Abstract
Effects of gas pressure on Ellingham diagrams are considered to evaluate the difference of sintering behavior of silicon nitride under normal, hot-press and encapsulated HIP sintering. Special attention was focussed on the stability of carbon impurities. The results are summarized as follows:
(1) Although exact internal pressures in capsules under HIP sintering are not possible to measure, the gas pressure is higher than hot-press sintering, if there are gas generation reactions. The minimum temperature of silicon carbide formation from carbon impurity in the HIP'ed silicon nitride increases with higher gas pressures. This can be explained by the free energy change of the reactions under high pressures, i.e. high pressure Ellingham diagrams proposed by the authors.
(2) The reaction, SiO2+3C→SiC+2CO was observed in the normal sintered and hot-pressed silicon nitride. This reaction was limited in HIP sintering, because carbon becomes stable under high gas pressures.
