Effect of Laser Remelting of Fe-Based Thermally Sprayed Coating on AZ91 Magnesium Alloy on Its Structural and Tribological Properties

Ci a ion: Buch ík, M.; Bˇ ezina, M.; M ˇna, L.; Palán, M.; Filipenský, J.; Doležal, P.; Neˇcas, D.; F ýza, J.; Kajánek, D.; Wasse baue , J.; e al. E ec o Lase Remel ing o Fe-Based The mally Sp ayed Coa ing on AZ91 Magnesium Alloy on I s S uc u al and T ibological P ope ies. Coa ings 2023,13, 1033. h ps://doi.o g/ 10.3390/coa ings13061033 Academic Edi o : Manuel An ónio Pe al a E a is o Recei ed: 12 May 2023 Re ised: 26 May 2023 Accep ed: 30 May 2023 Published: 2 June 2023 Copy igh : © 2023 by he au ho s. Licensee MDPI, Basel, Swi ze land. This a icle is an open access a icle dis ibu ed unde he e ms and condi ions o he C ea i e Commons A ibu ion (CC BY) license (h ps:// c ea i ecommons.o g/licenses/by/ 4.0/). coa ings A icle E ec o Lase Remel ing o Fe-Based The mally Sp ayed Coa ing on AZ91 Magnesium Alloy on I s S uc u al and T ibological P ope ies Ma in Buch ík1,*, Ma ˇej Bˇ ezina 1, Libo M ˇna 2,3, Ma ek Palán4, Jan Filipenský5, Pa el Doležal 1,3 , Da id Neˇcas 3, Jose F ýza 3, Daniel Kajánek 6, Ja omí Wasse baue 1and Leoš Doskoˇcil 1 1Ma e ials Resea ch Cen e, Facul y o Chemis y, B no Uni e si y o Technology, Pu kyno a 464/118, 612 00 B no, Czech Republic; [email p o ec ed] (M.B.); [email p o ec ed].cz (P.D.); [email p o ec ed] (J.W.); [email p o ec ed] (L.D.) 2Ins i u e o Scien i ic Ins umen s o he Czech Academy o Sciences, K alo opolska, 147, 612 64 B no, Czech Republic; [email p o ec ed] 3Facul y o Mechanical Enginee ing, B no Uni e si y o Technology, Technicka 2896/2, 602 00 B no, Czech Republic; [email p o ec ed] (D.N.); [email p o ec ed].cz (J.F.) 4Depa men o Ma e ials and Enginee ing Me allu gy, Facul y o Mechanical Enginee ing, Uni e si y o Wes Bohemia, Uni e zi ni 22, 301 00 Plzen, Czech Republic; [email p o ec ed] 5PLASMAMETAL, L d., To a ni 917/1e, 643 00 B no, Czech Republic; [email p o ec ed] 6Resea ch Cen e, Uni e si y o Zilina, Uni e zi na 8215/1, 010 26 Zilina, Slo akia; [email p o ec ed] *Co espondence: [email p o ec ed]; Tel.: +420-541-149-469 Abs ac : An Fe-based coa ing was he mally sp ayed on o he su ace o AZ91 magnesium alloy ia he High-Veloci y-Oxygen-Fuel (HVOF) me hod. The he mally sp ayed coa ing wi h a hickness o 530 ± 25 µ m and a po osi y o 0.7 ± 0.1% did no show any mac os uc u al de ec s and did no cause any deg ada ion o he AZ91 alloy. Lase emel ing o he su ace laye o he sp ayed coa ing esul ed in he ec ys alliza ion o he s uc u e and he disappea ance o p esen ed po es, spla bounda ies, and o he de ec s. This led o an inc ease in he ha dness o he emel ed laye om he o iginal 535 ± 20 HV0.3 up o 625 ± 5 HV0.3. Howe e , du ing he lase emel ing a a lase powe o 1000 W, s ess c acking in he coa ing occu ed. The ibological p ope ies we e e alua ed by he ball-on-pla e me hod unde d y condi ions. Compa ed o he uncoa ed AZ91 magnesium alloy, a highe alue o ic ion coe icien (COF) was measu ed o he as-sp ayed coa ing. Howe e , he e was a dec ease in wea a e and weigh loss. The emel ing o he su ace laye o he as-sp ayed coa ing led o a u he dec ease in he wea a e and weigh loss. Based on he ob ained da a, i has been shown ha he applica ion o lase - emel ed he mally sp ayed Fe-based coa ings on AZ91 Mg alloy imp o es ha dness and ibological p ope ies compa ed o ba e Mg alloy and as-sp ayed Fe-based coa ings. Keywo ds: AZ91; magnesium alloy; HVOF coa ing; lase emel ing; ibological p ope ies; wea beha io 1. In oduc ion Magnesium alloys a e used in indus y, especially in he au omo i e indus y, he ae ospace indus y, elec ical enginee ing, and elecommunica ions, due o hei low den- si ies, which a e he lowes o any cu en ly known me allic cons uc ion ma e ials, and due o hei high s eng h- o-weigh a ios [ 1 – 4 ]. Cu en applica ions in he au omo i e indus y include hei use o componen s such as pedals, engine block cas ings, gea box housings, sea ames, and some bodywo k componen s, e c. [ 1 , 5 ]. In indus ial enginee - ing, such as in ex iles o p in ing, magnesium alloys a e used o componen s ope a ing a high speeds and which he e o e need o be ligh weigh o minimize ine ial o ces [1,5]. Coa ings 2023,13, 1033. h ps://doi.o g/10.3390/coa ings13061033 h ps://www.mdpi.com/jou nal/coa ings Coa ings 2023,13, 1033 2 o 16 Howe e , a common ea u e o all magnesium alloys is hei low esis ance o co o- sion and wea [ 1 , 6 – 9 ]. These disad an ages can be a oided by he applica ion o p o ec i e coa ings [ 7 , 9 – 12 ]. Many ypes o coa ings ha e been de eloped o inc ease he co osion esis ance o magnesium ma e ials, o example, plasma elec oly ic oxida ion (PEO) coa - ings [ 13 ], laye ed double hyd oxides (LDH), supe hyd ophobic coa ings [ 14 , 15 ], elec oless coa ings o elec opla ed coa ings [ 16 – 18 ], phospha e coa ings o coa ings deposi ed ia ho sp ay echnology [ 3 , 19 – 21 ]. Al hough hese coa ings inc ease co osion esis ance, hey achie e low adhesion, ha dness and ab asion esis ance, which limi s hei p ac ical applica ion [22,23]. In addi ion o he abo e-men ioned applica ions o p o ec i e coa ings, he addi ion o alloying elemen s o he use o hea o mechanical ea men (e.g., sho peening [ 24 – 26 ], lase sho peening [ 27 ], lase cladding [ 28 ] o ic ion s i welding [ 29 ] a e also used o imp o e he su ace p ope ies o Mg alloys. As epo ed in publica ions [ 30 – 33 ], he addi ion o an alloying elemen such as Nd, Sc o Sm o AZ91 alloy esul ed in imp o ed co osion p ope ies and inc eased ha dness, educed wea a e and inc eased s eng h. A sui able al e na i e could also be he appli- ca ion o coa ings by mechanical alloying ollowed by lase cladding echnique [ 28 ]. The applica ion o FeCoNiC Al/AlSi12-based composi e coa ings by he mechanical alloying me hod can inc ease he ha dness and nanoha dness o he sample su ace. Subsequen lase cladding o he FeCoNiC Al/AlSi12 coa ing esul ed in a u he signi ican inc ease in ha dness. The coa ing exhibi ed a e ined and homogeneous mic os uc u e wi hou s uc u al de ec s such as po es and c acks. In ecen yea s, he mos p omising me hods o he p epa a ion o coa ings appea o be he High-Veloci y-Oxygen-Fuel (HVOF) and Cold Sp ay (CS) me hods [ 20 , 22 , 34 ]. Bo h HVOF and CS a e he mal sp aying echniques, allowing he deposi ion o dense, ha d, and low po ous coa ings [ 35 , 36 ]. The HVOF echnique is sui able o he deposi ion o bo h me al and me al–ce amic-based coa ings and allows he applica ion o ab asion- esis an coa ings [ 22 , 37 , 38 ]. Ga cía-Rod íguez e al. [ 39 ] in es iga ed he applica ion o 316L-base HVOF-sp ayed coa ings on ZE41 magnesium alloy and s udied he su ace p ope ies. I was ound ha he applica ion o 316L-based coa ings educed he wea a e (a low wea a es and lowe loads) by up o 93% compa ed o ba e ZE41 magnesium alloy. A he same ime, he ha dness was inc eased mo e han six- old om 65 HV0.1 o 433 HV0.1 by he applica ion o he coa ings. Koga e al. [ 40 ] epo ed ha he applica ion o amo phous Fe60C 8Nb8B24 (a .%) HVOF-sp ayed coa ings can lead o a signi ican inc ease in su ace ha dness, om 222 ± 5 HV0.3 o s eel subs a e o 838 ± 23 HV0.3 o amo phous coa ing. As epo ed by he au ho s [ 40 ], he coa ings exhibi excellen ab asion p ope ies, mainly due o he highe p opo ion o he amo phous phase. On he basis o wea measu emen s, i was obse ed ha he wea a e o he amo phous coa ings was app oxima ely wo o de s o magni ude lowe han ha o he s eel subs a e. Wi h espec o s uc u al de ec s o HVOF he mally sp ayed coa ings (po es, mic oc- acks, spla bounda ies, e c.), se e al me hods o pos - ea men ha e been s udied in he pas o imp o e su ace p ope ies. Among hem, hea ea men and lase ea men ha e been used o emo e hese s uc u al de ec s [ 41 – 43 ]. Such de ec s can ha e a nega i e e ec on bo h he esul ing unc ional p ope ies and he beha io o he coa ed samples. The lase emel ing p ocess can imp o e he p ope ies o he coa ings by elimina ing hei de ec s (po es, c acks, e c.), which can lead o an inc ease in densi y, ha dness, wea esis ance, s ess elie , e c. The aim o his pape was o desc ibe he e ec o he lase emel ing o Fe-based he mally sp ayed coa ings deposi ed on he su ace o AZ91 magnesium alloy. Using lase powe s o 650 W and 1000 W, he e ec s o lase emel ing on he s uc u e, ha dness, and ibological p ope ies o coa ings we e e alua ed. The lase emel ing p ocess can be a use ul echnique o he pos - ea men o he mally sp ayed coa ings and o imp o ing he pe o mance and ex ending he li e o he applied coa ing o coa ed pa . This issue has no ye ecei ed su icien a en ion in he li e a u e. The expe imen al esul s showed ha Coa ings 2023,13, 1033 3 o 16 he applica ion o lase - emel ed he mally sp ayed Fe-based coa ings on AZ91 Mg alloy imp o es ha dness and ibological p ope ies compa ed o ba e Mg alloy and as-sp ayed Fe-based coa ings. 2. Ma e ials and Me hods 2.1. Ma e ial W ough AZ91 magnesium alloy was used as he subs a e o he deposi ion o he - mally sp ayed coa ings. The elemen al composi ion o AZ91 alloy was (w %): 8.80 Al ; 0.81 Zn ; 0.32 Mn ; 0.04 Si; 0.01 Z ; balance Mg. The ma e ial was ecei ed as cas AZ91 mag- nesium alloy shee s wi h he dimensions o 100 × 100 × 7 mm. Be o e he sp aying p ocess, he su ace o each shee was co undum blas ed wi h F36 b own co undum ( 500–600 µm ) using a Hunzike ST 1403 (Hunzike , Die Cas Machine y, Waukegan, IL, USA) blas ing uni a oom empe a u e unde a p essu e o 3 ba and hen d y ai -cleaned. The su ace oughness o he blas ed samples (R a ) was de e mined o be app oxima ely 5.2 µm using a con ocal mic oscope (Lex OLS 3000, Olympus, Tokyo, Japan). The used eeds ock powde (Figu e 1) was a omized Fe-based powde o he mal sp aying (Diamalloy 1010; Oe likon Me co, Lomm, The Ne he lands) wi h a nominal size dis ibu ion o − 45 + 16 µ m. The chemical composi ion o he eeds ock powde speci ied by he supplie was (w %): 28 C ; 16 Ni; 4.5 Mo; 1.5 Si; 1.75 C; balance Fe. Coa ings 2023, 13, x FOR PEER REVIEW 3 o 17 a use ul echnique o he pos - ea men o he mally sp ayed coa ings and o imp o ing he pe o mance and ex ending he li e o he applied coa ing o coa ed