±0.1
Coa ings 2019,9, 461 4 o 9
Figu e 1shows he mic os uc u e o as-deposi ed and hea - ea ed Ni–P. The mic os uc u es
o as-deposi ed and hea - ea ed Ni–P coa ings we e simila , ega dless o he chemical composi ion.
The a e age hickness o all coa ings was app oxima ely 30
µ
m. The coa ing was uni o m wi hou
s uc u al de ec s and he e was no undesi able in e laye be ween he magnesium alloy subs a e and
Ni–P coa ing. The hea ea men did no a ec he hickness o o e all chemical composi ion o he
deposi ed Ni–P coa ings.
Coa ings 2019, 9, x FOR PEER REVIEW 4 o 9
s uc u al de ec s and he e was no undesi able in e laye be ween he magnesium alloy subs a e
and Ni–P coa ing. The hea ea men did no a ec he hickness o o e all chemical composi ion o
he deposi ed Ni–P coa ings.
Figu e 1. Mic os uc u e o Ni–P coa ings wi h 7.4 w .% P (a) as-deposi ed; (b) hea - ea ed.
As shown in Figu e 2a, he mic os uc u e o he cas AZ91 magnesium alloy consis s o (1) α
solid solu ion o Al in Mg, (2) discon inuous p ecipi a es o in e me allic Mg
17
Al
12
-β
D
phase, and (3)
eu ec ic α + β [1,2].
In he case o hea - ea ed samples, he mic os uc u e o he AZ91 alloy was changed. As seen
in Figu e 2b, he p esence o he discon inuous p ecipi a e o he Mg
17
Al
12
phase was no obse ed.
This inding can be explained by he ac ha he discon inuous p ecipi a e was dissol ed in α solid
solu ion o Al in Mg du ing he hea ea men a 400 °C o 1 h. Due o he as cooling in he ai a e
being emo ed om he u nace, discon inuous p ecipi a es o he Mg
17
Al
12
-β
D
phase we e no
p esen [2]. Howe e , he Mg
17
Al
12
-β phase and eu ec ic α + β was s ill obse ed in he
mic os uc u e.
Because o he dissolu ion o he discon inuous p ecipi a es o he Mg
17
Al
12
-β
D
phase a 400 °C,
he con en o Al in he α solid solu ion inc eased, which may lead o imp o emen o some
mechanical p ope ies due o he solid solu ion s eng hening.
Figu e 2. Mic os uc u e o he AZ91 Mg alloy (a) as-cas , (b) hea - ea ed a 400 °C o 1 h.
3.2. Mic oha dness o Ni–P Coa ings
As s a ed in he li e a u e [10,18], low-phospho us Ni–P coa ings a e c ys alline,
medium-phospho us coa ings a e mic oc ys alline, and high-phospho us Ni–P coa ings a e
amo phous. The mic os uc u e o deposi ed Ni–P coa ings s ic ly a ec s hei p ope ies [12,13]. In
gene al, he mic oha dness dec eases wi h he inc easing P con en .
F om he esul s o mic oha dness, he measu emen can be s a ed ha he highes
mic oha dness alue was obse ed in he case o he Ni–P coa ing wi h he lowes P. Wi h he
inc easing P con en , he mic oha dness dec eased in he case o bo h he as-deposi ed and in he
hea - ea ed coa ings (Figu e 3), which is in co ela ion wi h he li e a u e [12,13,19]. Ash iani e al.
Figu e 1. Mic os uc u e o Ni–P coa ings wi h 7.4 w .% P (a) as-deposi ed; (b) hea - ea ed.
As shown in Figu e 2a, he mic os uc u e o he cas AZ91 magnesium alloy consis s o (1)
α
solid solu ion o Al in Mg, (2) discon inuous p ecipi a es o in e me allic Mg
17
Al
12
-
βD
phase, and
(3) eu ec ic α+β[1,2].
Coa ings 2019, 9, x FOR PEER REVIEW 4 o 9
s uc u al de ec s and he e was no undesi able in e laye be ween he magnesium alloy subs a e
and Ni–P coa ing. The hea ea men did no a ec he hickness o o e all chemical composi ion o
he deposi ed Ni–P coa ings.
Figu e 1. Mic os uc u e o Ni–P coa ings wi h 7.4 w .% P (a) as-deposi ed; (b) hea - ea ed.
