Final surface modification for better wear resistance of ceramic coating on cast AlSi10Mg alloy

Ce amics In e na ional 48 (2022) 37433–37447 A ailable online 21 Sep embe 2022 0272-8842/© 2022 The Au ho (s). Published by Else ie L d. This is an open access a icle unde he CC BY license (h p://c ea i ecommons.o g/licenses/by/4.0/). Final su ace modi ica ion o be e wea esis ance o ce amic coa ing on cas AlSi10Mg alloy Roman Gabo a , * , Tom´ aˇ s P ymus a , Ladisla C ˇ cek b , V´ acla Nehasil c , Jose Hlinka d , e , Ma ˇ ej Buˇ il b , Michaela Toka ˇ cíko ´ a a , Jana Seidle o ´ a a , a Nano echnology Cen e, CEET, VSB – Technical Uni e si y o Os a a, 17. Lis opadu 15/2172, 708 00, Os a a-Po uba, Czech Republic b Depa men o Ma e ials Enginee ing, Facul y o Mechanical Enginee ing, Czech Technical Uni e si y in P ague, Ka lo o N´ amˇ es í 293/13, 120 00, P ague 2, Czech Republic c Depa men o Su ace and Plasma Science, Cha les Uni e si y, P ague, Czech Republic d Depa men o Ma e ials Enginee ing and Recycling, Facul y o Ma e ials and Technology, Vˇ SB-Technical Uni e si y o Os a a, 17. Lis opadu 2172/15, 708 00, Os a a-Po uba, Czech Republic e Cen e o Ad anced Inno a ion Technologies, VSB-Technical Uni e si y o Os a a, 17. Lis opadu 2172/15, 708 00, Os a a-Po uba, Czech Republic Depa men o Chemis y and Physico-Chemical P ocesses, Facul y o Ma e ials and Technology, Vˇ SB-Technical Uni e si y o Os a a, 17. Lis opadu 2172/15, 708 00, Os a a-Po uba, Czech Republic ARTICLE INFO Keywo ds: Mic o-a c oxida ion Design o expe imen Coa ing Wea Al–Si alloys ABSTRACT Using he design o expe imen (DOE) me hod and he mic o-a c oxida ion (MAO) echnique, ce amic laye s on AlSi10Mg alloy we e sys ema ically p epa ed o design op imal p ocess condi ions o achie ing he bes ibological p ope ies o he ce amic laye s. The lowes concen a ions o he applied 6 g/l NaOH and 12 g/l Na 2 SiO 3 esul ed in he p epa a ion o uni o m MAO laye s wi h he lowes a ed pa ame e s R a , R z and hickness achie ed unde mic o-a c discha ge condi ions a 500 V and 60 min. Wi h he inc easing hickness o he coa ings, he e was an inc ease o Si in he MAO coa ing. Full ac o ial DOE was used o op imize he ibological p ope ies in a polyalphaole in (PAO) en i onmen a 80 ◦C. The mos signi ican in luence o he p epa a ion o ab asion- esis an laye s o he in es iga ed ac o s was iden i ied on he AlSi10Mg alloy by he NaOH con en in he elec oly e. The ic ion coe icien s o MAO coa ings eached an a e age alue o 0.15. Ae o-lap polishing echnology was applied o inc eased wea esis ance equi emen s o elimina e he de iciencies o MAO coa - ings, leading o dec ease wea ack by almos double compa ed o polished silumin. Remo al o he ou e MAO laye by polishing led o a educ ion in he high co osion esis ance o he MAO coa ing, demons a ing he in luence o he ou e laye no only on he ibological p ope ies bu also on he co osion esis ance o MAO coa ings. 1. In oduc ion Al–Si alloys a e one o he mos impo an aluminium-based ound y alloys, commonly used in au omo i e and ae ospace indus ies due o hei excellen p ope ies such as cas abili y, s eng h- o-weigh a io and good he mal and elec ical conduc i i y. As a esul , Al–Si alloys a e inding mo e applica ions in he men ioned sec o s, pa icula ly in he manu ac u ing o cylinde s, pis ons, engine blocks and b ake calli- pe s [1–3]. Due o he equi emen s o he p oduce s o indi idual p oduc s o he long- e m du abili y o cas ings, he applica ion o Al–Si alloys is signi ican ly limi ed due o