pa . This issue has no ye ecei ed sufficien a en ion in he li e a u e. The expe imen al esul s showed ha he applica ion o lase - emel ed he mally sp ayed Fe-based coa ings on AZ91 Mg alloy imp o es ha dness and ibological p ope ies compa ed o ba e Mg alloy and as- sp ayed Fe-based coa ings. 2. Ma e ials and Me hods 2.1. Ma e ial W ough AZ91 magnesium alloy was used as he subs a e o he deposi ion o he - mally sp ayed coa ings. The elemen al composi ion o AZ91 alloy was (w %): 8.80 Al; 0.81 Zn; 0.32 Mn; 0.04 Si; 0.01 Z ; balance Mg. The ma e ial was ecei ed as cas AZ91 magne- sium alloy shee s wi h he dimensions o 100 × 100 × 7 mm. Be o e he sp aying p ocess, he su ace o each shee was co undum blas ed wi h F36 b own co undum (500–600 µm) using a Hunzike ST 1403 (Hunzike , Die Cas Machine y, Waukegan, IL, USA) blas ing uni a oom empe a u e unde a p essu e o 3 ba and hen d y ai -cleaned. The su ace oughness o he blas ed samples (Ra) was de e mined o be app oxi- ma ely 5.2 µm using a con ocal mic oscope (Lex OLS 3000, Olympus, Tokyo, Japan). The used eeds ock powde (Figu e 1) was a omized Fe-based powde o he mal sp aying (Diamalloy 1010; Oe likon Me co, Lomm, The Ne he lands) wi h a nominal size dis ibu ion o −45 + 16 µm. The chemical composi ion o he eeds ock powde speci ied by he supplie was (w %): 28 C ; 16 Ni; 4.5 Mo; 1.5 Si; 1.75 C; balance Fe. (a) (b) Figu e 1. The su ace mo phology o eeds ock powde : (a) Diamalloy 1010 pa icles and (b) pa icle de ail. 2.2. The mal Sp aying P ocess The mally HVOF-sp ayed coa ings (sp ayed in 4 o ch passes) we e sp ayed on he blas ed AZ91 magnesium alloy samples using a JP-5000 liquid- uel HVOF gun connec ed o an IRB-2400/16 obo (ABB, Zü ich, Swi ze land), Figu e 2a. The o iginal 100 × 100 mm blas ed samples (Figu e 2b) we e HVOF sp ayed along a s ip o 100 × 70 mm. The samples we e ixed wi h s eel b acke s on each side o a dis ance o 15 mm. Figu e 1. The su ace mo phology o eeds ock powde : ( a ) Diamalloy 1010 pa icles and ( b ) pa i- cle de ail. 2.2. The mal Sp aying P ocess The mally HVOF-sp ayed coa ings (sp ayed in 4 o ch passes) we e sp ayed on he blas ed AZ91 magnesium alloy samples using a JP-5000 liquid- uel HVOF gun connec ed o an IRB-2400/16 obo (ABB, Zü ich, Swi ze land), Figu e 2a. The o iginal 100 ×100 mm blas ed samples (Figu e 2b) we e HVOF sp ayed along a s ip o 100 × 70 mm. The samples we e ixed wi h s eel b acke s on each side o a dis ance o 15 mm. 2.3. Lase T ea men P ocess Lase ea men was pe o med on he HVOF-sp ayed coa ings. The coa ed magne- sium samples we e i s p ehea ed o 200 ◦ C o 30 min o educe he hea shock. The lase beam powe ed by a YLS-2000 ibe lase sou ce (IPG, Ox o d, MA, USA) was led by a 100 µm- hick ibe o a Fibe Rhino (A ges, Wacke sdo , Ge many) scanning head wi h an S4LFT1330/328 - he a lens (Sill Op ics, Wendels ein, Ge many). The diame e o he beam was 325 µ m, and he lase powe s we e se a 650 W and 1000 W. The linea scanning speed was 4 m · s −1 wi h a line spacing o 60 µ m. The lase ea men p ocessing pa e n was laid down in he same way as in ou p e ious s udy [19]. Coa ings 2023,13, 1033 4 o 16 Coa ings 2023, 13, x FOR PEER REVIEW 4 o 17 (a) (b) Figu e 2. The mal sp aying p ocess; (a) sample placemen o he mal sp aying and (b) sample holde . 2.3. Lase T ea men P ocess Lase ea men was pe o med on he HVOF-sp ayed coa ings. The coa ed magne- sium samples we e i s p ehea ed o 200 °C o 30 min o educe he hea shock. The lase beam powe ed by a YLS-2000 ibe lase sou ce (IPG, Ox o d, MA, USA) was led by a 100 µm- hick ibe o a Fibe Rhino (A ges, Wacke sdo , Ge many) scanning head wi h an S4LFT1330/328 - he a lens (Sill Op ics, Wendels ein, Ge many). The diame e o he beam was 325 µm, and he lase powe s we e se a 650 W and 1000 W. The linea scanning speed was 4 m·s−1 wi h a line spacing o 60 µm. The lase ea men p ocessing pa e n was laid down in he same way as in ou p e ious s udy [19]. 2.4. Cha ac e iza ion The eeds ock powde and he HVOF-sp ayed Fe-based coa ings we e cha ac e ized by a JEOL JSM-7600 FESEM mic oscope equipped wi h an Ul im® Max ene gy-dispe si e X- ay spec oscope (Ox o d Ins umen s plc, Abingdon, UK). The hicknesses o he di - e en samples we e measu ed om a c oss-cu using he Sma SEM® Use In e ace scan- ning elec on mic oscope so wa e. Measu emen s we e aken a h ee loca ions. EBSD (Elec on backsca e diff ac ion) analysis o he coa ings was pe o med using a JEOL JSM-7600 (JEOL) FESEM mic oscope equipped wi h a No dlys EBSD de ec o . The accel- e a ing ol age o EBSD analysis was se o 20 kV in o de o achie e he bes signal:noise a io and he s ep size was 0.5 µm. Fo he EBSD cha ac e iza ion, he samples we e ion polished using IB-19500CP C oss Sec ion Polishe (JEOL). The po osi y was e alua ed by means o image analysis using ImageJ so wa e. The phase composi ions o AZ91 magnesium alloy, eeds ock powde , and as- sp ayed coa ings we e de e mined using an Empy ean X- ay diff ac ion (XRD) spec om- e e (Panaly ical, Mal e n, UK) wi h Cu Kα adia ion. The 2θ scan ange anged om 10 o 85°; he scan s ep