As shown in Figu e 2a, he mic os uc u e o he cas AZ91 magnesium alloy consis s o (1) α
solid solu ion o Al in Mg, (2) discon inuous p ecipi a es o in e me allic Mg
17
Al
12
-β
D
phase, and (3)
eu ec ic α + β [1,2].
In he case o hea - ea ed samples, he mic os uc u e o he AZ91 alloy was changed. As seen
in Figu e 2b, he p esence o he discon inuous p ecipi a e o he Mg
17
Al
12
phase was no obse ed.
This inding can be explained by he ac ha he discon inuous p ecipi a e was dissol ed in α solid
solu ion o Al in Mg du ing he hea ea men a 400 °C o 1 h. Due o he as cooling in he ai a e
being emo ed om he u nace, discon inuous p ecipi a es o he Mg
17
Al
12
-β
D
phase we e no
p esen [2]. Howe e , he Mg
17
Al
12
-β phase and eu ec ic α + β was s ill obse ed in he
mic os uc u e.
Because o he dissolu ion o he discon inuous p ecipi a es o he Mg
17
Al
12
-β
D
phase a 400 °C,
he con en o Al in he α solid solu ion inc eased, which may lead o imp o emen o some
mechanical p ope ies due o he solid solu ion s eng hening.
Figu e 2. Mic os uc u e o he AZ91 Mg alloy (a) as-cas , (b) hea - ea ed a 400 °C o 1 h.
3.2. Mic oha dness o Ni–P Coa ings
As s a ed in he li e a u e [10,18], low-phospho us Ni–P coa ings a e c ys alline,
medium-phospho us coa ings a e mic oc ys alline, and high-phospho us Ni–P coa ings a e
amo phous. The mic os uc u e o deposi ed Ni–P coa ings s ic ly a ec s hei p ope ies [12,13]. In
gene al, he mic oha dness dec eases wi h he inc easing P con en .
F om he esul s o mic oha dness, he measu emen can be s a ed ha he highes
mic oha dness alue was obse ed in he case o he Ni–P coa ing wi h he lowes P. Wi h he
inc easing P con en , he mic oha dness dec eased in he case o bo h he as-deposi ed and in he
hea - ea ed coa ings (Figu e 3), which is in co ela ion wi h he li e a u e [12,13,19]. Ash iani e al.
Figu e 2. Mic os uc u e o he AZ91 Mg alloy (a) as-cas , (b) hea - ea ed a 400 ◦C o 1 h.
In he case o hea - ea ed samples, he mic os uc u e o he AZ91 alloy was changed. As seen
in Figu e 2b, he p esence o he discon inuous p ecipi a e o he Mg
17
Al
12
phase was no obse ed.
This inding can be explained by he ac ha he discon inuous p ecipi a e was dissol ed in
α
solid
solu ion o Al in Mg du ing he hea ea men a 400
◦
C o 1 h. Due o he as cooling in he ai
a e being emo ed om he u nace, discon inuous p ecipi a es o he Mg
17
Al
12
-
βD
phase we e no
p esen [
2
]. Howe e , he Mg
17
Al
12
-
β
phase and eu ec ic
α
+
β
was s ill obse ed in he mic os uc u e.
Because o he dissolu ion o he discon inuous p ecipi a es o he Mg
17
Al
12
-
βD
phase a 400
◦
C,
he con en o Al in he
α
solid solu ion inc eased, which may lead o imp o emen o some mechanical
p ope ies due o he solid solu ion s eng hening.
Coa ings 2019,9, 461 5 o 9
3.2. Mic oha dness o Ni–P Coa ings
As s a ed in he li e a u e [
10
,
18
], low-phospho us Ni–P coa ings a e c ys alline,
medium-phospho us coa ings a e mic oc ys alline, and high-phospho us Ni–P coa ings a e amo phous.
The mic os uc u e o deposi ed Ni–P coa ings s ic ly a ec s hei p ope ies [
12
,
13
]. In gene al,
he mic oha dness dec eases wi h he inc easing P con en .
F om he esul s o mic oha dness, he measu emen can be s a ed ha he highes mic oha dness
alue was obse ed in he case o he Ni–P coa ing wi h he lowes P. Wi h he inc easing P con en ,
he mic oha dness dec eased in he case o bo h he as-deposi ed and in he hea - ea ed coa ings
(Figu e 3), which is in co ela ion wi h he li e a u e [
12
,
13
,
19
]. Ash iani e al. [
19
] epo ed ha in
he case o he Ni–P coa ings hea - ea ed a 400
◦
C, he mic oha dness dec eased wi h he inc easing
P con en , which is in ag eemen wi h p esen ed da a. Deposi ed Ni–P coa ing wi h 9.35 w .% o P
eached he mic oha dness o 970 HV 50 g . Meanwhile, he coa ings wi h 10.31 and 11.45 w .% o P
eached he mic oha dness o 856 HV 50 g and 788 HV 50 g , espec i ely.
