Origins and avoidance of colour inhomogeneities in silicon nitride ceramics

Gas pressure sintering of silicon nitride ceramics can lead to the formation of different coloured near surface layers.Grey level analyses suggest, that the morphology of these layers was influenced by the sintering parameters, like nitrogen pressure, sintering temperature and holding time.For obtaining a homogeneous coloured material a critical specific weight loss should be avoided, which was found to be the dominant mechanism, leading to the optical inhomogeneity.The interaction between the surface and the gas-phase during sintering was the most important factor in the formation of the dark-coloured layer and the outermost grey-coloured layer.