Tribological Characteristics, Phase Composition and Microhardness of Subsurface Regions of WC–(Fe–Mn–C) Composites after High-Speed Sliding on Steel

Abstract

The authors investigated tribological characteristics, phase composition of worn surfaces and microhardness of subsurface regions of WC–(Fe–Mn–C) composites with a two-phase (γ + α′) matrix containing 4 wt % Mn (WC–80G4), and a single-phase matrix of γ-iron containing 20 wt % Mn (WC–80G20) after sliding against a disk of high-speed steel at a contact pressure of 5 MPa and sliding speeds in the range from 10 to 37 m/s. The wear intensity of WC–80G4 and WC–80G20 increased with increasing sliding speed, while the wear rate of WC–80G20 at fixed sliding speeds was approximately three times higher than that of WC–80G4. The values of the friction coefficient decrease with increasing sliding speed in such a way that at fixed sliding speeds the values of the friction coefficient of WC–80G4 were lower than those of WC–80G20. The amount of mixed oxide FeWO4 formed during tribo-oxidation of the composites’ worn surface increased with the sliding speed and was directly proportional to the wear intensity and inversely proportional to the friction coefficient values. At fixed sliding speeds tribooxidation of WC–80G4 leads to the formation of a larger amount of FeWO4 on the friction surface compared to the WC–80G20 composite. Indentation of worn surfaces with a Vickers pyramid showed that the nature of indentation resistance of tribolayers formed at high sliding speeds (30 m/s and 37 m/s) differs from that for tribolayers obtained at relatively low sliding speeds (10 and 20 m/s), namely, the friction surfaces after high sliding speeds were characterized by a more tough behavior. Measurement of microhardness values of the WC–80G4 and WC–80G20 composites obtained after indentation below tha worn surfaces recorded the fact of hardening of the near-surface regions of the WC–80G4 composites and, on the contrary, softening in the case of WC–80G20. Thus, under conditions of intensive heating and severe plastic deformation of the surface, structural-phase state of the substrate of WC–(Fe–Mn–C) composites, on which this viscous protective tribolayer is formed, turns out to be a very important factor. It is the two-phase (γ + α′) steel matrix that, under conditions of intensive frictional heating, provides the conditions for effective formation of a heterophase composite layer that reduces the friction coefficient and has a high resistance to fracture upon indentation.