Coa ings 2019, 9, x FOR PEER REVIEW 5 o 9
[19] epo ed ha in he case o he Ni–P coa ings hea - ea ed a 400 °C, he mic oha dness
dec eased wi h he inc easing P con en , which is in ag eemen wi h p esen ed da a. Deposi ed Ni–P
coa ing wi h 9.35 w .% o P eached he mic oha dness o 970 HV 50 g . Meanwhile, he coa ings
wi h 10.31 and 11.45 w .% o P eached he mic oha dness o 856 HV 50 g and 788 HV 50 g ,
espec i ely.
Figu e 3b shows ha he hea - ea ed Ni–P coa ings had highe alue o mic oha dness when
compa ed o he as-deposi ed coa ings wi h he same P con en . Du ing he hea ea men , all he
coa ings (depending on he P con en ) became mo e c ys alline due o he ea angemen o he
s uc u e and he ans o ma ion o he non-equilib ium solid solu ion P in he Ni-β phase (low
phospho us), o al amo phous γ phase (high phospho us), o hei mix u e (medium phospho us) o
equilib ium c ys alline solid solu ion P in Ni-α phase. Simul aneously, hea ea ing lead o he
o ma ion o he ha d body cen e ed e agonal Ni
3
P phase. The p esence o Ni
3
P esul s in an o e all
inc ease o he mic oha dness o he coa ings [9,10]. The p esence o he Ni
3
P phase in he
hea - ea ed coa ings was con i med by XRD analysis.
Figu e 3. Mic oha dness dependence on he phospho us con en o Ni–P coa ings, (a) as-deposi ed,
(b) hea - ea ed.
3.3. Phase Analysis o Ni–P Coa ings
The XRD pa e ns co esponding o he indi idual measu emen s ep esen ing Ni–P coa ings
a e p o ided in Figu e 4. As seen in he Figu e 4a, he peak co esponding o he cc nickel c ys al
la ice (1 1 1) can be obse ed nea he di ac ion angle 2θ ≈ 45°. A b oad peak co esponding o he
Ni di ac ion was obse ed in he case o high-phospho us coa ings, and wi h he dec easing P
con en , he peak o Ni became sha pe . This e ec indica es a mo e o de ed in e nal mic os uc u e
[18]. The highes in ensi y o Ni di ac ion was measu ed o he Ni–P coa ing wi h 5.5 ± 0.1 w .% o
P. Meanwhile, he lowes in ensi y and he b oades peak was obse ed in he case o he Ni–P
coa ing wi h 10.8 ± 0.1 w .% o P.
Excep o he di ac ion o Ni, he e we e clea di ac ions be ween 30°–40° and a ound he
angle 2θ ≈ 48°. Gu [20] lis ed ha hese XRD peaks co espond o he p ima y α-Mg phase and he
Mg
17
Al
12
phase. The α-Mg phase was also de ec ed in he wo k o Hu [21].
The p esence o he phase pa icles can be explained by he ac ha he e was a join sepa a ion
o he Mg alloy oge he wi h he Ni–P coa ing du ing he mechanical sepa a ion. Howe e , he
p esence o hese phases in he es ed powde did no a ec he mic os uc u al changes obse ed in
deposi ed Ni–P coa ings.
Figu e 4b shows he pa e ns o hea - ea ed Ni–P coa ings wi h di e en P con en . The
di ac ion o Ni (1 1 1) can be seen nea he di ac ion angle 2θ ≈ 44.4°. Ano he di ac ion o Ni (2 0
0) can be seen a 2θ ≈ 51.8°. Fo bo h o he Ni di ac ions, i can be obse ed ha hei in ensi y
inc eases and peaks become mo e sha p wi h he dec easing P con en (excep he P con en 5.2 ±
0.2 w .%, which is sligh ly lowe han he peak o he Ni–P coa ing wi h he P con en 5.5 ± 0.1 w .%).
Figu e 4b shows ha he p esence o he Ni
3
P s able phase was ob ious o all he coa ings and
he in ensi y o he Ni
3
P phase inc eased wi h he inc easing P con en . F om pa e ns shown in
Figu e 3.
