Effect of heat treatments on microstructure and creep behaviour of silicon nitride based ceramics

Silicon nitride based ceramics were exposed to reducing and/or oxidising atmospheres and subsequently creep-tested in uniaxial tension. The microstructure was examined before and after creep testing. All post-densification treatments caused grain growth. Additionally, treatments in nitrogen atmosphere lead to a reduction of the total oxygen content. Grain coarsening and reduction of the oxygen content were less pronounced for materials containing submicron-sized silicon carbide particles. The creep rates at 1500 °C of materials heat-treated in nitrogen atmosphere were found to be reduced by up to about one order of magnitude. The reduction, however, was only partially due to grain growth. As an additional effect, modifications of the grain-boundary structure were identified. Furthermore, the treatments caused an increase in the activation energy for creep leading to a more pronounced reduction of the creep rate at lower temperatures. Oxidised materials showed only minor changes of the creep resistance. Chemical modifications of the intergranular phase, grain growth, and a pronounced interlocking of the grain facets in the vicinity of SiC particles are suggested to explain the increased creep resistance of the Si3N4/SiC nanocomposites, the latter two phenomena being considered to have the largest effects on the creep resistance.