size was 0.013°; he ime pe s ep was se o 39 s; he gene a o ol age was 40 kV wi h he ube cu en 30 mA. HighSco e Plus so wa e was used o he e alu- a ion o measu ed da a. The Vicke s mic oha dness o all samples wi h he mally sp ayed coa ings and lase - emel ed coa ings was measu ed unde an applied load o 300 g o a du a ion o 10 s (AMH 55, LECO, S . Joseph, MI, USA) acco ding o ASTM E384. T ibological measu emen s we e pe o med using he UMT T ibolab (B uke , Bille - ica, MA, USA) uni e sal mechanical es e pla o m using he ball-on-pla e me hod ha conside s ecip oca ing mo ion unde an applied no mal pin load o 20 N (Mean con ac p essu e 1150 MPa). The du a ion was se o 600 s, wi h a o al sliding dis ance o 12 m a a equency o 2 Hz, and a wo king dis ance (s oke leng h) o 10 mm. The sliding speed was 20 mm·s−1. The d y sliding wea es s we e pe o med a labo a o y empe a u e on Figu e 2. The mal sp aying p ocess; ( a ) sample placemen o he mal sp aying and ( b ) sample holde . 2.4. Cha ac e iza ion The eeds ock powde and he HVOF-sp ayed Fe-based coa ings we e cha ac e ized by a JEOL JSM-7600 FESEM mic oscope equipped wi h an Ul im ® Max ene gy-dispe si e X- ay spec oscope (Ox o d Ins umen s plc, Abingdon, UK). The hicknesses o he di e en samples we e measu ed om a c oss-cu using he Sma SEM ® Use In e ace scanning elec on mic oscope so wa e. Measu emen s we e aken a h ee loca ions. EBSD (Elec on backsca e di ac ion) analysis o he coa ings was pe o med using a JEOL JSM-7600 (JEOL) FESEM mic oscope equipped wi h a No dlys EBSD de ec o . The accele a ing ol age o EBSD analysis was se o 20 kV in o de o achie e he bes signal:noise a io and he s ep size was 0.5 µ m. Fo he EBSD cha ac e iza ion, he samples we e ion polished using IB-19500CP C oss Sec ion Polishe (JEOL). The po osi y was e alua ed by means o image analysis using ImageJ so wa e. The phase composi ions o AZ91 magnesium alloy, eeds ock powde , and as-sp ayed coa ings we e de e mined using an Empy ean X- ay di ac ion (XRD) spec ome e (Pan- aly ical, Mal e n, UK) wi h Cu K α adia ion. The 2 θ scan ange anged om 10 o 85 ◦ ; he scan s ep size was 0.013 ◦ ; he ime pe s ep was se o 39 s; he gene a o ol age was 40 kV wi h he ube cu en 30 mA. HighSco e Plus so wa e was used o he e alua ion o measu ed da a. The Vicke s mic oha dness o all samples wi h he mally sp ayed coa ings and lase - emel ed coa ings was measu ed unde an applied load o 300 g o a du a ion o 10 s (AMH 55, LECO, S . Joseph, MI, USA) acco ding o ASTM E384. T ibological measu emen s we e pe o med using he UMT T ibolab (B uke , Bille ica, MA, USA) uni e sal mechanical es e pla o m using he ball-on-pla e me hod ha conside s ecip oca ing mo ion unde an applied no mal pin load o 20 N (Mean con- ac p essu e 1150 MPa). The du a ion was se o 600 s, wi h a o al sliding dis ance o 12 m a a equency o 2 Hz, and a wo king dis ance (s oke leng h) o 10 mm. The sliding speed was 20 mm · s −1 . The d y sliding wea es s we e pe o med a labo a o y empe a u e on polished samples (Ra ≈ 0.05 µ m). The coun e -ball was Si 3 N 4 wi h a diame e o 7.5 mm. A o al o 2 epe i ions we e pe o med o each sample. 3. Resul s and Discussion 3.1. S uc u al Cha ac e iza ion Figu e 3a–c show he mic os uc u e o HVOF coa ings on AZ91 Mg alloy. F om he mac oscopic poin o iew, i can be seen ha he as-sp ayed coa ing and he coa ing ea ed by he 650 W lase powe we e homogeneous and ee o s uc u al de ec s (Figu e 3a,b). Fo he 1000 W lase emel ed coa ing, ex ensi e c acks we e isible, ex ending o abou hal o Coa ings 2023,13, 1033 5 o 16 he coa ing (Figu e 3c). As discussed in ou p e ious s udy [ 19 ], he p esence o hese c acks can be explained by he high s esses in he coa ing associa ed wi h he olume change in he emel ed laye . Du ing emel ing, po es, spla bounda ies and o he inhomogenei ies in he su ace laye disappea ed. This led o ensile s esses and c ack o ma ion in he coa ing. The e was no isible delamina ion, deadhesion, o isible oxida ion a he coa ing/Mg subs a e in e ace in any o he coa ings, indica ing he po en ially high adhesion o he coa ings o he Mg subs a e. Coa ings 2023, 13, x FOR PEER REVIEW 5 o 17 polished samples (Ra ≈ 0.05 µm). The coun e -ball was Si3N4 wi h a diame e o 7.5 mm. A o al o 2 epe i ions we e pe o med o each sample. 3. Resul s and Discussion 3.1. S uc u al Cha ac e iza ion Figu e 3a–c show he mic os uc u e o HVOF coa ings on AZ91 Mg alloy. F om he mac oscopic poin o iew, i can be seen ha he as-sp ayed coa ing and he coa ing ea ed by he 650 W lase powe we e homogeneous and ee o s uc u al de ec s (Figu e 3a,b). Fo he 1000 W lase emel ed coa ing, ex ensi e c acks we e isible, ex ending o abou hal o he coa ing (Figu e 3c). As discussed in ou p e ious s udy [19], he p esence o hese c acks can be explained by he high s esses in he coa ing associa ed wi h he olume change in he emel ed laye . Du ing emel ing, po es, spla bounda ies and o he inhomogenei ies in he su ace laye disappea ed. This led o ensile s esses and c ack o ma ion in he coa ing. The e was no isible delamina ion, deadhesion, o