The effect of structural parameters on the tribological properties of structural-coupling self-lubricating coatings

In this paper, a Cu-C/HDS structural-coupling self-lubricating coating (SSC) was successfully prepared on an H13 hot-work die steel (HDS) by wire electrical discharge machining and vacuum pressure thermal-diffusion welding. The effect of structural parameters as well as microstructure, composition and microhardness on the tribological properties of the SSC was investigated in detail. In the SSC, the self-lubricating units (SUs) distribute alternately with the HDS units (HDSUs) to form a striped coupling structure. The HDSUs have a microstructure consisting of gamma-Fe, martensite, (Fe, Cr, V) and Cr-rich carbides, and the SUs consist primarily of the Cu-matrix and graphite. Distinctive microstructures determine that the hardness of the HDSUs is much higher than that of the SUs. As a result, microhardness with alternating softness and hardness occurs on the surface of the SSC. The tribological properties of the SSCs were evaluated through the reciprocating friction and wear test and compared with the HDS substrate and the Cu-C coating at room temperature. The results show that the SSCs exhibit considerably better antifriction properties compared to the HDS substrate. Even some samples are better than the pure Cu-C coating. This result is attributed mainly to the positive role of the SUs and the synergistic effect between the coupling units. The tribological properties of SSCs have a close relationship with the fill ratio and the width ratio. With the increase in the fill ratio and the width ratio, the friction coefficient of the SSC first decreases and then increases slightly, while the wear rate presents mainly an overall upward trend. It mainly involves the supply of lubricants and the wear dominance of the SUs. Moreover, when the self-lubrication property of the SSC is determined by the fill ratio or the width ratio, choosing a smaller width for the SU is beneficial to improve the wear resistance.