Carbon behavior in sintered silicon nitride grain boundaries

Abstract

The behavior of carbon in the grain boundaries of both hot-pressed and hot isostatically pressed silicon nitride is reported. The silicon nitride powder was coated with a carbon film. The conditions used for hot pressing were a temperature of 1873 K, a uniaxial pressure of 20 MPa and a nitrogen atmosphere of 0.9 MPa. The hot isostatic pressing conditions were 60 and 200 MPa at 1823, 1873 and 1973 K. The hardness, density, fracture toughness and crystalline phase were evaluated. The grain boundaries were investigated by Auger electron spectroscopy and scanning electron microscopy. The results are summarized as follows: (1) in the case of hot press sintering, the amount of β-silicon nitride increases with carbon content. But carbon reduces the α-to-β transition for hot isostatic press sintering; (2) the mechanical properties of hot-pressed silicon nitride depend on the carbon content; (3) for the hot-pressed silicon nitrides with carbon film, silicon carbide precipitates in the grain boundary but it does not do so in the case of the hot isostatically pressed silicon nitrides. The results are explained by the free energy of formation under high pressures.