Effect of Tungsten Carbide Addition on the Wear Resistance of Flame-Sprayed Self-Lubricating Ni-Graphite Coatings

Ni-graphite coatings are widely used in industrial fields to improve the self-lubrication property of metallic surfaces. In this study, the effect of tungsten carbide (WC) addition on the microstructure, hardness and wear resistance of flame-sprayed Ni-graphite coatings was investigated. Microstructure examination of coatings shows that WC particles adhered well to Ni matrix but tended to come into contact with each other or graphite particles with the increasing addition amount. The hardness of the Ni-based graphite/WC composite coating depended on the WC content and reached its maximum at the WC addition amount of 30 wt.%. Tribological tests reveal that when the WC content was increased from 0 to 30 wt.%, the wear rate decreased about 42% (from about 5.0 × 10⁻⁵ to 2.9 × 10⁻⁵ mm³ N⁻¹ m⁻¹), while the maximum friction coefficient increased only about 19% (from about 0.16 to 0.19). The graphite in the coatings contributed to low friction coefficient with the formation of self-lubricating tribofilm. The wear mechanism of the coatings was mainly delamination, and well-embedded WC particles inhibited the propagation of delamination cracks. Results show that the Ni-based graphite/WC composite coating with 30 wt.% WC achieved a good balance between the wear resistance and the self-lubrication property.