Tribological and structural properties of amorphous B-N-C coatings

Amorphous BxNyCz:H films were prepared in a capacitively coupled r.f. PACVD reactor at deposition temperatures <200 degrees C starting from CH4-N-2-B2H6 (10% diluted in H-2) gas mixtures. The composition (B,N,C,H) was analysed with electron probe microanalysis and elastic recoil detection. Tribological film properties (wear and friction) were investigated by ball-on-disc testing under dry and humid N, atmospheres. Hardness and Young's modulus were determined by depth-sensing indentation measurements. It will be shown that among the wide range of compositions investigated, the pure amorphous carbon films (a-C:H) remain unsurpassed with respect to wear and friction behaviour. The addition of B and/or N to a-C:H films reduces their hardness and wear resistance, while the friction coefficient remains low (0.2) and increases for BN-rich films (> 0.5). As an exception, a striking similarity is found between a-B:H and a-C:H films with respect to their mechanical properties (hardness, Young's modulus). Nevertheless, these a-B:H films also show a much lower wear resistance in a ball-on-disc test due to different friction mechanisms related to the formation of transfer layers. These results are correlated with the microstructural properties of the coatings as determined from infrared spectroscopy and Raman spectroscopy.