Influence of low nitrogen gas pressures (< 1.0 MPa) and additives on the nitridation, formation and densification of B and O-SiAlON ceramics produced by reaction bonding and silicothermal reduction

Abstract

β and O-SiAlON materials have been synthesised by both reaction bonding and silicothermal reduction techniques under different nitrogen pressures. These two methods which use Si and/or halloysite clay in place of Si3N4 in the starting mixture represent a cost efficient alternative to the traditional liquid phase sintering typically employed to produce SiAlONs. The influence of the initial compositions and of different additives on the nitridation behaviour and SiAlON phase formation have been studied. All of the additives did not have the same effects on the nitridation of SiAlONs as on the nitridation of pure Si. Indeed, typically, CaO had a negative effect on the nitridation of SiAlONs while Y2O3, Dy2O3 and ZrO2 enhanced the nitridation. The two cost efficient processing routes led to similar final products after sintering under several combinations of the following parameters: nitriding and sintering additives, temperature, N2 pressures. The work was therefore focused on the silicothermal reduction route since it facilitates forming by pressing, slip casting and extrusion, and gives a greater flexibility in the size and shape of SiAlONs that can be produced. Nitrogen overpressures typically enhanced the nitridation of Si in SiAlONs. For SiAlONs that started to densify at low temperature and that reached open porosity as low as 10%, nitrogen overpressures were essential to lead to full nitridation. SiAlONs with lower open porosity could not be nitrided under overpressure up to 1.0 MPa. The possibility of quickly nitriding Si and forming β and O-SiAlON materials has been studied through their reaction sequence of formation. β and O-SiAlONs were fully nitrided and formed after a 1-h hold at 1400°C under 0.7 MPa of N2 while this was not achievable under static atmospheric pressure. The amount of SiO2 in the starting material played an important role during the formation of SiAlONs. It could react with Si and lead to significant mass losses by evaporation, but it could also lower the eutectic temperature in the ternary systems leading to early densification and pore elimination, explaining why O-SiAlONs start to densify at lower temperature than β-SiAlONs and why N2 pressures have a greater influence on their formation. Sintering of β and O-SiAlONs under nitrogen pressure led to materials which exhibited hardness as high as typically reported for similar techniques (15 and 13 GPa respectively). Longer sintering times can be used to further densify β-SiAlONs since their decomposition at high temperature leads to the formation of a α-SiAlON surface layer with low porosity, which then stops such decomposition. In contrast O-SiAlONs have to be densified with shorter dwelling times as their decomposition into β-SiAlON leads to the formation of a highly porous surface layer which allows further decomposition. In conclusion, the use of low nitrogen pressures during nitriding and sintering of SiAlON materials produced by silicothermal reduction is of interest with regard to the production of low cost SiAlON refractory materials.