Controlled porosity in silicon nitride

The significant disadvantage of hard-wearing engineering ceramics, such as silicon nitrides, is their difficulty in machining after sintering. Although decades of work have focused on developing this dense silicon nitride for engine applications, many researchers in the past few years have investigated the possibilities offered by introducing weak interfaces to deflect growing cracks, resulting in improved thermal shock resistance. While porous ceramics can be machined more easily, the strength of such ceramics decreases exponentially with increases in porosity. A layered structure of alternating dense and porous silicon nitride was proposed by Shigegaki et al., [1] fabricated by tape-casting and laminating techniques. This was achieved by the addition Of beta-Si3N4 whiskers to the porous layers and resulted in layers with well-oriented and tightly tangled elongated grains where porosity is represented by anisotropically shaped pores. The present investigation looks at the possibilities offered by controlled porosity in silicon nitrides, concentrating on the optimisation of processing methods to achieve desired shape, size and distribution of pores. Various different strategies are followed: partial sintering for different times and temperatures, sintering using fugitive particles and materials consisting of dense and porous layers in laminate. The properties are reported.