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

The amorphous hydrogenated carbon coatings (a-C:H) deposited by plasma-assisted 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 nitride intermediate layers. The most important parameters affecting the load-carrying 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 coatings, 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 rather stable, in the range of 0.10 to 0.19 in humid air (50% relative humidity). Some graphitization on the wear surface of the ta-C coating was observed, but the amount of graphite detected was rather small due to the stable sp3 structure of the ta-C coating. When the ta-C coatings were tested in dry air (0% relative humidity) the coefficient of friction increased to the value of 0.71. However, by doping the ta-C coating with hydrogen or methane, the coefficient of friction reduced as the hydrogen content of the coating increased. These friction properties of a-C:H and ta-C coatings can be related to the friction properties of graphite, which verifies the importance of graphite formation and the availability of hydrogen for low friction properties of the a-C:H and ta-C coatings. In water-lubricated tests, the ta-C coating showed excellent tribological performance with a dramatic drop in the coefficient of friction from 0.07 to 0.03, but the a-C:H coating was susceptible to sliding wear in water-lubricated conditions, which resulted in an early failure of the coating. The performance of the a-C:H coating in water could be improved by alloying the coating with titanium. In oil-lubricated conditions, the a-C:H and particularly ta-C coatings showed further improved tribological performance compared to unlubricated sliding conditions. In oil-lubricated conditions the tribofilm formation typical for a-C:H and ta-C coatings in dry sliding conditions was prevented and the oil chemistry governs the tribocontact. The DLC coatings are not only suggested for use in dry sliding conditions, but also in boundary lubricated conditions to provide safe operation in demanding conditions.