Page - 47 - in 3D Printing of Metals

Metals 2016,6, 284 Theglobalwearrate (Wglobal)ofaTi64samplecanbedepictedbythecombinationofoxidative wear (Woxidative)anddelaminationwear (Wdelamination)andisgivenby[16,26,27]: Wglobal=Woxidative+Wdelamination= ArC2A vzcexp ( − QRTf )+kFN H (1) whereAr is therealareaofcontact,C is thematerialconstant,A is theArrheniusconstant foroxidation, v is theslidingspeed,zc is thecritical thicknessofoxidefilm,Q is theactivationenergyforoxidation, R is themolar gas constant,Tf is theflash temperature, k is thewear coefficient,FN is thenormal load, andH is thehardness. Figure 11a,b reveal theSEMimagesof theweardebris and thewear trackof the0.5mmsamplerespectively. The indicationof the transfer layersandoxideparticles in Figure11aand thesurfacecracks thatareperpendicular to theslidingdirection inFigure11bgive avaluable insight into thewearmechanismofTi64,which is consistentwith theglobalwear rate proposedbyAlMolinarietal. [16].Nevertheless, asimplecalculationof the theoreticaldelamination wear rate using the sample’s respective averagehardness value obtained from the earlierVickers hardnessmeasurementasshowninTable1,evenwithoutconsiderationofoxidativewear, revealed ahigherwearrateascomparedto theexperimentalobtainedglobalwearratevaluebytwofactors. Thismaybeowningtotheformationofatribo-layerthatavoidsthedirectmetal-metalcontactbetween theasperitiesof thesteelballandtheasperitiesof theTi64sample, therebypreventingfurtherwear throughsliding. Figure11. (a)SEMmicrographof the transfer layersgeneratedbyslidingonthe0.5mmEBM-built sample; (b)SEMmicrographof thewear trackonthe0.5mmEBM-built sampleafterwear test; (c)EDX spectrumandinsertedXRDpatternsof the transfer layersand(d)EDXspectrumandinsertedXRD patternsof theoxideparticles thatweregeneratedduringslidingonthe0.5mmEBM-built sample. Thedetection locationsof (c) and(d)were indicated in (a). 47