coa ings
A icle
Room and High Tempe a u e T ibological
Pe o mance o Mul ilaye ed TiSiN/TiN and
TiSiN/TiN(Ag) Coa ings Deposi ed by Spu e ing
Filipe Fe nandes 1,2 , Abbas AL-Rjoub 1,* , Diogo Ca alei o 1, Tomas Polca 2and
Albano Ca alei o 1
1
Depa men o Mechanical Enginee ing, CEMMPRE—Cen e o Mechanical Enginee ing Ma e ials and P ocesses,
Uni e si y o Coimb a, Rua Luís Reis San os, 3030-788 Coimb a, Po ugal; ilipe. e [email protected] (F.F.);
diogoaca alei [email protected] (D.C.); albano.ca alei [email protected] (A.C.)
2Depa men o Con ol Enginee ing, Czech Technical Uni e si y in P ague, Technicka 2, 6 166 27 P ague,
Czech Republic; [email p o ec ed]
*Co espondence: [email p o ec ed]
Recei ed: 7 No embe 2020; Accep ed: 4 Decembe 2020; Published: 6 Decembe 2020


Abs ac :
In his s udy, we compa e he ibological pe o mance o a mul ilaye TiSiN/Ti(Ag)N
coa ing wi h a TiSiN/TiN coa ing wi h a simila Si con en in o de o demons a e he e ec o
he solid lub ican phase, sil e . Fo Al
2
O
3
balls, he ha dness and educed modulus de e mine
he ibological pe o mance o he coa ings o es s conduc ed a oom empe a u e (RT) agains
Al
2
O
3
balls. A 550
◦
C, he TiSiN/TiN coa ing ailed, whe eas he Ag-con aining coa ing pe o med
be e due o he p esence o Ag in he con ac , which dec eased he shea s ess and, consequen ly,
he ic ion. Fo es s agains TiAl6V4 balls, he Ag-con aining coa ing was always be e han he
TiSiN/TiN one. A 550
◦
C, Ag in he wea ack p e en ed he adhesion o he oxidized Ti-alloy wea
deb is in he con ac , a o ing he adhesion o wea deb is om he coa ing o bo h he coa ing and
coun e pa su aces. No wea could be measu ed o he 700
◦
C es s o bo h coa ings due o di e en
easons: (i) he p esence o oxidized adhe ed ma e ial om he ball o he e e ence TiSiN/TiN coa ing
su ace p o ec ed om wea and (ii) he p esence o Ag-agglome a ed pa icles dec eased he ic ion
and minimized he adhesion wea o he coun e pa o he TiSiN/TiN(Ag) coa ing.
Keywo ds:
TiSiN/TiN(Ag) coa ings; high- empe a u e ibology; wea esis ance; wea mechanisms
1. In oduc ion
The con inuous g ow h in mobili y has placed an inc easing demand on he anspo indus y o
manu ac u e ehicles a a lowe cos , while ensu ing ha hey ope a e e icien ly, a e iendly o he
en i onmen and mee sa e y equi emen s [
1
]. This is pa icula ly impo an in he ae ospace and
au omo i e indus ies, whe e he use o ligh weigh ai ames, as well as he global imp o emen o
ma e ials’ mechanical p ope ies, encou age he in eg a ion o composi e ma e ials in a la ge numbe
o s uc u al componen s [
1
,
2
]. Nowadays, he demand o i anium alloys has s eadily inc eased
in such indus ies due o hei excellen s eng h- o-weigh a io and elec ochemical compa ibili y.
Ti anium alloys ha e ou s anding physical-mechanical p ope ies, bu hey a e di icul o machine
due o hei high chemical eac i i y, poo hea conduc i i y and low modulus o elas ici y, which lead
o lowe p oduc ion a es and inc eased ool wea [
1
,
3
–
6
]. The o ma ion o buil -up edges [
5
,
7
],
sp ingback e ec [
2
], high hea s ess [
3
] and a ia ions in he chip hickness [
3
] a e he main e ec s
leading o he p ema u e deg ada ion o he ools. Se e al solu ions ha e been sugges ed o imp o e
he machinabili y o Ti alloys. These include (i) he use o s anda d coolan s and lub ican s o c yogenic
cooling, (ii) use o ools wi h a high he mal conduc i i y, (iii) op imiza ion o he machining condi ions
Coa ings 2020,10, 1191; doi:10.3390/coa ings10121191 www.mdpi.com/jou nal/coa ings
Coa ings 2020,10, 1191 2 o 13
by e alua ing he s ess, empe a u e and ib a ion du ing p oduc ion, and (i ) use o coa ed ools.
In he la e , coa ings deposi ed by spu e ing p ocesses could double he li e ime o he ools as
compa ed o he uncoa ed ones [
1
–
5
,
8
]. Howe e , he cu en ly used ool coa ings o machining
i anium alloys canno sa is y he equi emen s o high-speed machining and g een manu ac u ing.
Recen ly, he e has been a signi ican inc ease o in e es o using sel -lub ican ha d coa ings o
p o ec and ex end he li e ime o machining ools used o p ocess ha d- o-machine ma e ials ope a ing
unde d y machining condi ions [
4
]. Any p omising coa ing o his pu pose should combine a
high oughness, low ic ion, good wea esis ance and he mal s abili y unde high- empe a u e
condi ions [
9
,
10
]. The mos common candida es a e high oxida ion esis an bina y o e na y ni ides
and ca boni ides o ansi ion me als such as Ti [
11
,
12
], C [
13
], W [
14
] o Ni [
15
] alloyed wi h: (i) so
me als (Ag, Au, Cu, e c.); (ii) luo ides (CaF
2
, BaF
2
and CeF
3
); and (iii) me al oxides (V
2
O
5
, Ag
2
Mo
2
O
7
).