Mic oha dness dependence on he phospho us con en o Ni–P coa ings, (
a
) as-deposi ed,
(b) hea - ea ed.
Figu e 3b shows ha he hea - ea ed Ni–P coa ings had highe alue o mic oha dness when
compa ed o he as-deposi ed coa ings wi h he same P con en . Du ing he hea ea men , all he
coa ings (depending on he P con en ) became mo e c ys alline due o he ea angemen o he s uc u e
and he ans o ma ion o he non-equilib ium solid solu ion P in he Ni-
β
phase (low phospho us),
o al amo phous
γ
phase (high phospho us), o hei mix u e (medium phospho us) o equilib ium
c ys alline solid solu ion P in Ni-
α
phase. Simul aneously, hea ea ing lead o he o ma ion o he
ha d body cen e ed e agonal Ni
3
P phase. The p esence o Ni
3
P esul s in an o e all inc ease o he
mic oha dness o he coa ings [
9
,
10
]. The p esence o he Ni
3
P phase in he hea - ea ed coa ings was
con i med by XRD analysis.
3.3. Phase Analysis o Ni–P Coa ings
The XRD pa e ns co esponding o he indi idual measu emen s ep esen ing Ni–P coa ings a e
p o ided in Figu e 4. As seen in he Figu e 4a, he peak co esponding o he cc nickel c ys al la ice
(1 1 1) can be obse ed nea he di ac ion angle 2
θ≈
45
◦
. A b oad peak co esponding o he Ni
di ac ion was obse ed in he case o high-phospho us coa ings, and wi h he dec easing P con en ,
he peak o Ni became sha pe . This e ec indica es a mo e o de ed in e nal mic os uc u e [
18
].
The highes in ensi y o Ni di ac ion was measu ed o he Ni–P coa ing wi h 5.5
±
0.1 w .% o P.
Meanwhile, he lowes in ensi y and he b oades peak was obse ed in he case o he Ni–P coa ing
wi h 10.8 ±0.1 w .% o P.

Coa ings 2019,9, 461 6 o 9
Coa ings 2019, 9, x FOR PEER REVIEW 6 o 9
Figu e 4b, i is e iden ha in he case o low-phospho us Ni–P coa ings, he Ni phase c ys allizes
mo e ( he peak o Ni is sha pe and wi h highe in ensi y) when compa ed o he high-phospho us
Ni–P coa ings. On he o he hand, he Ni
3
P phase p ecipi a ed and g ew mo e in he case o he
high-phospho us Ni–P coa ing. The p esence o he Ni
12
P
5
me as able phase was obse ed in he case
o high-phospho us Ni–P coa ings (10.2 and 10.8 w .% o P) a ound he di ac ion angle 2θ ≈ 47° o
48°. Acco ding o he li e a u e [22,23], he Ni
12
P
5
me as able phase should comple ely disappea
a ound he empe a u e o 350 °C. Howe e , Keong [24] showed ha he Ni
12
P
5
phase may s ill be
p esen a 400 °C. The p esence o his phase could be caused by he incomple e ans o ma ion om
he o iginally amo phous ma ix o he mix u e o c ys alline Ni and he Ni
3
P s able phase.
Figu e 4. XRD pa e ns o (a) as-deposi ed and (b) hea - ea ed Ni–P coa ings wi h di e en
phospho us con en .
3.5 C ys alli e Size
Figu e 5 shows he e ec o he phospho us con en on he Ni c ys alli e size in as-deposi ed
coa ings and hea - ea ed coa ings. Only one di ac ion plane Ni (1 1 1) was obse ed (see Figu e 4a)
and wo di ac ion planes (1 1 1) and (2 0 0) o Ni we e obse ed (Figu e 4b) in he case o
as-deposi ed and hea - ea ed coa ings by XRD, espec i ely. Figu e 5 shows ha wi h he inc easing
P con en , he c ys alli e size o nickel dec eases, bo h in he case o he as-deposi ed and hea - ea ed
coa ings. This ac can be explained due o he inc easing la ice diso de (a g ea e p opo ion o he
amo phous phase) wi h he inc easing P con en in he Ni–P ma ix [12,24].
As can be seen in Figu e 5a, he c ys alli es o Ni in low-phospho us as-deposi ed Ni–P coa ings
eached app oxima ely 40 Å, whe eas he c ys alli es o Ni in he Ni–P coa ing wi h 10.8 w .% o P
eached he size o 13.1 Å. A e he hea ea men , he c ys alli e size o Ni subs an ially inc eased
o mo e han 300 Å in he case o low-phospho us coa ings in he plane (1 1 1). A simila end was
obse ed o he di ac ion plane (2 0 0).
