1. triboanalysis in industry for pvd-coated stamping...

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Vol.36,No.1(2014)3‐8

TribologyinIndustry

www.tribology.fink.rs

TriboanalysisinIndustryforPVD‐coatedStampingDies

E.Zdravecká

a,M.Martonb,A.Gmiterko

a,J.Tkáčováa

a

TechnicalUniversityofKošice,FacultyofMechanicalEngineering,Letná9,Košice,Slovakia.bSlovak Technical University, Faculty of Electrical Engineering and Information Technology, Ilkovičova 3,Bratislava,Slovakia.

Keywords:

PVDcoatingsWearStampingdiesIndustrialprocessconditions

ABSTRACT

Theapplicationofhardwear‐resistantcoatingsisexpandingintovarioussectors of industry to protect tools and other components from failurecaused by wear and corrosion. However, in forming industry it is stillratheranexceptionthanaruletofindcoatedformingdies.ThisstudyhasobjectivestoinvestigatewearbehaviourofP/MVanadis6ledeburiticsteelwith PVD‐TiN and TiCN coatings. The tests were carried out in thelaboratoryand industrialprocessconditions.Theobtained resultsunderoperatingconditionsshowthattheapplicationofPVDcoatingsappearsasaveryeffectivemethodforincreasingthelifetimeandwearresistanceofcoldstampingdies.

©2014PublishedbyFacultyofEngineering

Correspondingauthor:

EvaZdraveckáTechnicalUniversityofKošice,FacultyofMechanicalEngineering,Letná9,Košice,SlovakiaE‐mail:[email protected]

1. INTRODUCTION

In the manufacturing industry, e.g. in theautomotive industry the on‐line production issteadilyshortened.Stampingdiesareoftenveryexpensive, therefore it is necessary to reducetheir production costs and alternatively toincreasetheirdurability.Formingprocessesarelinked to tribological contact conditions thatoccur between thematerials and dies. Inmanycases the die lifetime can be increased bydepositionof thin andhard coatingsondiebutmechanical and tribological behavior of thecoateddiemustbeoptimized[1‐4].The application of hardwear‐resistant coatingsis expanding intovarious sectorsof industry toprotecttoolsandothercomponentsfromfailure

causedbywearandcorrosion.However,incoldstamping industry it is still rather an exceptionthan a rule to find coated forming dies. Theimprovementsincludecoatingbyaceramicthinfilm, which is produced by physical vapourdeposition(PVD).SeveralPVDcoatingsarenowavailableto increasethetoolwearperformancein boundary conditions that are usuallyencountered during stamping. The basicrequirements are good adhesion to the basesteelinordertocounteracttheshearingstresseswithoutmicrocrackingordelaminatingandhighhardnessinordertoinducelowwearratesandalowfrictioncoefficient[5‐7].Thetraditionalsinglelayerhardcoatingssuchastitaniumnitride,titaniumcarbideandchromiumnitride,depositedbyphysicalvapourdeposition

RESEARCH

E.Zdraveckáetal.,TribologyinIndustryVol.36,No.1(2014)3‐8

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(PVD) or chemical vapour deposition (CVD),cannot meet these requirements in manyapplications, such as blanking of carbon‐steel,because of their insufficient hardness or highfrictioncoefficient.Among advanced coatings the multilayerconcept seems to be the most versatile andpromising with respect to properties andperformanceinalmostallfieldsofapplication.Duetodifferentcontactconditionsatdies‐blankinterfaces,andthewiderangeofappliedcontactpressures,theselectionofthecorrectcoatingisvery important and also complex problem. Toselect the suitable coating the results ofexperimental tests can be helpful. Butwemustconsider fact, that methods for wearinvestigations like the pin‐on‐disk tribometerand the calotest cannot fully reflect the realconditionsofcoldformingprocesses[8,9].Thereareanumberofpublicationsoncomparativediewear studies, however, they do not givequantitativeinformationregardingdielifeunderpractical conditions [10‐12].Wear is caused bysliding and may be characterized by adhesion,abrasion,transferphenomenaandalsobybrittlesurfacemicrocracking.Suchdamageimpairsthequalityofthestampedproduction.Thepresented investigationsareobtained fromthe production of mechanical parts for heatingunits. The stamping dies are made of powdermetallurgy the P/M Vanadis 6 steel, a verypopular tool steel used for different cold workapplications[13].Thestampingoperationtakesplace under constant lubrication. Ourinvestigation is divided into two stages. In thefirststage,bothPVD‐TiCNandTiNcoatingsweredepositedontoVanadis6steel.Thelaboratorytestsgivethebasicinvestigationson the tribologicalbehaviourof coatings. In thesecond stage, considering the results ofexperimental tests it was decided to coat thestamping die segments with PVD‐TiCN coatingand to analyse under operating conditions byopticalmicroscopyandSEManalysis.The aim of this study was to show theimprovement to increase the lifetime of coldstampingdie.

