LIQUID-PHASE SINTERING OF METAL-MATRIX COMPOSITES CONTAINING SOLID LUBRICANTS

Attempts to improve the wear properties of sintered high speed steels have been made by the simultaneous addition of hard ceramic particles (TIG) or (NbC) alongside particles with solid lubricative properties s uch as (MnS) or (CaF2). Preliminary investigations carried out to stud y any interactions between such particles and a sintered high speed st eel matrix indicated that the steel matrix carbides were modified by a chemical reaction which occurred between the added ceramic particle a nd its high speed steel matrix; the result of which was to give a subs tantial improvement in hardness. The solid lubricant remained chemical ly unaltered by the sintering process, and tended to reduce hardness. Both types of particulate addition raised the sintering temperature ne eded to achieve full density due to their effect on solidus/liquidus t emperatures. Studies of the sintering kinetics in such materials confi rmed that densification was due to a liquid phase sintering mechanism and that solution/re-precipitation of carbide phases played a major ro le in the densification process. Coarsening of the solid lubricant par ticles into large agglomerates occurred during sintering by a process of the transportation of smaller particles into pore interstices by th e liquid phase.