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AbstractTheresidualstressstateofcoatingsinfluencesthehighhardnessandwearresistancewiththermalstabilityenablecoatingstoincreasetoollifesignificantly[1,2].Thethermo-mechanical loads are process-specific. Thus, effi-cient coatings have to be designed inpidually withadjustedpropertiesforthecaseofapplication.Inpreviousworks approaches of the coating properties thickness [3],hardness [4] and residual stress [5–7] related to the per-formance of PVD-coated tools have been investigated.PostiandNieminen[3]foundthatwearresistanceincrea-ses with increasing coating thickness. According toBouzakisetal.[4],coatingswithhigherthicknessesexhibitlower compressive residual stresses and a reduced hard-ness. With increasing compressive residual stress up to acriticalvaluetoollifeincreases,shownbyKlockeetal.[7].Among the composition and the structure of coatings theresidual stress influences wear behavior and thus the per-formance of cutting tools. High compressive residualcoatings stress leads to an increasing wear resistance ofcoatings but also can decrease the adhesion betweencoating and substrate and weaken the structure of thecoating[8,9].However,accordingtoVoglietal.[10]anincrease of compressive residual stresses in the substrateincreasesadhesion.Itcanbepointedoutthatadhesiveandcohesive damage processes in the coating-substrate-inter-faceareinfluencedbytheresidualstressesinthecoatingsand can promote or prevent coating failures [11]. 61724
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Spruteetal.[12]showedthat,dependingonthepretreatmentandthe residual stresses of the substrate, the residual stressstates and gradients in the coating change. Also coatingsresidual stresses and properties are affected by the layerstructure and the design of multilayer coatings. Quantityandcompositionoflayerschangeadhesion,toughnessandhardness of coatings as well as their residual stress statesandgradients[13,14].Inthesestudiestheoptimizationofcoatings residual stress state with respect to different1 Introduction& B.Richterrichter@ifw.uni-hannover.de1InstituteofProductionEngineeringandMachineTools(IFW),LeibnizUniversita¨t Hannover,AnderUniversita¨t 2,30823Garbsen,Germany2OerlikonMetaplasGmbH,AmBoettcherberg30-38,51427BergischGladbach,Germany123 tool life and performancein machining significantly. Duetothermo-mechanicalloadsduringthecuttingprocessthecoatings require tailored properties. Beside the coatingstructurethetwomostimportantpropertiesaretheresidualstress state and the chemical composition of the coating.Therefore, the influence of these two properties is inves-tigatedinthisstudy,comparingthecuttingperformanceincontinuous and interrupted cutting of 42CrMo4 (AISI4140)andTi–6Al–4V.Itisshownthattheresidualstressstates of the coating close to the surface and close to thesubstrate are important for the wear behavior. High com-pressiveresidualstressesnearthesubstratecombinedwitha material-optimized composition increase the resistanceagainstchipping.Flankwearresistanceincreaseswithhighcompressive residual stresses near the surface anddecreasingstressestowardsthesubstrate.Hardcoatingsdepositedbyphysicalvapormethods(PVD)areappliedtocuttingtoolswithincreasingapplications.Incombination with tailored cutting edge microgeometries, 148Prod.Eng.Res.Devel.(2016)10:147–155Table1
ClassificationofthecoatingsCompositionAlTiN1AlTiN2AlTiN3GroupAThickness(lm)Hardness(HV)4.23.73.9350024003600CompositionAlCrNCAlCrNTGroupBResidualstressesThickness(lm)Hardness(HV)Compressive4.9Tensile4.922001900CompositionAlTiVXNGroupCThickness(lm)Hardness(HV)3.41200CompositionTiAlSiXNTiAlSiXNGroupD2 ExperimentalLayerDual4.0Multi3.4Thickness(lm)Hardness(HV)2.1 Classificationoftheusedcoatings39002500CompositionAl O32GroupEThickness(lm)Hardness(HV)13.11100Table2 Mechanicalpropertiesofmaterials42CrMo4Ti–6Al–4V505055456733TensilestrengthRmYieldstrengthRp0.2ElongationA(%)Hardness(HV1)(MPa)(MPa)10339058481267926162253702.2 CuttingexperimentsIn this study, external continuous and interrupted turningexperiments without cooling were performed on a Gilde-meisterCTX520lathe.Asworkpiecematerials42CrMo4and Ti–6Al–4V were used. The mechanical properties oftheusedmaterialsaresummarizedinTable2.Hardnessofthe materials is the average of four measurements afterVickers with a spherical diamond-tipped cone of 120°angle. The chromium and molybdenum containing steel42CrMo4 is widely used, e.g. for automotive crankshafts.Tool wear is dominated by crater wear [17, 18]. Ti–6Al–4V is used for advanced applications in aerospace andautomotiveindustries[19,20].Inmachining,thermaland23123 cutting conditions like interrupted cut was subordinatelyconsidered.Duetotheknowninteractionofresidualstressandwearbehavioranoptimizedresidualstressstateanditsdepth distribution can increase the performance of coat-ings.Thereforetheresidualstressstateofmoderncoatingswhichexhibitacomplexstructurewithstacksofalternat-ingsublayersanddifferentcoatingcompositionshastobecharacterized with depth resolved information. A possi-bility for non-destructive depth resolved residual stressdetermination of coatings as well as the substrate subsur-face is the scattering vector method by Genzel [5, 15],which is used in this paper as well. The influence of theresidual stress state and its depth distribution of coatingswithvariouscompositionsindifferentcuttingconditionsisinvestigated in this paper. To determine the significantinfluences of residual stresses on cutting performance,hardness,thermalpropertiesandespeciallythestructureofthecoatingsaresubordinatelyconsidered.Nine different coatings were investigated. The coatingswere classifiedinfivegroups accordingto theircomposi-