Thm Sohd Fdms, 96 (1982)93-101 METALLURGICALANDPROTECTIVECOATINGS93 ABRASI VEWEAROFI ON- P L AT E DT I T ANI UMNI T RI DE COATI NGSONP L AS MA- NI T RI DE DSTEELSURFACES*E. H. SIRVIOANDM.SULONEN LaboratoryofMetalFormmg andHeatTreatment,HelsmktUmversttyof Technology,Vuorzmlehentte 2 A, 02120 Espoo12(Finland) H.SUNDQUIST Metals Laboratory,TechnicalResearchCentreofFmland,Revontulentle7, 02100 Espoo10 (Fmland) (Received Mar ch 9,1982,accepted Aprd5,1982) Theabr asi vewear of i on- pl at edt i t ani umni t ndecoat i ngsonhar denedt oolsteelandonhar denedandpl asma- ni t r i dedt ool steelwasstudied.Wear testswere madewi t hadr ys a nd- r ubbe r wheelset-up, andtheinitialsurfacer oughnessof the speci menwasoneofthetest par amet er s.Awear mechani s mforhar dcoat i ngsonsoft ersubst r at esunder abr asi vewearcondi t i onswassuggested. Thewear initiatesonthet opsof t heasperi t i eswherethe basemat er i al is exposedfirst.Wear pr oceedswi t hthef or ma t mnof crat ersat these sites andt he gr owt hofthese cr at er sby fract uresontheridges f or med by thecoat i ng mat eri al .The wearr at e ofcoat ed steel i ncreases wi t h i ncreasi ng initialsurface roughness.Aconsi der abl ei ncreaset akespl acewhenthescaleofthesurface r oughnessis of the sameor der ofmagni t udeasthecoat i ngthickness. Thi si ncreasecanbeshiftedtoa hi ghersurfacer oughnessifasubsurfacehar deni ngprocess, inthiscasepl as ma mt rl di ng, is used. 1.INTRODUCTION Ha r d t i t ani umni t ri decoat i ngshaverecent l ybeenusedsuccessfullyaswear- resi st ant coat i ngsfor met al cut t mgandf or mi ng tools. Thedevel opment ofpl asma-enhanceddeposi t i onprocesses has madeIt possi bl e to deposi t t i t ani umni t ri deont o t ool steelsusi ngl owdeposi t i ont emper at ur es 1'2.I nmet al cut t i ngma n y wor k mat er i al s cont ai npart i cl esofphases whi ch have a muchhi gher har dnesst hant hat ofthewor kpl eceasawhole.Ma nyof thesesecondar ypart i cl esarehar der t hanthe mat r i xmhi ghspeedt ool sandret ai nt hei rhar dnesstohi ghert emper at ur es. Such mat er ml sint hewor kmat er i al ma ycauseabr asi on, especi al l yonsteeltools, undercondi t i onsofseizureaswellasunder slidingcondi t i ons, andmmos t publ i shed paper soncut t i ngt ool wear t he maj or i t y of fl ankwear,inpart i cul ar, is at t r i but edt o abr asi on 3. * Paper presented at the Internatmnal Conference on Metallurgical Coatings and Process Technology, San Diego, CA, U S A,, Aprd 5-8,1982 0040-6090/82/0000-0000/$02.75OElsevier Sequola/Pnnted In The Netherlands 94E,HSIRVIO, MSULONEN, H.SUNDQUIST TltanIum-nltrlde-coatedhighspeedsteeltwistdrillshaveshownimproved wearresistanceespecially whenhardabrasivesteelisusedasaworkpiece 4' 5. Also, whenductileironcontainingalargeamount ofhardparticlesandahighly work- hardenedmatrixismachined,titaniumnltrldecoatingshaveprolongedthelifeof high speed steel wedge cutters,spur cutters,hobs andthread cutters 6. Because of the abrasive wear resistanceof titaniumnitride it has also been used asahardcoatingforwatchcasesInthisapphcatlonaverythincoatingisused which requiresa hardenedsubsurface, e . g . a hardchromium-plated base material ~. 