Figu e 5. The e ec o Ni c ys alli e size on he P con en in (a) as-deposi ed Ni–P coa ings and (b)
hea - ea ed Ni–P coa ings.
Figu e 4.
XRD pa e ns o (
a
) as-deposi ed and (
b
) hea - ea ed Ni–P coa ings wi h di e en
phospho us con en .
Excep o he di ac ion o Ni, he e we e clea di ac ions be ween 30
◦
–40
◦
and a ound he
angle 2
θ≈
48
◦
. Gu [
20
] lis ed ha hese XRD peaks co espond o he p ima y
α
-Mg phase and he
Mg17Al12 phase. The α-Mg phase was also de ec ed in he wo k o Hu [21].
The p esence o he phase pa icles can be explained by he ac ha he e was a join sepa a ion o
he Mg alloy oge he wi h he Ni–P coa ing du ing he mechanical sepa a ion. Howe e , he p esence
o hese phases in he es ed powde did no a ec he mic os uc u al changes obse ed in deposi ed
Ni–P coa ings.
Figu e 4b shows he pa e ns o hea - ea ed Ni–P coa ings wi h di e en P con en . The di ac ion
o Ni (1 1 1) can be seen nea he di ac ion angle 2
θ≈
44.4
◦
. Ano he di ac ion o Ni (2 0 0) can be
seen a 2
θ≈
51.8
◦
. Fo bo h o he Ni di ac ions, i can be obse ed ha hei in ensi y inc eases and
peaks become mo e sha p wi h he dec easing P con en (excep he P con en 5.2
±
0.2 w .%, which is
sligh ly lowe han he peak o he Ni–P coa ing wi h he P con en 5.5 ±0.1 w .%).
Figu e 4b shows ha he p esence o he Ni
3
P s able phase was ob ious o all he coa ings and he
in ensi y o he Ni
3
P phase inc eased wi h he inc easing P con en . F om pa e ns shown in Figu e 4b,
i is e iden ha in he case o low-phospho us Ni–P coa ings, he Ni phase c ys allizes mo e ( he peak
o Ni is sha pe and wi h highe in ensi y) when compa ed o he high-phospho us Ni–P coa ings.
On he o he hand, he Ni
3
P phase p ecipi a ed and g ew mo e in he case o he high-phospho us Ni–P
coa ing. The p esence o he Ni
12
P
5
me as able phase was obse ed in he case o high-phospho us
Ni–P coa ings (10.2 and 10.8 w .% o P) a ound he di ac ion angle 2
θ≈
47
◦
o 48
◦
. Acco ding o he
li e a u e [
22
,
23
], he Ni
12
P
5
me as able phase should comple ely disappea a ound he empe a u e o
350
◦
C. Howe e , Keong [
24
] showed ha he Ni
12
P
5
phase may s ill be p esen a 400
◦
C. The p esence
o his phase could be caused by he incomple e ans o ma ion om he o iginally amo phous ma ix
o he mix u e o c ys alline Ni and he Ni3P s able phase.
3.4. C ys alli e Size
Figu e 5shows he e ec o he phospho us con en on he Ni c ys alli e size in as-deposi ed
coa ings and hea - ea ed coa ings. Only one di ac ion plane Ni (1 1 1) was obse ed (see Figu e 4a)
and wo di ac ion planes (1 1 1) and (2 0 0) o Ni we e obse ed (Figu e 4b) in he case o as-deposi ed
and hea - ea ed coa ings by XRD, espec i ely. Figu e 5shows ha wi h he inc easing P con en ,
he c ys alli e size o nickel dec eases, bo h in he case o he as-deposi ed and hea - ea ed coa ings.
This ac can be explained due o he inc easing la ice diso de (a g ea e p opo ion o he amo phous
phase) wi h he inc easing P con en in he Ni–P ma ix [12,24].
