Room and High Temperature Tribological Performance of Multilayered TiSiN/TiN and TiSiN/TiN(Ag) Coatings Deposited by Sputtering

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. Coa ings 2020,10, 1191 7 o 13 Coa ings 2020, 10, x FOR PEER REVIEW 7 o 14 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. Coa ings 2020,10, 1191 8 o 13 Coa ings 2020, 10, x FOR PEER REVIEW 8 o 14 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. Coa ings 2020,10, 1191 9 o 13 Coa ings 2020, 10, x FOR PEER REVIEW 9 o 14 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 .