6.5 Material Variables

Particle Size: In terms of the basic stages of sintering, decreasing particle size leads to increased sintering. The smaller particle size has a greater pore/solid interfacial area producing a greater driving force for sintering. It promotes all types of diffusion transport, e.g. greater surface area leads to more surface diffusion, small grain size promotes grain boundary diffusion and a larger interparticle contact area to volume diffusion.

Particle Shape: The factors that lead to greater intimate contact between particles and increased internal surface area promote sintering. These factors include decreasing sphericity and increasing macro- or microsurface roughness.

Particle Structure: A fine grain structure within the original particles can promote sintering because of its favourable effect on several material transport mechanisms.

Particle Composition: Alloying additions or impurities within a metal can affect the sintering kinetics. The effect can either be deleterious or beneficial depending upon the distribution and reaction of the impurity. Surface contamination, such as oxidation is usually undesirable. Dispersed phases within the matrix may promote sintering by inhibiting grain boundary motion. Reaction between impurities and either the base metal or alloying additions at the relatively high sintering temperature may be undesirable.

Green Density: A decreasing green density signifies an increasing amount of internal surface area and consequently, a greater driving force for sintering. Although the percentage change in density, increases with decreasing green density, the absolute value of the sintered density remains highest for the higher green density material.