Independen ly o he coa ings’ con igu a ion (monolaye , mul ilaye o nanocomposi e), he ic ion
was dec eased and he wea esis ance was imp o ed (see comp ehensi e pape s in he li e a u e:
Voe odin e al. [
5
], Zhu e al. [
16
] and Aouadi e al. [
17
]). Howe e , in mos cases, hei pe o mance
ailed a a high empe a u e due o he low oxida ion esis ance, and s ong elemen al di usion o he
su ace wi h he consequen apid deple ion o he lub icious agen and he consequen loss o he
low- ic ion ibolaye a e sho ope a ing pe iods [
10
]. The con ol o he me al ou -di usion is now
one o he majo challenges o achie ing sui able long-du a ion wea and ic ion p ope ies wi hou
comp omising he o iginal p ope ies o he hos bina y and e na y ilms. The e o e, one possible
solu ion o his p oblem will be o use a dual phase s uc u e, in which one o he phases could ac as
a di usion ba ie o he me al, which should be esponsible o he lub ica ion. The TiSiN sys em is a
possible solu ion. When his sys em is deposi ed as ei he a nanocomposi e s uc u e (nanosized TiN
c ys alli es su ounded by an amo phous ma ix o Si-N) o a mul ilaye ed s uc u e (TiSiN laye s
in e cala ed wi h laye s con aining he lub icious elemen ), he Si-N phase can wo k as an an i-di usion
ba ie [18].
In ou p e ious s udies [
11
,
19
], we epo ed he in luence o Ag alloying on he mo phology,
s uc u e, mechanical p ope ies, he mal s abili y and oxida ion esis ance o monoli hic TiSiNAg
and mul ilaye ed TiSiN/TiN(Ag) coa ings deposi ed by spu e ing. The he mal s abili y and
oxida ion esis ance esul s e ealed ha he TiSiN sys em allowed he con ol o he di usion
o he lub icious elemen (Ag) o he su ace, being a p omising sys em o use as sel -lub ican coa ing
o cu ing applica ions.
This wo k compa es he ibological pe o mance o a TiSiN/TiN(Ag) mul ilaye ed coa ing wi hou
and wi h Ag alloying con aining a simila Si concen a ion. Ou aim is o demons a e he e ec o a
solid lub ican phase, sil e , on he ibological p ope ies a a high empe a u e. The ibological es s
we e pe o med using a high- empe a u e pin-on-disk equipmen sliding agains wo ypes o balls,
Al2O3and TiAl6V4, a oom empe a u e, 550 and 700 ◦C.
2. Ma e ials and Me hods
TiSiN/TiN and TiSiN/TiN(Ag) coa ings wi h a simila Si concen a ion and pe iod hickness
(
~34 nm
) we e deposi ed by di ec cu en (DC) eac i e magne on spu e ing, wo king in unbalanced
mode, o compa e hei ibological pe o mance and he po en ial o Ag-alloyed TiSiN/TiN as a
sel -lub ican coa ing. Two a ge s (20 cm
×
10 cm) we e used in he deposi ions: (i) a high pu i y Ti
(99.9%) a ge wi h 19 d illed holes o 5-mm diame e dis ibu ed in he e osion zone: 11 holes we e
illed wi h Ag pelle s, and he emaining holes we e illed wi h Ti pelle s; and (ii) a high pu i y (99.9%)
composi e Ti
88
Si
12
(a .%) a ge . The a ge s we e posi ioned pa allel o each o he . WC-6 w .% Co
subs a es we e used o he ibological es s and we e ul asonically cleaned in ace one and alcohol
o 15 and 10 min, espec i ely.
The base p essu e o he chambe p io o he deposi ion was ~ 4
×
10
−4
Pa. Be o e he deposi ion,
he subs a es we e e ched o 50 min in an A a mosphe e (pulsed bias o 500 V and equency o
250 kHz applied o he subs a e’s holde ), wi h a shu e in on o he a ge s and applying 200 W
Coa ings 2020,10, 1191 3 o 13
o each o he a ge s. The in e laye ( o imp o ing he adhesion o he coa ings o he subs a es)
consis ed o wo s eps: (i) a 300-nm TiSi adhesi e laye p oduced om he TiSi composi e a ge when
applying a powe densi y o 6.5 W/cm
2
, an A low o 11 sccm, a DC pulsed bias o
−
60 V and 250 kHz,
and (ii) a 250-nm g adien TiSiN laye p oduced wi h an inc easing N
2
low un il he inal p essu e o
0.35 Pa was achie ed, wi h a o a ion speed o 18 pm and applying a powe densi y o 6.5 W/cm
2
,
A low o 11 sccm, and DC pulsed bias o
−
60 V and 250 kHz. The main mul ilaye s uc u e was
g own by applying 6.5 W/cm
2
o each a ge , using 11 sccm o A and 13 sccm o N, applying a DC
pulsed bias o
−
60 V wi h a subs a e o a ion o 0.9 pm o ensu e a mul ilaye s uc u e wi h a pe iod
o ~34 nm. The deposi ion ime o he main coa ings was se a 1 h and 15 min o bo h coa ings in o de
o p oduce coa ings wi h app oxima ely 3 µm o o al hickness including he in e /g adien laye s.
A pin-on-disk ibome e om CSM Ins umen s was used o e alua e he ibological pe o mance
o he coa ings a oom empe a u e (RT) and 550
◦
C agains Al
2
O
3
balls; u he es s we e also
pe o med wi h TiAl6V4 balls a RT, 550 and 700
◦
C. Tempe a u es o 550 and 700
◦
C we e selec ed as
hey we e close o he empe a u es gene a ed a con ac du ing machining. A 5 N load and Al
2
O
3
balls we e used as a s anda d o e alua e he wea o he coa ings a high empe a u es. The e o e,
his p ocedu e allowed o he di ec compa ison wi h many o he s udies on ha d coa ings. TiAl6V4
balls we e used o e alua e he po en ial use o hese coa ings o he machining o he TiAl6V4 alloy.
The diame e o he balls was 6 mm. The es s we e pe o med wi h a load o 5 N, a sliding speed o
10 cm·s−1and a du a ion o 2500 cycles, wi h a o al dis ance o 62.8 m. In he case o es s conduc ed
agains Al
2
O
3
balls a 550
◦
C, he leng h o he es s was dec eased o 12.6 m wi h 500 cycles o a oid
he comple e wea ing ou o he coa ing and he subs a e exposu e. The ic ion coe icien (COF) was
con inuously acqui ed du ing he es s. The olume loss due o wea was calcula ed wi h he help o a
3D p o ilome e , and he speci ic wea a e was calcula ed acco ding o A cha d’s law [
20
]. A e he
es s, he wea mechanisms and wea deb is we e cha ac e ized by scanning elec on mic oscopy (SEM)
and ene gy dispe si e X- ay spec oscopy (EDS), and hey we e co ela ed wi h he ic ion coe icien
and wea a e alues o he coa ings and balls.
