The role of graphitic carbon nitride in the formulation of copper-free friction composites designed for automotive brake pads

  Ci a ion: Ma ˇejka, V.; Leona di, M.; P aus, P.; S a elini, G.; Gialanella, S. The Role o G aphi ic Ca bon Ni ide in he Fo mula ion o Coppe -F ee F ic ion Composi es Designed o Au omo i e B ake Pads. Me als 2022, 12, 123. h ps://doi.o g/10.3390/ me 12010123 Academic Edi o s: Slobodan Mi o ic and Pa el K akhmale Recei ed: 25 No embe 2021 Accep ed: 6 Janua y 2022 Published: 9 Janua y 2022 Publishe ’s No e: MDPI s ays neu al wi h ega d o ju isdic ional claims in published maps and ins i u ional a il- ia ions. Copy igh : © 2022 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/). me als A icle The Role o G aphi ic Ca bon Ni ide in he Fo mula ion o Coppe -F ee F ic ion Composi es Designed o Au omo i e B ake Pads Vlas imil Ma ˇejka 1,2,* , Ma a Leona di 3, Pe P aus 1,2 , Gio anni S a elini 4and S e ano Gialanella 4 1 Depa men o Chemis y, Facul y o Ma e ials Science and Technology, VSB-Technical Uni e si y o Os a a, 17. Lis opadu 2172/15, 708 33 Os a a, Czech Republic; pe [email p o ec ed] 2Ins i u e o En i onmen al Technology, CEET, VSB-Technical Uni e si y o Os a a, 17. Lis opadu 2172/15, 708 33 Os a a, Czech Republic 3Ad anced R&D Depa men , B embo S.p.A., 24040 S ezzano, I aly; Ma a_Leona di@b embo.i 4Depa men o Indus ial Enginee ing, Uni e si y o T en o, Via Somma i e 9, Po o, 38123 T en o, I aly; [email p o ec ed] (G.S.); [email p o ec ed] (S.G.) *Co espondence: [email p o ec ed]; Tel.: +420-597325293 Abs ac : In his s udy, g aphi ic ca bon ni ide (g-C 3 N 4 , labelled as gCN) was es ed in he o - mula ion o coppe - ee (Cu- ee) ic ion mix u es, which a e po en ially in e es ing o b ake pad manu ac u ing. Th ee o mula ions o ic ion composi es we e p epa ed s a ing om a common Cu- ee mas e ba ch: (i) wi hou g aphi e, (ii) wi h g aphi e and (iii) wi h gCN. The mix u es we e p essed in he o m o pins by ho -p ess moulding. The ic ion-wea pe o mance o he p epa ed pins was in es iga ed using a pin-on-disc (PoD) es a oom empe a u e (RT), high empe a u e (HT) (400 ◦ C) and, again, a oom empe a u e (H-RT). The alues o he ic ion coe icien ( µ ) o he composi es wi h gCN (o g aphi e) we e as ollows: (i) RT es , µRT = 0.52 (0.47); (ii) HT es , µHT = 0.37 (0.37); (iii) RT a e he HT es s, µH-RT = 0.49 (0.39). Wi h espec o wea esis ance, he samples wi h g aphi e pe o med be e han he samples wi hou his solid lub ican . To he bes o ou knowledge, his is he i s epo ega ding he e alua ion o he ole o gCN in ic ion composi es designed o au omo i e b ake lining applica ions. The esul s indica e he main ole o gCN as a so ab asi e. Keywo ds: ic ion composi es; g aphi ic ca bon ni ide; ic ion-wea p ope ies; pin-on-disc es 1. In oduc ion F ic ion ma e ials designed o au omo i e b ake linings a e usually classi ied in o h ee main g oups: semi-me allic, non-asbes os o ganic (NAO) and ce amic [ 1 ]. In addi ion, coppe me al ma ix composi es should also be aken in o accoun , e en i he e is an e o o comple ely emo e he coppe om he b ake pad o mula ion [ 2 ]. The ic ion-wea p ope ies o b ake linings a e es ed on he “ma e ial le el” using pin-on-disc es e s [ 3 ], subscale dyno-bench es e s [ 4 ] o a he “sys em le el” using a ull-scale dynamome e [ 5 ]. F ic ion mix u es used o manu ac u e au omo i e b ake pads consis o se e al com- ponen s belonging o ou main g oups: (i) ab asi es, (ii) ille s, (iii) solid lub ican s and (i ) binde s. Usually, he mul iple unc ionali ies o each componen a e obse ed. Thousands o aw ma e ials ha e al eady been es ed as componen s o ic ion mix u es [ 6 ]. Alumina, silicon ca bide, and zi con a e well-known ab asi es [ 7 – 9 ], and hei ole is o main ain a ce ain ic ion coe icien ( µ ) and p e en i s dec ease wi h inc easing empe a u e (a phenomenon called ading). The ole o he ab asi es is o enew he ic ion laye o med on he su aces o bo h pads and discs. Al hough he main ole o ab asi es in ic ion composi es is o a ec he ic ion coe icien , he e a e also s udies ela ed o hei e ec on wea pa icle p oduc ion as epo ed by Pa k e al. [ 10 ]. G aphi e and laye ed sulphides a e he mos commonly used solid lub ican s [ 11 – 14 ], and hei ole is o s abilise he Me als 2022,12, 123. h ps://doi.o g/10.3390/me 12010123 h ps://www.mdpi.com/jou nal/me als Me als 2022,12, 123 2 o 15 ic ion coe icien du ing b aking (smoo hing o he b ake ac ion) and educe he wea a e. The ille s can be u he subdi ided in o pa icula e ille s o ib es, bo h syn he ic and na u al. The ille s a e in ended o c ea e he p ima y con ac pla eaus, s eng hen he en i e ic ion composi e, and ill up i s olume [ 15 – 19 ]. Phenolic esin is he mos equen ly used binde , and i is u ilised o hold all he componen s oge he . Al hough o he ypes o binde s ha e been es ed, phenolic esin emains o da e he mos equen ly used [ 20 ]. The p ope combina ion o he componen s om all he g oups and he selec ion o he ho -p essing p ocess a e he key ac o s ha ensu e he equi ed unc ionali y o he inal b ake lining [21–23]. The g owing nano echnology sec o has in oduced a numbe o componen s wi h unique p ope ies, mainly owing o he nanome e-leng h scale o hese ma e ials. Se e al o hese ma e ials ha e al eady been es ed as componen s o ic ion ma e ials. The e ec o g aphene on he ic ion-wea pe o mance o semi-me allic ic ion composi es con aining b ass o use in ca b ake linings was s udied by Rajan e al. [ 24 ]. In compa ison o adi ional g aphi e, he au ho s obse ed se e al posi i e e ec s o g aphene on he ic ion s abili y, ade pe o mance and wea a e educ ion. The e ec o nanosized ab asi es o Al 2 O 3 , SiC, and SiO 2 on he ic ion-wea pe o mance o Cu-con aining ic ion composi es was in es iga ed by Bijwe e al. [ 25 ]. The au ho s epo ed he posi i e e ec o he nanosized ab asi es in e ms o educed wea a e and uning o he ic ion pe o mance. Mahale e al. [ 26 ] in es iga ed he e ec o po assium i ana e nano- and mic o-pa icles on he ic ion wea pe o mance o NAO ic ion composi es con aining b ass. Imp o ed wea esis ance and ic ion pe o mance we e ob ained o composi es wi h nanosized po assium i ana e. E o s a e con inuously made o eplace Cu in he o mula ions [ 27 – 29 ]. Fo example, Bha e al. [ 30 ] s udied he e ec o hyd a ed calcium silica e on NVH pe o mance in Cu- ee o mula ions and indica ed he posi i e e ec o his componen on mos o he ibological p ope ies o p epa ed samples. Recen ly, gCN has a ac ed esea ch in e es o pho oca aly ic applica ions [ 31 ], o he p epa a ion o anodes o ba e ies [ 32 ], and as senso s [ 33 ]. gCN, usually in he o m o a yellow powde , shows a laye ed s uc u e, and he laye s a e made o a ne o hep azine ings bonded oge he by an de Waals o ces [ 34 ]. The e a e se e al epo ed me hods o he p epa a ion o gCN, mainly based on he he mal polycondensa ion o a sui able p ecu so , wi h melamine [ 35 ] being he mos widely used p ecu so . The s acked laye s o bulk gCN can be sepa a ed, and nano lakes o gCN can be ob ained [ 36 ]. Bulk gCN is simila o g aphi e, while isola ed nano lakes, in some espec s, a e simila o g aphene. The e ec o gCN in polyme composi es based on polyimide was es ed by Zhu e al. [ 37 ]. The au ho s obse ed ha he use o bulk gCN dec eased he ic ion coe icien and wea a e o he p epa ed polyme ma ix composi es. Duan e al. [ 38 ] es ed nanos uc u ed gCN as he ille in a polyimide ma ix and, simila o Zhu e al. [ 37 ], he au ho s obse ed imp o ed wea esis ance o he esul ing composi es, which inc eased wi h inc easing es ing empe a u e. Howe e , he ic ion coe icien also sligh ly inc eased wi h inc easing es ing empe a u e. Zhang e al. [ 39 ] s udied he e ec o mic o-sized and ex olia ed gCN on he wea esis ance o polyme ma ix composi es based on a poly–e he –e he –ke one ma ix unde oil lub ica ion condi ions. The au ho obse ed ha e en a low addi ion o nanos uc u ed gCN o he polyme ma ix signi ican ly imp o ed he wea esis ance o he composi e. The cha ac e is ics o gCN, as well as p e iously published esul s, sugges ha gCN should beha e as a solid lub ican in ic ion ma e ials dedica ed o au omo i e b ake pads. In his pape , a p elimina y assessmen o he unc ionali y o gCN in he o mula ion o Cu- ee ic ion composi es is epo ed. Composi es wi h 9 w .% o syn he ic g aphi e and 9 w .