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materials

Article

Surface Damaging of Brass and Steel Pins when Sliding over Nitrided Samples Cut by Finishing and Roughing EDM Conditions

Vitaliy Martynenko 1,2, Daniel Martínez Krahmer 1,2, Amelia Nápoles Alberro 3, Amado Cabo 4, Daniela Pérez 1,2, Enrique E. Zayas Figueras 3, Hernán A. Gonzalez Rojas 3 and Antonio J. Sánchez Egea 3,5,*

1 Center for Research and Development in Mechanics, National Institute of Industrial Technology (INTI), Avenida General Paz 5445, Buenos Aires 1650, Argentina; vmart@inti.gob.ar (V.M.); dmartinez@inti.gob.ar (D.M.K.); danielap@inti.gob.ar (D.P.)

2 Faculty of Engineering, Universidad Nacional de Lomas de Zamora, Juan XXIII y Camino de Cintura, Buenos Aires 1832, Argentina

3 Department of Mechanical Engineering, Universitat Politècnica de Catalunya, C. Jordi Girona, 1–3, 08034 Barcelona, Spain; amelia.napoles@upc.edu (A.N.A.); enrique.zayas@upc.edu (E.E.Z.F.); hernan.gonzalez@upc.edu (H.A.G.R.)

4 IONAR S.A. Avenida Arias, 342, Ciudad Autónoma de Buenos Aires C1430CRB, Argentina; cabo@ionar.com.ar

5 Department of Mechanical and Metallurgical Engineering, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, Región Metropolitana 7820436, Chile

* Correspondence: antonio.egea@upc.edu

Received: 26 June 2020; Accepted: 15 July 2020; Published: 17 July 2020

Abstract: In the forging industry, surface quality and surface treatments of dies are crucial parameters to extend their life. These components are usually machined by milling or by Electrical Discharge Machining (EDM), and the final surface roughness depends on the machining techniques and operational conditions used in its fabrication. After milling, a nitriding treatment is widely applied to extend its service life. Nevertheless, no scientific report that informs about nitriding after EDM has been found. Accordingly, this work focuses on the wear and friction behavior of pins made of brass and medium carbon steel sliding over AISI H13 discs, made by wire EDM in the conditions of finishing and roughing. The discs are plasma nitride, and their effect on the wear during pin-on-disc tests is evaluated. In this sense, the analysis of the surface damage for the different pins will help us to understand the service life and wear evolution of the forging dies. The results show that plasma nitride reduces the friction and prevents the degradation of the pin, independently of the material of the pin, when sliding over finishing and roughing EDM conditions.

Keywords: plasma nitriding; EDM; pin-on disc test; surface quality; wear and friction



1. Introduction

Forging is a metal forming process widely used in industries, such as automotive, aerospace, railway, naval, oil, mining, and health [1,2]. Several parts are involved in this process: press, dies, material to be forged, lubrication system, and type of lubricant. Press and dies have to bear fatigue stresses, and dies are commonly manufactured with H series tool steel, where AISI H13 grade is the most universally used. This tool steel is hard to machine because of the high cutting tension due to its alloy content [3]. Replacing dies represent between 10% and 30% of the cost of a forged part [4], which enforces the need to decrease this percentage. In that sense, a proper forging process must take into account how the die is manufactured and during the forging process, protect and lubricate.



Materials 2020, 13, 3199; doi:10.3390/ma13143199



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The die’s surface quality is relevant in the forging process to manufacture the forged part without surface damage or scratches. Accordingly, different options exist to manufacture dies, and depending on the option, different surface features can be achieved. One option is milling the dies. Here, parameters such as type of tool and its geometry [5–8], the machining strategy [9,10] and the operational conditions [11,12] play an essential role on its final surface properties, and these parameters are tightly related to the service efficiency of the die (for example, fatigue resistance). Another option is the Electrical Discharge Machining (EDM) of the dies [13]. EDM is capable of making complex shapes and deep cavities; the only requirement is to work with an electrically conductive material. The necessary conditions to take into account in the EDM process that can affect the EDM removal rates and surface finishing of dies are the following: the material of the electrodes [14], the frequency of pulses and depths [15] and the addition of external powders [16] to modify surface properties and minimize the roughness by filling pores and cracks. Once the dies are manufactured, different surface treatments are used to extend their service life [17]; the most used treatment is the diffusion of nitrogen (nitriding), which achieves a significant increase of surface hardness and increase up to 125% in fatigue life [18]. The plasma nitriding is a modern technology that depends mainly on: gas composition, voltage and duty cycle, pressure, time, and temperature. For example, Solis-Romero et al. [19] used pin-on-disk tests at room temperature with several axial loads. In all cases, the friction coefficients were reduced for nitrided samples. A similar nitriding procedure was performed by Leite et al. [20] at 400 ◦C for 4 h, 9 h, 16 h, and 36 h. Then, during the ball-on disk test, a decrease in wear rate was found by up to 50% when the nitriding time passed from 4 to 36 h. Other experiments were made, modifying the internal pressure at the plasma nitriding chamber [21]. These authors found that the friction coefficients went from 0.55 to 0.30 for nitrided samples with pressures of 200 to 300 Pa, respectively.

Besides the technology of manufacturing the dies and the type of their surface treatment, it is necessary to consider the medium or lubricant used during the forging operation. A graphite-based lubricant is commonly used due to the high temperature reached in hot and warm forging processes. Dual-phase lubricants, spreading over the surface, reduce the friction coefficient during the forging process. Previous works [22–24] analyzed the behavior of several commercial graphite-based lubricants in hot forging, employing wear tests like pin-on-disc and friction tests like ring compression test, performed in different surface conditions. The experience shows that the size of the embedded graphite particles, the graphite concentration, and the lubricant’s kinematic viscosity are parameters that influence the friction coefficient at the interface, independent of the testing temperature. This work aims to study the initial surface properties to describe their influence on surface wear. These surface properties of the discs are defined by wiring EDM in finishing and roughing conditions and using plasma nitride. Then pin-on-disc tests are performed to compare the friction coefficient and wear rate for pins made of brass and medium carbon steel, which slide over discs made of AISI H13.