low co osion esis ance and su ace ha dness [4]. In o de o imp o e he ab asion and co osion esis ance o Al–Si alloys, hei su aces a e modi ied using anodic oxida ion o he subs a e [5–7] o he ela i ely newly de eloped me hod o he mal sp aying [8,9]. The widesp ead implemen a ion o con ex ha d anodiza ion o su ace ea men o AlSi10Mg alloys is limi ed by he mic os uc u e o he alloy, which is o med by α (Al) dend i es in addi ion o he p ima y α (Al) dend i es and α (Al)+Si eu ec ic [10]. The esul ing oxide laye s p epa ed by ha d anodiza ion do no ha e su icien ab asion and co osion esis ance. The e o e he possibili y o using he ela i ely new echnique o mic o-a c oxida ion (MAO) a ises o he su ace ea men o Al–Si alloys which has been e i ied in se e al wo ks [11–13]. This * Co esponding au ho . E-mail add ess: [email p o ec ed] (R. Gabo ). Con en s lis s a ailable a ScienceDi ec Ce amics In e na ional jou nal homepage: www.else ie .com/loca e/ce amin h ps://doi.o g/10.1016/j.ce amin .2022.09.224 Recei ed 21 July 2022; Recei ed in e ised o m 16 Sep embe 2022; Accep ed 18 Sep embe 2022 Ce amics In e na ional 48 (2022) 37433–37447 37434 echnique is used o p epa e a ce amic adhesion coa ing wi h a compac co osion- esis an and ab asion- esis an inne laye and an ou e po ous laye unde ele a ed s ess condi ions o p oduce a dielec ic discha ge [14]. Key pa ame e s o he p epa a ion o MAO laye s include he choice o elec oly e, which signi ican ly a ec s he g ow h mechanism o he oxide laye and i s co osion and mechanical p ope ies [15]. The mos commonly used elec oly es o he p epa a ion o he oxide laye in Al–Si alloys include silicon-con aining elec oly es [16–18]. Howe e , he p esence o eu ec ic and silica e ions in he elec oly e in luences he esul ing opog aphy o he oxide laye due o he eme gence o “c a e s” on he su ace o he coa ing ep esen ing apidly solidi ied mel o he oxidized subs a e lowing h ough discha ge channels o he su ace [19]. This leads o de e io a ion o he ibological p ope ies and an inc ease o he coe icien o ic ion leading o a signi ican limi a ion in he use o MAO coa ings o Al–Si alloys and hei applica ion in he ield o lub ica ed engine componen s. As a esul , inc eased a en ion has been paid o he de elopmen o sel -lub ica ing MAO coa ings based on Al 2 O 3 /PTFE composi e coa ing [20], mul iphase MAO coa ing con ain- ing g aphi e [21], Si 3 N 4 /TiO 2 nanocomposi e coa ing [22], o syn he- sized MoS 2 laye on he su ace o MAO coa ing [23]. Howe e , an op imal, economically easible p ocedu e is s ill being sough o enable he e icien p epa a ion o MAO coa ing, mee ing he equi emen s o b oade Al–Si applica ions in e ms o ibological and co osion esis- ance p ope ies. To his end, s a is ical analysis me hods such as he sys ema ic design o expe imen (DOE) app oach a e used o op imize and desc ibe he indi idual inpu ac o s’ in luence [24]. This a icle p esen s a p ocedu e o he p epa a ion o ab asion and co osion- esis an MAO coa ing using DOE. Main esul s o his wo k a e (1) de e mina ion o he op imum p ocess condi ions o he MAO p ocess including he impo ance o he main ac o s in luencing he ibological beha iou o he coa ing in polyalphaole in (PAO) a 80 ◦C, (2) desc ip ion o he applica ion p ocedu es o Ae o-lap polishing o imp o e he ibological p ope ies o MAO coa ings, (3) e alua ion o he ibological and co osion p ope ies o he ou e and inne laye o MAO coa ings. The esul s indica e he po en ial applica ion o he abo e-men ioned echnique in he ield o