2ABRASIVEWEAROFTHINHARDCOATINGS Abrasionisawearprocessinwhichdetachmentof wearparticlesfromthe surface may occur by ploughing, microchipping or fracturing 7. During ploughing a highhydrostaticstresscomponent isimposedonthewearsurfaceinfrontofan abrasiveparticleenablingplasticdeformationevenintypicallybrittlematerials. Plastic deformation also extends into the subsurface because of the cumulative effect of strainhardeningandwear.Inasurface coatedwithathinhardlayer subsurface plasticdeformationmayreadilytakeplaceandbrittlefractureof thecoating may occur. Whenhardthincoatingsareconsideredasprotectionagainstabrasivewear, the initialsurface roughnessof thespecimenis important.In two-body as wellas in three-bodyabrasionthewearbeginsatthetipsof theasperitieswheretheloadis highThe running-in of the surface proceeds ata high rateuntilan equilibriumstate is reached,andthesurfaceroughnessbecomestypical fortheabrasionparameters used,i . e. load,sizeandshapeof theabrasiveparticles,materialsinvolvedetc.The wear rate is thenlower thanthatinitiallyandremains constant. Ifthedimensionsoftheinitialsurfaceroughnessareofthesameorderof magnitude as the coating thickness,the coating wearsoffat the tipsof theasperities revealing the base material.The wear rateof the softer base materialIS higher, anda cratersurroundedbysharpedgesof thecoatingmaterialisformed.Consequently theseridgesform sharpnew tipswitha highload concentration andareabradedor fracturedoff; thus,the size of the wear crater continues togrow quickly. If theinitialsurfaceroughnessisconsiderably lessthanthecoatingthickness, thewearof thetipsdoesnotrevealthebasematerialduringtherunning-inperiod. Consequently,thewearrateofthesurfacewillbecharacteristicofthecoating materialused,andforhardcoatingsthiswillbemarginalcomparedwiththewear rateof the base material. Duringploughingabrasion,theplasticdeformationof thebasematerialmay cause cracks inthecoating andcrater-type wear may takeplace.A thirdpossibility for thisrapidwear is the existenceof imperfections inthe thinhardcoating or poor adhesion of the coating tothesubstrate. Hardeningtreatmentspenetratingtogreaterdepthsthanthecoatingmaybe usedasapretreatmentbeforecoatingtopreventplasticdeformationofthe subsurface.Plasma nltrlding is suggested as a suitablepretreatmentas it can be done mthe same equipmentas the deposition In thisworkaninvestigationontheabrasivewearof titaniummtrldecoatmgs on tool steelspecimens was madeusing a dry sand-rubberwheeltest 8. Themm was ABRASIVEWEAROFION-PLATEDTiNITRIDECOATINGS95 tostudytheeffectoftheinitialsurfaceroughnessontheabrasivewearofthin coatings.Also aseriesof spectmenswerepreparedtostudytheeffect of subsurface hardening,by using plasma nitridlng as a pretreatment. Thereportedhardnessesof thequartz sandused in thesand-wheeltestsvaries between800 and1200 HV,andthe hardnessof ion-plated titaniumnitridecoatings between2000 and3000 HV.Hencetheratioof thehardnessof thewearsurfaceto thatof theabrasivevaries from1.7 to3.5, andavery low wear rate is to be expected as long as thebase materialis not exposed. 