2. Methodology

This section is divided into several subchapters: characteristics of the lubricant, machining conditions and surface treatment of the discs, and pin-on-disc tests at room temperature. Throughout these subsections, the experimental protocols, equipment, and facilities used to investigate the friction and wear of steel and brass pins on EDMed discs with different types of surfaces conditions are described.

2.1. Characteristics of the Lubricant

The graphite-based lubricant was diluted up to 5% in water, which corresponds to a typical average lubricant used by Argentine forging companies. This lubricant had a density between 1.10 and 1.20 g/cm3. A scanning electron microscopy (FEI Model: QUANTA 250 FEG, FEI, Eindhoven, the Netherlands) was used to determine the elemental chemical composition of the lubricant and the size of the graphite particles. Also, the kinematic viscosity was determined with an oscillating rheometer (Anton Paar Physica Model MCR301, Anton Paar, Ostfildern, Stuttgart, Germany). Figure 1a



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of the graphite particles. Also, the kinematic viscosity was determined with an oscillating rheometer



(Anton Paar Physica Model MCR301, Anton Paar, Ostfildern, Stuttgart, Germany). Figure 1a shows sthheowass-rtehceeiavse-rdegceriavpehditgerlaupbhriicteanlut bmraictarinxtams asutrpixplaiesds,uapnpdliFedig,uarned1bFipgruerseen1tbs pthreeskeinntesmthaetickivniescmoasittiyc vcuisrcvoesiftoyrctuhrevewfaoterrthdeiluwtaetderludbirluictaendt.luTbarbilcean1t.quTaanbtliefi1esqtuhaenativfieersagtheesiazveeroafgtehesigzeraopfhtihtee pgarratpichlietse, pwahrticichleasr, ewohbicthainareedofbrtoamine2d0 fmroemas2u0remmeeanstusr.eAmlesnot,st.hAelsaov,etrhaegeavpeerracgeenptaegreceonftatghee owf tehigehwt eoifghetacohf ecahcehmcihcaelmeilceaml eelnetmcoenrrtecsoprornesdpsotnodtshetorethsuelrteosfuflitvoef mfiveeasmureeamsuernetms.ents.



FFiigguurree 11.. ((aa)) GGrraapphhiittee--bbaasseedd lluubbrriiccaanntt mmaattrriixx aanndd ((bb)) tthhee kkiinneemmaattiicc vviissccoossiittyy ccuurrvvee ooff tthhee wwaatteerr ddiilluutteedd lluubbrriiccaanntt..



TTaabbllee 11.. CChheemmiiccaall ccoommppoossiittiioonn aanndd ppaarrttiiccllee ssiizzee ooff tthhee ggrraapphhiittee ppaarrttiicclleess..



LubriLcuabnrtican(twCt.%C )(wt.%()wOt.O%)(wt.%)(wNt.a%(wN)t.a%)



Al Al (wt(.%wt).%)



AveraAgveerage 80.6780.67 11.7811.78 2.362.36 0.920.92



Si Si (wt(.w%t.)%)

3.235.25



SS(wt.%) (wt.%)