lub ica ed mo o componen s made o Al–Si alloy equi ing high wea and co osion esis ance. 2. Expe imen al 2.1. Ma e ials and sample p epa a ion Su aces o AlSi10Mg ound y alloy samples (20 mm (L)*10 mm (W)*5 mm (H)) we e ea ed by umbling in HV 20 ube ib a o (OTEC, Ge many) o homogenise hem. S eel sa elli es (5 ×3 mm) we e used as p ocess media o 24 h, ollowed by 72 h o lapping in plas ic g inding chips M 10. The lapping p ocedu es we e ca ied ou unde we condi- ions. Finally, he alloy was diges ed in an acid mix u e in a Miles one E hos Up mic owa e diges ion plan (I aly). Selec ed elemen s (Table 1) we e de e mined by a omic emission spec oscopy wi h induc i ely coupled plasma (AES-ICP, Spec o A coss, Ge many). 2.2. Su ace modi ica ion p ocess The MAO coa ing p epa a ion was pe o med acco ding o he combina ion based on he DOE on he semi-ope a ional uni shown in Fig. 1. The swi ching powe supply (DEHOR-spec. Li íno s. .o., Czech Republic) wi h a pulse uni was con olled by a cons an ol age o 500 V o 60 min. The MAO p ocess was ca ied ou wi h he sample as anode and a s ainless s eel shee (1.4301) o 2 mm hickness se ing as a ca hode. The elec oly e composi ion was de ined by he designed DOE wi h empe a u e con olled up o 25 ◦C du ing he MAO p ocess. N. 1) Mixed deg easing ba h (1 M NaOH; 45 ◦C); N. 2,3) Mixed insing ba h (dis illed wa e ; conduc i i y <10 μ S/cm); N. 4) Pickling ba h (20 w % HNO 3 +2 w % HF); N. 5,6) Mixed insing ba h (dis illed wa e ; conduc i i y <10 μ S/cm); N. 7) Mixed insing elec oly e ba h wi h coun e - elec ode (elec oly e; pH ≥12). N. 8,9) Mixed insing ba h (dis illed wa e ; conduc i i y <10 μ S/cm). To imp o e he ibological p ope ies, a lapping echnique was applied o he op imal DOE expe imen using an ae o-lap polishing machine (AERO LAP YT300-OE, Japan). Ve i ica ion o he ibological p ope ies imp o emen was ca ied ou a 1, 2 and 3 min ime, wo king p essu e 0.7 MPa, wo king dis ance 100 mm, and ubbe g anules (Mul icon) o size 0.5–2 mm wi h diamond pas e as p ocess media. 2.3. Design o expe imen (DOE) The design o expe imen s, including he selec ion o ac o s and hei limi alues, was based on he empi ical expe ience o he au ho s. Table 1 Chemical composi ion AlSi10Mg. Sample Elemen (w . %) Si Fe Cu Mn Mg Ni Zn Pb Ti AlSi10Mg 10.0 0.33 0.21 0.03 0.26 0.01 0.07 0.01 0.08 Fig. 1. MAO p ocess. R. Gabo e al. Ce amics In e na ional 48 (2022) 37433–37447 37435 Selec ed h ee main ac o s we e es ed o hei in luence on he esul ing ibological p ope ies in wo le els (2 3 ac o ial design). The selec ed limi alues o each ac o (see Table 2) we e applied wi h epe i ion wi hou cen al poin s. The ull ac o ial design, including he combina ion o he le els o each ac o , is shown in Table 3. 2.4. Su ace cha ac e isa ion Su ace and c oss-sec ion images o he samples we e aken using a JEOL JSM-7610F Plus scanning elec on mic oscope (JEOL, Japan) in seconda y (SE) and backsca e ed elec on (BSE) mode. The chemical composi ion o MAO coa ings and elemen al mapping we e s udied using a dispe si e X- ay spec ome e (EDS, ULTIM MAX 65 mm 2 , Ox- o d Ins umen s, England) a ached o he SEM. The su ace mo phology o he samples was e alua ed u he using co ela i e analysis p o iding da a om AFM Li eScopeTM (Neno ision s. .o., Czech Republic) and SEM allows 3D-CPEM iew (Co ela i e P obe and Elec on Mic oscopy), op ical 3D IF-Senso R25 ins umen (B uke Ali- cona, Aus ia), p o ilome e Talysu 50 (Taylo Hobson, England). Analysis o elemen s and hei chemical s a es was pe