3.EXPERIMENTALPROCEDURE 3.1Preparation ofthe coatmgs A reactive ion plating process with a discharge supported by a negative filament wasusedtoproducetitaniumnitridecoatings 1. Titaniumwasevaporatedwithan electronbeamgunatanevaporationrateof0.8 g min~.Thechamberpressure during coat mg was maintainedat5 mTorr.The atmosphere consisted of argon and nitrogengases.Thecathodevoltagewasbiasedafewhundredvoltsnegative,and theioncurrentwas250 mA.Thetotalareaof thespecimenscoatedwas120 cm 2, gtvinganaveragecurrentdensityofabout 2 mA cm2.Thetemperatureofthe specimenduringthecoatingwascontinuouslymonitoredandremainedbelow 280 C.Figure1 showsascannmg electronmicrographof theresultantcoating on anedgeof afracturedspecimen.Plasmanltridingwasmadeinthesamechamber usingthelow pressureplasmanltrldingprQcess 9. Thepressureusedwas40 mTorr,theatmosphereconsistingofnitrogenandhydrogengases.Thespecimenswere biased4 kVnegative,givingacurrentof90 mAandthusacurrentdensityof 0.75mAcm2.Undertheseconditionsthetemperatureofthespecimenswas maintamedat 450 C, andthe nltridlng process was conducted for 4 h. The resulting diffusionzonewasabout 50 ~tm thick,ascanbeseeninFig.2whichisanoptical micrograph of the surface. F~g1Scanning electronmJcrographof anIon-platedt~tanlumintrudecoatingonhardenedtoolsteel (speclrnen1 3) 96E. H. SIRVIO, M.SULONEN, H.SUNDQUIST Fig.2Opticalmlcrographof aplasma-nltrldedandtltanlum-nltrlde-coatedtoolsteelsurface(Nltal etched, specimen 3 5) The test specimens were made of AISI D3 tool steel in a hardenedand tempered (450 C)condition.Threedifferentsurfaceroughnessesweremachinedforeach case. The R a values of the surface roughnesses used varied from 0.06 to11.6 gmThe propertiesof thespecimens and the wear testresultsaregiven in TableI. 3.2.Weartests Figure3isaschematicIllustrationof thetestequipmentusedTheabrasive (quartzsand)isintroducedbetweenthetestspecimenandarotatingwheelwhich has a chlorobutyl rubber rim with a hardnessof Duromet er A-60. The testspecimen as pressed against the rotating wheel with a force of 130 Nwhile a flow of sand (about TABLEI PROPERTIESOFTHETESTSPECIMENSANDWEARTESTRESULTS SpecimenTreatmentBulkSurfaceWearRelative numberhardnessroughness(mg)wear (HRC150)Ra (~tm) 1 1Coated5495310 23 125 7 118006 135 8 00 6 6004 21Plasma539 51210 90 2 2nltnded55!.3970 72 2 3540 061070 79 31Coatedand57116620 46 3 2nltrlded573 5370 27 3 3561 4340 25 3 4541 4320 24 3 55800724018 3 6580 07310 23 ReferenceUncoated565801-121351 samples ABRASIVEWEAROFI ON- PLATEDTi NI TRI DECOATI NGS97 HOPPER QUA R]ZSAND WEIONTS RUBBERLINEDWHEEL Fig3Schematic view of the set-up used for the dry sand-rubber wheel wear test 300gmm-1) a b r a d e s t het est sur f ace. The s pe c i me ns wer ewe i ghe dbef or ea n d af t ert het est , a n d t hel ossmma s s was r e c o r d e d .Th e t es t s wer ema d e a c c o r d i n g t oa nAS T Ms t a n d a r d8wi t hs o me modi f i -c a t i o n s The d~fferencesf r oms t a n d a r d p r o c e d u r e s wer easf ol l ows( t heval ues oft he c o r r e s p o n d i n g s t a n d a r d t est p a r a me t e r s a r ei nbr acket s ) . The d i a me t e r oft he r u b b e rwheel was 185mm(228. 