% o gCN we e p epa ed in he o m o pins by ho -p ess moulding. A composi e wi hou g aphi e o gCN was used as a e e ence. The d y sliding beha iou o he samples was assessed using a pin-on-disc es ig. Me als 2022,12, 123 3 o 15 2. Ma e ials and Me hods 2.1. Mas e Ba ch and G aphi e A Cu- ee mas e ba ch o he ic ion mix u e (B embo S.p.A., S ezzano, I aly), labelled M0, was used as a e e ence. The ic ion mix u e M0 did no con ain any g aphi e o be e unde s and he ole o gCN in ic ion composi es. The main componen s o M0, hei oles, and hei es ima ed amoun s a e gi en in Table 1. Table 1. Main componen s o M0, hei oles, and es ima ed con en s in he e e ence mas e ba ch wi hou g aphi e. Cons i uen s o M0 Main Role Con en (w .%) S eel Rein o cing ib es 30 Aluminium oxide, silicon ca bide, and magnesium oxide Ab asi es 25 Tin sulphide, sphale i e, and zinc oxide Lub ican s 13 Ve miculi e and o he s Fille s 24 Phenolic esin Binde 8 The cha ac e is ics o he mas e ba ch s udied using scanning elec on mic oscopy (SEM) a e shown in Figu e 1a. Some o he main cons i uen s we e iden i ied using ene gy- dispe si e X- ay (EDX) local mic oanalysis, and he esul s showed he p esence o s eel ib es, e miculi e, aluminium oxide, magnesium oxide, and in sulphide. Me als 2022, 12, x FOR PEER REVIEW 3 o 16 wi hou g aphi e o gCN was used as a e e ence. The d y sliding beha iou o he sam- ples was assessed using a pin-on-disc es ig. 2. Ma e ials and Me hods 2.1. Mas e Ba ch and G aphi e A Cu- ee mas e ba ch o he ic ion mix u e (B embo S.p.A., S ezzano, I aly), la- belled M0, was used as a e e ence. The ic ion mix u e M0 did no con ain any g aphi e o be e unde s and he ole o gCN in ic ion composi es. The main componen s o M0, hei oles, and hei es ima ed amoun s a e gi en in Table 1. Table 1. Main componen s o M0, hei oles, and es ima ed con en s in he e e ence mas e ba ch wi hou g aphi e. Cons i uen s o M0 Main Role Con en (w .%) S eel Rein o cing ib es 30 Aluminium oxide, silicon ca bide, and magnesium oxide Ab asi es 25 Tin sulphide, sphale i e, and zinc ox- ide Lub ican s 13 Ve miculi e and o he s Fille s 24 Phenolic esin Binde 8 The cha ac e is ics o he mas e ba ch s udied using scanning elec on mic oscopy (SEM) a e shown in Figu e 1a. Some o he main cons i uen s we e iden i ied using en- e gy-dispe si e X- ay (EDX) local mic oanalysis, and he esul s showed he p esence o s eel ib es, e miculi e, aluminium oxide, magnesium oxide, and in sulphide. Figu e 1. SEM mic og aphs: (a) e e ence mas e ba ch (M0) wi h some iden i ied cons i uen s (1— s eel ib e, 2— e miculi e, 3—aluminium oxide, 4—magnesium oxide and 5— in sulphide); (b) syn- he ic g aphi e pa icles in he M0_G o mula ion. Comme cial syn he ic g aphi e (labelled as G) ob ained om Ime ys G aphi e and Ca bon (Bi onico, Swi ze land) was selec ed as he s anda d solid lub ican o addi ion o mas e ba ch M0. The mic os uc u e o he ine black G powde is shown in Figu e 1b. The pa icle size dis ibu ions o he G pa icles, aken om he echnical da a shee , a e lis ed in Table 2. Table 2. Pa icle size dis ibu ion o he syn he ic g aphi e (sie ing analysis). F ac ion con en 0.4% 23% 37% 72% 91% F ac ion size >800 µm >600 µm >550 µm >250 µm >150 µm Figu e 1. SEM mic og aphs: ( a ) e e ence mas e ba ch (M0) wi h some iden i ied cons i uen s (1—s eel ib e, 2— e miculi e, 3—aluminium oxide, 4—magnesium oxide and 5— in sulphide); (b) syn he ic g aphi e pa icles in he M0_G o mula ion. Comme cial syn he ic g aphi e (labelled as G) ob ained om Ime ys G aphi e and Ca bon (Bi onico, Swi ze land) was selec ed as he s anda d solid lub ican o addi ion o mas e ba ch M0. The mic os uc u e o he ine black G powde is shown in Figu e 1b. The pa icle size dis ibu ions o he G pa icles, aken om he echnical da a shee , a e lis ed in Table 2. Table 2. Pa icle size dis ibu ion o he syn he ic g aphi e (sie ing analysis). F ac ion con en 0.4% 23% 37% 72% 91% F ac ion size >800 µm >600 µm >550 µm >250 µm >150 µm 2.2. G aphi ic Ca bon Ni ide Sample gCN was p epa ed by he mal polycondensa ion o melamine in wo s eps. In he i s s ep, melamine (Sigma–Ald ich, S . Louis, MO, USA) was hea ed o 10 min in a semi-closed c ucible a 475 ◦ C (hea ing a e 5 ◦ C/min) in a mu le u nace LAC LMH (LAC, Me als 2022,12, 123 4 o 15 B no, Czech Republic). The samples we e hen emo ed om he u nace and allowed o cool unde