o med by X- ay pho oelec on spec oscopy (XPS) in a UHV chambe wi h a base p essu e o a ound 3‧10 −7 Pa. The measu emen sys em consis s o an Omic on EA 125 mul i-channel analyse and a dual anode (Mg/Al) X- ay sou ce. The p ima y ene gy sou ce was he Al K α line (1486.6 eV). The measu emen sys em was calib a ed agains he binding ene gy o C1s (284.5 eV). Table 2 Le els main ac o s. Pa ame e s Fac o Low le el (−1) High le el (+1) A NaOH (g/l) 6 12 B Na 2 SiO 3 (g/l) 9 18 C F equency (Hz) 95 130 Table 3 Full ac o ial design. Tes NaOH (g/l) Na 2 SiO 3 (g/l) F equency (Hz) 1 6 12 130 2 9 12 130 3 6 18 130 4 9 18 130 5 6 12 95 6 9 12 95 7 6 18 95 8 9 18 95 Fig. 2. Op ical images wi h 3D-CPEM. R. Gabo e al. Ce amics In e na ional 48 (2022) 37433–37447 37436 2.5. T ibological es s A ball-on-disk CSM THT ibome e (CSM Ins umen s, Swi ze land) was used o de e mine he ibological p ope ies. The ic ion pai consis ed o he MAO coa ing on a plana sample and an Al 2 O 3 ball 6 mm in diame e . Polyalphaole in (PAO) liquid was chosen as he es en i- onmen . PAO is a non-pola syn he ic hyd oca bon liquid ha is he main syn he ic base componen o mos oils used in indus ial and au omo i e lub ican s. PAO p o ide supe io lub ica ion pe o mance o e a wide ope a ing empe a u e ange han pe oleum oils and is less ola ile. Speci ically, PAO Labo ac 14 (Welch, Ge many) was chosen as a sui able e e ence o ibological measu emen s. The manu ac u e s a es a iscosi y index o 29.0 mm 2 /s a 40 ◦C and 5.6 mm 2 /s a 100 ◦C. Since he iscosi y index dec eases signi ican ly wi h inc easing em- pe a u e, ibological es s we e pe o med a a empe a u e o 80 ◦C o ideally simula e ypical engine ope a ing empe a u es. Measu emen s we e pe o med wice a a no mal load o 2 N, a numbe o laps 10000, linea sliding speed 50 mm/s and a adius o 4 mm. The ic ion coe icien ( μ ) was calcula ed om he a io o he angen ial ic ion o ce and he no mal o ce. The wid h o he wea ack was e alua ed using a digi al mic oscope Olympus DSX1000 (Olympus Co po a ion, Japan). Fig. 3. SEM images o MAO coa ings. Fig. 4. Su ace oughness o MAO coa ings: Pa ame e s R a , R z . R. Gabo e al. Ce amics In e na ional 48 (2022) 37433–37447 37437 2.6. Adhesion es s The CSM Re e es Xp ess +de ice (CSM Ins umen s, Swi ze land) was used o he sc a ch es . The sc a ch es e equipped wi h a Rock- well diamond inden e ( ip adius 200 μ m) was used. The sc a ch es load was se o linea ly inc ease om 1 N o 50 N along he 5 mm sc a ch pa h and linea speed 10 mm/min. 2.7. Co osion es s Samples we e exposed o neu al sal sp ay using a co osion chambe SKB 400 A-TR (Geb . Liebisch GmbH, Ge many) acco ding o ˇ CSN EN ISO 9227 o 300 h. The concen a ion o he sp ayed NaCl solu ion was 50 ±5 g/l. The a e age pH o he accumula ed saline solu ion was 7.0. The samples we e s o ed in he co osion chambe a an inclina ion o 20◦±5◦. The samples we e u he es ed in a h ee-elec ode sys em (Vol alab PGZ 100, SAS O igaLys Elec oChem, F ance). The sample was con- nec ed as a wo king elec ode; he calomel elec ode was used as a e lec ion elec ode, and he ca bon od as an auxilia y elec ode. Po en iodynamic pola isa ion es s we e pe o med in a 3.5 w % NaCl solu ion o e a sample a ea o 0.5 cm 2 . The ini ial po en ial o he po en iodynamic measu emen s was se o −150 mV s open ci cui po en ial (OCP) a e s abilisa ion o he co osion equilib ium wi h a pola isa ion a e o 5 mV/s. Fig. 5. C oss sec ion o MAO samples. R. Gabo e al. Ce amics In e na ional 48 (2022) 37433–37447 37438 2.8. S a is ical analyses Da a ela ed o su ace mo phology o each es we e compa ed wi h each o he using One-way ANOVA ollowed by he Tukey es . The s a is ical analyses we e ca ied ou using Mini ab® 17 s a is ical so - wa e. Resul s we e p esen ed as mean ±s anda d de ia ion (S.D.). Di e ences be ween es s we e e alua ed a a s a is ical signi icance p <0.01. 