Corrosion behaviour of silicon nitride ceramics in aqueous solutions

Silicon nitride ceramics are used under conditions were high strength, hardness and wear resistant is necessary. The increasing use of Si3N4-ceramics as ceramic ball bearings and valves in chemical apparatus demand an understanding of the relations between microstructure and corrosion behaviour of silicon nitride ceramics in different environments. The stability of the ceramics against corrosion in acids and bases is mostly controlled by the stability of the grain boundary. Therefore, the influence of different rare earth ions and the influence of the crystallisation of the grain boundary on the stability in sulphuric acid were investigated. The results showed that the nature of the rare earth has a minor influence on the corrosion behaviour. The stability against corrosion of the ceramics with amorphous grain boundaries is mostly controlled by the amount of the SiO2 in the grain boundary. The crystallisation of the grain boundary can improve or not change the corrosion behaviour depending on the amount of crystalline grain boundary phases and the composition (stability) of the residual amorphous grain boundary phase. The results are discussed in the context with previous investigation and some systematisation of the corrosion behaviour of Si3N4 in acids and bases is given.