isible oxida- ion a he coa ing/Mg subs a e in e ace in any o he coa ings, indica ing he po en ially high adhesion o he coa ings o he Mg subs a e. The hicknesses o as-sp ayed and lase emel ed coa ings was abou 500 µm. The ha dness o he as-sp ayed coa ing was de e mined o be 535 ± 20 HV0.3. In he case o he emel ed coa ing a a lase powe o 650 W, he e was an inc ease in he ha dness o he 30 µm-wide emel ed laye on he su ace o 610 ± 8 HV0.3. In he case o he emel ed coa ing a a lase powe o 1000 W, he e was an inc ease in he ha dness o he 45 µm wide emel ed laye on he su ace o 625 ± 5 HV. This inc ease in ha dness may be asso- cia ed wi h he disappea ance o he spla bounda ies, po es, and o he mic os uc u al de ec s. The indi idual coa ing hickness alues a e summa ized in Table 1. The X- ay diff ac og am (Figu e 3d) o he subs a e (AZ91 alloy) showed he p es- ence o wo main phases, namely he α-Mg ma ix phase (a solid solu ion o alloying ele- men s in Mg), and he Mg17Al12 in e me allic phase. In he case o he eeds ock powde , he as-sp ayed coa ing, and he lase - emel ed coa ings, only wo phases we e always de- ec ed, namely aus eni e and M7C3 ca bide, which is a mixed ca bide, whe e M = Fe, C , Mo [44,45]. Apa om hese peaks, no o he phases appea ed in he coa ings. Compa ed o he peaks o he eeds ock powde , he indi idual peaks co esponding o he coa ings ha e a lowe in ensi y, which is ela ed o he se e e plas ic de o ma ion o he pa icles du ing he sp aying p ocess, and he p esence o amo phous a eas associa ed wi h he impac o ully mel ed eeds ock powde pa icles on he Mg subs a e and hei apid solidi ica ion [20,22]. (a) (b) Coa ings 2023, 13, x FOR PEER REVIEW 6 o 17 (c) (d) Figu e 3. Mic os uc u e o he mally sp ayed coa ings, (a) as-sp ayed, (b) lase - emel ed a 650 W, (c) lase - emel ed a 1000 W. XRD pa e ns (d) o AZ91 alloy, eeds ock powde , and he mally sp ayed coa ings. As can be obse ed in Figu e 4, he mic os uc u e o he as-sp ayed coa ing consis ed o non-mel ed, pa ially mel ed, and ully mel ed a eas. In Figu e 4a, he diffe en egions a e clea ly dis inguishable and include spla bounda ies and po es. Figu e 4b shows a non-mel ed pa icle sepa a ed om i s su oundings by spla bounda ies. As shown by he XRD analysis (Figu e 3d), wo phases we e de ec ed in he coa ing, and hus in he non-mel ed a ea–aus eni e and M7C3 ca bide. In he non-mel ed a ea, he dend i es o aus- eni e (da ke egions) and ca bide (ligh e egions) we e well esol ed. A diffe en mic os uc u e was obse ed o pa ially mel ed a eas and signi ican ly de o med a eas (Figu e 4c). As can be seen in Figu e 4c, hese a eas we e composed o ine ca bide pa icles and ca bides in he in e dend i ic space which did no o m a con inuous ne wo k, as in he case o non-mel ed a eas. I appea s ha du ing he apid solidi ica ion p ocess, he ca bide did no ha e ime o ully c ys allize. A high deg ee o plas ic de o - ma ion is e iden compa ed o he non-mel ed egion (see Figu e 4a). Fully mel ed a eas we e homogeneous. Acco ding o he li e a u e [22,40,46], such ully mel ed a eas (Figu e 4c) can be assumed o be comple ely amo phous. These amo - phous a eas we e p obably o med as a esul o he apid solidi ica ion o he ully mel ed pa icle [22,47]. Table 1. Mic os uc u al and mechanical ea u es o he coa ings and Mg subs a e. Sample Thickness (µm) Thickness o Remel ed Laye (µm) Ha dness HV0.3 Po osi y (%) AZ91 alloy - - 85 ± 9 - As-sp ayed coa ing 530 ± 25 - 535 ± 20 0.7 ± 0.1 Lase - emel ed 650 W 517 ± 10 30 ± 5 610 ± 8 0.0 ± 0.0 Lase - emel ed 1000 W 515 ± 9 45 ± 5 625 ± 5 0.0 ± 0.0 Figu e 3. Mic os uc u e o he mally sp ayed coa ings, ( a ) as-sp ayed, ( b ) lase - emel ed a 650 W, ( c ) lase - emel ed a 1000 W. XRD pa e ns ( d ) o AZ91 alloy, eeds ock powde , and he mally sp ayed coa ings. The hicknesses o as-sp ayed and lase emel ed coa ings was abou 500 µ m. The ha dness o he as-sp ayed coa ing was de e mined o be 535 ± 20 HV0.3. In he case o he emel ed coa ing a a lase powe o 650 W, he e was an inc ease in he ha dness o he 30 µ m-wide emel ed laye on he su ace o 610 ± 8 HV0.3. In he case o he emel ed coa ing a a lase powe o 1000 W, he e was an inc ease in he ha dness o he 45 µ m wide emel ed laye on he su ace o 625 ± 5 HV. This inc ease in ha dness may be associa ed wi h he disappea ance o he spla bounda ies, po es, and o he mic os uc u al de ec s. The indi idual coa ing hickness alues a e summa ized in Table 1. Coa ings 2023,13, 1033 6 o 16 Table 1. Mic os uc u al and mechanical ea u es o he coa ings and Mg subs a e. Sample Thickness (µm) Thickness o Remel ed Laye (µm) Ha dness HV0.3 Po osi y (%) AZ91 alloy - - 85 ±9 - As-sp ayed coa ing 530 ±25 - 535 ±20 0.7 ±0.1 Lase - emel ed 650 W 517 ±10 30 ±5 610 ±8 0.0 ±0.0 Lase - emel ed 1000 W 515 ±9 45 ±5 625 ±5 0.0 ±0.0 The X- ay di ac og am (Figu e 3d) o he subs a e (AZ91 alloy) showed he p esence o wo main phases, namely he α -Mg ma ix phase (a solid solu ion o alloying elemen s in Mg), and he Mg 17 Al 12 in e me allic phase. In he case o he eeds ock powde , he as-sp ayed coa ing, and he lase - emel ed coa ings, only wo phases we e always de ec ed, namely aus eni e and M 7 C 3 ca bide, which is a mixed ca bide, whe e M = Fe, C , Mo [ 44 , 45 ]. Apa om hese peaks, no o he phases appea ed in he coa ings. Compa ed o he peaks o he eeds ock powde , he indi idual peaks co esponding o he coa ings ha e a lowe in ensi y, which is ela ed o he se e e plas ic de o ma ion o he pa icles du ing he sp aying p ocess, and he p esence o amo phous a eas associa ed wi h he impac o ully mel ed eeds ock powde pa icles on he Mg subs a e and hei apid solidi ica ion [ 20 , 22 ]. As can be obse ed in Figu e 4, he mic os uc u e o he as-sp ayed coa ing consis ed o non-mel ed, pa ially mel ed, and ully mel ed a eas. In Figu e 4a, he di e en egions a e clea ly dis inguishable and include spla bounda ies and po es. Figu e 4b shows a non-mel ed pa icle sepa a ed om i s su oundings by spla bounda ies. As shown by he XRD analysis (Figu e 3d), wo phases we e de ec ed in he coa ing, and hus in he non-mel ed a ea–aus eni e and M 7 C 3 ca bide. In he non-mel ed a ea, he dend i es o aus eni e (da ke egions) and ca bide (ligh e egions) we e well esol ed. A di e en mic os uc u e was obse ed o pa ially mel ed a eas and signi ican ly de o med a eas (Figu e 4c). As can be seen in Figu e 4c, hese a eas we e composed o ine ca bide pa icles and ca bides in he in e dend i ic space which did no o m a con inuous ne wo k, as in he case o non-mel ed a eas. I appea s ha du ing he apid solidi ica ion p ocess, he ca bide did no ha e ime o ully c ys allize. A high deg ee o plas ic de o ma ion is e iden compa ed o he non-mel ed egion (see Figu e 4a). Fully mel ed a eas we e homogeneous. Acco ding o he li e a u e [ 22 , 40 , 46 ], such ully mel ed a eas (Figu e 4c) can be assumed o be comple ely amo phous. These amo phous a eas we e p obably o med as a esul o he apid solidi ica ion o he ully mel ed pa icle [22,47]. The mic os uc u es o he lase - emel ed laye s a e shown in Figu e 5. The hicknesses o he emel ed laye s a lase powe s o 650 W and 1000 W we e abou 30 µ m and 45 µ m, espec i ely. A high-magni ica ion BSE SEM mic og aph (Figu e 5b,d) shows ha he coa ing was p edominan ly c ys allized a e lase emel ing. This was also con i med by he EBSD analysis (see Figu e 6a). This was due o he ac ha he HVOF coa ing abso bed a la ge amoun o hea du ing he emel ing p ocess. The mic os uc u e in he lase - emel ed laye consis ed o columna g ains wi h a subg ain dend i ic mic os uc u e. Indi idual g ains we e o med by dend i es o aus eni e (da ke a eas) and M 7 C 3 ca bide (ligh e a eas) in he in e dend i ic a ea (Figu e 5b,d). The e o e, emel ing did no esul in he o ma ion o new phases. This was also con i med by he XRD analysis abo e (Figu e 3d). As can be seen om he de ails in Figu e 5b,d, he dend i ic s uc u e in he emel ed laye is simila o he dend i ic s uc u e o non-mel ed pa icles in he as-sp ayed coa ing (Figu e 4b). As can be obse ed in Figu e 5d, in he case o he coa ing emel ed a a lase powe o 1000 W, he aus eni e dend i es and ca bides in he emel ed laye we e coa se han hose in he case o he coa ing emel ed a a lase powe o 650 W (Figu e 5b). This may be due o he ac ha mo e hea was in oduced in o he coa ing du ing emel ing wi h he highe lase powe , which esul ed in a longe cooling ime. Coa ings 2023,13, 1033 7 o 16 Coa ings 2023, 13, x FOR PEER REVIEW 7 o 17 (a) (b) (c) Figu e 4. Highe magni ica ion mic og aph o as-sp ayed coa ing, (a) c oss-sec ion, (b) de ail o non- mel ed pa icle, (c) de ail o mel ed, de o med and ully mel ed a ea. The mic os uc u es o he lase - emel ed laye s a e shown in Figu e 5. The hick- nesses o he emel ed laye s a lase powe s o 650 W and 1000 W we e abou 30 µm and 45 µm, espec i ely. A high-magni ica ion BSE SEM mic og aph (Figu e 5b,d) shows ha he coa ing was p edominan ly c ys allized a e lase emel ing. This was also con i med by he EBSD analysis (see Figu e 6a). This was due o he ac ha he HVOF coa ing ab- so bed a la ge amoun o hea du ing he emel ing p ocess. The mic os uc u e in he lase - emel ed laye consis ed o columna g ains wi h a subg ain dend i ic mic os uc- u e. Indi idual g ains we e o med by dend i es o aus eni e (da ke a eas) and M7C3 ca bide (ligh e a eas) in he in e dend i ic a ea (Figu e 5b,d). The e o e, emel ing did no esul in he o ma ion o new phases. This was also con i med by he XRD analysis abo e (Figu e 3d). As can be seen om he de ails in Figu e 5b,d, he dend i ic s uc u e in he emel ed laye is simila o he dend i ic s uc u e o non-mel ed pa icles in he as- sp ayed coa ing (Figu e 4b). As can be obse ed in Figu e 5d, in he case o he coa ing emel ed a a lase powe o 1000 W, he aus eni e dend i es and ca bides in he emel ed laye we e coa se han hose in he case o he coa ing emel ed a a lase powe o 650 W (Figu e 5b). This may be due o he ac ha mo e hea was in oduced in o he coa ing du ing emel ing wi h he highe lase powe , which esul ed in a longe cooling ime. As shown in Figu e 6, he as-sp ayed coa ing is p edominan ly non-c ys alline, as e y ew diff ac ions we e