0.209.29



GGrarpaphhitieteSize Size(μ(µmm)) 55.6.363±±2.22.727



22..22.. Discs and Pins Preparation

A PPrroommeecor SSMMT 119//550000 nnuummerical ccontrol laathe ((PProomecor,, CCóórrddoobbaa, AArrggeenntina) wwaas uusseed ttoo mmachine 16 AISI H13 steel discs with an external diameter of 63 mm, an internal diameter of 19 mm, aand 8 mmmm oofftthhicickknneesss. .AAllldl idsciscws ewrerheahrdarednendeadt 1a0t 41004°C0 ◦aCndantedmtpeemrepderfeodr 2fohr a2t h57a0t°5C7.0B◦oCth. Bfaoctehs foafcetsheofdthisecsdiwscesrewegrreogurnoduenddeodn oan taantagnegnetniatliagl rgirnidnidnigngmmacahchininee((DDaavvoonnisis mmooddeell SGS-1230AHR, DDavonis, Billinghurst, Buenos Aires, Argentina), using an AA446I10V ggrrinding wwheel wwith aan aavverage ggrraainn sizze of 0.38 mm. A hardness tester (INSTRON WOLPERT, Model:: S8-233971, Rockwell C Scale, IInnssttrroonnCCoorrppoorraattiioonn,,CCaannttoonn,,MMAA,,UUSSAA))wwaassuusseedd ttoo mmeeaassuurree tthhee haarrddnneessssooff tthhee disscc beforee the wiree EEDM and niittrriiddiinngg ttrreeaattmmeenntt,,bbeeininggoof f5511.6.6±±1.51.H5 RHCRCforfofrinfiisnhiisnhgincgoncdonitdiointioanndan5d1.151±.12.±1 2H.1RCHRfoCr froorurgohuingghicnogncdoitnidonit.ioFnin. aFlliny,atlhlye, tshuerfsaucrefsaocefsthoef dthisecds iwscesrwe werierewEirDeMED(NMo(vNicokvAicRk 3A5RM3A5 )MuAsi)nugstiwngo towpoeroaptieornaatliocnoanldciotinodnist:iohnalsf: ohfatlhfeomf tihnema finisahfiinngischoingdictioonnd(itoinon= 5(toμns;=to5ff µ=s3;4toμffs=; T34= µ39s;μTs;=f 3=92µ5.s6; fH=z;2t5o.n6/TH=z;0t.o1n3/;TV==04.1V3;)V, a=nd4 tVh)e, aonthdetrhheaolfthinerahraolfuignhainrgoucgohnidnigtioconn(dtointi=on50(tμons;=to5ff0=µ1s8; 0toμff s=; T18=02µ3s0; Tμs=; f2=304.µ3sH; fz;=to4n./3TH= z0;.2t2on; /VT==60V.2)2. ;FVro=m6thVe)s.eFtrwomo gtrhoeuspest,whoalfgroofuthpes,dhisaclsf owfetrheepdlaiscmsawneirteridpilnasgmbay nIOitrNidAiRngS.bAy. ITOhNe AniRtriSd.iAn.gTchoendniittiroidnsinwgecroen: tdeimtiopnesrawtuerrees: otef m50p0e°rCat,uerxepsoosfur5e00tim◦Ce,oefx1p3oshu, rperotitmecetivoef 1g3ash,mparodteecotfiv2e0%gasNm2 +ad8e0%of H202%, aNnd2 +a p80re%ssHur2e, aonfd6ahpPrae.sFsuinraelloyf,6ahtPraan. sFvienraslalyl,saectrtiaonnsvoefrtshael sneicttriiodne ocof mthbeinneidtriwdeithcofminbisihniendgwanitdh rfionuigshing aEnDdMrocuognhdintigonEsDwMascoanadliytizoends iwn aasVaincakleyrzsemd iicnroahaVridcknerss mteisctreorh(SahrdimneasdsztuesHteMr (VS-h2i0m00a,dSzhuimHaMdVzu-2C00o0r,pSohraimtioand,zKuyCootorp, oJarpaatino)n. ,RKeygoartod,inJagptahne).pRinesg, athrdeiyncgotmhe pfrionms, tdhreaywcnombaerfsroomf 6d.3r5awmnmbianrsdoiaf m6.3et5emr. mSixintedeniam(16et)epr.inSsixwteeerne (m16a)dpeinosf wbrearsesmAaSdTeMofBb1r6ashsaArdSnTeMss BH1R6Bha7r6d, naensds aHnRoBth7e6r,1a6ndpiannsowtheerre1m6apdinesowf setreeeml AadISeI o1f0s4t5eehlaArdISnIe1ss04H5RhBar8d7n.eAssllHoRf tBhe8m7. Awlelroefmthaedme wweitrhe hmeamdiespwhietrhichael mtipissp4hmermicadliatimpset4ermamt bodtihamenedtesrthatatbwotehreenpdolsistheadt wweitrhe apborlaishiveedpwapitehrsaobfragsriavine psiazpeesr1s0o0f, g6r0a0i,nasnidze1s010000., 600, and 1000.



2.3. Pin-on Disk Test at Room Temperature



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2.3. Pin-on Disk Test at Room Temperature

TMhaeteriaplsi2n0-2o0,n1-3d, xisFcOsR PtEeEsRtsREVwIEeWre carried out with equipment made in-house (Figu4roef 102) at INTI-Mechanical Center in Argentina. The tangential force during the tests was recorded with a data logTgheer (pViner-onnie-driMscsodteesltsLawbeQreuceasrtr,iVederonuiet rw, Bitehaveqeurtiopnm,eOntRm, UadSeAi)n, -whohuicseh (hFaigsuarelo2a)dartaInNgTeI-up to 50 N.MPeinchsaannicdaldCiesnktserwineArergfiexnetdinain. Tthheetamngaecnhtiinalefotorcecadrurryinogutht ethteesttsewstasswreictohrdthede wapitphraodpartiaaltoegagmerount of luba(rVnicedrandniites.rkTsMhwoeedrleeulbfLirxaiebcdQaniuntetwsht,eaVsmeaardcnhdieienr,deBtoeonacvatehrrretyodno,iusOtct’Rhs,esUuteSrsAftas)c,wew,ithchihctehhcehkaaipns pgartolohpaarditaarteaunangmeifoouurpnmttoodf5il0supbNer.ircsPaiinontns. had occurTrehde lbuebfroicraentthwe atsesatdbdeegdaonn. tThhe edaisxci’salsulorafadcse,ocnhetchkeinpginthsawt aeruen4if.o5rmN dfoisrptehrseiobnrahsasdaoncdcu6r.r5edN for the stbeeeflo, rteo trheeptreosdt ubecgeaan.cTohnetaacxtiaplrleosasdusroensitmheilpairnstowtehraet4w.5hNicfhorocthceurbsraisns aanfodr6g.i5nNg pforrotcheessstweeilt,htothese materrieaplsro(d14u0ceaandco2n0t0acMt pPraesrseusrpeescitmivilealryt[o25th])a.t Twhheicthanogcceunrtsiailnsapefoerdgiinngthperopcienss-dwisitkhctohnetsaecmt wataersia0l.s2 m/s, and th(1e40toatnadl e2x00peMrPima reenstpdecutirvaetliyon[25o]f).eTahcehttaensgtewntaiasl 2sp0emedinin(tehqeupivina-ldeinskt tcon2t4a0ctmw)a.s T0.h2emse/se, axnpderthime ents were troetpaleaextepder3im2etnimt deusrraetisounltoinf geafcrhomtesthweacso2m0 bmininat(ieoqnuiovafltewntot-opi2n40mma)t.eTrihaelsse, tewxpoerdimisecnstms wacehreining condirteiopneaste(dfin3is2htiinmgeas nrdesruoltuingghifnrgomwitrheeEcDomMb)i,ntawtioonsuorffatcweot-rpeiantmmeantetrciaolns,dtiwtioondsi(sncsonm-nacithriindinegd and nitridceodnddiitsiocns)s,(afinndishfionugrarnedpreotiutgiohninsgpweirreeaEcDhMc)o,ntwdiotisounrf.aFcerotrmeattmheenetxcpoenrdimitioennsts(,nfornic-ntiiotrnidceudravneds and weighntitlroisdseodfdthisecsp),inasndweforuerdreetpeermtitiinonesd.pTerheeawcheicgohntdditiiffoner. eFnrocemwthaes mexepaesruimreedntws,iftrhicatiRonadcwuravgeselaencdtronic scale,ewmleeocigtdrhoetlnlAiocsSssc2ao2lef0,.tRmh2eod(pReilanAds wSw2ae2gr0e,.RRd2aedt(eRorammd,iwnPeaodgl,.aRnTadhde)o, mwwe,itiPhgohaltannddaifc)f,cewurerintahcceyanowafac0sc.u1mrmaecagys.uoFrfei0nd.a1lwmlyig,tht.hFaeinaRadlaldhyw,etshaiogens of mata2t1er±awidae3lhroee◦snCpioedanrifnssocdorsmfrwmeeldeaarttaeeitvrsi2eat1ulh±douni3emd°dCiidstcoaistndywdeoersfercelr5ais0tbtivue±edt1hihe0eud%mstu.oidrdfiateycsecorfpib5reo0pt±he1er0ts%iuer.sf.aTceheprpoipne-rotnie-sd. iTshcetepsitns-owne-rdeispcetrefsotsrmed



