Tribological properties of hydrogenated and hydrogen-free diamond-like carbon coatings

The amorphous hydrogenated carbon coatings (a-C:H) deposited by plasmaassisted chemical vapour deposition (PACVD), and tetrahedral amorphous carbon coatings (ta-C) deposited by pulsed vacuum arc discharge were evaluated by scratch adhesion testing, load-carrying capacity testing and tribological testing in unlubricated and lubricated sliding conditions. The wear surfaces of the coatings and counterparts were analyzed by optical and scanning electron microscopy, secondary ion mass spectroscopy (SIMS), Auger electron spectroscopy (AES), and micro-Raman spectroscopy. The adhesion of the a-C:H coating was improved by titanium and titanium carbide intermediate layers. The most important parameters affecting the loadcarrying capacity of the a-C:H and ta-C coated systems proved to be the hardness and Young ́s modulus of the substrate, and the Young ́s modulus and internal stresses of the coating. In general, the ta-C coating was more wear-resistant compared to the a-C:H coating due to the higher hardness of the coating. The wear of the counterparts sliding against the ta-C coating was one order of magnitude higher compared to the a-C:H coating due to higher surface roughness combined with the high hardness of the ta-C coating. For the a-C:H coating, the increase in normal load and sliding velocity reduced the coefficient of friction from 0.42 to 0.1 against steel, and from 0.13 to 0.02 against alumina. The formation of graphite in the tribocontact of the a-C:H coating was observed by micro-Raman analyses when high normal loads and sliding velocities were applied causing high contact pressures and increased contact temperatures. The coefficient of friction of the ta-C coating was low and