THE ROLE OF HARD 2ND PHASES IN THE MILD OXIDATIONAL WEAR MECHANISM OF HIGH-SPEED STEEL-BASED MATERIALS

In the case of dry unlubricated wear of metals, an oxidational wear mechanism can be established under certain conditions, with oxide films being developed on the sliding surface which markedly influence the friction and wear behaviour. For high-speed steel-based materials containing various hard second phases, an a-Fe2O3 film is formed during pin-on-disc testing (under mild conditions of load and sliding speed), which reaches a critical thickness of 1-2 mum before breaking up in loose wear debris. The role of the second phases in the oxidational wear mechanism was studied in this investigation. The size of the hard second phase particles appears to be the most important parameter determining the possibility for the particles to provide protection against oxidational wear of the matrix. Particles of a size less than or equal to the critical thickness are carried away when the oxide breaks up, while particles larger than the critical oxide thickness remain in place. In this case, their ability to protect the metallic matrix from the loads imposed by the counterbody depends on their mechanical resistance to these loads, as well as the strength of their cohesion with the metallic matrix.