Influence of substrate material and topography on the tribological behaviour of submicron coatings

Submicron diamond-like carbon films (DLC) were prepared on substrate materials with different Young's modulus (silicon, glass, polymers) by PACVD using a Ar/C 2H 2 gas mixture at temperatures below 150 °C. The microtribological behaviour of these thin films were investigated by different methods. For wear evaluation we used a tape ball-cratering test, that was especially developed to characterise very thin films ( d∼100 nm). The friction coefficient was measured by pin-on-disc tests with spherical surfaces of 4.76 and 10 mm diameter as well as tests under single asperity contact using AFM-methods, where the applied load resulted in areal pressures of 1 and 10 GPa, respectively. In case of a sandwich-like system consisting of hard coatings and soft substrates a plate bending effect occurred, which caused an increase of friction for softer materials for spherical pin-on-disc test. In contrast to this, single asperity investigations by means of AFM methods led to a considerable decrease of the friction coefficient with decreasing Young's modulus of the substrate. Friction coefficients as low as 0.05 have been measured for DLC coatings. The same tendency was found by a reduction of the tip radius of the indentor, which resulted in a reduced contact area. On the other hand, the wear resistance could be increased considerably by use of soft substrates. It turned out that the wear resistance of DLC on SU8 photoresist was nearly doubled in comparison to silicon. As a result of this investigation we will show that even polymeric microparts, which can be fabricated at a low cost scale, can be tribologically optimised by use of ultra-thin coatings. This rises important perspectives for future use of ultra-thin coatings for micromachines and micro-electromechanical system (MEMS) applications.