Seite - 45 - in 3D Printing of Metals

Metals 2016,6, 284 Figure7.XPSsurveyspectra for (a) the20mmEBM-builtTi64sampleand(b) theTi64powder. Figure8.XPSspectraof the (a)Ti2ppeakand(b)O1speakonthesurfaceof the20mmEBM-built Ti64sample. 3.3.DrySlidingWearBehaviour Therepresentativecoefficientof friction(COF)curvesaredisplayedinFigure9a. Similarwear characteristicswereobserved, regardlessof thedifferentmicrostructuresexisted inside theEBM-built andtheas-cast samples. In the insert showninFigure9a, it isnotedthat thewearcharacteristicsof the Ti64samplecanbeclassifiedinto fourmajorstages: (I) Wearing-in period,where the friction coefficient increases rapidlywhen the ball is in direct contactwith theoxide layerofsamplesurface. (II) Cushioningof theoxide layer,which ischaracterizedbyaslightdecrease inCOF,probablydue tosurfaceoxide thatpreventsdirectmetal-metal contactbetweentheballandtheTi64sample. (III) Breakageof theoxide layer into fragments,whichwas indicatedbyasignificant increase inCOF. In this stage, theoxidedebrisgeneratedwere fragmentedasaresultof repeatedcyclesof stress andloadduringsliding. This, coupledwith thecompleteremovalof theoxide layer,allowsthe directmetal-metal contactand,assuch,causes the increase inCOF. (IV) StabilizationofCOF,whichoccursafter~10,000 laps. 45