Colour variations in silicon nitride

Silicone nitride is a general-purpose structural ceramic material used for example in gas turbines, bearings, seals and various wear parts. After densification of silicon nitride with sintering additives, variations in the colour or nuance occasionally appear on the material. To investigate the origin of the colour variations, differently coloured hot-isostatically pressed silicone nitrides with yttria additives were analysed with respect to composition, microstructure, hardness, fracture toughness and wear resistance. The results suggest that the differences in colour originate from the degree of crystallinity of the intergranular silicate phase. The colour variations were also accompanied by differences in the mechanical properties.