3. Resul s and discussion 3.1. Su ace opog aphy o MAO coa ings Di e en mo phology o he inpu g ound su ace o he subs a es and oxide laye s p epa ed using di e en p ocess condi ions we e obse ed in Fig. 2 (Table 3) using op ical imaging and co ela i e CPEM analysis. The subs a e su aces o each es (shown in Fig. 3) we e analysed wi h SEM, which con i med he p esence o mic o-de ec s o MAO coa ings. The e olu ion o he coa ing ( ollowing he eaching o he dielec ic discha ge) is accompanied – besides he s uc u al changes – by he o ma ion o so-called olcanic c a e s (Fig. 3, Tes 4). These s uc u es a e o med in he p esence o mic o-discha ge o o m mel ha lows ou h ough so-called discha ge channels owa ds he su ace, whe e i is apidly cooled in he elec oly e en i onmen . A eas wi h a su ace mic os uc u e exhibi a g ea e p esence o hese c a e s and di ec ly a ec he esul ing oughness o MAO su aces [25]. Du ing he MAO p ocess, gas de elopmen occu s; he gas passes h ough he mic ochannels and, oge he wi h he solidi ying mel , o ms a po ous s uc u e wi h localised mic oc acks (Fig. 3, Tes 1). The in ensi y o he in e nal s esses eleased by he coa ing co esponds o he mic o-a c discha ge condi ions wi h subsequen apid cooling in he elec oly e en i onmen unde he indi idual DOE es s [26]. Su ace oughness was e alua ed using he pa ame e s R a (a e age oughness o su ace), R z (a e age o he absolu e alues o he heigh s and dep hs) and hei esul ing alues we e compa ed o each DOE es using one-way analysis o a iance (ANOVA). The esul s o he ela i e posi ions o he obse ed g oups o pa ame e s R a , R z a e shown in Fig. 4. The esul s con i m he s a is ically signi ican e ec o he DOE pa- ame e s on he oughness o he su aces when he null hypo hesis is ejec ed (p <0.01), whe e a leas one mean o he R a and R z pa ame e s is s a is ically di e en . The s a is ical ag eemen was demons a ed o he pa ame e s R a , R z o es s 5 and 1. In he case o he R z su ace pa ame e , es s 6 and 3, as well as es s 7 and 6 we e ound o be s a- is ically indis inguishable. The su ace pa ame e R a was simila o es s 8 and 3. In se e al s udies [27,28], he in luence o silica e con en on he esul ing su ace oughness was con i med. The DOE expe imen con i med he e ec o NaOH addi ion and he p esence o silica es on he esul ing oughness (Fig. 4). As bo h he concen a ion o NaOH, Na 2 SiO 3 and he conduc i i y o he elec oly e inc eases, he e was a s onge mic o-a c discha ge obse ed, mo e mel ans e owa ds he su ace and an inc easing ep esen a ion o he so-called c a e s on he sample su aces, which in u n a ec ed he esul ing su ace oughness. Using he lowes NaOH, Na 2 SiO 3 con en s (Tes 1, 5) o he selec ed DOE combina ions, uni o m su aces wi h he lowes e alua ed pa ame e s R a , R z we e achie ed wi hou any obse ed e ec o he applied sou ce equency. The esul ing quali y o he su aces, in e ms o he e- quency o mic ode ec s, is c ucial o he co osion and ibological p ope ies o he su aces. 3.2. C oss sec ion and chemical composi ion Du ing he MAO p ocess (60 min a 500 V), an inc ease in he oxide laye (Fig. 5) was obse ed, accompanied by a dec ease in cu en . Di e