Vacuum tribological properties in Cr-doped H-DLC self-lubricating radial spherical plain bearings

Abstract

To improve the vacuum tribological properties of the self-lubricating radial spherical plain bearings (SSPBs), the multi-layered Cr-doped hydrogenated diamond-like carbon films (Cr-H-DLC) and non-doped H-DLC films as the lubricating materials were prepared on the inner ring with an outer spherical surface and the outer ring with an inner spherical surface by unbalanced magnetron sputtering. The microstructure, phase composition, tribological properties, and failure mechanism of Cr-H-DLC SSPBs and H-DLC SSPBs were investigated. The results showed that the microstructure of the outer and inner spherical surfaces for the two kinds of SSPBs was dense. The microgram indicated that the self-lubricating film thickness of the outer spherical surface was greater than that of the inner spherical surface. What is more, the Raman spectra showed that the self-lubricating coating on the outer spherical surface contains more graphite characteristics of sp2 hybrid bonds based on the value of ID/IG. According to the vacuum tribological test, the oscillating life of H-DLC SSPBs was higher than that of Cr-H-DLC SSPBs. Moreover, the change law of friction torque and friction temperature rise showed that the testing process of coated SSPBs could be divided into running-in, stable, and failure periods. Compared with the ID/IG values before and after the vacuum test, it could be inferred that the graphitization of the sp3 hybrid bond in the self-lubricating film on both spherical occurred. Based on the microscopic wear morphology, the failure mechanism of H-DLC SSPBs and Cr-H-DLC SSPBs were abrasive wear and fatigue wear. When the lubricating layer of the friction interface was worn, the metal substrate of the inner and outer rings would contact, resulting in adhesive wear.