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The sialon research programme at Industrial Research Ltd., New Zealand, comprises fundamental and strategic studies funded by central government that underpin an applied science and technology transfer programme designed in association with industry partners and supported through joint government-industry funding initiatives. The paper surveys current and recent research into a range of sialon fabrication processes, bodies and composites with particular emphasis on O-sialon.

A new phase in the Si-Al-O-N system has been identified, following syntheses based on the nitridation of silicon/clay mixtures at low temperatures (<1350°C). The structure of the new phase was determined using a combination of diffraction and high-resolution imaging techniques, and this new phase possessed the same sheet structure as O-SiAlON (Si 2−x Al x O 1+x N 2−x ) but with a different stacking arrangement. It is considered to be a low-temperature polymorph of O-SiAlON and transforms to conventional O-SiAlON at temperatures greater than ∼1350°C.

O1-sialon is a low temperature polymorph of O-sialon that can be synthesised in good yield at less than 1350°C, but rapidly transforms to O-sialon at higher temperatures, particularly in the presence of an aluminosilicate liquid phase. The O1-sialon to O-sialon phase transformation was investigated by spark plasma sintering (SPS). Dense O-sialon bodies (composition: Si 1.85 Al 0.15 O 1.15 N 1.85 , with 0 to 6 wt% Y 2 O 3 ) with nano-scale microstructures and strengths of up to 860 MPa (ball on ring method) were obtained by sintering at 1400 to 1700°C for 0 to 5 minutes under an applied uniaxial pressure of 50 to 135 MPa. The effect of the O1-sialon to O-sialon phase transformation on the microstructure and properties (strength, hardness and toughness) of sintered O-sialon ceramic is examined.