obse ed. The EBSD esul s (Figu e 6b) sugges ed ha he black a eas may be amo phous o po es and spla bounda ies, while he non-mel ed pa icles and he pa ially mel ed and plas ically de o med pa icles e ained hei c ys allini y. Ad- di ionally, i may be e y ine g ains/a eas ha a e smalle han he EBSD cha ac e iza ion s ep size [48]. The e was an app oxima ely 5 µm- hick ansi ion band be ween he as- Figu e 4. Highe magni ica ion mic og aph o as-sp ayed coa ing, ( a ) c oss-sec ion, ( b ) de ail o non-mel ed pa icle, (c) de ail o mel ed, de o med and ully mel ed a ea. Coa ings 2023, 13, x FOR PEER REVIEW 8 o 17 sp ayed coa ing and he lase - emel ed laye (Figu e 6a,c). This was p obably he hea - affec ed zone below he le el o he mel pools. (a) (b) (c) (d) Figu e 5. Highe magni ica ion mic og aph o lase - emel ed coa ings, (a) coa ing emel ed a 650 W lase powe , (b) coa ing emel ed a 650 W lase powe –de ail, (c) coa ing emel ed a 1000 W lase powe , (d) coa ing emel ed a 1000 W lase powe –de ail. (a) (b) Figu e 5. Highe magni ica ion mic og aph o lase - emel ed coa ings, ( a ) coa ing emel ed a 650 W lase powe , ( b ) coa ing emel ed a 650 W lase powe –de ail, ( c ) coa ing emel ed a 1000 W lase powe , (d) coa ing emel ed a 1000 W lase powe –de ail. Coa ings 2023,13, 1033 8 o 16 Coa ings 2023, 13, x FOR PEER REVIEW 8 o 17 sp ayed coa ing and he lase - emel ed laye (Figu e 6a,c). This was p obably he hea - affec ed zone below he le el o he mel pools. (a) (b) (c) (d) Figu e 5. Highe magni ica ion mic og aph o lase - emel ed coa ings, (a) coa ing emel ed a 650 W lase powe , (b) coa ing emel ed a 650 W lase powe –de ail, (c) coa ing emel ed a 1000 W lase powe , (d) coa ing emel ed a 1000 W lase powe –de ail. (a) (b) Coa ings 2023, 13, x FOR PEER REVIEW 9 o 17 (c) (d) Figu e 6. EBSD analysis o LT650W sample, (a) o e iew image o he lase - emel ed laye , ansi- ion band, and non-affec ed HVOF coa ing, (b) de ail o lase - emel ed a ea, (c) de ail o (1) non- affec ed as-sp ayed coa ing and (2) ansi ion band, (d) pole igu es o he emel ed a ea. 3.2. T ibological Beha iou Figu e 7 summa izes he ic ion and wea esul s ob ained by he ball-on-pla e me hod pe o med on AZ91 alloy, he as-sp ayed coa ing su ace, and lase - emel ed coa - ings. I can be seen om Figu e 7a,b ha he lowes alue o ic ion coefficien (COF), 0.24 ± 0.03, was measu ed o he uncoa ed AZ91 alloy. In he case o he coa ed alloy (as- sp ayed coa ing), i was obse ed ha he COF was abou 0.45 a he beginning o he es ing, bu as he sliding dis ance inc eased, he COF inc eased o app ox. 0.70. In he case o he lase - emel ed coa ings a lase powe s o 650 and 1000 W, he COF alues we e compa able—speci ically, 0.50 ± 0.05 and 0.48 ± 0.04, espec i ely. Figu e 7a shows ha he COF can be conside ed cons an du ing he measu emen . The highe alues o he COF o coa ings may be due o he highe p opo ion o ca bides in he coa ing and in he lase - emel ed laye . These ca bides do no ha e good sliding p ope ies bu inc ease wea esis ance [21,49]. In he case o he as-sp ayed coa ing, he g adual inc ease in he ic ion coefficien can be caused by ab asi e and ibo-oxida i e wea , whe e debu ed and oxidized pa icles and pa icles apped in he open po es o he coa ing and pi s can inc ease he ic ion. The COF ends o inc ease due o he ex a ene gy in ol ed in d ag- ging and pulling deb is ou o con ac be ween coun e -ball and wo n su ace; hus, he wea ends o inc ease [50,51]. E en hough he lowes COF alue was ob ained o he uncoa ed AZ91 alloy, he wea a e, pene a ion dep h, and weigh loss we e signi ican ly highe han o he coa ed samples (Figu e 7c,d). The pene a ion dep h a e he wea es was app oxima ely 4–5 imes lowe o he coa ed samples han o he uncoa ed AZ91 alloy. The weigh loss was also signi ican ly lowe o he coa ed samples. In he case o AZ91 alloy, he weigh loss was de e mined o be 2.7 × 10−4 g, while in he case o he as- sp ayed coa ing, he weigh loss was 1 × 10−4 g. In he case o lase - emel ed coa ings, he weigh loss was e en lowe , a 3.3 × 10−5 g. Figu e 6. EBSD analysis o LT650W sample, ( a ) o e iew image o he lase - emel ed laye , ansi ion band, and non-a ec ed HVOF coa ing, ( b ) de ail o lase - emel ed a ea, ( c ) de ail o (1) non-a ec ed as-sp ayed coa ing and (2) ansi ion band, (d) pole igu es o he emel ed a ea. As shown in Figu e 6, he as-sp ayed coa ing is p edominan ly non-c ys alline, as e y ew di ac ions we e obse ed. The EBSD esul s (Figu e 6b) sugges ed ha he black a eas may be amo phous o po es and spla bounda ies, while he non-mel ed pa icles and he pa ially mel ed and plas ically de o med pa icles e ained hei c ys allini y. Addi ionally, i may be e y ine g ains/a eas ha a e smalle han he EBSD cha ac e iza ion s ep size [ 48 ]. The e was an app oxima ely 5 µ m- hick ansi ion band be ween he as-sp ayed coa ing and he lase - emel ed laye (Figu e 6a,c). This was p obably he hea -a ec ed zone below he le el o he mel pools. 