PHASE REACTIONS AND CHEMICAL-STABILITY OF CERAMIC CARBIDE AND SOLID LUBRICANT PARTICULATE ADDITIONS WITHIN SINTERED HIGH-SPEED STEEL MATRIX

Efforts to improve the wear properties of sintered high speed steels have made use of the simultaneous addition of hard ceramic particles (TiC or NbC) alongside particles which might act as a solid lubricant (MnS or CaF2). Preliminary investigations carried out to study interactions between such particles and a sintered high speed steel matrix indicated that the steel matrix carbides were modified by a chemical reaction which occurred between the added ceramic particles and the high speed steel matrix the result of which was to give a substantial improvement in hardness. The solid lubricant remained chemically unaltered by the sintering process, and tended to reduce hardness. Both types of particulate addition raised the sintering temperature needed to achieve full density due to their effect on solidus and liquidus temperatures. Studies of the sintering kinetics in such materials confirmed that densification was due to a liquid phase sintering mechanism and that solution-reprecipitation of carbide phases played a major role in the densification process. Coarsening of the solid lubricant particles into an agglomerated form occurred during sintering by a process of the transportation of smaller particles into pore interstices by the liquid phase.