labo a o y condi ions. In he second s ep, he c ucible was placed in he same mu le u nace, p ehea ed o 550 ◦ C, and held o 2 h a his empe a u e. A e y ine yellow powde was ob ained di ec ly and assigned as gCN. The p epa ed sample o gCN was cha ac e ised ia X- ay di ac ion (XRD) using a MiniFlex600 di ac ome e equipped wi h a Co ube and a D/ eX Ul a de ec o (Rigaku, Tokio, Japan). The di ac ion pa e n was eco ded in he 2 θ ange o 10–80 ◦ wi h a s ep size o 0.01 ◦ and a speed o 2 ◦ /min. The in a ed spec a o he gCN sample we e eco ded using he a enua ed o al e lec ion (ATR) mode o a The mo Scien i ic Nicole 6700 Fou ie ans o m in a ed (FTIR) spec ome e (The mo Fishe Scien i ic, Wal ham, MA, USA). The spec a we e eco ded in he ange o 400–4000 cm −1 wi h a esolu ion o 2 cm −1 , and he ob ained spec um was he a e age o 64 scans. 2.3. Modi ica ion o he Mas e Ba ch wi h gCN and G To e eal he e ec o gCN on he ic ion-wea pe o mance o he Cu- ee ic ion composi es, wo new o mula ions we e p epa ed om he e e ence mas e ba ch M0 by he addi ion o : 1. 9 w .% o G: o mula ion labelled as M0_G; 2. 9 w .% o gCN: o mula ion labelled as M0_CN. The con en o 9 w .% o g aphi e was selec ed in en ionally based on ou p e ious expe ience wi h his componen [ 40 ]. The o mula ions we e mixed o 20 min using he shake mixe TURBULA ® T 2 F (Willy A. Bacho en AG, Mu enz, Swi ze land). The powde mix was p essed o 10 min in a cylind ical mould using a BUEHLER ho -moun ing p ess (Buehle , Lake Blu , IL, USA) a a cons an p essu e o 17 MPa and a empe a u e o 150 ◦ C o ob ain he specimens o he ibological es s. Cylind ical samples wi h a diame e o 10 mm and a heigh o 10 mm we e successi ely pos -cu ed in a labo a o y o en UN55 (Memme GmbH + Co. KG, Schwabach, Ge many) o 4 h a 200 ◦ C in ai . The bulk densi ies o he pins, 2.25 ± 0.05 g · cm −3 , we e de e mined based on hei weigh (m) and olume (V). 2.4. Cha ac e isa ion o he The mal S abili y o gCN, G, and he P epa ed F ic ion Composi es The mog a ime ic (TG) analysis was pe o med o s udy he he mal s abili y o g aphi ic ca bon ni ide, g aphi e, and he ic ion composi es M0, M0_G and M0_CN. TG measu emen s we e pe o med on an SDT650 simul aneous he mal analyse (TA Ins umen s, New Cas le, DE, USA) in a dynamic ai a mosphe e (10 L · min −1 ). The samples (10 mg) we e placed in an alumina c ucible and hea ed in he empe a u e ange o 25–900 ◦C a a hea ing a e o 10 ◦C·min−1. 2.5. Pin-On-Disc Tes s T ibological es s we e pe o med unde d y sliding condi ions using a Ducom pin- on-disc (PoD) appa a us (Ducom Ins umen s P . L d., Bengalu u, India). A pea li ic g ey cas i on disc 60 mm in diame e wi h a Vicke s ha dness o 235 HV10 measu ed on QNESS 60 A+ EVO (ATM Qness GmbH, Golling, Aus ia) was used as he coun e ace. A sliding eloci y o 1.50 m · s −1 and a nominal con ac p essu e o 1 MPa we e kep cons an du ing he es s. The selec ed sliding eloci y and con ac p essu e co esponded o mild b aking condi ions; i scaled o a small passenge ca , he es ing eloci y app oxima ely co esponded o a ehicle speed o 13 km·h−1. The du a ion o each es was 90 min. Fi s , es s we e pe o med a oom empe a u e (RT). A p elimina y 30 min long bedding p ocedu e was conduc ed o allow con o mal con ac be ween he pin and he disc su aces and o emo e he coa se su ace aspe i ies. To assess he ic ion-wea pe o mance o he samples a ele a ed empe a u es, es ing o he composi es a high empe a u e (HT) was also ca ied ou ollowing he p ocedu e es ablished by Leona di e al. [ 40 ]. The high- empe a u e es s we e ca ied ou using an induc ion hea ing appa a us enclosing he pin-on-disc es ing chambe o hea he disc a 400 ◦ C. To e eal he eco e y Me als 2022,12, 123 5 o 15 pe o mance, u he es s we e conduc ed a oom empe a u e (H_RT) on he same specimens ob ained a e he HT es s we e conduc ed. This sequence o es ing condi ions (RT and HT ollowed by H_RT) was adop ed o in es iga e he eco e y capabili y o all o he p epa ed ic ion ma e ials unde in es iga ion (i.e., M0, M0_G and M0_CN). Two pins we e es ed o each ma e ial, and he a e age ic ion coe icien was calcula ed. The coe icien o ic ion was con inuously eco ded du ing each es . The wea o he pins was e alua ed by weighing he sample