Comparative study of the tribological moisture sensitivity of Si-free and Si-containing diamond-like carbon films

Diamond-like carbon (DLC) has become one of the most widely used wear-protective low-friction coatings due to its outstanding mechanical and tribological properties. However, a major disadvantage of DLC is the deterioration of its lubricating properties at high relative humidity, limiting its use in certain tribological applications for which ambient humidity may be subject to significant variations. The addition of small amounts of Si to DLC has been reported as an effective method for reducing friction coefficient at high humidities. In this paper we have characterised the tribological behaviour of a reference DLC coating prepared using a PACVD technique in the relative humidity range 5–85%, sliding against steel, alumina and DLC counterfaces, and directly compared this to similar coatings alloyed with small amounts of Si (<6 at.%). Commercially available DLC coatings were also studied for comparison. Our results confirm that the addition of Si can significantly improve the humidity sensitivity of a given coating and show that by adjusting the Si content, the friction behaviour can be tailored to suit the expected humidity operating range or counterface materials. However, the addition of Si also produces a reduction in coating wear resistance by a factor as high as 4. Results indicate that the nature of the counterface is an important factor for frictional behaviour, and the DLC/steel friction pair was shown to be particularly sensitive to the presence of Si, displaying a threefold reduction in friction coefficient at high relative humidities and an inversion in humidity sensitivity (i.e. higher friction coefficients under dry conditions).