Quanshun Luo

Braking discs play an important role for the safety of vehicles whereas severe frictional contacts in repeated braking operations result in thermal and tribological failures. This paper reports comprehensive surface and subsurface analyses of a used grey cast iron braking disc to investigate its failure mechanisms as well as related microstructure evolution. Thermal cracking, spalling wear and tribo-oxidation have been found to contribute to the wear failure. The flake-type graphite of the grey cast iron triggered propagation and internal oxidation of the metallic matrix. In a depth of 3 mm beneath the rubbing surface, extensive microstructure evolution occurred by severe plastic deformation of the ferritic matrix, nodularisation and partial dissolution of the lamellar carbides, and internal oxidation. The microstructure evolution resulted in decreased surface hardness to HV_0.3 247 as compared to HV_0.3 284 of the bulk cast iron.

Microhardness testing is a widely used method for measuring the hardness property of small-scale materials. However, pronounced indentation size effect (ISE) causes uncertainties when the method is used to estimate the real hardness. In this paper, three austenitic Hadfield steel samples of different plastic straining conditions were subjected to Vickers microhardness testing, using a range of loads from 10 to 1000 g. The obtained results reveal that the origin of ISE is derived from the fact, that the indentation load P and the resultant indent diagonal d do not obey Kick’s law (P = A · d²). Instead, the P and d parameters obey Meyer’s power law (P = A · dⁿ) with n < 2. The plastically strained samples showed not only significant work hardening, but also different ISE significance, as compared to the non-deformed bulk steel. After extensive assessment of several theoretical models, including the Hays-Kendall model, Li-Bradt model, Bull model and Nix-Gao model, it was found that the real hardness can be determined by Vickers microhardness indentation and subsequent analysis using the Nix-Gao model. The newly developed method was subsequently utilised in two case studies to determine the real hardness properties of sliding worn surfaces and the subsurface hardness profile.

This paper reports the sliding wear properties and wear mechanisms of Hadfield high-Mn austenitic steel in a dry-sliding reciprocal tribotest against a WC counterpart. The associated wear mechanisms were studied through extensive characterisation of the obtained worn surface using analytical SEM, TEM, XRD and micro-hardness test. The tribotest revealed a coefficient of wear in the scale of 10⁻¹⁴ m³·N⁻¹·m⁻¹ and a coefficient of friction of 0.5–0.6. The steel encountered severe plastic deformation beneath the worn surface leading to a gradient of hardness profile, including the extreme hardening of the worn surface up to HV_0.1 8.2 GPa. Despite the severe deformation and significant strain hardening, the steel still retained its austenitic structure without any detectable martensite. The combined surface and cross-sectional microscopic observations and extensive analysis of XRD peak breadth revealed the formation of nano-heterogeneous microstructure including nano-laminate, nanotwins and nanocrystalline beneath the worn surface. Spalling wear was found as the predominant wear mechanism. The spalling wear was caused by the embrittlement of the extremely hardened and nanocrystallised worn surface. Tribo-oxidation was also observed in the resultant wear debris.