3. Resul s
An ex ensi e discussion o he in luence o Ag alloying on he s uc u e and mechanical p ope ies
o mul ilaye ed TiSiN/TiN coa ings can be seen in ou p e ious publica ion [
19
]. Howe e , Table 1
p esen s a summa y o he coa ings’ denomina ion, chemical composi ion and mechanical p ope ies.
In his wo k, we will ocus on he ibological pe o mance.
Table 1. Coa ings’ denomina ion, chemical composi ion and mechanical p ope ies.
Sample Designa ion TiSiN/TiN TiSiN/TiN(Ag)
Chemical composi ion (a .%)
Ti—46.1 Ti—42.3
Si—4.6 Si—4.3
N—49.3 N—50.2
– Ag—3.2
Ha dness (H) (GPa) 26 ±4 18 ±4
Reduced modulus (E) (GPa) 334 ±28 286 ±23
Elas ic s ain o ailu e—H/E0.078 0.062
Onse poin o oxida ion (◦C) 700 ◦C 700 ◦C
Oxida ion weigh gain a 800 ◦C o 2 h mg/cm20.025 0.16
Coa ings 2020,10, 1191 4 o 13
3.1. F ic ion Coe icien
3.1.1. Al2O3Balls
Figu e 1a,b shows he ic ion coe icien (COF) o he coa ings as a unc ion o he numbe o
cycles o he es s pe o med wi h Al
2
O
3
balls a RT and 550
◦
C, espec i ely. A oom empe a u e,
wo di e en s ages o he ic ion coe icien could be obse ed, he i s o which could be ela ed o a
unning-in, while in he second one a s eady-s a e could be achie ed. The ic ion coe icien a he
unning-in s age inc eased s eeply om he ini ial alue o 0.2 o 0.65 and 0.85 a e 700 and 1000 cycles
o TiSiN/TiN and TiSiN/TiN(Ag) coa ings, espec i ely. Fo he la e , COF emained app oxima ely
cons an un il he end o he es , wi h a alue close o 0.9. Fo he e e ence coa ing, a slow inc easing
end was obse ed, eaching a alue o 0.85, which was kep cons an in he las 500 cycles o he es .
Globally, he a e age ic ion coe icien o he Ag-con aining coa ing was sligh ly highe han he
COF o he e e ence coa ing. In he second s age o he cu e, he e e ence coa ing showed small
luc ua ions in he ic ion coe icien alue un il he end o he es , which, acco ding o he li e a u e,
could be a ibu ed o he occu ence o plas ic de o ma ion o he ilm [
21
], as will be discussed la e .
The a e age COF o his mul ilaye ell in he ange o he alues epo ed in he li e a u e o TiN
and TiSiN deposi ed as monolaye ilms [
22
–
25
]. The Ag-con aining coa ing displayed a e y smoo h
and s able coe icien o ic ion du ing he s eady-s a e when compa ed wi h he e e ence coa ing.
In spi e o he di e en shapes o he cu es, in he inal s eady-s a e le el he COF was e y simila o
bo h coa ings.
Coa ings 2020, 10, x FOR PEER REVIEW 4 o 14
Figu e 1a,b shows he ic ion coe icien (COF) o he coa ings as a unc ion o he numbe o
cycles o he es s pe o med wi h Al2O3 balls a RT and 550 °C, espec i ely. A oom empe a u e,
wo di e en s ages o he ic ion coe icien could be obse ed, he i s o which could be ela ed o
a unning-in, while in he second one a s eady-s a e could be achie ed. The ic ion coe icien a he
unning-in s age inc eased s eeply om he ini ial alue o 0.2 o 0.65 and 0.85 a e 700 and 1000
cycles o TiSiN/TiN and TiSiN/TiN(Ag) coa ings, espec i ely. Fo he la e , COF emained
app oxima ely cons an un il he end o he es , wi h a alue close o 0.9. Fo he e e ence coa ing, a
slow inc easing end was obse ed, eaching a alue o 0.85, which was kep cons an in he las 500
cycles o he es . Globally, he a e age ic ion coe icien o he Ag-con aining coa ing was sligh ly
highe han he COF o he e e ence coa ing. In he second s age o he cu e, he e e ence coa ing
showed small luc ua ions in he ic ion coe icien alue un il he end o he es , which, acco ding
o he li e a u e, could be a ibu ed o he occu ence o plas ic de o ma ion o he ilm [21], as will
be discussed la e . The a e age COF o his mul ilaye ell in he ange o he alues epo ed in he
li e a u e o TiN and TiSiN deposi ed as monolaye ilms [22–25]. The Ag-con aining coa ing
displayed a e y smoo h and s able coe icien o ic ion du ing he s eady-s a e when compa ed
wi h he e e ence coa ing. In spi e o he di e en shapes o he cu es, in he inal s eady-s a e le el
he COF was e y simila o bo h coa ings.
Figu e 1. F ic ion coe icien o he TiSiN/TiN mul ilaye coa ings wi h and wi hou Ag addi ion as a
unc ion o he numbe o cycles es ed agains Al2O3 balls a : (a) RT and (b) 550 °C.
Fo he es a 550 °C, he e e ence coa ing was immedia ely wo n ou . The Ag-con aining
coa ing could su i e a his empe a u e, bu due o he ex ensi e wea he numbe o cycles had o
be dec eased in o de o a oid subs a e exposu e. Simila o he RT es s, wo s ages could be
obse ed in he COF cu e. Howe e , he unning-in pe iod was e y sho , only 20 cycles, due o
he easie adap ion o he su ace p omo ed by he empe a u e and he p esence o Ag a con ac , as
will be discussed la e . The a e age COF alue was lowe han when es ed unde RT condi ions.