2. EXPERIMENTALPROCEDUREThe objective of the investigation was theoptimization of the lifetime of stamping die inthe production of stamped parts for heatingunits. In manufacturing industry the problemswith failure of stamping die occurred after0.5x106strokes.2.1 InvestigatedmaterialsforstampingdiesThe P/M Vanadis 6 steel is taken as thesubstrateonwhichthecoatingisdepositedandapplied forcoldstampingdie.FortheVanadis6(16MnCr5 EN 10084‐94, EN 84‐70) steel withchemicalcomposition:2.1%C,1.0%Si,0.4%Mn,6.8%Cr,1.5%Mo,5.4%VandtheresidualofFe,thehardnessinasdeliveredstateisapproximatelyHB 255 (HRC 23), and after heat treatment ahardness of HV 750 (HRC 62) can be easilyachieved.Materialofstampedparts iscoldrolledsteel sheet according to Slovak Standard STN 411321(ENDC01,1.0330),thicknessof2.2mm.ColdstampingdieisshowninFig.1.Dimensionsof stamping die segment are given in Fig. 2.a.FatiguefailureofdiesegmentisshowninFig.2b(after 5 x 103 strokes). Microstructure of diesegmentisshowninFig.3.

Fig.1.Viewonstampingdie.

Fig.2a.Stampingdiesegment.


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Fig.2b.Failureofdiesegment.

Fig.3.Microstructureofdiesegment.2.2 Characteristicsofthecoatings

PVDcoatingshaveagreaterpotentialtoreducewear, crack extension and improve the lifetimeof cold stamping die. The study was toinvestigate the wear mechanisms of PVDcathodic arc system TiN and TiCN coated diematerials under laboratory conditions and todiscusscomparativewearproperties.Analysis of both TiN monolayer and TiCNmultilayer coatings (hardness and thickness)was carried out. The thicknesswasdeterminedusingcalotest(CSMballwithdiameter10mm).ThemechanicalpropertiesofPVDTiCNcoatingwere determined by depth‐sensing nano‐indentation under the load of 30 mN, loadingrate 20 mN/min, indentor of Berkowich type–Fig.4.

Fig.4.CalotestofTiCNandnanohardnessload‐depthgraph.

The primary tribological tests were performedon modified Amsler wear machine with thecontactpairgeometryblocks–on–disc(Fig.5).The test block was loaded against the rotatingsteeldisc.Thisprovidesanormalforce700Nforthe contact pair, under 5 x 103 strokes andsliding speed 100 mm/min. Normal load andfriction moment were measured continuouslyusing the measuring system byMATLAB/Simulink(Fig.6) [14].The testblocks(20 x 30 x 6 mm) were made of tool steelVanadis 6 coated with PVD TiN and TiCNcoatings.Thecounterpartcylindricaldiscs(50x10 mm) in contact pair geometry of block‐on‐disc test were made of steel EN ISO 21MnCr5(1.7147) heat hardened, tempered andcarburizedof800HV(62HRC).

Fig.5.ModifiedAmslerwearmachine.

Fig.6.ProcessingthedatabyMATLAB/Simulink3. RESULTSANDDISCUSIONThickness of coatings was determined bycalotestas follows:TiN in the range2,0‐2,5μmand TiCN in the range 2,5‐3,5 μm. Table 1summarizes main mechanical properties oftestedcoatings.


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Table1.Mechanicalcharacteristicsofcoatings.

Substrate/coating HIT[GPa] EIT[GPa]Vanadis6/TiN 20.40 216Vanadis6/TiCN 32.82 320

When TiCN coated blocks were applied thefriction coefficient was reduced to (0.670.73)comparetofrictioncoefficientoftheTiNcoatedblocks (0.810.85). With decrease of frictioncoefficient value in the contact zone of slidingtestspecimensthesteeltransfer(lightcontrast)wasobservedasverylow.Theoccurrenceofwearisattributedtothefourwear mechanisms adhesion, abrasion, surfacefatigue and tribochemical reaction [15]. Thepeeling of the TiN coating was observed, thewear debris were smeared on the coatingsurface (Fig. 7). The transfer and back transferphenomena may occur due to the chemicalreactionswiththeenvironment,oxidationofthesurfacesandthetransferredmaterial.