6r am) a n d t hen u mb e r ofr e v o l u t i o n s us edi ne a c ht est was 420(100).Thi s gi ves as l i di ng d i s t a n c e of 244m(718m). Th e f or ceont hes pe c i me nwa s t hes a me ast ha t speci f i edi nt hes t a n d a r d , i.e. 130N. Th e si zeoft heq u a r t z s a n d pa r t i c l e sus edvar i ed, ma i n l y (80%)b e t we e n si eve si zes50a n d 150c o r r e s p o n d i n g t od i a me t e r s f r om300t o100I~m 4RESULTS 4.1.Wear o f t i t amummt rl decoatings Fi g u r e 4s hows ap h o t o g r a p h of t het es t eds peci mens . The p h o t o g r a p h wast a k e n wi t hanmc l i n e d hght s our c e a n d t hewe a r t r a c k isc l e a r l yseen. The b r i g h ta r e a s int hewe a r t r a c k a r e t hea r e a s whe r e t he c o a t mg ha s beenwo r n t hr ough. As la)(b)~c) Fi g4We a r t r acks ont het est edh t a n i u m- mt r l d e - c o a t e d s pe c i me ns(a)11, (b)1 2a nd(c)1 3 9 8 EHSIRVIO,M.SULONEN, H, SUNDQUI ST Fi g5Sc a nni ng el ect r onml c r ogr a phoft hewear surfaceof s peci men11Coa t i ngIsstillpr es ent att he sites of t heval l eys of t he m~nal sur f ace canbeseen,coating1.3hashardlybeenwornthrough,andcoating1.2hasworn through on the topsof thehighest aspentles.Coating1.1 has been worn through on the tops of the asperities in a large area of the wear track. In spite of th~s all the valleys of them~tlalsurfaceretainthecoating,ascanalsobeseenmFig.5 whichisof the same surface. The new surface profile obtained as a result of abrasive wear is a mirror ~mage of theoriginal,astheinitialvalleys havechangedtohpsof newasperitieswithhard coating on the top.On the s~te of an ininalt~p there~s now a valley. Thewearof thetestspecimensandtheirrelanvewearcomparedwiththatof untreatedspecimensispresentedinTableI.Thewearofcoatedsampleswas between4% and23% of thatof the untreatedspecimens ThetotalwearincreasedconsiderablywhenthesurfaceroughnessR,was increasedfrom1 to10 lam.Thisisexpectedaccordingtothewearmechanismof hardcoatings proposedearherWhenthe surface roughness ~s 10 gin, thethickness of the coating (6 gm) is exceeded andthe wear rate is h~gh where the base materialis revealed.In thisspecimenmostof the wear (31 rag) iswear of tool steelbeneaththe initialasperitiesWhenthesurface roughness is less thanthe film thickness,wear ~s moremoderate,anditseemsthatevenwiththeselowroughnessesacoatingona smoother surface gives betterwear protection. 4.2Wear ofplasma-mtrlded surfaces Thesurfacehardnessof plasma-nitndedhardenedtoolsteelsurfaceisabout 1000HV,i.e. aboutthesameasthatof theabrasivesand.Thusthetreatmentdoes not give fullprotection against abrasive wear. Nevertheless, the increase in hardness lowersthewearrateatthebeginningof thetest,asthewearof metalsisinversely proportional to their hardness. This had only a slight effect on the wear results in this study,asthethicknessof thediffusionlayerwasonly50 ;amandwaspenetrated during the test.The wear of allthe plasma-nltridedspecimens was between70% and 90%of thatof theuntreatedspecimens,andthewearscar wassmooth withoutany differences dueto the initialsurface roughness ABRASIVEWEAROFION-PLATEDTtNITRIDECOATINGS99 4.3Wear oftitanium nitrtdecoattngson plasma-mtrtded steel Tabl eIshowst hat inthisst udythewear ofsteelspeci menscoat edwi t h t i t ani umni t ri deandpl as mani t rl dedwashi ghert hant hat of speci mensonl y coat ed wi t ht i t ani umni t nde. Thi sbehavi our wasf oundt obeduet othedifferencesint he st ruct uresoft hecoat i ngsonhar denedandonpl asma- ni t r i dedsteel.Fi gur e6 shows scanni ngel ect ronmi cr ogr aphs of Vi ckersi ndent at i onsmadeoncoat i ngsforthe t wocases.Itcanbe seent hat thecoat i ngonthepl asma- ni t r i dedsurfacehasal arger grai nsizet hanthecoat mgontheunt r eat edsurface.It isal somor ebri t t l easitis cr acki ngattheedgesoft heVi ckersi ndent at i on. Thel argergrai nsizecanbe expl amedasduet othehi ghert emper at ur e(450 C)at thebegi nni ngofthecoat i ng processor i gi nat i ngf r om t hepr el i mi nar ypl as ma ni t rt di ngprocess.