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2.4. S2u.r4f.aScuerRfaocue gRhonuegshsness

As thAesEtDheMEDpMrocperosscedssoedsoensontohtahvaeveaapprereddoommiinnaanntt ssuurrffaacceeroruoguhgnhenssesdsirdeicrteiocnti[o2n6][,2t6h]e, stuhrefascuerface rmoeutgehrmmrno(Teeueatasgenyshrlsnmo(ueTrrseafHsayasmclooueberrraseoHosmuunogrebehSnsmnuotersenstnwrsStos(uenRwrriatecre)orm3aen+niamdc,dTat3ehad+eyie,nltTioonrartaayrHnlalonhdordeobioHmgsmohontb((mo,smofLunutehl,littecLiidedresoiiirtrcueeeegcrcst,httieEoinornne,nsagEsall)nlap)dgnrdioldrafien)ircledmetic)o(teRnmiaotss)en.,uaAwsrs.euhpAdreoerrtdepthaotetbhhrleateearcribautorlhtuie-tmoghrfhmoefntuaeiencgtsidhscmneeasns surfactheereovuagluhantieosnsl(eRngat)hawnedrethseettaott0a.8l hmemigahntdo4f mthme rreosupgechtniveeslsy.pTrhoefiAleIS(IRHt)1,3wstheeelrdeitshcsewceurte-otuffrnaendd the evaluqautieonnchleedn,gttehmwpeerreed saentdaftin0a.l8lymgmrouanndded4umpmto areRsapoefct0i.v51el±y0. .T08hµemA.ITSaIbHle123shstoewels davisecrasgwe evraeluteusrned quencahneddt,hteemstapnedraerddadnedvifiatnioanllyofgtrhoeusnudrfeadceurpoutoghanResas of 0th.5e1d±isc0s.0m8eµasmur.eTdaobnleth2esshuorwfasceasvoebrtaagineevdalues and thfreomstafnindisahridngdaenvdiartoiounghoifngthEeDsMurfwacitehraonudgwhnitehsosuot fthtehepldaissmcsa mniteraidsuinrgedtreoantmtheents.uArftaocteals oofb9t6ained from mfineaisshurinemgeanntsdwroeruegmhaindge tEoDdMeterwmiitnheaanridthwmietthicomuteatnhesuprlfaacsemraoungihtrniedsisn(gRat)reaantdmtheentto. tAal htoeitgahl tof 96 measuorfetmheernotusgwhneeressmpraodfeileto(Rdt)e.termine arithmetic mean surface roughness (Ra) and the total height of the roughness profile (Rt).



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Taabbllee 2. SuSrufarcfaecreouroguhgnhesnseassndanmdatmeraiatel rhiarldhnaersdsnfoerssthfeorditfhfeerednitffEeDreMnt cEoDndMitioconns dwititiohnasndwwithithaonudt wniittrhidoiuntgn. itriding.



SamplSeample



RRaa±±SSDD



Rt ±SRDt ± SD HardnesHs ardness



((µμmm±±SSDD) ) (μm ±(µSmD±) SD) (HV ±SD(H)V ± SD)



FinishFiinngisEhDinMg(EsuDbMstra(steu)bstrate) 33..7744±±00.3.388 26.852±6.38.53±9 3.35956.0 ± 2555.26.(0*±) 25.2 (*)



RougRhionuggEhDinMg(EsuDbMstra(steu)bstrate) 77..8888±±11.5.511 54.955±4.89.59±3 8.95343.1 ± 3554.43.(1*±) 35.4 (*)



NitridNeditrfiindiesdhinfigniEsDhMing EDM NitridNeidtrrioduegdhirnoguEgDhMing EDM



22..3311±±00.4.400 18.091±8.20.96±0 2.601130.3 ± 17113.10.3 ± 71.1 55..4433±±00.7.766 43.074±3.60.70±8 6.08853.3 ± 2865.43.3 ± 26.4