Fig.7.WeardebrissmearedonTiN.The wear type was predominantly of an abrasivenature for TiN coated blocks. Adhesive weargenerateweardebriswithTiNmicroparticleswhichcausesfurtherwearbytwoorthreebodyabrasion.ThesomeveryfinescratcheswerefoundontheTiCN surface (Fig. 8). The polishing of thefunctional surfaces was the dominantmechanism of wear. It was demonstrated that,the TiCN coating showed a slight degradation.Duetoitshighhardness,PVDTiCNcoatingwithincreased thickness compare to TiN improvesthe adhesive and abrasive wear resistance ofsurfaces.BasedonresultsofexperimentaltestsitwasrecommendedtoapplyPVD‐TiCNcoating

on thestampingdiesegments.Notablyminimalwear performance was observed for all coatedblockswithPVD‐TiCNunderslidingconditions.

Fig.8.ThewornTiCNsurface.Therefore, in this study an attempt is made toinvestigate the effect of PVD TiCN coating toindustrialapplicationonstampingdies.Insecondphase,thesurfacesuncoatedstampingdie segmentandcoatedwithPVD‐TiCN underoperatingconditions in industrywereexplored.In Fig. 9 the surface of uncoated die along theradiusafter0.5x106strokesisshown.Primarilythe adhesive wear occurs under the contactpressure between sliding surfaces. Abrasivewear,however,occursbetweentwohardslidingsurfaceswhenharddebrisparticlesareindentedandmake grooves in the sliding surface of thematerial. In the area of the radius cracks aredeveloping during the stamping operationshowing the linear striations visible orientedparalleltoslidingdirectiononthesurface.

Fig.9.SEMmicrographofuncoateddiesegment.


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Figure 10 shows carbide particles of the substratematerialprotrudingonthesurfaceofdie.

Fig.10. SEMmicrograph of cross‐section of wornuncoateddiesegment.The TiCN coated die segment surface after1.8x106strokesisshowninFig.11.

Fig. 11. SEMmicrograph of worn TiCN coated diesegment.SEM cross‐section analysis (Fig. 12) of wornsurface showed good adhesion of the TiCNcoatingtosubstrate.Creating the best die for a stamping operationinvolves analysing tool steels to find one thatprovides the proper balance of wear, strength,and toughness characteristics for a particularapplication. Whatever grade of tool steel isselected must be heat‐treated properly tocapitalizeonthosequalitiesandprovideoptimalresults in production. A variety of surfacetreatmentsalsoareavailabletoincreasesurfacehardnessandwearresistancewhilereducingthecoefficient of friction of tool steels, helping to

prolongdielife.Understandingtheavailabletoolsteeloptionsisthefirststepinachievingqualityresults.

Fig. 12. SEM micrograph of cross‐section of wornTiCNcoateddiesegment.The tribosystem i.e. the contact of sheet metaland the die is affected by several tribologyaspects, including microgeometry andmechanical properties of dies materials.Although themechanicalproperties of stampedmaterials are important for forming processes,the final behaviour is determined by the ratiosbetweenthestampedmaterialanddie.TheaveragehardnessandelasticmodulusoftheTiCN coating are about 38.82GPa and 320GPa,respectively. For the TiN coating, the averagehardnessandelasticmodulusdecreasedto20.40GPaand216GPa.Theresultsareconsistentwiththe literature [16]. The higher hardness of theTiCN coating compared to TiN may be partlyattributedtothesolidsolutioneffectofcarboninthe TiN lattice. Due to its high hardness, PVDTiCN coating can improve the adhesive andabrasivewearresistanceofmetalsurfaces.Obtained results under operating conditionsshowed that TiCN coating on the surface ofstamping die improved the lifetime four‐timescomparetotheuncoateddie.4. CONCLUSIONConsidering the results of experimental tests itwas decided to coat the stamping die insertswith PVD‐TiCN coating and to analyse underoperating conditionsbyopticalmicroscopyandSEManalysis.


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TiCNcoatingsweredepositedontomechanicallypolished P/M Vanadis 6 substrates by cathodicarc method. The results obtained can besummarizedasfollows: Ceramic coatings like TiCN improve the

wear resistance of the system because ofhighhardness,goodfrictioncharacteristics,dense coating morphology and improvedadhesionproperties.

Wear damage of the TiCN coatings isminimal. Good adhesion between coatingand substrate does not allow the coating tobedelaminatedeasilyandremainingcoatingcontinuestheprotectivefunctiontothelast.

The multilayer structure of TiCN provides aneffective barrier to fissure propagation causedby dynamic stress, inhibiting surface fracturesfrom propagating to the die and/or to wear ofdie. Application of PVD TiCN coating on diesurfaceresultedinimprovedqualityofstampingoperation, the lifetime of TiCN coated die wasincreased four‐times, wear and stickingproblemswere reduced and also eliminationoflong‐lastingandexpensiveproductionfailures.AcknowledgementThiscontributionwascreatedundersupportoftheGrantAgencyoftheMinistryofEducationofthe Slovak Republic through the projects VEGA1/0264/11andAPVV‐0365‐12.REFERENCES[1] M. Wieland, M. Merklein: Wear Behavior of

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