Coa ings 2019,9, 461 7 o 9
Coa ings 2019, 9, x FOR PEER REVIEW 6 o 9
Figu e 4b, i is e iden ha in he case o low-phospho us Ni–P coa ings, he Ni phase c ys allizes
mo e ( he peak o Ni is sha pe and wi h highe in ensi y) when compa ed o he high-phospho us
Ni–P coa ings. On he o he hand, he Ni
3
P phase p ecipi a ed and g ew mo e in he case o he
high-phospho us Ni–P coa ing. The p esence o he Ni
12
P
5
me as able phase was obse ed in he case
o high-phospho us Ni–P coa ings (10.2 and 10.8 w .% o P) a ound he di ac ion angle 2θ ≈ 47° o
48°. Acco ding o he li e a u e [22,23], he Ni
12
P
5
me as able phase should comple ely disappea
a ound he empe a u e o 350 °C. Howe e , Keong [24] showed ha he Ni
12
P
5
phase may s ill be
p esen a 400 °C. The p esence o his phase could be caused by he incomple e ans o ma ion om
he o iginally amo phous ma ix o he mix u e o c ys alline Ni and he Ni
3
P s able phase.
Figu e 4. XRD pa e ns o (a) as-deposi ed and (b) hea - ea ed Ni–P coa ings wi h di e en
phospho us con en .
3.5 C ys alli e Size
Figu e 5 shows he e ec o he phospho us con en on he Ni c ys alli e size in as-deposi ed
coa ings and hea - ea ed coa ings. Only one di ac ion plane Ni (1 1 1) was obse ed (see Figu e 4a)
and wo di ac ion planes (1 1 1) and (2 0 0) o Ni we e obse ed (Figu e 4b) in he case o
as-deposi ed and hea - ea ed coa ings by XRD, espec i ely. Figu e 5 shows ha wi h he inc easing
P con en , he c ys alli e size o nickel dec eases, bo h in he case o he as-deposi ed and hea - ea ed
coa ings. This ac can be explained due o he inc easing la ice diso de (a g ea e p opo ion o he
amo phous phase) wi h he inc easing P con en in he Ni–P ma ix [12,24].
As can be seen in Figu e 5a, he c ys alli es o Ni in low-phospho us as-deposi ed Ni–P coa ings
eached app oxima ely 40 Å, whe eas he c ys alli es o Ni in he Ni–P coa ing wi h 10.8 w .% o P
eached he size o 13.1 Å. A e he hea ea men , he c ys alli e size o Ni subs an ially inc eased
o mo e han 300 Å in he case o low-phospho us coa ings in he plane (1 1 1). A simila end was
obse ed o he di ac ion plane (2 0 0).
Figu e 5. The e ec o Ni c ys alli e size on he P con en in (a) as-deposi ed Ni–P coa ings and (b)
hea - ea ed Ni–P coa ings.
Figu e 5.
The e ec o Ni c ys alli e size on he P con en in (
a
) as-deposi ed Ni–P coa ings and
(b) hea - ea ed Ni–P coa ings.
As can be seen in Figu e 5a, he c ys alli es o Ni in low-phospho us as-deposi ed Ni–P coa ings
eached app oxima ely 40 Å, whe eas he c ys alli es o Ni in he Ni–P coa ing wi h 10.8 w .% o P
eached he size o 13.1 Å. A e he hea ea men , he c ys alli e size o Ni subs an ially inc eased
o mo e han 300 Å in he case o low-phospho us coa ings in he plane (1 1 1). A simila end was
obse ed o he di ac ion plane (2 0 0).
On he o he hand, in he case o he hea - ea ed Ni–P coa ings, he Ni
3
P c ys alli e size inc eased
wi h he inc easing P con en , as seen in Figu e 6. The Ni
3
P c ys alli e size dependence on he P con en
was s udied o di ac ions wi h he highes in ensi y. The mos dis inc i e di ac ion angles we e
2θ≈41.7◦,
42.8
◦
, 43.6
◦
, 45.3
◦
, 46.6
◦
, and 52.7
◦
, which co esponds o he di ac ion planes (3 2 1),
(3 3 0),
(1 1 2), (4 2 0), (1 4 1), and (3 1 2), espec i ely. Inc easing c ys alli e size o Ni
3
P is ela ed o
inc easing P con en . Wi h a highe P con en , a Ni
3
P phase ac ion is o med and combined o o m
coa se pa icles. Meanwhile, in he case o low phospho us coa ings, he o med Ni
3
P phase is in he
o m o ine-g ained p ecipi a es dis ibu ed in he Ni–P ma ix.