be o e and a e each es using an analy ical balance Ke n ADJ (KERN & SOHN GmbH, Balingen, Ge many) wi h a p ecision o 10 −4 g. F om hese da a, he speci ic wea coe icien (K a ), calcula ed using Equa ion (1), was e alua ed. Ka= V/(Fn·s) (1) whe e V (m 3 ) is he measu ed wea olume, F n (N) is he applied load, and s (m) is he sliding dis ance. A scanning elec on mic oscope (SEM) JEOL IT300 (JEOL L d., Tokyo, Japan) ope a ed a an accele a ed ol age o 20 kV was used o s udy he mo phology o G and gCN pa icles, as well as o cha ac e ise he ic ion su aces a e he PoD es . SEM images o G and CN we e ob ained using a seconda y elec on de ec o , and he images o he ic ion su aces we e ob ained in he backsca e ed elec on mode. Fo local chemical analysis o he wo n su aces a he end o he PoD es , an ene gy-dispe si e X- ay spec oscopy (EDXS) sys em was used. 3. Resul s and Discussion 3.1. Cha ac e isa ion o gCN SEM mic og aphs o he syn hesised gCN powde a e shown in Figu e 2. Me als 2022, 12, x FOR PEER REVIEW 5 o 16 disc su aces and o emo e he coa se su ace aspe i ies. To assess he ic ion-wea pe - o mance o he samples a ele a ed empe a u es, es ing o he composi es a high em- pe a u e (HT) was also ca ied ou ollowing he p ocedu e es ablished by Leona di e al. [40]. The high- empe a u e es s we e ca ied ou using an induc ion hea ing appa a us enclosing he pin-on-disc es ing chambe o hea he disc a 400 °C. To e eal he eco e y pe o mance, u he es s we e conduc ed a oom empe a u e (H_RT) on he same spec- imens ob ained a e he HT es s we e conduc ed. This sequence o es ing condi ions (RT and HT ollowed by H_RT) was adop ed o in es iga e he eco e y capabili y o all o he p epa ed ic ion ma e ials unde in es iga ion (i.e., M0, M0_G and M0_CN). Two pins we e es ed o each ma e ial, and he a e age ic ion coe icien was calcula ed. The coe icien o ic ion was con inuously eco ded du ing each es . The wea o he pins was e alua ed by weighing he sample be o e and a e each es using an ana- ly ical balance Ke n ADJ (KERN & SOHN GmbH, Balingen, Ge many) wi h a p ecision o 10−4 g. F om hese da a, he speci ic wea coe icien (Ka), calcula ed using Equa ion (1), was e alua ed. Ka = V/(Fn·s) (1) whe e V (m3) is he measu ed wea olume, Fn (N) is he applied load, and s (m) is he sliding dis ance. A scanning elec on mic oscope (SEM) JEOL IT300 (JEOL L d., Tokyo, Japan) ope - a ed a an accele a ed ol age o 20 kV was used o s udy he mo phology o G and gCN pa icles, as well as o cha ac e ise he ic ion su aces a e he PoD es . SEM images o G and CN we e ob ained using a seconda y elec on de ec o , and he images o he ic- ion su aces we e ob ained in he backsca e ed elec on mode. Fo local chemical analy- sis o he wo n su aces a he end o he PoD es , an ene gy-dispe si e X- ay spec os- copy (EDXS) sys em was used. 3. Resul s and Discussion 3.1. Cha ac e isa ion o gCN SEM mic og aphs o he syn hesised gCN powde a e shown in Figu e 2. Figu e 2. SEM mic og aphs o gCN powde : (a) image o he ypical gCN pa icle; (b) de ails o he po es obse ed on he gCN su ace. The s uc u e o gCN is a g aphi e-like laye ed ma e ial (co alen bonds o ca bon and ni ogen eplace he ca bon and ca bon bonds in g aphi e). A la ge plana ne wo k s uc u e is clea ly isible in he SEM image in Figu e 2a. The de ail o ano he obse ed pa icle documen ing he p esence o po es o igina ing du ing gCN syn hesis is shown in Figu e 2b. EDXS analyses we e pe o med o quali a i ely check he composi ion o he gCN pa icles (Table 3). Figu e 2. SEM mic og aphs o gCN powde : ( a ) image o he ypical gCN pa icle; ( b ) de ails o he po es obse ed on he gCN su ace. The s uc u e o gCN is a g aphi e-like laye ed ma e ial (co alen bonds o ca bon and ni ogen eplace he ca bon and ca bon bonds in g aphi e). A la ge plana ne wo k s uc u e is clea ly isible in he SEM image in Figu e 2a. The de ail o ano he obse ed pa icle documen ing he p esence o po es o igina ing du ing gCN syn hesis is shown in Figu e 2b. EDXS analyses we e pe o med o quali a i ely check he composi ion o he gCN pa icles (Table 3). Table 3. EDXS spec a acqui ed on he gCN powde . Elemen s C N Al O Con en (w .