The effect of Al2O3-MgO additives on the microstructure of spark plasma sintered silicon nitride

It was shown that spark plasma sintered silicon nitride with a high content of Al2O3 and MgO consists of alpha and beta silicon nitride, the main phase being a silicon nitride. The increase in the sintering temperature did not lead to significant changes in the phase composition as occurs in silicon nitride added with Al2O3-Y2O3. It was found that increasing in SPS temperature above 1650 degrees C leads to an insignificant increase in the density. A complex shaped equiaxed grain microstructure was shown in both cases. However, doping with aluminum and yttrium oxides allows obtaining an elongated grain microstructure. The Hall-Petch effect was observed for the micro-hardness of the investigated SPSed silicon nitride. The microhardness of the described ceramics was rather high and more than 1900 HV compared to the pressureless sintered at 1800 degrees C silicon nitride with the microhardness equal to 1511 HV.