Low Temperature Sintering Additives for Silicon Nitride

This work deals with sintering additives for the low temperature pressureless sintering of Si3N4, via a transient liquid technique. Much attention has been given to the development of a sintering process using low-cost Si3N4 powder (Silzot HQ, Germany) as the starting material and employing sintering additives which are also both economically attractive and enable sintering at low temperatures. Two new additives based on the Li2O-Y2O3 system (LiYO2), and on the Li2O-Al2O3-SiO2 system (LiAlSiO4) were investigated in this study. The grain boundary liquid phases that form from the two additive systems possess similar eutectic temperatures (<1300°C) enabling liquid phase sintering of Si3N4 at temperatures as low as 1500°C. Different procedures for powder homogenization were investigated first. This was necessary because the grain size and shape distribution of the Si3N4 powder under investigation differ significantly from those of powders normally used for the liquid phase sintering of Si3N4. Among three mechanical mixing processes for the introduction of additives in Si3N4 powder, namely attrition milling, ball milling and vibratory milling, best results were obtained with attrition milling. This method yielded a uniform dispersion of the additive powder in a fine unagglomerated Si3N4 matrix without contamination. It also produced good sintering characteristics. The densities obtained for sintered samples with 15 and 20 wt% of LiYO2 and with 10 wt% of LiAlSiO4 additives, respectively, were more than 98% of the theoretical density. Li2O is removed in gaseous form during sintering and thus, the liquid phase is partially transient. This creates some potential for good high temperature properties in spite of the low sintering temperature. The densification step is followed by a phase transformation from α−Si3N4 to β−Si3N4 in both cases. The lag between these two stages in the sintering process is more pronounced in the Li2O-Al2O3-SiO2 additive system due to the high viscosity of the transient liquid. The degree of α→β phase transformation increases during prolonged sintering. The reaction kinetics of the phase transformation were found to be of first-order in both systems. The microstructure of Si3N4 materials with the both additives sintered at 1600°C for 8 h is characterized by a homogeneous distribution of elongated β−Si3N4 grains and glassy phase, located in thin layers at grain boundaries and triple points. The β−Si3N4 grains are surrounded by a glassy phase at almost all grain contacts indicating a good wetting behavior.