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 h ave made use of the simultaneous addition of hard ceramic particles (T iC or NbC) alongside particles which might act as a solid lubricant (M nS or CaF2). Preliminary investigations carried out to study interacti ons between such particles and a sintered high speed steel matrix indi cated that the steel matrix carbides were modified by a chemical react ion which occurred between the added ceramic particles and the high sp eed steel matrix the result of which was to give a substantial improve ment in hardness. The solid lubricant remained chemically unaltered by the sintering process, and tended to reduce hardness. Both types of p articulate 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 d ensification was due to a liquid phase sintering mechanism and that so lution-reprecipitation of carbide phases played a major role in the de nsification process. Coarsening of the solid lubricant particles into an agglomerated form occurred during sintering by a process of the tra nsportation of smaller particles into pore interstices by the liquid p hase.