3.1.2. TiAl6V4 Balls
The ic ion coe icien alues o he coa ings as a unc ion o he numbe o cycles es ed agains
TiAl6V4 balls a di e en empe a u es (RT, 550 and 700 °C) a e shown in Figu e 2. No di e ences in
he COF alues a e obse ed a oom empe a u e o bo h coa ings. Ne e heless, he ic ion alues
a e lowe han when es ed agains Al2O3 balls. Fu he mo e, he unning-in s age o he e e ence
coa ing las ed 250 cycles, while i was almos nonexis en o he Ag-con aining coa ing. Fo he high-
empe a u e es s, he COF o he Ag-con aining coa ing was always lowe han ha o he e e ence
coa ing by 29% and 24% o 550 and 700 °C, espec i ely. In bo h cases, a sligh dec ease in he COF
o he coa ings was obse ed when es ing om 550 o 700 °C.
0 500 1000 1500 2000 2500
0.2
0.4
0.6
0.8
1.0
1.2
TiSiN/TiN
TiSiN/TiN(Ag)
(a) Agains Al2O3 a RT
Numbe o cycles
F ic ion Coe icien
0 100 200 300 400 500
0.2
0.4
0.6
0.8
1.0
1.2
TiSiN/TiN(Ag)
TiSiN/TiN - Delamina ed
(b) Agains Al
2
O
3
a 550
F ic ion Coe icien
Numbe o cycles
°C
Figu e 1.
F ic ion coe icien o he TiSiN/TiN mul ilaye coa ings wi h and wi hou Ag addi ion as a
unc ion o he numbe o cycles es ed agains Al2O3balls a : (a) RT and (b) 550 ◦C.
Fo he es a 550
◦
C, he e e ence coa ing was immedia ely wo n ou . The Ag-con aining coa ing
could su i e a his empe a u e, bu due o he ex ensi e wea he numbe o cycles had o be
dec eased in o de o a oid subs a e exposu e. Simila o he RT es s, wo s ages could be obse ed
in he COF cu e. Howe e , he unning-in pe iod was e y sho , only 20 cycles, due o he easie
adap ion o he su ace p omo ed by he empe a u e and he p esence o Ag a con ac , as will be
discussed la e . The a e age COF alue was lowe han when es ed unde RT condi ions.
3.1.2. TiAl6V4 Balls
The ic ion coe icien alues o he coa ings as a unc ion o he numbe o cycles es ed agains
TiAl6V4 balls a di e en empe a u es (RT, 550 and 700
◦
C) a e shown in Figu e 2. No di e ences
in he COF alues a e obse ed a oom empe a u e o bo h coa ings. Ne e heless, he ic ion
alues a e lowe han when es ed agains Al
2
O
3
balls. Fu he mo e, he unning-in s age o he
e e ence coa ing las ed 250 cycles, while i was almos nonexis en o he Ag-con aining coa ing.
Fo he high- empe a u e es s, he COF o he Ag-con aining coa ing was always lowe han ha o
Coa ings 2020,10, 1191 5 o 13
he e e ence coa ing by 29% and 24% o 550 and 700
◦
C, espec i ely. In bo h cases, a sligh dec ease
in he COF o he coa ings was obse ed when es ing om 550 o 700 ◦C.
Coa ings 2020, 10, x FOR PEER REVIEW 5 o 14
Figu e 2. F ic ion coe icien (COF) o he e e ence and Ag-alloyed coa ings as a unc ion o he
numbe o cycles es ed agains TiAl6V4 balls a (a) RT, (b) 550 °C and (c) 700 °C.
3.2. Speci ic Wea Ra e o Coa ings and Balls
The speci ic wea a e o he coa ings es ed agains he wo coun e pa s a RT, 550 and 700 °C
( he las empe a u e o he es s conduc ed agains TiAl6V4 balls) is shown in Figu e 3, along wi h
he wea a e o he co esponding balls. The wea a es we e calcula ed om he analysis o he wea
sca s o bo h he coa ed disks (2D p o iles—Figu e S1 o he Supplemen a y Ma e ial) and he balls
(op ical mic oscopy—Figu e S2 o he Supplemen a y Ma e ial). As expec ed, and in acco dance wi h
he ic ion coe icien a ia ion desc ibed abo e, he speci ic wea a e depends on he chemical
composi ion o he coa ings, es ing empe a u e and coun e pa ma e ial used.
Figu e 3. (a) Speci ic wea a e o he coa ings es ed agains di e en balls, and (b) speci ic wea a e
o Al
2
O
3
es ed a RT and 550 °C and TiAl6V4 balls es ed a RT, 550 and 700 °C.
3.2.1. Al
2
O
3
Balls
The e e ence coa ing displayed a be e wea pe o mance a RT, which was expec ed due o i s
highe ha dness and ac u e oughness and low ic ion coe icien . A simila endency was obse ed
by Dang e al. [26] o mul ilaye TiSiN/Ag ilms con aining a low Ag concen a ion (sliding agains
WC-6 w % Co balls) and by Ca alei o e al. [27] o TiSiN(Ag) monolaye ilms deposi ed by HiPIMS
in DOMS mode sliding agains Al
2
O
3
balls. Despi e he p esence o Ag in he TiSiN/TiN(Ag) coa ing,
he absence o a con inuous Ag- ich ibolaye in he con ac does no allow a dec ease in he ic ion
a RT and, consequen ly, is no able o coun e ac he in luence o i s wo se mechanical p ope ies on
he wea esis ance.
A 550 °C, he Ag addi ion had a posi i e e ec on he ibological pe o mance o he coa ing.
Despi e he high speci ic wea a e, as compa ed o he RT es , he coa ing clea ly ou pe o med he
e e ence, which was wo n ou immedia ely. A a high empe a u e, he speci ic wea a e o he ball
was almos negligible o he Ag-con aining coa ing, which was p ima ily a ibu ed o he dec ease
in he numbe o cycles and he o ma ion o a p o ec i e Ag- ich ibolaye .
0
2
4
6
8
10
550
°
C
RT
700
°
C
550
°
C
RT
No wea
No wea
TiSiN/TiN(Ag)
TiSiN/TiN
TiSiN/TiN(Ag)
TiSiN/TiN(Ag)
TiSiN/TiN
TiSiN/TiN
TiSiN/TiN
TiSiN/TiN(Ag)
TiSiN/TiN(Ag)
TiSiN/TiN
Wo n ou
Speci ic wea a e (10-6 mm3/N.m)
a) Coa ings
Al2O3 balls TiAl6V4 balls
0
10
20
400
480
560
1300
1320
1340
RT
RT
550 °C RT
700
°
C
TiSiN/TiN
TiSiN/TiN(Ag)
TiSiN/TiN(Ag)
TiSiN/TiN(Ag)
TiSiN/TiN(Ag)
TiSiN/TiN(Ag)
TiSiN/TiN
TiSiN/TiN
TiSiN/TiN
TiSiN/TiN
wo n ou
b) Balls
Speci ic wea a e (10-6 mm3/N.m)
Al
2
O
3
balls
TiAl
6
V
4
balls
550
°
C
Figu e 2.