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TThhee fifninisihshiningganadnrdourgohuignhginEgDMEDcoMndcitoionndsithioanvse shiganvieficsaingtndifiiffcaenretncdeisffienrethnecessurifnacethreousguhrnfaecses, rboeuingghhniegshs,ebr efoinrgthheigrhoeurgfhoirngthceornoduigtihoinnogfcEoDndMit.iTonheonf EitDridMe.dTshpeecniimtreidnesdpsrepseecnimtedenlsowpreersvenaltuedesloowf Rear vanalduRest,oafsRwaeallnads lRotw, aesr dwiespllearssiloonw. eSrimdiilsapretrrseinodns. SwimereilaarlstorefnoudnsdwbeyreSoallissoRfoomunedrobeyt aSlo.l[i1s9R].oFmigeuroree3t ashl.o[w19s]t.hFeigsuurrefa3cesmhoowrpshtohleogsuiersfafocer fimnoisrhpihnogloangidesrofuogr hfiinnigshEiDnMg asnudrfraocuesgwhiinthgoEuDt aMndsuwriftahcethsewpiltahsomuat annitdriwdiinthg tthreeaptmlasemnta. nRitergidarindgintgretahtemmenatt.eRreiagla’srdhianrgdtnheessm, athteerisapl’eschimarednntersesa, ttehde isnpeacifimneisnhtirnegatEedDMin acofnindiisthioinngpEreDseMntceodnadhitaiordnnpersess3e2n.t5e%d haihgahredrntehsasn3t2h.e5%rouhgighhinerg tEhDanMthcoenrdoiutigohni.nNg iEtrDidMedcofinndisihtiionng. NEDitMridsepdecfiinmisehnisngreaEcDhMed sspimeciilmarehnasrdrenaecshsevdalsuimesiltaor [h1a8r].dnReesgsarvdailnugesthtoe E1D8M. RsuegrfaarcdeinmgotrhpehoElDogMy (sFuirgfuacree 3mao,cr)p, hthoelorgesyu(lFtsigpurreese3nat,ct)h,ethseamreesufelatstuprreessaesntththeewsoarmkerefpeaotruterdesinas[1th3e], wwohrekrerethpeorstaemdpinle[w13a]s, wEDhMereedthwe istahmshpolertwpauslsEeDs Maneddawloitnhgsdhourrtatpiounls.es and a long duration.



Figure 3. (a) Surface morphology of EDM in roughing condition; (b) Surface morphology of EDM in roughing condition and nitriding; (c) Surface morphology of EDM in ffiinishing condition; (d) Surface morphology of EDM in fifinishing condition and nitriding.

33.. RReessuullttss aanndd DDiissccuussssiioonn TThhiiss sseeccttiioonnaannaallyyzzeesstthheefrfircictitoionncucurvrevsestotodedsecsrcibriebtehtehienflinufelunecne coef othfethsuersfuacrfearcoeurgohungehsns easnsdatnhde

tnhiterindiitnrigdtirnegattmreeantmt oenntthoenfrtihcetiofrnicctoioenfficcoieenfftisc.ieBnetssid. Bese,stihdeesw, ethigehwt leoisgshotflothses poifntshaefpteirntshaeftpeirn-tohne-pdiinskotens-tdsiisskatneasltyszisedantoalmyzeeadsutoremtheeaswuerearthoef twooeal rtiopfstfooorltthipespfionrs tmheadpeinosfmbraadses aonf dbrsatsesela.nd steel.



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33..11.. PPiinn--oonn DDiisskk TTeessttss TThhee eexxppeerriimmeennttaall tteessttss aarree rreeppeeaatteedd fifivvee ttiimmeess ppeerr eeaacchh ssuurrffaaccee ccoonnddiittiioonn ttoo aannaallyyzzee tthhee ffrriiccttiioonn

ccooeeffiffMicciaieteenrnitatlsaa2tt0t2th0h,ee13ss,ttxaaFttOiiooRnnPaaErrEyyRpRpEhhVaaIsEseWe.. FFiigguurree 44 eexxhhiibbiittss tthhee bbooxxpplloott ooff tthhee ffrriiccttiioonn ccooeeffifficciieennttss ffoo6rroftth1h0ee ttwwoo ssuurrffaa3cc.ee1.ccPooinnnd-doiinttiiDooninssks aTannesddtswwiitthh tthhee pprreesseennccee oorr nnoottoofftthheenniittrriiddiinnggttrreeaattmmeenntt..



The experimental tests are repeated five times per each surface condition to analyze the friction coefficient at the stationary phase. Figure 4 exhibits the boxplot of the friction coefficients for the two surface conditions and with the presence or not of the nitriding treatment.



(a)



(b)



FFiigguurree44.. BBooxxpplloottss ooff tthhee ffrriiccttiioonn ccooeefffificciieennttss aatt tthhee ssttaattiioonnaarryy pphhaassee ffoorr tthhee ddiiffffeerreenntt ssuurrffaaccee ccoonnddiittiioonnss wwiitthhaannddwwiitthhoouutttthheenniitt(rraiid)diinnggttrreeaattmmeennttffoorr((aa))ppininssmmaaddeeooffsstteeeellaanndd(b((bb))p)pininssmmaaddeeooffbbrraassss. .