en oxide laye hicknesses we e ob ained by combining he pa- ame e s based on he DOE design. The esul s p esen ed in Fig. 6 con i m he in luence o he chosen pa ame e s on he esul ing coa ing hickness and, hus, lead o he ejec ion o he null hypo hesis (p <0.01) ha all diame e s a e equal. The use o one-way Analysis o Va iance (ANOVA) also con i med ha – based on he compa ison o he indi idual es s – he a e ages o he 5-1, 6-3, and 8-3 es s a e s a is- ically indis inguishable (p >0.01). The smalles hicknesses we e achie ed a he lowes concen a ions o NaOH and Na 2 SiO 3 used; he e ec o he equency used was no e iden in he case o he smalles laye hickness. On he con a y, he g ea es hickness was ob ained when he elec oly e had he highes NaOH and Na 2 SiO 3 con en and when he equency used was 130 Hz. These esul s co espond wi h he obse ed oughness pa ame e s and can be explained based on he mechanism o MAO laye o ma ion [29]. As he ionic con en o he solu ion inc eases, a mo e in ense discha ge occu s, accompanied by a linea inc ease in he laye hickness due o a la ge olume o mel passing h ough he discha ge channels deposi ing on he cooled oxide su ace wi h an inc easing p opo ion o c a e s and po es. The elemen al composi ion o he MAO coa ings shown in Table 4 was s udied using EDX a 50x magni ica ion due o he p esence o he eu ec ic α (Al)+Si and he inhomogenei y o he coa ings. The inc eased Si con en s co espond o es s wi h highe Na 2 SiO 3 con en in he elec oly e; he elemen al dis ibu ion o he esul ing Al–O–Si sys em (Fig. 7) con i ms he p esence o Si in he laye . Acco ding o Chao e al. [30], silicon is mainly inco po a ed in he ou e laye o he coa ing du ing he MAO p ocess om elec opho esis and di usion o he p e- sen SiO2− 3 om he elec oly e. Du ing he MAO p ocess, se e al e- ac ions occu a he anode, leading o he o ma ion o a complex Al–Si–O sys em unde plasma discha ge condi ions (>3500 K) [31–33]. 2OH−−2e−→ H2O+1/2O2(1) Al → Al3++3e−(2) Fig. 6. Thickness MAO coa ings. Table 4 Chemical analysis o su aces by EDX. Tes w . % O Al Si 1 41.87 ±0.34 39.45 ±0.28 18.68 ±0.23 2 39.37 ±0.37 33.08 ±0.26 27.55 ±0.27 3 45.55 ±0.35 17.42 ±0.19 37.03 ±0.28 4 40.90 ±0.37 26.60 ±0.24 32.51 ±0.28 5 39.92 ±0.34 35.14 ±0.25 24.94 ±0.24 6 38.48 ±0.38 39.09 ±0.29 22.43 ±0.25 7 42.29 ±0.38 21.09 ±0.22 36.62 ±0.30 8 38.89 ±0.35 33.22 ±0.25 27.89 ±0.25 R. Gabo e al. Ce amics In e na ional 48 (2022) 37433–37447 37439 2Al3++3O2−→ Al2O3+6e−(3) SiO2− 3−4e−→ 2SiO2+O2(4) ySiO2+xAl2O3→ySiO2.xAl2O3(5) The esul s o he XPS analysis o he chemical s a es o he c ucial elemen s (C, Al, Si, O) p esen on he su ace o he laye s a e shown in Fig. 8. All binding ene gies we e calib a ed o he main C 1s peak (284.5 eV). The p esence o ca bon can be explained by he p esence o g aphi ic ca bon admix u e due o he wid h o he de e mined peak. The Si 2p binding ene gy (103.8 ±0.2 eV) co esponds o he p esence o SiO 2 , which is o med du ing he MAO p ocess om silica es p esen in he elec oly e (Eq. (4)). The Al 2p line (75.4 eV) in he spec um indica es he p esence o an oxide phase. A shi o highe alues o abou 1 eV compa ed o he abula ed alue o Al 2 O 3 indica es he p obable p esence o he aluminosilica e (Eq. (5)) [34]. The O 1s peak (530.6 ±0.1 eV) is in he ange o binding ene gies ha co espond o he p esence o an oxide phase [35]. 