Mechanical and tribological evaluation of PVD WC/C coatings

Five different WC/C coatings deposited by physical vapour deposition (PVD) on high speed-steel (HSS) have been evaluated with respect to their mechani cal and tribological properties. For all coatings a chromium layer was depo sited first to enhance coating adhesion. The carbide phase (WC) and the car bon (C) phase were deposited simultaneously by direct-current magnetron spu ttering of a WC target and plasma-assisted chemical vapour deposition using hydrocarbon gas, respectively. The influence of the chromium interface lay er thickness, the amount of WC phase and the how of hydrocarbon gas on the mechanical and tribological properties of the coatings have been investigat ed. The coatings have been characterised with respect to their chemical com position (glow discharge optical emission spectroscopy), hardness (Vickers microhardness), morphology (scanning electron microscopy, SEM), roughness ( profilometry), residual stress (beam bending), critical load (scratch testi ng) and abrasive wear resistance (the "dimple grinder test"). Furthermore, a ball-on-plate test was employed to obtain information about the frictiona l properties and sliding wear resistance of the coatings. The wear mechanis ms and wear debris were analysed by SEM, Auger electron spectroscopy and el ectron spectroscopy for chemical analysis. All WC/C coatings displayed a th ickness between 2 and 4 mu m and a surface roughness in the range of IO to 70 nm. The hardness varied between 1500 and 1800 HV. The coating residual s tress was found to range from -2.5 to -0.5 GPa. The scratch test revealed a relatively high critical normal load, i.e., a relatively good adhesion of the WC/C coatings to the HSS. The abrasive wear resistance was found to be very high, in fact equally as high as that of PVD TiN. In the sliding wear test it could be seen that the coating containing the lowest amount of carb ide phase (WC), i.e., the highest amount of carbon phase (C), and which had the highest compressive residual stress yielded the lowest friction and we ar rate against steel. In addition, this coating was also found to yield th e lowest wear rate of the counter material. In summary, a WC/C coating with overall good mechanical and tribological properties was obtained provided a relatively thin chromium layer was deposited first and if a relatively hi gh acetylene gas flow was utilised during deposition of the WC/C layer. (C) 1999 Elsevier Science S.A. All rights reserved.