(a)(b) Fag6Scanningelectronmlcrographs ofVlckerslndentataonsmthesurfaceof specimens (a)1 3and (b) 3 5 105 LU ~ 0 0 SQ:: 001 Fag7 JrI~ x l - - " "@ i I I _ _ _ _ ~ L ~ t01051510 SURFACEROUGHNESS,Ra, lam Relattvewear of thetestedspecimens asafunctionof theinitialsurfaceroughnessx, plasma mtrlded,C), tltamummtrldecoated,0, bothtreatmentsThewear is comparedwiththatof untreated AISI D3steel mahardened condmon 100E. H. SIRVIO, MSULONEN,HSUNDQUIST Whenalltheresultsof thisstudyareplottedinagraph(Fig7) itcanbeseen thatthe increase in wear of the tltanlum-nltrlde-coatedsamplestakes place at lower surface roughnesses thanwhenthe base materialhas been plasma nltrlded. Consequently, it is suggested that,if the specimens were cooled after the plasma nltndlngprocessandbeforecoating,thewearresistanceof thecoatingcouldbe furtherimproved,particularlyif thesurfaceroughnesswereof thesameorderof magnitude as the coating thickness. 5CONCLUDINGREMARKS TheabrasivewearofhardthincoatingsisclearlydependentontheInitial surfaceroughnessif thecoatingissubstantiallyharderthantheabrasivegritThe wearof titaniumnltrldecoatings onsurfaces witharoughness of thesameorderof magnitude as the coating thickness canbe improvedby subsurface hardening of the base material,if the coating structureandadhesionare thesame Abrasive wear of hard coatings on softer substratestakes place according to the suggestedwearmechanism.Wearstartswiththeformationof acraterof exposed basematerial.Thegrowthintheareaofthecraterproceedsbyfracturesofthe coatingattheridges,andthegrowthinitsdepthbyconventionalabrasivewear ThisisseenmFig.8, whereascratchby anabrasiveparticleextendstotheridgeof thecratercausing fracturesatimpactHowever, nowear groove hasformed onthe coatingunderthetrackofthisabrasweparticlebeyondthecrateredge,andno ploughing or mlcrocuttlng abraswe wear hastakenplace Fig8Trackof abrading asperityon exposed base materialThe particle has impinged on the edge of the wear crater,~ethe coating, causmg fractures and deformalton ACKNOWLEDGMENTS Theauthorswishtoacknowledgethecont nbuUonofD.GTeerand AMatthews,whodevelopedtheionplatingprocess usedhere.Thankarealsodue toAKorhonen for invaluable discussions ABRASIVEWEAROFION-PLATEDTiNITRIDECOATINGS101 REFERENCES 1AMat t hewsandDGTeer,ThmSohdFt l ms, 72(1980)541-549 2AKSun,RRNl rnmagaddaand RFBunshah, ThznSohdFtlms, 64(1979)191-203 3EMTrent,Treatise Mat erSctTechnol,13 (1979) 443-489 4RBuhl, HKPul ker andEMoll, ThmSohdFdm~, 80( 1981) 265- 270 5RRNlmmagadda, HJDoerrandRRBunshah, ThmSohdFtlms, 84 (1981)303-306 6HSundqmst,EH.SlrvloandMKurkmen,ProcConf onlon-asststedSurfaceTreatments- Techmques and Apphcattons,Warwtck, September1982 7K-HZum Gahr,Wear ofMatertals1981,American Society of Mechamcal Engineers, New York, 1981,pp396405 8A S T MSt andG65-80,1980 9ASKorhonen and EHSlrvlo,ProcInt.Conf on Metallurgtcal Coatmgs and Process Technology, San Dtego,1982,inThin Solid Ftlms, 96 (1982)103