M3 Particulate Metal Matrix Composite Materials (MMC, Hardmetals)

Topic

This particulate metal-matrix composite—exemplified by cobalt-bonded tungsten carbide—consists of a particulate ceramic reinforcement embedded in a ductile metal matrix; both components are densified together via liquid-phase sintering. Because this composite densifies through liquid-phase sintering, the model describing the process employs the same liquid-phase densification mechanism used for the same purpose by another model in this set: here, the metal binder itself melts at the sintering temperature and acts as the liquid phase, wetting the ceramic reinforcement particles and allowing the composite to densify just as a ceramic would with a molten flux between its particles.

Once densification is complete, the binder's mean free path—the average distance separating neighboring ceramic particles through the metal binder—determines the composite's hardness, in accordance with the Lee-Gurland relationship. A shorter binder mean free path—meaning ceramic particles are closer together with less metal binder between them—results in higher composite hardness, as the hard ceramic reinforcement exerts greater influence over the composite's behavior; conversely, a longer binder mean free path results in lower hardness, as the softer, ductile metal binder plays a more dominant role in that behavior.

This same binder mean free path also determines the composite's toughness, though through an empirical relationship inverse to that governing hardness: a shorter binder mean free path results in lower toughness, because less ductile metal binder remains available to absorb energy during fracture; A larger mean free path of the binder results in higher toughness, as more ductile metallic binder remains available for energy absorption. The fact that this same parameter governs hardness and toughness in opposing directions means that adjusting one—by varying the proportion of ceramic reinforcement to metallic binder—inevitably comes at the expense of the other.

Consequently, this model—which begins with liquid-phase sintering—fully adopts the densification mechanism already described by two distinct models: one within this same group and another from the equivalent group for metals. Since, according to Lee-Gurland, the binder's mean free path is the parameter determining both the composite's hardness and toughness, adjusting one of these properties in this composite always entails a trade-off with the other.

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