Tribological behaviour of thin a-C and a-C:H films with different topographic structure under rotating and oscillating motion for dry lubrication

Fluid lubrication can hardly be realized for microparts and MEMS components. Therefore, coatings with low friction will be applied. Due to the small size of these parts, only nanofilms are applicable. For further improvement of the microtribological behaviour, we investigated the influence of a geometrical well-defined surface structure. For standard characterization, pin-on-disc tests were performed. Beside the study of the friction coefficient under rotating motion, also, the microtribological behaviour of an areal microcontact was studied. A variation of width to spacing of concentric circles for the pin-on-disc test showed the existence of an optimum contact area resulting in minimized friction coefficients. Microstructures of different shape and depth led also to a reduction of friction. In this case, the resulting depth of the structures played a major role in tribological optimization. Shallow structures (d: 1.5 μm) showed only slight differences in the friction coefficient, while deeper structures (d: 4.5 μm) led to a significant reduction. It turned out that the main effect of reduction was dependent on the lateral size of these structures. Besides investigations on the microtribological behaviour under single asperity contact based on nanoindentation, we used an oscillating test for microstructured areal microprobes. First, investigations on silicon micro-samples sliding against a silicon counterpart showed a linear dependence of sliding distance and resulting wear. Also, a dependence of the friction coefficient on the roughness was observed. Therefore, different roughnesses (polished, Ra=50 nm, 200 nm) were investigated.