Coa ings 2019, 9, x FOR PEER REVIEW 7 o 9
On he o he hand, in he case o he hea - ea ed Ni–P coa ings, he Ni3P c ys alli e size
inc eased wi h he inc easing P con en , as seen in Figu e 6. The Ni3P c ys alli e size dependence on
he P con en was s udied o di ac ions wi h he highes in ensi y. The mos dis inc i e di ac ion
angles we e 2θ ≈ 41.7°, 42.8°, 43.6°, 45.3°, 46.6°, and 52.7°, which co esponds o he di ac ion planes
(3 2 1), (3 3 0), (1 1 2), (4 2 0), (1 4 1), and (3 1 2), espec i ely. Inc easing c ys alli e size o Ni3P is
ela ed o inc easing P con en . Wi h a highe P con en , a Ni3P phase ac ion is o med and
combined o o m coa se pa icles. Meanwhile, in he case o low phospho us coa ings, he o med
Ni3P phase is in he o m o ine-g ained p ecipi a es dis ibu ed in he Ni–P ma ix.
Figu e 6. The e ec o Ni3P c ys alli e size on he P con en in hea - ea ed Ni–P coa ings.
Based on he li e a u e [10,14], i is e iden ha he chemical composi ion and he size o he
Ni3P phase can a ec he esul ing mic os uc u e and ha dness o he Ni–P coa ings.
Higgs [18] s a es ha he hea - ea ed coa ings showed ine-g ained in e me allic p ecipi a es o
Ni3P in he Ni–P ma ix. The au ho also poin ed ou ha he size o Ni3P p ecipi a es in he Ni–P
ma ix depended on he empe a u e. Acco ding o ou s udy, he size o Ni3P p ecipi a es is also
ela ed o he P con en (Figu e 6).
Fine p ecipi a es o Ni3P may be esponsible o he inc eased ha dness o he imp o emen o
o he mechanical p ope ies [14,25]. Wi h e e ence o measu ed esul s o mic oha dness o he Ni–
P coa ings (Figu e 3) and hei mic os uc u al cha ac e is ics (Figu es 5,6), i is e iden ha he
mic oha dness depends no only on he P con en , bu also on he size and dis ibu ion o Ni3P
p ecipi a es in he case o hea - ea ed coa ings. The e o e, he hea ea men leads o a signi ican
inc ease o mic oha dness o he Ni–P coa ings.
Howe e , he hea ea men also a ec s he subs a e. The empe a u e o 400 °C in luences he
s uc u e o Mg alloys mo e han in he case o s eels. The hea ea men o he AZ91 alloy led o he
dissolu ion o he discon inuous p ecipi a e Mg17Al12. The di e ence in he he mal expansion
coe icien o AZ91 Mg alloy and he Ni–P coa ing may lead o ension a he in e ace. Howe e ,
he e was no obse able impac ( isible c acks o delamina ion) a he Mg subs a e/Ni–P coa ing
in e ace.
4. Conclusions
The elec oless Ni–P coa ings wi h a ious P con en we e deposi ed on AZ91 Mg alloys and
subsequen ly hea - ea ed a 400 °C o 1 h.
As-deposi ed Ni–P coa ings showed he dec ease in mic oha dness wi h inc easing P con en .
Hea - ea ed Ni–P coa ings showed a simila end. Howe e , he hea - ea ed coa ings eached
signi ican ly highe mic oha dness alues.
F om he XRD analysis, i was de e mined ha he mic os uc u e o he as-deposi ed
high-phospho us coa ings was amo phous, and wi h he dec easing P con en hey become mo e
c ys alline. Hea - ea ed Ni–P coa ings we e comple ely c ys alline, and a p esence o c ys alline Ni
and he in e media e Ni3P phase in he coa ing was obse ed. I was obse ed ha he Ni c ys alli e
size in he coa ing dec eased wi h he inc easing P, bo h o as-deposi ed and hea - ea ed Ni–P
Figu e 6. The e ec o Ni3P c ys alli e size on he P con en in hea - ea ed Ni–P coa ings.
Based on he li e a u e [
10
,
14
], i is e iden ha he chemical composi ion and he size o he Ni
3
P
phase can a ec he esul ing mic os uc u e and ha dness o he Ni–P coa ings.
Higgs [
18
] s a es ha he hea - ea ed coa ings showed ine-g ained in e me allic p ecipi a es o
Ni
3
P in he Ni–P ma ix. The au ho also poin ed ou ha he size o Ni
3
P p ecipi a es in he Ni–P
ma ix depended on he empe a u e. Acco ding o ou s udy, he size o Ni
3
P p ecipi a es is also
ela ed o he P con en (Figu e 6).
