Effects of amount and type of cation species on the phase formation of single and codoped α-SiAlONs

Abstract

The phase formation of single phase α-SiAlON ceramics based on α-Si3N4 was investigated using different rare earth (RE) or metallic cations to stabilise the α-phase. The effects of sintering temperature, size and type of stabilising cation and the addition of extra liquid forming additives on the phase transformation of α-Si3N4 to α-SiAlON were considered. The influence of the chosen parameters on properties such as density and hardness is furthermore discussed for Ca-codoped silicon aluminium oxynitride (SiAlON) ceramics (50% RE cations combined with 50% calcium cations). For smaller and larger RE systems different intermediate phases occur and the dissolution of sintering additives takes place at different temperatures, resulting in different amounts of SiAlON at a given sintering temperature. For smaller cations, the SiAlON formation is favoured below 1,750 °C compared to larger cations. The addition of extra liquid to the starting composition supports the SiAlON formation above the eutectic temperature. Contrary to the RE systems the calcium-doped samples did not show an intermediate phase during sintering. The cation solubility for sintering additives is higher and therefore the amount of SiAlON created below 1,750 °C increased. The final amount of SiAlON at 1,850 °C was about the same for all systems. Mechanical properties are also influenced by the composition of the starting powder. In general, increasing temperature leads to higher density and hardness.