3.3. T ibological p ope ies o MAO coa ings in oil The decisi e pa ame e o selec ing he mos sui able ype o MAO laye was he e alua ion o i s wea . The combina ion o a ha d and b i le MAO laye on a ough silumin base ma e ial can be sensi i e o he ini ial poin p essu e du ing unning-in. The e o e, he es s we e pe o med in PAO oil o make he unning-in phase as gen le as possible. A he same ime, a small 2 N load, a s anda d linea sliding speed o 50 mm/s, a adius o 4 mm, and 10000 cycles (co esponds o he ack 251 m) we e used. Fig. 9a shows a compa ison o he ic ion coe icien s o he MAO laye s (Tes 1–8), whe e he ic ion coun e pa was an Al 2 O 3 ball. The MAO laye s showed ela i ely high po osi y (Fig. 3) and oughness (Fig. 4). In some cases, ab asi e pa icles o med du ing he unning-in phase, which caused an uns able cou se o ic ion. The excep ion was Tes 5 laye , which beha ed e y s ably, and i s coe i- cien o ic ion was also he lowes o all es ed MAO laye s. Howe e , i was s ill highe han ha o pu e silumin, which se ed as a e e ence sample. In his case, howe e , i was a polished su ace. Fe n´ andez- L´ opez e al. [12] epo ed a simila cou se o he ic ion coe icien o silumin wi h and wi hou MAO ea men wi h a polished su ace. The si ua ion was simila in he case o he wea ack wid h e alu- a ion. I can be seen in Fig. 9b ha he Tes 5 sample had he smalles ack wid h and sligh ab asi e wea o he ball (Fig. 10a). Addi ionally, he hickness o he Tes 5 laye was he lowes o all he laye s (Fig. 6), and i s ab asion should mani es i sel much ea lie han he o he laye s. Howe e , i mus be no ed ha he ack wid h was only sligh ly smalle han ha o polished silumin (Fig. 9b), whe e adhesi e ic ion p e- domina ed be ween he ball and he sample (Fig. 10c). Fo a p ecise de e mina ion, i would be necessa y o compa e samples wi h simila su ace oughness. On he con a y, he Tes 8 sample showed he g ea es wea ack, whe e ab asi e ic ion p e ailed be ween he ball and he sample (Fig. 10b). 3.4. Adhesion o MAO coa ing The sc a ch es me hod was used o compa e he adhesion o he MAO coa ings. A linea ly inc easing load was se o es ing om 1 N o 50 N. The load was chosen so ha he esul ing sc a ch dep h was Fig. 7. SEM/EDX mapping images o MAO coa ings. R. Gabo e al. Ce amics In e na ional 48 (2022) 37433–37447 37440 compa able o he sc a ch dep h in he s anda d sc a ch es , whe e he ha d base ma e ial and load up o 100 N a e used. The es s pe o med did no show ailu es in he cohesion o adhesion o he MAO coa ings. This is consis en wi h he g ow h mechanism o MAO coa ings, which a e o med by su ace oxida ion and o m a compac sys em wi h he base ma e ial. The po osi y o he MAO coa - ings a ied g ea ly, as con i med in Fig. 3. Mo e po ous MAO coa ings could ha e o med ab asi e pa icles o a g ea e ex en and inc eased wea . This assump ion was e i ied in MAO coa ings wi h he lowes and highes wea . Fig. 11 compa es he sc a ches o he samples wi h he highes wea esis ance (Tes 5) and he lowes wea esis ance (Tes 8). The e was no damage o he cohesion o adhesion o he MAO coa ings a ound he sc a ches. The e o e, he deg ada ion o he wea esis ance o he Tes 8 sample is no caused by adhesion ailu e, bu by he Fig. 8. XPS spec um o MAO coa ings. Fig. 9. Compa ison o a) he ic ion coe icien s and b) he wea acks o he MAO laye s Tes 1–8. Polished silumin was used as a e e ence. R. Gabo e al. Ce amics In e na ional 48 (2022) 37433–37447 37441 Fig. 10. Compa ison o he wea o he Al 2 O 3 ball (le ) and he wea ack o sample ( igh ) o : a) Tes 5 sample, b) Tes 8 sample, and c) silumin. Fig. 11. Sc a ch es o MAO coa ings: a) Tes 5 sample and b) Tes 8 sample (load 1–50 N, dis ance 50 mm). R. Gabo e al.