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

In the case of dry unlubricated wear of metals, an oxidational wear me chanism can be established under certain conditions, with oxide films being developed on the sliding surface which markedly influence the fr iction and wear behaviour. For high-speed steel-based materials contai ning various hard second phases, an a-Fe2O3 film is formed during pin- on-disc testing (under mild conditions of load and sliding speed), whi ch 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 me chanism was studied in this investigation. The size of the hard second phase particles appears to be the most important parameter determinin g the possibility for the particles to provide protection against oxid ational wear of the matrix. Particles of a size less than or equal to the critical thickness are carried away when the oxide breaks up, whil e particles larger than the critical oxide thickness remain in place. In this case, their ability to protect the metallic matrix from the lo ads imposed by the counterbody depends on their mechanical resistance to these loads, as well as the strength of their cohesion with the met allic matrix.