Fabrication and tribological properties of a self-lubricating wear-resistant coating based on structural coupling

Abstract To improve the traditional laser cladding self-lubricating coating technique, a new type of self-lubricating composite coating is proposed in this study. This novel structural-coupling self-lubricating wear-resistant coating (SSWC) is fabricated on stainless steel using a laser cladding + vacuum pressure thermal-diffusion welding (VPTW) process. The coating has an interlaced-stripe structure from an organic combination of separated wear-resistant units (WUs) and self-lubricating units (SUs), which are prepared using NiCrSiB powder and Cu-coated graphite composite powder, respectively. The WUs have a microstructure consisting of a Ni-rich γ- (Ni, Fe) matrix and Cr-rich carbide and boride precipitates, and the SUs primarily consist of Cu matrix and graphite. Distinctive microstructures determine the obvious increase in the hardness of the WUs compared to the substrate and the decrease in the hardness of the SUs. As a result, microhardness with alternating softness and hardness occurs on the surface of the SSWC. The tribological properties of the SSWC are evaluated through the reciprocating friction and wear test compared with the as-received substrate, NiCrSiB coating and Cu-G coating at room temperature. The results exhibit considerably better antifriction properties of the SSWC compared to other wear specimens. This property can be attributed to the positive role of the SUs contained in the SSWC, which have a self-lubricating property and promote the formation of tribofilm with a lubricity and protection on the worn surface of WUs by sliding action. The wear mechanism of SUs is mainly abrasive wear. In contrast, the main wear mechanism of WUs varies with the sliding time: abrasive wear at the initial stage that shifts to delamination wear and abrasive wear after long-term sliding due to the generation of tribofilm. Additionally, a synergistic relationship between the WUs and SUs was observed during the wear process, which can lead to the specific tribological properties of the SSWC.