Figure 4. Boxplots of the friction coefficients at the stationary phase for the different surface conditions

RReeggaawrrdidthiinnaggndtthhweeitphpoiinunststmhmeaandditereidooiffnbgbrrtraaessasst,m, ttehhneet ffforrriicc(atti)ioopnnincscoomeeaffifdfieccioiefensnttteesslhhaoonwwd s(sbn)nopoinsssiiggmnnaiidfifeiccoaafnnbttrcacshhsa.annggeess wwiitthh aanndd wwiitthhoouuttthtehpelapslmasamnaitrindiitnrigdtirnegatmtrenatm. Henotw. eHveorw, aesveexrp, eactsede,xtphecltoewde, rtfhriectilonwceoreffifrcicietinotncocroreesfpfiocniednst tcootrhressppoenRcdiemgsaetrnodsintphgreothcseepspesicenidsmmbeyandfisenpoisrfhobcirneagssssE,etDdhMebfyrciocftmiinopinsahcroienedfgfitcoEieDrnoMtusghhcooiwnmsgpnEoaDrseMigdn. iOtfoincartnohtuecghchaoinnngtgreasErwyD,ittMhhea. nfOdrinctitohne ccooenffitrwcairietyhn,otutohtfepthfierniscptmilaoasnmdecaooenffifstritciedieeilnnigts ontrfoetapitcimneaesnbmtl.yaHrdeoedwuoecfveesdrt,eweahlseisnexntphoeectitsceuedra,fbatlhcyee rilseodnwuietcrreidfreidwct,ihoinennpcatohretefifcisucuilearnrft,a7ce.6%is fnoirtrtihdceoerdfir,nesiinpshopninadrgstiEtcoDultMahre,as7np.d6e%c1im9f.oe4nr%sthfpoerrofcirneosiusseghdhinibngygEfEDinDMisMhai.nnIgdnEt1Dh9iM.s4%sceonfmosrep,raSoreoudlgishtoRinorgomuEgeDrhoiMneg.tIEanlD.tM[h1i.9s]Osuennsitsnheeg, Spoinlis tmeRomoaomdebercnRteyaooirotinofrmnptiAdreaemaetrpIrodSiyearI,,rlei.5sottnh2[ra1ap,elfi19.na0fn]r[dr01tiiuis9ccohdst]uini,oiulnaafnsAgormi,nuIcp7oSgon.iIne6dnpf%Hdfisinac1fmspiod3eamrneadstctdathirsedeoeecefasfotsipoofnpeifinoAsoAshlfIbiIisSmnS3thIaI3gaei%55nddE22eD,,owm11Mof00ifi0ft0rhrasrointoocsnedtnrieilAoli1Acfn9IiiosSn.I4ncISin%osIHcoheaHtf1ffi,ior3cb1rfeco3diarideouibsdneulncyitgsedshwmcrpeisanohdelgpreuidysnocEheelDpeticdhsdarMhepwwey.aehdIsritnteesrhnwetahoasnitiitfthsledhiecds3odes3stnni%uhlasircemecfa,aoodroScnbfienosidldcfciirasessiprcwbatiisiodtthnee pcloaesfmfeicmaieennriytripwdaihpneegnr.s Ctahonendysedtqriaeumaetonentdldy,tphthaesetdeloistwocseoswbttfiartihinctpimolianrsrcmooraefffiinnciistierhin,dtfionruegsn.udCltaeodndsieencqraeunaesnaetvloeyfr,a3tg3he%eolofof0w.f2rei2sc,ttifoofnrricbtoiothn tcyopeeffscicooiefefnfmitciaretenestruiwaltlhesed,nwintihtaheynatanrvdeeawtreadigthethooeuf td0ni.s2ic2tsr,iwdfoiirnthgbopinltahsfimtnyaipsenhsiitnroigfdimEnDga.tMeCroicanolssne,qdwuiteintohtnlya.,nAtdhsewaliortwehfoeesurtet nfnrciicettr,iiosdniimngilainr ffriincitsiohcnoinevfgfaiEcluiDeenMst wrecesoruneldtfeiodtuioinnnd.anAbyasvLaeerriaetgefeerotefna0lc..e2[,2s,0if]mo, rwilbahorethnfrtiycetpsiteoisngovfSamil3uaNetes4riwbaaleslr,lsewofiotnhupanondldisbwhyeiLtdheosiutetrfenatictareilsd. io[n2fg0n]ii,ntwridheedn

dteisctisnfmigniaSsdih3eiNnog4fbEAaDlIMlSsIocHonn1pd3oitslitiosenhe.leA.dssaurreffaecrenscoef, sniimtriildarefdridctiisocnsvmalaudese woferAeIfSoIuHnd1b3ystLeeeitle. et al. [20], when testing Si3N4 balls on polished surfaces of nitrided discs made of AISI H13 steel.

33..22.. WWeeiigghhtt LLoossss ooff tthhee PPiinnss 3.2. Weight Loss of the Pins DDuurriinngg tthhee ppiinn--oonn--ddiisscc tteesstt,, tthhee ppiinn iiss eexxppeecctteedd ttoo wweeaarr bbeeccaauussee tthhee mmaatteerriiaall ooff tthhee ddiisscc iiss hhaarrddeerr

tacthhfotaanennrdctttihhotthihaneoenednppDisittpinihunoie.arnn-ifoAns.tpnegciAarn-ctfd.ohctteicerhAsrdopcecirticndhtnopege-irsonildntynpgsi,--ni.loFdnygAn-ii,lsgoy-cnudFc, t-lriiedFegsesiaicutgs5,ncrutteiehrentxeesegh5tsps5tipsbi.en.reixAtoxiAshshciteeicbhcbxdlliiepeettussaaewrncnettteiihehnniodeggegfhtw1opwtp5erwlerooimgoeicsghaecsihdrtenodutblfooeruletcsforhasuesoeusolfstdofer1oifatfft5fhhs1eoeemt5rnhmedinienmcaitftfdibeopenrfairiifnetafuohnlselftortawrefnpauntisidhlttnothersnspaduiiscsaiirnosonfcbsnadpaicirstcaesohhnupcabrdowarfadannitcstedhoheuriltrwiwfoainacthsse piseorpforirosmoppaerndoopilnanweoaalcswhpasesarpfmoerrpfmolerembdeeidfnoirneeaemcahcehsaassamumrpipnllege bbtheeffeoowrreeemmigeehaastusluorirsnisgn. gthtehwe ewigehitglhotsslo. ss.