F ic ion coe icien (COF) o he e e ence and Ag-alloyed coa ings as a unc ion o he numbe
o cycles es ed agains TiAl6V4 balls a (a) RT, (b) 550 ◦C and (c) 700 ◦C.
3.2. Speci ic Wea Ra e o Coa ings and Balls
The speci ic wea a e o he coa ings es ed agains he wo coun e pa s a RT, 550 and 700
◦
C
( he las empe a u e o he es s conduc ed agains TiAl6V4 balls) is shown in Figu e 3, along wi h
he wea a e o he co esponding balls. The wea a es we e calcula ed om he analysis o he
wea sca s o bo h he coa ed disks (2D p o iles—Figu e S1 o he Supplemen a y Ma e ial) and he
balls (op ical mic oscopy—Figu e S2 o he Supplemen a y Ma e ial). As expec ed, and in acco dance
wi h he ic ion coe icien a ia ion desc ibed abo e, he speci ic wea a e depends on he chemical
composi ion o he coa ings, es ing empe a u e and coun e pa ma e ial used.
Coa ings 2020, 10, x FOR PEER REVIEW 5 o 14
Figu e 2. F ic ion coe icien (COF) o he e e ence and Ag-alloyed coa ings as a unc ion o he
numbe o cycles es ed agains TiAl6V4 balls a (a) RT, (b) 550 °C and (c) 700 °C.
3.2. Speci ic Wea Ra e o Coa ings and Balls
The speci ic wea a e o he coa ings es ed agains he wo coun e pa s a RT, 550 and 700 °C
( he las empe a u e o he es s conduc ed agains TiAl6V4 balls) is shown in Figu e 3, along wi h
he wea a e o he co esponding balls. The wea a es we e calcula ed om he analysis o he wea
sca s o bo h he coa ed disks (2D p o iles—Figu e S1 o he Supplemen a y Ma e ial) and he balls
(op ical mic oscopy—Figu e S2 o he Supplemen a y Ma e ial). As expec ed, and in acco dance wi h
he ic ion coe icien a ia ion desc ibed abo e, he speci ic wea a e depends on he chemical
composi ion o he coa ings, es ing empe a u e and coun e pa ma e ial used.
Figu e 3. (a) Speci ic wea a e o he coa ings es ed agains di e en balls, and (b) speci ic wea a e
o Al
2
O
3
es ed a RT and 550 °C and TiAl6V4 balls es ed a RT, 550 and 700 °C.
3.2.1. Al
2
O
3
Balls
The e e ence coa ing displayed a be e wea pe o mance a RT, which was expec ed due o i s
highe ha dness and ac u e oughness and low ic ion coe icien . A simila endency was obse ed
by Dang e al. [26] o mul ilaye TiSiN/Ag ilms con aining a low Ag concen a ion (sliding agains
WC-6 w % Co balls) and by Ca alei o e al. [27] o TiSiN(Ag) monolaye ilms deposi ed by HiPIMS
in DOMS mode sliding agains Al
2
O
3
balls. Despi e he p esence o Ag in he TiSiN/TiN(Ag) coa ing,
he absence o a con inuous Ag- ich ibolaye in he con ac does no allow a dec ease in he ic ion
a RT and, consequen ly, is no able o coun e ac he in luence o i s wo se mechanical p ope ies on
he wea esis ance.
A 550 °C, he Ag addi ion had a posi i e e ec on he ibological pe o mance o he coa ing.
Despi e he high speci ic wea a e, as compa ed o he RT es , he coa ing clea ly ou pe o med he
e e ence, which was wo n ou immedia ely. A a high empe a u e, he speci ic wea a e o he ball
was almos negligible o he Ag-con aining coa ing, which was p ima ily a ibu ed o he dec ease
in he numbe o cycles and he o ma ion o a p o ec i e Ag- ich ibolaye .
0
2
4
6
8
10
550
°
C
RT
700
°
C
550
°
C
RT
No wea
No wea
TiSiN/TiN(Ag)
TiSiN/TiN
TiSiN/TiN(Ag)
TiSiN/TiN(Ag)
TiSiN/TiN
TiSiN/TiN
TiSiN/TiN
TiSiN/TiN(Ag)
TiSiN/TiN(Ag)
TiSiN/TiN
Wo n ou
Speci ic wea a e (10-6 mm3/N.m)
a) Coa ings
Al2O3 balls TiAl6V4 balls
0
10
20
400
480
560
1300
1320
1340
RT
RT
550 °C RT
700
°
C
TiSiN/TiN
TiSiN/TiN(Ag)
TiSiN/TiN(Ag)
TiSiN/TiN(Ag)
TiSiN/TiN(Ag)
TiSiN/TiN(Ag)
TiSiN/TiN
TiSiN/TiN
TiSiN/TiN
TiSiN/TiN
wo n ou
b) Balls
Speci ic wea a e (10-6 mm3/N.m)
Al
2
O
3
balls
TiAl
6
V
4
balls
550
°
C
Figu e 3.
(
a
) Speci ic wea a e o he coa ings es ed agains di e en balls, and (
b
) speci ic wea a e
o Al2O3 es ed a RT and 550 ◦C and TiAl6V4 balls es ed a RT, 550 and 700 ◦C.
3.2.1. Al2O3Balls
The e e ence coa ing displayed a be e wea pe o mance a RT, which was expec ed due o i s
highe ha dness and ac u e oughness and low ic ion coe icien . A simila endency was obse ed
by Dang e al. [
26
] o mul ilaye TiSiN/Ag ilms con aining a low Ag concen a ion (sliding agains
WC-6 w % Co balls) and by Ca alei o e al. [
27
] o TiSiN(Ag) monolaye ilms deposi ed by HiPIMS
in DOMS mode sliding agains Al
2
O
3
balls. Despi e he p esence o Ag in he TiSiN/TiN(Ag) coa ing,
he absence o a con inuous Ag- ich ibolaye in he con ac does no allow a dec ease in he ic ion a
RT and, consequen ly, is no able o coun e ac he in luence o i s wo se mechanical p ope ies on he
wea esis ance.