Fine p ecipi a es o Ni
3
P may be esponsible o he inc eased ha dness o he imp o emen
o o he mechanical p ope ies [
14
,
25
]. Wi h e e ence o measu ed esul s o mic oha dness o he
Ni–P coa ings (Figu e 3) and hei mic os uc u al cha ac e is ics (Figu es 5and 6), i is e iden ha
he mic oha dness depends no only on he P con en , bu also on he size and dis ibu ion o Ni
3
P
Coa ings 2019,9, 461 8 o 9
p ecipi a es in he case o hea - ea ed coa ings. The e o e, he hea ea men leads o a signi ican
inc ease o mic oha dness o he Ni–P coa ings.
Howe e , he hea ea men also a ec s he subs a e. The empe a u e o 400
◦
C in luences
he s uc u e o Mg alloys mo e han in he case o s eels. The hea ea men o he AZ91 alloy
led o he dissolu ion o he discon inuous p ecipi a e Mg
17
Al
12
. The di e ence in he he mal
expansion coe icien o AZ91 Mg alloy and he Ni–P coa ing may lead o ension a he in e ace.
Howe e , he e was no obse able impac ( isible c acks o delamina ion) a he Mg subs a e/Ni–P
coa ing in e ace.
4. Conclusions
The elec oless Ni–P coa ings wi h a ious P con en we e deposi ed on AZ91 Mg alloys and
subsequen ly hea - ea ed a 400 ◦C o 1 h.
As-deposi ed Ni–P coa ings showed he dec ease in mic oha dness wi h inc easing P con en .
Hea - ea ed Ni–P coa ings showed a simila end. Howe e , he hea - ea ed coa ings eached
signi ican ly highe mic oha dness alues.
F om he XRD analysis, i was de e mined ha he mic os uc u e o he as-deposi ed
high-phospho us coa ings was amo phous, and wi h he dec easing P con en hey become
mo e c ys alline. Hea - ea ed Ni–P coa ings we e comple ely c ys alline, and a p esence o c ys alline
Ni and he in e media e Ni
3
P phase in he coa ing was obse ed. I was obse ed ha he Ni c ys alli e
size in he coa ing dec eased wi h he inc easing P, bo h o as-deposi ed and hea - ea ed Ni–P coa ings.
On he o he hand, he c ys alli e size o Ni3P inc eased wi h inc easing P con en in he coa ing.
In e ms o he p ecipi a ion ha dening p ocess, he hea - ea ed Ni–P coa ings eached highe
mic oha dness alues han he as-deposi ed coa ings. This is due o he p esence o a la ge numbe o
in e me allic p ecipi a es o Ni3P.
The in luence o empe a u e du ing he hea ea men led o he dissolu ion o he discon inuous
p ecipi a e Mg
17
Al
12
. Despi e hesubs a emic os uc u al changesanddi e ence in he malexpansion
coe icien s, his did no lead o he delamina ion o isible c acking o he coa ing. The p epa ed
coa ings we e uni o m and wi h no isible de ec s.
Au ho Con ibu ions:
Concep ualiza ion, M.B. and M.K.; me hodology M.B., J.M. and J.W.; alida ion, M.B.,
M.K. and J.W.; o mal analysis, M.B. and M.K.; in es iga ion, M.B. and M.K.; esou ces, M.B. and J.W.; da a
cu a ion, M.B. and J.M.; w i ing—o iginal d a p epa a ion, M.B.; w i ing— e iew and edi ing, M.B. and M.K.
isualiza ion, M.B. and M.K.; supe ision, J.W.; p ojec adminis a ion, J.W.; unding acquisi ion, J.W.
Funding:
This wo k was suppo ed by p ojec N . LO1211, Ma e ials Resea ch Cen e a FCH BUT- Sus ainabili y
and De elopmen (Na ional P og am o Sus ainabili y I, Minis y o Educa ion, You h and Spo s).
Con lic s o In e es : The au ho s decla e no con lic o in e es .
Re e ences
1.
F ied ich, H.; Mo dike, B.L. Magnesium Technology: Me allu gy, Design Da a, Applica ions; Sp inge :
Be lin, Ge many, 2006.
2. Cze winski, F. Magnesium Alloys: Design, P ocessing and P ope ies; InTech: Rijeka, C oa ia, 2011.
3.
Buch
í
k, M.; Kos
á
, P.; Wasse baue , J.; Tkacz, J.; Doležal, P. Cha ac e iza ion o elec oless Ni–P coa ing
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