Influence of surface topography of HF-CVD diamond films on self-mated planar sliding contacts in dry environments

Surface roughness of self-mated CVD diamond coatings have a decisive effect on coefficient of friction (COF) and wear especially under dry ambient conditions. Therefore diamond films with four different grain size domains were deposited onto silicon carbide front faces and tested in unlubricated planar sliding contacts. In accordance to the different growth morphologies, the roughness of the coarsest diamond films is about four times higher in average than that of the nanocrystalline films. The diamond film quality, expressed in terms of sp3-hybridised diamond fraction, decreases from nearly 100% to about 73% with falling grain size due to an increase in grain boundaries. The topography of the respective diamond coatings differs also considerable. While the coarse-grained films exhibit a pronounced wedge shape and a strong two-waviness, the fine-grained diamond films display a good flatness with a maximum profile height of only 280 nm. All results obtained from the dry sliding tests are visualised in a tribo map. The best tribological behaviour in terms of low COF and little wear is achieved by the fine-grained diamond coatings. The worst results reveal the nanocrystalline films with Cauliflower structure, mainly due to the highest number of local film delaminations in this test series. For high-end sealing applications, smooth fine-grained microcrystalline CVD diamond coatings therefore seem to be the most promising approach.