A 550
◦
C, he Ag addi ion had a posi i e e ec on he ibological pe o mance o he coa ing.
Despi e he high speci ic wea a e, as compa ed o he RT es , he coa ing clea ly ou pe o med he
e e ence, which was wo n ou immedia ely. A a high empe a u e, he speci ic wea a e o he ball
was almos negligible o he Ag-con aining coa ing, which was p ima ily a ibu ed o he dec ease in
he numbe o cycles and he o ma ion o a p o ec i e Ag- ich ibolaye .

Coa ings 2020,10, 1191 6 o 13
3.2.2. TiAl6V4 Balls
A RT, he speci ic wea a e o he coa ings es ed agains TiAl6V4 balls was lowe when compa ed
o Al
2
O
3
balls, due o ei he he less se e e con ac p omo ed by he so e and less igid TiAl6V4 ball o
he p esence o adhe en ma e ial a he wo n ack, as will be shown la e . The ball wea was ex ensi e,
leading o a apid educ ion o he con ac p essu e and hus minimizing he coa ing wea (Figu e 3).
The ball showed a sligh ly be e wea esis ance when sliding agains he Ag-alloyed coa ing han
agains he e e ence one.
A 550
◦
C, in spi e o he masking e ec o he adhe en ma e ial, i was possible o obse e deepe
nega i e zones in he 2D p o iles (Figu e S1b om he Supplemen a y Ma e ial) when compa ed o
he RT- es ed samples, sugges ing ha he wea esis ance was dec eased. In his sense, Ag-alloyed
coa ings ou pe o med he e e ence coa ing. Su p isingly, he wea a e o he balls was wo o h ee
o de s o magni ude lowe han ha o he RT es s. Again, he ball slide agains he Ag-alloyed coa ing
displayed a lowe wea han he one es ed agains he e e ence one.
A 700
◦
C, he wea o he coa ing was no measu able; con e sely, he balls wo e ou ex ensi ely,
pa icula ly he one sliding agains he e e ence coa ing. The inc ease in he wea a e o he ball a
his empe a u e should be a ibu ed o he signi ican dec ease o he mechanical s eng h o he
Ti-alloy a his empe a u e [
28
]. In summa y, o bo h ball ma e ials he Ag-alloyed coa ing pe o med
be e han he e e ence coa ing wi h an inc easing empe a u e. Al hough he e was no coa ing wea
a 700
◦
C, e en in his case he addi ion o Ag was bene icial. I signi ican ly limi ed he amoun o
adhe ed ma e ial on he wo n coa ing su ace and hus educed he TiAl6V4 ball wea .
4. Discussion
To unde s and he beha io s o he ic ion and he wea desc ibed abo e, a de ailed analysis
o he wea mechanisms, including a wea deb is cha ac e iza ion, was pe o med, and a de ailed
discussion ollows below.
4.1. Al2O3Balls
Figu e 4shows he ypical wea ack mic og aphs o TiSiN/TiN and TiSiN/TiN(Ag) coa ings
es ed a RT. An EDS spec um o he wea deb is p esen a he e e ence coa ing wea ack is
also shown. Bo h coa ings displayed ela i ely smoo h wea acks (see 2D p o iles in Figu e S1 in
he Supplemen a y Ma e ial) p edominan ly indica ing a polishing wea mechanism. Addi ionally,
ish-like wea deb is can be obse ed on he wea ack o he e e ence coa ing, sugges ing he plas ic
de o ma ion o he coa ing; he EDS analysis e ealed small peaks o O and Al, sugges ing a mix u e o
ma e ials om he coa ing and deb is om he ball. The ball wea sca was comple ely co e ed wi h
deb is o he same ype (see Figu e S2 o he Supplemen a y Ma e ial). On he o he hand, such wea
pa icles we e almos absen om he wea ack o he Ag-alloyed coa ing. The con inuous sliding
mo emen o he ball will d ag he wea deb is pa icles along he coa ing su ace, con ibu ing o he
con inuous emo al o he coa ing. The wea o bo h coa ings is go e ned by mic oab asion. In he
case o he e e ence TiSiN/TiN coa ing, plas ic de o ma ion o big-size deb is can also occu , which is
co obo a ed by he luc ua ions o he ic ion shown in Figu e 1a. Howe e , he highe ha dness and
esis ance o plas ic de o ma ion (see Table 1) esul in lowe wea when compa ed o he Ag-con aining
coa ings. In ibological con ac , when an ab asion mechanism is obse ed, he mechanical s eng h
de e mines he wea esis ance. A RT, Ag is no expec ed o di ec ly in luence he ic ional beha io
ia he o ma ion o a low- ic ion ibolaye . Duc ile ma e ials, such as Ag, only p o ide low ic ion a
high empe a u es [
25
,
26
]. The e o e, he wea a e is highe han o he e e ence coa ing. Howe e ,
by con ibu ing o he so ening o he mul ilaye coa ing, i allows an easie p oduc ion o smalle
wea deb is, a oiding he luc ua ion in he COF obse ed in he TiSiN/TiN coa ing. Ha ing o med
simila ibolaye s, wi h he excep ion o he p esence o Ag in he TiSiN/TiN(Ag) coa ing, he COF
alues a he end o he es s a e simila o bo h coa ings.
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Figu e 4. SEM mic og aphs o he wea acks o he (a) e e ence and (b) Ag-alloyed coa ings es ed
a RT agains Al2O3 balls. In (a), he EDS spec um o he wea deb is isible on he wea ack o he
e e ence coa ing is also shown.
