Abrasion Resistance of Cermets Containing Co/Ni Binders
Prior work in these Laboratories(1, 2, 3) has shown that the properties of the binder phase play a crucial role in determining the mechanical behavior of cermets. In this work we have investigated the effect of alloying the binder phase of a model VC/Ni cermet with cobalt. The results indicate that the abrasion resistance of the cermets exhibits a pronounced maximum when the binder phase composition reaches the value Co/Ni ≃ 85/15. The effect appears to be related to the presence and stability of the h.c.p. phase in the binder. For Co/Ni > 85/15, the h.c.p. and f.c.c. phases are originally intermixed, but X-ray diffraction of abraded surfaces indicates that the binder converts fully to the h.c.p. phase in the near surface regions upon deformation. For Co/Ni < 85/15, the f.c.c. phase is stabilized and does not transform during abrasion. Accordingly, we suggest that the maximum in abrasion resistance can be accounted for in the following manner: at high Co contents, the abrasion resistance is somewhat degraded because of the limited ductility of the Co - h.c.p. phase. At high nickel content, the abrasion resistance is again degraded because the binder becomes relatively soft. The maximum then appears at a binder composition where the f.c.c. phase becomes fully stabilized (allowing ductile behavior) but where the binder hardness is still high due to its large cobalt content. Similar results obtained for WC/Co with Ni additions suggest a means for conserving critical cobalt supplies while improving abrasion resistance for this important cutting tool material.
KeywordsAbrasion Resistance Binder Phase High Nickel Content Binder Composition Cleavage Energy
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