The Role of Oxygen in Non-Oxide Engineering Ceramics

Abstract

Non-oxide ceramics (borides, nitrides, carbides and silicides) are important engineering materials because in comparison with oxides they offer increased refractoriness, higher strength and stiffness and often improved thermal shock resistance at elevated temperatures. Unless elaborate precautions are taken, oxygen is present at every stage of production from the processing of the raw materials to the firing of the final product, in some cases helping, but more often hindering the success of each step of the process. Most non-oxide starting powders contain up to 8 w/o of oxides and larger amounts may be introduced during powder handling and processing. In the case of nitrogen ceramics the oxygen is indispensible if a fully dense product is to be achieved but the resulting glassy phase degrades the properties of the final product. Current technology therefore aims to minimize the levels of oxygen required for densification and techniques such as HIPing are useful for this purpose. Preparation of non-oxide fibres and whiskers, often carried out via organic routes, can result in residual oxygen, especially on the fibre surface. This causes problems during the firing of composite materials containing these fibres, and of course the quality of the fibre-matrix interface is critical in controlling the subsequent mechanical performance of the material.

The role of oxygen in the final product depends to some extent on its location. Thus, for example, oxygen dissolved in the lattice of aluminium nitride significantly reduces the thermal conductivity. Oxygen present as a grain-boundary glass in silicon nitride-based ceramics is helpful as regards fracture toughness and room temperature strength but detrimental as far as high-temperature mechanical properties are concerned. Residual oxygen on the surface on many non-oxide ceramics is helpful in forming a passivating film which protects against oxidation at both room and elevated temperatures.