A 550 °C, he e e ence coa ing was comple ely delamina ed a e he es s a , esul ing in a
ca as ophic coa ing ailu e (Figu e 5a). On he o he hand, he wea ack o he Ag-alloyed coa ing
(Figu e 5b) showed a se e e ab asion wea mechanism, wi h he majo i y o he wea deb is ejec ed
ou o he wea ack bo de . Simila o he RT es s, he wea ack o his coa ing was also smoo h
(see Figu e 5b). None heless, some wea deb is pa icles adhe ed in he wea ack, wi h he ball
coun e pa comple ely co e ed wi h hem (see he Supplemen a y Ma e ial). The EDS analysis
conduc ed on such wea deb is e ealed Ti, Si, Al and Ag, as well as a low amoun o O, sugges ing
he pa ial oxida ion o he coa ing deb is and/o deb is om he alumina ball, oge he wi h Ag.
Al hough a empe a u e highe han 550 °C, mo i a ed by he mo emen o he ball ( lash con ac
empe a u e), was expec ed on he ball con ac du ing he ibological es s, he pa ial oxida ion o
he wea deb is ag eed well wi h he highe onse poin o oxida ion measu ed o he coa ings (~700
°C, see Table 1). The coun e body was ully co e ed wi h wea deb is o his ype. The p oduc ion
o he coa ing wea deb is necessa y o o ming he ibolaye inc eased he coa ing wea a e in
ela ion o he RT alues, as could be shown by he highe amoun o adhe ed ma e ial o he ball a
550 °C (compa e he RT and 550 °C esul s in he Supplemen a y Ma e ial). As shown be o e, he
p esence o Ag in he con ac led o a dec ease o he COF as compa ed o he RT es due o he less
globally mechanical s eng h o he ibolaye a a high empe a u e. The p esence o Ag in he con ac
will also dec ease he shea s esses needed o he sliding, leading o a be e ibological beha io
o he Ag- ich coa ing as compa ed o he e e ence TiSiN/TiN coa ing, despi e i s lowe mechanical
p ope ies.
Figu e 4.
SEM mic og aphs o he wea acks o he (
a
) e e ence and (
b
) Ag-alloyed coa ings es ed a
RT agains Al
2
O
3
balls. In (
a
), he EDS spec um o he wea deb is isible on he wea ack o he
e e ence coa ing is also shown.
A 550
◦
C, he e e ence coa ing was comple ely delamina ed a e he es s a , esul ing in a
ca as ophic coa ing ailu e (Figu e 5a). On he o he hand, he wea ack o he Ag-alloyed coa ing
(Figu e 5b) showed a se e e ab asion wea mechanism, wi h he majo i y o he wea deb is ejec ed
ou o he wea ack bo de . Simila o he RT es s, he wea ack o his coa ing was also smoo h
(
see Figu e 5b
). None heless, some wea deb is pa icles adhe ed in he wea ack, wi h he ball
coun e pa comple ely co e ed wi h hem (see he Supplemen a y Ma e ial). The EDS analysis
conduc ed on such wea deb is e ealed Ti, Si, Al and Ag, as well as a low amoun o O, sugges ing he
pa ial oxida ion o he coa ing deb is and/o deb is om he alumina ball, oge he wi h Ag. Al hough
a empe a u e highe han 550
◦
C, mo i a ed by he mo emen o he ball ( lash con ac empe a u e),
was expec ed on he ball con ac du ing he ibological es s, he pa ial oxida ion o he wea deb is
ag eed well wi h he highe onse poin o oxida ion measu ed o he coa ings (~700
◦
C, see Table 1).
The coun e body was ully co e ed wi h wea deb is o his ype. The p oduc ion o he coa ing wea
deb is necessa y o o ming he ibolaye inc eased he coa ing wea a e in ela ion o he RT alues,
as could be shown by he highe amoun o adhe ed ma e ial o he ball a 550
◦
C (compa e he RT and
550
◦
C esul s in he Supplemen a y Ma e ial). As shown be o e, he p esence o Ag in he con ac led
o a dec ease o he COF as compa ed o he RT es due o he less globally mechanical s eng h o he
ibolaye a a high empe a u e. The p esence o Ag in he con ac will also dec ease he shea s esses
needed o he sliding, leading o a be e ibological beha io o he Ag- ich coa ing as compa ed o
he e e ence TiSiN/TiN coa ing, despi e i s lowe mechanical p ope ies.
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Figu e 5. SEM mic og aphs o he wea acks o he (a) e e ence and (b) Ag-alloyed coa ings es ed
a 550 °C agains Al2O3 balls. The magni ica ion o he wea deb is on he wea ack o he Ag-doped
coa ing; he co esponding EDS spec um o hose wea deb is is plo ed oo.
4.2. TiAl6V4 Balls
Figu e 6 shows he wea ack o he coa ings es ed agains TiAl6V4 balls a RT. I is clea ha
he wea is di e en o ha om he es ing agains Al2O3 balls; he wea acks a e much oughe .
The wea ack o bo h coa ings displayed ypical adhesi e wea wi h a e y high amoun o adhe ed
ma e ial (see also he 2D p o iles in Figu e S1 o he Supplemen a y Ma e ial). The EDS analysis o
bo h coa ings e ealed ha he wea deb is mainly consis ed o Ti, Al and V, wi h mino aces o O.
In some cases, he wea pa icles also included elemen s om he coa ing, wi h signals om Ag in
he case o he Ag-alloyed coa ing, as can be obse ed in Figu e 6b. Since his ball ma e ial is much
so e han Al2O3, he much ha de ma e ial om he coa ing causes ex ensi e ball wea . The su ace
acies o he ball c a e indica e a ypical ab asion mechanism. The wea olumes in he balls we e
simila in he es s wi h bo h coa ings. As he es p og esses, ma e ial om he ball will be ans e ed
o he coa ing su ace, c ea ing a con inuous adhesi e laye a he coa ing su aces ha p o ec s hem
om u he wea . The low amoun o O indica es ha he adhe ed ma e ial is nonoxidized. Thus,
he es ablished con ac du ing sliding is be ween he ma e ial om he ball adhe ed o he coa ing
and he ball ma e ial, i.e., Ti6Al4V sliding agains i sel . Then, simila COF alues a e measu ed, as
shown in Figu e 2. As he con ac was simila and he wea mainly occu ed in he ball, simila ball
wea a es we e also achie ed in he es s wi h bo h coa ings.
Figu e 5.
SEM mic og aphs o he wea acks o he (
a
) e e ence and (
b
) Ag-alloyed coa ings es ed a
550
◦
C agains Al
2
O
3
balls. The magni ica ion o he wea deb is on he wea ack o he Ag-doped
coa ing; he co esponding EDS spec um o hose wea deb is is plo ed oo.