3.2. T ibological Beha iou Figu e 7summa izes he ic ion and wea esul s ob ained by he ball-on-pla e me hod pe o med on AZ91 alloy, he as-sp ayed coa ing su ace, and lase - emel ed coa ings. I can be seen om Figu e 7a,b ha he lowes alue o ic ion coe icien (COF), 0.24 ± 0.03, was measu ed o he uncoa ed AZ91 alloy. In he case o he coa ed alloy (as-sp ayed coa ing), i was obse ed ha he COF was abou 0.45 a he beginning o he es ing, bu as he sliding dis ance inc eased, he COF inc eased o app ox. 0.70. In he case o he lase - emel ed coa ings a lase powe s o 650 and 1000 W, he COF alues we e compa able—speci ically, 0.50 ± 0.05 and 0.48 ± 0.04, espec i ely. Figu e 7a shows ha he COF can be conside ed cons an du ing he measu emen . The highe alues o he COF o coa ings may be due o he highe p opo ion o ca bides in he coa ing and in he lase - emel ed laye . These ca bides do no ha e good sliding p ope ies bu inc ease wea esis ance [ 21 , 49 ]. In he case o he as-sp ayed coa ing, he g adual inc ease in he ic ion coe icien can be caused by ab asi e and ibo-oxida i e wea , whe e debu ed and oxidized pa icles and pa icles Coa ings 2023,13, 1033 9 o 16 apped in he open po es o he coa ing and pi s can inc ease he ic ion. The COF ends o inc ease due o he ex a ene gy in ol ed in d agging and pulling deb is ou o con ac be ween coun e -ball and wo n su ace; hus, he wea ends o inc ease [50,51]. Coa ings 2023, 13, x FOR PEER REVIEW 10 o 17 (a) (b) (c) (d) Figu e 7. Resul s o ibological es ing o indi idual samples: (a) eco d o he ic ion coefficien s (COF), (b) COF alues, (c) pene a ion dep h alues, (d) weigh loss alues. In he case o Figu e (d), he e o ba s a e no shown because he measu emen e o (weigh change) was close o ze o. Wea acks and wea p o iles o he es ed samples a e shown in Figu e 8. The indi- idual dimensions o he wea acks co ela e wi h he weigh loss esul s. As shown in Figu e 8a, in he case o alloy AZ91, he pene a ion dep h o he wea ack a he lowes poin was abou 30 µm. The wid h o he wea ack was app oxima ely 1200 µm a e he wea es . The sp aying o he coa ing inc eased he wea esis ance o samples and hus dec eased he pene a ion dep h. The pene a ion dep h was de e mined o be 5 µm in he case o he as-sp ayed coa ing, and he wea ace wid h was de e mined o be 650 µm (Figu e 8b). The emel ing o he he mally sp ayed (as-sp ayed) coa ing a a lase powe o 650 o 1000 W led o a u he inc ease in wea esis ance. The pene a ion dep h a e he wea es o he emel ed coa ings was de e mined o be app oxima ely 4 µm in bo h cases, and he wid h o he wea acks was app oxima ely 430 µm (Figu e 8c,d). (a) (b) Figu e 7. Resul s o ibological es ing o indi idual samples: ( a ) eco d o he ic ion coe icien s (COF), ( b ) COF alues, ( c ) pene a ion dep h alues, ( d ) weigh loss alues. In he case o Figu e ( d ), he e o ba s a e no shown because he measu emen e o (weigh change) was close o ze o. E en hough he lowes COF alue was ob ained o he uncoa ed AZ91 alloy, he wea a e, pene a ion dep h, and weigh loss we e signi ican ly highe han o he coa ed sam- ples (Figu e 7c,d). The pene a ion dep h a e he wea es was app oxima ely 4–5 imes lowe o he coa ed samples han o he uncoa ed AZ91 alloy. The weigh loss was also signi ican ly lowe o he coa ed samples. In he case o AZ91 alloy, he weigh loss was de e mined o be 2.7 × 10 −4 g, while in he case o he as-sp ayed coa ing, he weigh loss was 1 × 10 −4 g. In he case o lase - emel ed coa ings, he weigh loss was e en lowe , a 3.3 ×10−5g. Wea acks and wea p o iles o he es ed samples a e shown in Figu e 8. The indi idual dimensions o he wea acks co ela e wi h he weigh loss esul s. As shown in Figu e 8a, in he case o alloy AZ91, he pene a ion dep h o he wea ack a he lowes poin was abou 30 µ m. The wid h o he wea ack was app oxima ely 1200 µ m a e he wea es . The sp aying o he coa ing inc eased he wea esis ance o samples and hus dec eased he pene a ion dep h. The pene a ion dep h was de e mined o be 5 µ m in he case o he as-sp ayed coa ing, and he wea ace wid h was de e mined o be 650 µ m (Figu e 8b). The emel ing o he he mally sp ayed (as-sp ayed) coa ing a a lase powe o 650 o 1000 W led o a u he inc ease in wea esis ance. The pene a ion dep h a e he wea es o he emel ed coa ings was de e mined o be app oxima ely 4 µ m in bo h cases, and he wid h o he wea acks was app oxima ely 430 µm (Figu e 8c,d). Coa ings 2023,13, 1033 16 o 16 58. B aszczy´nska-Malik, K.N. Sphe ical shape o γ -Mg17Al12 p ecipi a es in AZ91 magnesium alloy p ocessed by equal-channel angula p essing. J. Alloys Compd. 2009,487, 263–268. [C ossRe ] 59. Myshkin, N.K.; G igo ie , A.Y. Scale ac o in ibology: Roughness and ex u e. 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Co osion and co osion- ic ion p ope ies o plasma cladding wea - esis an laye on Fe-based alloy. Ma e . Res. Exp ess 2018,5, 026525. [C ossRe ] Disclaime /Publishe ’s No e: The s a emen s, opinions and da a con ained in all publica ions a e solely hose o he indi idual au ho (s) and con ibu o (s) and no o MDPI and/o he edi o (s). MDPI and/o he edi o (s) disclaim esponsibili y o any inju y o people o p ope y esul ing om any ideas, me hods, ins uc ions o p oduc s e e ed o in he con en .