%) 44.9 41.1 12.1 1.9 Me als 2022,12, 123 6 o 15 As expec ed, he gCN consis ed o ca bon and ni ogen. The de ec ed aluminium came om he ma e ial used o suppo he powde s du ing SEM obse a ions, and a small amoun o oxygen was ound as a ypical con aminan o gCN [41]. The X- ay di ac ion pa e n and FTIR spec a o he gCN sample a e shown in Figu e 3a,b, espec i ely. Me als 2022, 12, x FOR PEER REVIEW 6 o 16 Table 3. EDXS spec a acqui ed on he gCN powde . Elemen s C N Al O Con en (w .%) 44.9 41.1 12.1 1.9 As expec ed, he gCN consis ed o ca bon and ni ogen. The de ec ed aluminium came om he ma e ial used o suppo he powde s du ing SEM obse a ions, and a small amoun o oxygen was ound as a ypical con aminan o gCN [41]. The X- ay di ac ion pa e n and FTIR spec a o he gCN sample a e shown in Fig- u e 3a,b, espec i ely. Figu e 3. (a) Di ac ion pa e n o gCN sample and (b) FTIR spec a o he gCN sample. The XRD pa e n o gCN (Figu e 3a) shows wo peak in ensi ies a (100) and (002), demons a ing he p esence o g-C3N4 [42]. The di ac ion peak (100) was ela ed o he in-plane o de ing o ni ogen-linked hep azine uni s, whe eas di ac ion peak (002) de- sc ibes he in e laye s acking o he melem planes. The FTIR spec a o gCN (Figu e 3b) exhibi ed bands wi h maxima a equencies ypical o g-C3N4. The bands in he egion om 1700 o 1100 cm−1 we e ela ed o he s e ching ib a ions o a oma ic C–N he e ocycles [43]. The sha p cha ac e is ic peak wi h a maximum a 802 cm−1 desc ibes he b ea hing ib a ion o he i-s- iazine ing sys em [44]. The bands obse ed in he egion o 3500–2500 cm−1 we e a ibu ed o he s e ching modes o N–H and O–H bonds o ee su ace amino g oups and adso bed hy- d oxyl species [45]. 3.2. The mal S abili y o he F ic ion Composi es The TG cu es o gCN p epa ed in his s udy and comme cially a ailable G a e shown in Figu e 4a. The igu e clea ly indica es he high he mal s abili y o G; i also shows he he mal s abili y o gCN up o 570 °C. Figu e 3. (a) Di ac ion pa e n o gCN sample and (b) FTIR spec a o he gCN sample. The XRD pa e n o gCN (Figu e 3a) shows wo peak in ensi ies a (100) and (002), demons a ing he p esence o g-C 3 N 4 [ 42 ]. The di ac ion peak (100) was ela ed o he in- plane o de ing o ni ogen-linked hep azine uni s, whe eas di ac ion peak (002) desc ibes he in e laye s acking o he melem planes. The FTIR spec a o gCN (Figu e 3b) exhibi ed bands wi h maxima a equencies ypical o g-C 3 N 4 . The bands in he egion om 1700 o 1100 cm −1 we e ela ed o he s e ching ib a ions o a oma ic C–N he e ocycles [ 43 ]. The sha p cha ac e is ic peak wi h a maximum a 802 cm −1 desc ibes he b ea hing ib a ion o he i-s- iazine ing sys em [ 44 ]. The bands obse ed in he egion o 3500–2500 cm −1 we e a ibu ed o he s e ching modes o N–H and O–H bonds o ee su ace amino g oups and adso bed hyd oxyl species [45]. 3.2. The mal S abili y o he F ic ion Composi es The TG cu es o gCN p epa ed in his s udy and comme cially a ailable G a e shown in Figu e 4a. The igu e clea ly indica es he high he mal s abili y o G; i also shows he he mal s abili y o gCN up o 570 ◦C. Me als 2022, 12, x FOR PEER REVIEW 7 o 16 Figu e 4. (a) TG cu es o componen s G and gCN and (b) o ic ion mix u es M0, M0_G and M0_CN. The he mal s abili y o he ic ion composi es is documen ed by he TG cu es shown in Figu e 4b. The he mal beha iou o all o he composi es was simila up o 580 °C; abo e his empe a u e, he TG cu es o all h ee ic ion composi es di e ed. The weigh loss o he M0_CN composi e abo e 580 °C was he mos p onounced. The o e all weigh loss o he M0_CN composi e in he empe a u e ange o 510–680 °C was 8%. Because all o he es ed ic ion composi es we e o complex composi ion, i is di icul o add ess he indi idual con ibu ion o each componen o he he mal deg ada ion o he ic ion composi es a a gi en empe a u e. Howe e , Figu e 4a clea ly shows he he mal s abili y o gCN up o 570 °C; hus, he weigh loss o he M0_CN composi e in he em- pe a u e ange o 510–680 °C e lec s he decomposi ion o gCN (TG cu e o sample M0_CN in Figu e 4b). The addi ion o g aphi e o he e e ence mix u e M0 (composi e M0_G) imp o ed he he mal s abili y o he M0_G composi e as shown in Figu e 4b. In e ms o he he mal s abili y, he ic ion composi e wi h g aphi e showed p e e able be- ha iou . 