4.2. TiAl6V4 Balls
Figu e 6shows he wea ack o he coa ings es ed agains TiAl6V4 balls a RT. I is clea ha
he wea is di e en o ha om he es ing agains Al
2
O
3
balls; he wea acks a e much oughe .
The wea ack o bo h coa ings displayed ypical adhesi e wea wi h a e y high amoun o adhe ed
ma e ial (see also he 2D p o iles in Figu e S1 o he Supplemen a y Ma e ial). The EDS analysis o
bo h coa ings e ealed ha he wea deb is mainly consis ed o Ti, Al and V, wi h mino aces o O.
In some cases, he wea pa icles also included elemen s om he coa ing, wi h signals om Ag in he
case o he Ag-alloyed coa ing, as can be obse ed in Figu e 6b. Since his ball ma e ial is much so e
han Al
2
O
3
, he much ha de ma e ial om he coa ing causes ex ensi e ball wea . The su ace acies
o he ball c a e indica e a ypical ab asion mechanism. The wea olumes in he balls we e simila
in he es s wi h bo h coa ings. As he es p og esses, ma e ial om he ball will be ans e ed o
he coa ing su ace, c ea ing a con inuous adhesi e laye a he coa ing su aces ha p o ec s hem
om u he wea . The low amoun o O indica es ha he adhe ed ma e ial is nonoxidized. Thus,
he es ablished con ac du ing sliding is be ween he ma e ial om he ball adhe ed o he coa ing and
he ball ma e ial, i.e., Ti6Al4V sliding agains i sel . Then, simila COF alues a e measu ed, as shown
in Figu e 2. As he con ac was simila and he wea mainly occu ed in he ball, simila ball wea a es
we e also achie ed in he es s wi h bo h coa ings.
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Figu e 6. SEM mic og aphs o he wea acks and wea deb is, and he co esponding EDS analysis
o he (a) e e ence and (b) Ag-alloyed coa ings es ed a RT agains TiAl6V4 balls.
The wea ack o he e e ence coa ing es ed a 550 °C poin s is simila o he wea mechanism
a RT, wi h adhe ed ma e ial in he wea ack. Howe e , he EDS analysis in Figu e 7a shows a
signi ican di e ence in he chemical composi ion: a s ong O peak is de ec ed, meaning ha he
adhe ed ma e ial is oxidized, sugges ing he p esence o a complex mix u e o oxides. Simila oxides
also adhe ed o he ball coun e pa (see Figu e S2 o he Supplemen a y Ma e ial). The e o e, in
ela ion o RT, he main di e ence in he sliding p ocess is ha he con ac is now be ween TiAlV-
oxide and TiAlV-oxide, which can explain bo h he signi ican changes encoun e ed o ei he he
COF and he wea a e. The COF inc ease o 0.8 is due o he change in he ma e ials in he con ac .
On he o he hand, he pa ial co e age o he ball wi h oxide deb is p o ec s he unde laye ma e ial
om wea , dec easing he wea a e o he ball in ela ion o he RT es .
Figu e 7. SEM mic og aphs o he wea acks and wea deb is, and he co esponding EDS analysis
o he (a) e e ence and (b) Ag-doped coa ings es ed a 550 °C agains Tial6V4 balls.
Figu e 6.
SEM mic og aphs o he wea acks and wea deb is, and he co esponding EDS analysis
o he (a) e e ence and (b) Ag-alloyed coa ings es ed a RT agains TiAl6V4 balls.
The wea ack o he e e ence coa ing es ed a 550
◦
C poin s is simila o he wea mechanism a
RT, wi h adhe ed ma e ial in he wea ack. Howe e , he EDS analysis in Figu e 7a shows a signi ican
di e ence in he chemical composi ion: a s ong O peak is de ec ed, meaning ha he adhe ed ma e ial
is oxidized, sugges ing he p esence o a complex mix u e o oxides. Simila oxides also adhe ed o he
ball coun e pa (see Figu e S2 o he Supplemen a y Ma e ial). The e o e, in ela ion o RT, he main
di e ence in he sliding p ocess is ha he con ac is now be ween TiAlV-oxide and TiAlV-oxide,
which can explain bo h he signi ican changes encoun e ed o ei he he COF and he wea a e.
The COF inc ease o 0.8 is due o he change in he ma e ials in he con ac . On he o he hand,
he pa ial co e age o he ball wi h oxide deb is p o ec s he unde laye ma e ial om wea , dec easing
he wea a e o he ball in ela ion o he RT es .
A dis inc wea beha io was ound o he Ag-alloyed coa ing. Se e al g oo es pa allel o he
di ec ion o sliding we e isible oge he wi h a eas co e ed by he adhe ed ma e ial, sugges ing a
mix u e o ab asion and adhesion wea mechanisms. The adhe ed pa icles we e mos ly composed o
Ti, Si, O and Ag; he nea -absence o Al sugges ed ha he ball ma e ial ans e and adhe ence o he
wea ack we e negligible. The low signal o O mean ha he wea deb is we e mainly no oxidized,
in good ag eemen wi h he highe onse poin o oxida ion o he coa ing, i.e., 700
◦
C (
see Table 1
).
In he smoo h pa s o he wea acks, Ag pa icles could be obse ed a he su ace (
see Figu e 7b
),
which should be he o igin o he nonadhe ence o wea deb is om he ball. Mo eo e , he ma e ial
ha adhe ed o he ball wea sca showed a simila composi ion, indica ing a ans e om he coa ing
o he ball. These obse a ions sugges ed ha he con ac was signi ican ly di e en om he e e ence
coa ing, i.e., i should be o a TiSiN(Ag)(O) and/o TiSiN(Ag)(O) +TiAlV(O) ype, which jus i ied he
much lowe COF. The p esence o deb is o oxidized ball ma e ial on he wea ack o he coa ed sample
should no be disca ded. As e e ed o abo e o he e e ence coa ing es ed a 550
◦
C, he TiAl6V4
ball es ed agains he Ag-con aining sample a his empe a u e also had a much lowe wea a e
when compa ed o he RT condi ions, since i was pa ially co e ed wi h wea deb is p o ec ing he
su ace agains he wea .