FigurFeig5u.rWe 5e.igWhetiglohst sloosfs tohfethpeinpsinms madadeeooffbbrraassss andd sstteeeellaaftfetrerthtehepipni-non-o-dni-sdcsistcesttienstthine ftohlelofwoilnlogwing sFuirgfuasrcuer5cfao.cnWedceiotinigodhnittsil:oonfisssn: ifosinfhiitsnhhgein(pglie(nflest)fmta)naadnderorooufugbghrhiainnsgsg (a(rrniiggdhhstt)t)eEEeDlDMaMftceocrnodtnhidteiiotpnioisnnw-soiwtnh-idathnisdacnswdtiethwsotiutinht notiuhtrteidnfiiontlrglio.dwinign.g surface conditions: finishing (left) and roughing (right) EDM conditions with and without nitriding.

The weight loss values show that the average weight loss is about 1.22 to 2.32 times higher for bTrhaesswaenidghstteleolsswvhaelnuecsomshpoawrintghantotnh-eniatrviedreadgediwscesigvhetrsluosssniistraidbeoduto1n.e2s2. tTohe2.3lo2wteimstessuhrfiagcheer for brass and steel when comparing non-nitrided discs versus nitrided ones. The lowest surface



Materials 2020, 13, 3199



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The weight loss values show that the average weight loss is about 1.22 to 2.32 times higher for brass



and steel when comparing non-nitrided discs versus nitrided ones. The lowest surface degradation



was found for a nitrided and roughing EDM condition, whereas the worst surface was denoted for non-niMtraitdereiadls 2a0n20d, 13fi, nx iFsOhRiPnEgERERDEVMIEWconditions. Das et al. [27] reported wear tests of non-ni7troifd10ed and



nitrideddegArIaSdIatHio1n3wdaissfcosupnrdefpoar raenditbriydesdtaannddarrodugmhientgalElDogMracponhdicitimone,twhohedrse,aas nthde pwionrsstmsuardfaeceofwaalsumina with a dseenmotie-sdpfhoer rneonm-noirtrpidheodloagnyd. fTinhisehyinfgoEuDnMd tchoantdtithioenws. eDigashettloals.s[2w7]arsepaobroteudt wtheraeretetsimts eosf nhoing-her for



non-nitnriitdrieddeddaisncds.niAtrlisdoe,dSAaIrSkIaHr1e3t dails.c[s2p8r]eppearefodrbmyesdtanpdinar-donm-detiaslclotgersatpshwicitmhepthiondsso, fanbdrapsisnsomveardde iscs of



steel. Tohf ealyumfoiunandwiathcaomsempai-rsapbhleerewmeoigrphhtolloogssy.tTohoeyurforuensdultthsa.t Itnhepwaeritgichut lloasrs, twhaesyadboeustcrthibreeewtimeiegsht loss values hloigwheerr ftohranno1n0-n0imtrigdefdordaisctsr.aAvelsloin, gSadrkisatraentcael.o[f28a]ppperrofoxrimmeadteplyin1-o5n0-dmisc(2t4es0tsmwiinththpiinsswoof rbkra).ssTable 3 shows wothveeeirgahdvtiselcorssasgovfeaswltueeeelsi.glTohhwteleyorsftoshuaannnd1d0a0itcsmomsgtpafoanrrdaabatlrerdawvdeeeliigvnhigat dtlioiosstnsantionceoiounfrcarrpeesapusrloitnxsg.imInoartpdealeyrrti1cf5uo0lramtrh, (te2h4ed0yiffmdeeirsnecnrtihtbiessurface conditiwonorsk()fi. Tnaisbhlein3gshoorwrsouthgehaivnegr)agaenwdetirgehattlmosesnatnsd(witsitshtaannddardwditehvoiauttionnitinridinicnrgea).sing order for the



different surface conditions (finishing or roughing) and treatments (with and without nitriding). Table 3. Average weight loss and its standard deviation in increasing order to identify which surface



conditiToanbslean3.dAtvreraatgme ewnetisghptrleossesnatnldoiwtsesrtawndeaarrd. deviation in increasing order to identify which surface



conditions and treatments present lower wear.



Configuration\\Sample Configuration\\Sample



Brass (mgB±raSsDs )

(mg ±SD)



NNNiiitttrrriiidddFeeeidddni& &&sNNhrfifiiiionttnnrurgiiiigdssdFhhheeiiididnnnni&gg&gshrfioinnugigshhiinngg



16.28312n3r1s±ndrdtsddt2.7



FinisNhiintrgided & finishing 241.86.±8 ±102..47



Roughing Finishing



24.68th± 10.4



Nitrided & rougRhoinugghing



192.7 6±th64.4



RougNhiitnrgided & roughing 23169.21.7±±6634.5.4



Roughing



236.1 ± 63.5



Steel



Steel (mg ± SD)



(mg ±SD)