3.3. F ic ion and Wea Beha iou a RT The ime e olu ion o he ic ion coe icien (µRT) o he M0, M0_G and M0_CN sam- ples es ed unde RT condi ions is shown in Figu e 5. In Figu e 5a, sample M0 displays unning-in (s abilisa ion o he ic ion pe o - mance) wi h high and uns able µRT alues ha dec ease wi h ime o a s eady-s a e condi- ion. Sample M0_G (Figu e 5b) shows a con inuous inc ease in µRT ha s abilises only in he la e pa o he es . Sample M0_CN (Figu e 5c) displays in e media e beha iou . Ini ial unning-in was obse ed in he ini ial pa o he es , cha ac e ised by an inc ease in µRT ollowed by a dec ease, simila o M0, ollowed by a con inuous inc ease in µRT, simila o M0_G. The a e age alues o he ic ion coe icien in he inal s eady-s a e pa o he es s ( he selec ed s eady s a e is ma ked by he ed, do ed lines in Figu e 5) a e lis ed in Table 4. Figu e 4. ( a ) TG cu es o componen s G and gCN and ( b ) o ic ion mix u es M0, M0_G and M0_CN. Me als 2022,12, 123 7 o 15 The he mal s abili y o he ic ion composi es is documen ed by he TG cu es shown in Figu e 4b. The he mal beha iou o all o he composi es was simila up o 580 ◦ C; abo e his empe a u e, he TG cu es o all h ee ic ion composi es di e ed. The weigh loss o he M0_CN composi e abo e 580 ◦ C was he mos p onounced. The o e all weigh loss o he M0_CN composi e in he empe a u e ange o 510–680 ◦ C was 8%. Because all o he es ed ic ion composi es we e o complex composi ion, i is di icul o add ess he indi idual con ibu ion o each componen o he he mal deg ada ion o he ic ion composi es a a gi en empe a u e. Howe e , Figu e 4a clea ly shows he he mal s abili y o gCN up o 570 ◦ C; hus, he weigh loss o he M0_CN composi e in he empe a u e ange o 510–680 ◦ C e lec s he decomposi ion o gCN (TG cu e o sample M0_CN in Figu e 4b). The addi ion o g aphi e o he e e ence mix u e M0 (composi e M0_G) imp o ed he he mal s abili y o he M0_G composi e as shown in Figu e 4b. In e ms o he he mal s abili y, he ic ion composi e wi h g aphi e showed p e e able beha iou . 3.3. F ic ion and Wea Beha iou a RT The ime e olu ion o he ic ion coe icien ( µRT ) o he M0, M0_G and M0_CN samples es ed unde RT condi ions is shown in Figu e 5. In Figu e 5a, sample M0 displays unning-in (s abilisa ion o he ic ion pe o mance) wi h high and uns able µRT alues ha dec ease wi h ime o a s eady-s a e condi ion. Sample M0_G (Figu e 5b) shows a con inuous inc ease in µRT ha s abilises only in he la e pa o he es . Sample M0_CN (Figu e 5c) displays in e media e beha iou . Ini ial unning-in was obse ed in he ini ial pa o he es , cha ac e ised by an inc ease in µ RT ollowed by a dec ease, simila o M0, ollowed by a con inuous inc ease in µRT , simila o M0_G. The a e age alues o he ic ion coe icien in he inal s eady-s a e pa o he es s ( he selec ed s eady s a e is ma ked by he ed, do ed lines in Figu e 5) a e lis ed in Table 4. 1 Figu e 5. E olu ion o he ic ion coe icien a RT o he ( a ) M0, ( b ) M0_G and ( c ) M0_CN samples. The ed, do ed pa s o he cu es indica e he ele an s eady s a e. Me als 2022,12, 123 8 o 15 Table 4. Expe imen al esul s o he PoD es s a oom empe a u e (RT), high empe a u e (HT) and a oom empe a u e a e he HT es s (H_RT): coe icien o ic ion ( µ ) and speci ic wea coe icien (Ka). Sample µ(-) Ka(×10−14 m2/N) RT HT H_RT RT HT H_RT M0 0.50 ±0.04 0.38 ±0.01 0.43 ±0.06 6.56 ±1.22 10.44 ±0.93 15.65 ±2.19 M0_G 0.46 ±0.01 0.37 ±0.01 0.39 ±0.02 3.85 ±0.37 8.02 ±0.52 4.44 ±1.58 M0_CN 0.52 ±0.01 0.37 ±0.04 0.49 ±0.04 6.57 ±0.08 11.10 ±0.21 12.20 ±0.42 The sample con aining g aphi e, M0_G, exhibi ed he lowes ic ion coe icien . The ma e ial wi h gCN, M0_CN, showed he highes alue o µRT in compa ison o bo h he M0 and M0_G samples. The mean alues o K a calcula ed o all h ee ic ion composi es a e also lis ed in Table 4. The e was no signi ican di e ence in he mean K a alues o M0 and M0_CN. M0_G exhibi ed he lowes wea a e. 3.4. F ic ion and Wea Beha iou o he Samples a HT and Reco e y o Thei F ic ion-Wea Pe o mance Figu e 6shows he e olu ion o µ ob ained om he es s conduc ed a HT and again a RT a e he HT es s (H_RT). A HT, a peculia un-in s age was obse ed o all h ee samples. I was cha ac e ised by an inc ease in µ HT wi h he a ainmen o a peak alue, a e which µHT dec eased o each a s eady s a e alue a e 2000–3000 s o sliding. Simila beha iou was epo ed by Leona di e al. [ 40 ]. The s eady-s a e alues o µHT we e in he ange o 0.37–0.38, independen o he ma e ial (Table 4), and hese alues we e lowe han he alues ob ained a RT. Figu e 6. E olu ion o he µ wi h ime o he ma e ials es ed a high empe a u e (HT) and a RT a e he HT es s (H_RT). (a,b) smaple M0, (c,d) M0_G, and (e, ) M0_CN). Me als 2022,12, 123 9 o 15 Du ing he subsequen H_RT es s (Figu e 6), which indica ed he abili y o he ic ion composi es o eco e he ic ion pe o mance, he µH_RT alues ob ained o all samples (i.e., M0, M0_G and M0_CN) exceeded he µHT alues ob ained o hose samples du ing HT es s (Table 4). As shown in he igu e, he ic ion coe icien cu e o M0 du ing he H_RT es exhibi ed a long un-in s age, ea u ing la ge luc ua ions in he ic ion ace, whe eas M0_G had a s able bu lowe alue o µH_RT . The bes beha iou du ing he H_RT es s was displayed by M0_CN, which showed an ini ial inc ease in µ , ollowed by s eady-s a e condi ions a a high alue o 0.49 (Table 4). The K a alues o he H_RT es s a e also lis ed in Table 4. Fo M0, an inc ease in he wea a e compa ed o he alue ob ained a HT was obse ed. The same compa ison showed a educ ion o K a in he case o M0_G, whe eas o M0_CN, compa able K a alues we e ob ained unde bo h H_RT and HT es condi ions (Table 4). 3.5. Analysis o Wo n Su aces SEM mic og aphs o he wo n su aces ob ained o he samples a e he es a RT a e shown in Figu e 7a–c. Figu e 7shows he wo n su aces o he es ed ma e ials obse ed a he end o he ele an PoD es s unde RT condi ions. All he samples displayed a ypical ic ion laye made o p ima y pla eaus (mainly s eel ib es) ha blocked he wea agmen s and, hus, p omo ed he o ma ion o seconda y pla eaus. The ic ion su ace o M0 consis ed o small seconda y pla eaus and many wea pa icles ha we e no well compac ed and we e dispe sed on o he su ace (indica ed as X in he mic og aphs in Figu e 7). The wo n su ace o M0_G showed la ge and mo e compac ed seconda y pla eaus coexis ing wi h poo ly compac ed wea agmen s, again ma ked as X. The wo n su ace o M0_CN showed he p esence o p ima y and seconda y pla eaus. The seconda y pla eaus o M0_CN we e well compac ed and did no display egions o loosened agmen s o wea pa icles. Me als 2022, 12, x FOR PEER REVIEW 10 o 16 mic og aphs in Figu e 7). The wo n su ace o M0_G showed la ge and mo e compac ed seconda y pla eaus coexis ing wi h poo ly compac ed wea agmen s, again ma ked as X. The wo n su ace o M0_CN showed he p esence o p ima y and seconda y pla eaus. The seconda y pla eaus o M0_CN we e well compac ed and did no display egions o loosened agmen s o wea pa icles. EDXS analyses we e conduc ed o e alua e he elemen al composi ion o he second- a y pla eaus, and he esul s a e p esen ed in Table 5. As expec ed, he dominan elemen o ming he seconda y con ac pla eaus was i on ha o igina ed om he wea o he s eel ib es and, mos impo an ly, om he wea o he cas i on disc. Figu e 7. SEM mic og aphs showing he su aces o he pins a e he PoD es s a RT: (a) M0, (b) M0_G and (c) M0_CN. P—p ima y pla eaus; S—seconda y pla eaus; X—non-compac ed wea pa - icles; SD—sliding di ec ion. Table 5 shows he lowes i on con en o M0_CN. The ca bon con en inc eased in he o de : M0, M0_G and M0_CN. The highe amoun o ca bon in M0_G wi h espec o M0 owed o he addi ion o g aphi e, which is known o en e he ic ion laye [40]. In he case o M0_CN, he la ge con en o ca bon in he seconda y pla eaus, compa ed o bo h M0 and M0_G, can be asc ibed o gCN, which, simila o g aphi e, ended o en e he seconda y pla eaus. This was also con i med by he p esence o ni ogen de ec ed in he seconda y pla eaus o M0_CN. I should be no ed ha some o he de ec ed ca bon may also ha e come om he decomposi ion o o ganic ing edien s o he ic ion ma e ials, such as he phenolic binde and ubbe agmen s. Table 5. Elemen al composi ion (EDXS) o seconda y pla eaus on he pin su aces. Elemen (w .%) M0 M0_G M0_CN Fe 60.19 60.0 54.76 C 6.72 8.91 9.73 Zn 4.41 3.59 4.94 Al 2.12 2.21 2.23 Figu e 7. SEM mic og aphs showing he su aces o he pins a e he PoD es s a RT: ( a ) M0, ( b ) M0_G and ( c ) M0_CN. P—p ima y pla eaus; S—seconda y pla eaus; X—non-compac ed wea pa icles; SD—sliding di ec ion.