1545...110±±±000..23.71455...1104±±t±h000..23.7



4th



5th



5th 34.8 ± 5.0



34.8 ± 5.0 7th



7th



8th



8th



3.3. Sur3f.a3.ceSuDrfaamceaDgaemoafgtehoefTthoeolTTooipl Tip

As expAescetexpde,ctthede, pthienspimnsamdeadoefobfrbarsasssshshooww hhiigghheerrtitpipdedgergadraadtioantioconmcpoamrepdatroetdhetostethelepsitnese. l pins. BesidesB,etshideessu, trhfeacsuerrfoacuegrhonugehssnepsrsepsreenstesntas da idffifefreerennttiinnflfluueenncceeoonnthtehteytpyepoef mofatmeraiatel.rAiaslm. Aoosthmsouorftahcesurface favors tfhaveodrsegthraeddaetgiorandoatfiothneobfrtahses bprians,swphini,lewlohwileerlodweegrrdadegartaiodnatiiosnfoius nfodufnodr ftohre tshteeesltepeilnp. iTnh. eThceontrary happencsonwtrhaeryn hthapepceonasrwsehseunrtfhaececoraorusegshunrefascseirsopurgehsneensts oisnptrheseesnut rofnactheeosfutrhfaecde iosfctsh. eFdigisucsr.eF6igeuxrhei6bits the SEM imexahgiebsitsofthtehSeEtMipsimaaftgeers solfidthine gtip(ps ianft-eorns-ldidiisncgte(psitn)-ovn-edrisncitreisdt)eodveisrcnsitwriditehdaisficsnwisihthinagfiEnDishMinpgrocess. The resEuDltMs sphroowcestsh. Tathethreesluolwts eshstowwethigathtthleolsoswwesatswfeoiughntdlofsosrwpains sfomunaddfeoropf isntseemlawdeitohf tshteeelfiwniitshhtihneg EDM processf,insilsihdiinnggEoDvMer pnriotrciedsse,dsldidisincsg. oOvnertnhietroidpepdodsiistecs,.thOenhthigehoepspt owsieteig, hthtelohsisghwesatswfoeuignhdt lfoosrs pwinass made of brasnfsooustilnciddeaifbnolgre phoiovnwseramfntaeodren2-0onfmibtirrniadossfesdslliidddiiinnsggcsteosivnteinrtghn,eothnre-onhuietgrmihdieisnpdghdeEirsiDccsaMlimn cothorepnhrdooilutoiggohynino.gfIttEheDistMinpsocomtincadediaetiboolnfeb. rhItaosisws after 20 minaorfe swlihdoilnlygrteemstoivnegd,.the hemispherical morphology of the tips made of brass are wholly removed.



(a)



(b)



FigureF6i.guPrien6t.ipPisnatfiptesrasftleidr isnligdiongn onnitnriitdrieddedddisicscssmmaaddee bbyyEEDDMMwwithithfinfiisnhiisnhgincogndcoitniodni.t(iao)nS.u(rafa)cSeurface damagdeaomfaagpe ionf ampaindemoadf estoefeslt;e(ebl;)(Sbu) Srufarcfaecedadmamaaggeeooff aa ppiinnmmadade eofobfrbasrsa.ss.



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Finally, an EDS analysis (Energy Dispersive X-rays Spectroscopy) was performed on the discs to determine if material adhesion was on its surface during the sliding tests. Table 4 shows the EDS values to estimate Cu and Zn on the surface of the discs tested with brass pins. The EDS analysis shows similar adhesion values when comparing finishing and roughing surfaces. However, a significant difference is observed when comparing the adhesion of Cu and Zn between nitrided and non-nitrided surfaces. Note that the amount of adhesion of copper-zinc significantly increased in the non-nitrided discs. This behavior can be justified by the compatibility chart of Rabinowicz et al. [29] to justify the adhesion of Cu and Zn on the discs of steel. Zn and Cu have soluble values between 0.1 and 1% and, consequently, have a high tendency to adhere to iron.



Table 4. EDS values of Cu and Zn on the surface of the wore discs for the four type of surface conditions and brass pins.



Sample

Cu (wt.%) Zn (wt.%)



Nitrided & Roughing

0.38 -



Roughing

1.46 0.89



Nitrided & Finishing

0.28 0.22



Finishing

2.02 1.10



4. Conclusions

The present study shows a thorough comparison of the friction and wear capability between two different EDM surfaces (finished and roughened) with and without nitriding treatment. Accordingly, some conclusions can be drawn:

• Nitriding significantly reduced the roughness, considering the arithmetic mean surface roughness (Ra) and the total height of the roughness profile (Rt). The roughing EDM process showed an increase of the surface roughness of 2.1 times in respect to the finishing EDM process. The nitriding treatment decreases 31% of the surface roughness on average.

• The material hardness of the nitrided disc machined with a finishing EDM was 32.5% higher than for the same disc machined with a roughing EDM condition.

• With respect to the pin weight loss, nitrided discs reduce the pin degradation at least between 18.4% and 19.6% for brass and steel, respectively.

• The friction coefficient exhibits lower values for nitride finishing surfaces and higher values for non-nitrided roughing surfaces, independently of pin material. However, for brass pins, significant differences are found for the surface condition (finishing and roughing EDM), and no significant differences are denoted in the disc treated or not with plasma nitriding.

Author Contributions: Conceptualization: D.M.K. and A.J.S.E. Data curation: V.M., A.N.A. and D.P. Formal analysis: D.P. and E.E.Z.F. Funding acquisition: D.M.K. and A.J.S.E. Resources: A.C. Methodology: V.M. and D.M.K. Software: D.P. and H.A.G.R. Supervision: E.E.Z.F. and H.A.G.R. Validation: V.M. and A.C. Writing original draft: A.N.A., D.M.K., A.J.S.E. and A.C. Writing, review and editing: A.J.S.E. and D.M.K. All authors have read and agreed to the published version of the manuscript.

Funding: This work is supported by the National Agency for Research and Development of Chile (ANID)—grant number: 3180006 and the Serra Húnter program (Generalitat de Catalunya)—reference number UPC-LE-304 (2018).

Acknowledgments: We are also grateful to Soledad Pereda and Andrea Romano for helping with Electron microscopy, Alejandro Bacigalupe for analyzing the kinematic viscosity, IONAR S.A. for nitriding the discs, Javier Pouton for metallographic tasks, Alberto Forcato for providing colloidal graphite and Nazareno Antunez for wire EDM.

Conflicts of Interest: The authors declare no conflict of interest.



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© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).