Characterisation of hot pressed commercial Si3N4 powders

Mechanical properties and microstructure of materials prepared from four commercial silicon nitride powders (hot pressed with Al2O3-Y2O3 as sintering aid) were compared. Taking into account the good mechanical properties obtained for powder A it has been decided to assess its sintering behaviour and the mechanical properties of the obtained ceramics using alternative additives chosen in the BaO-Al2O3-SiO2 system. These additives lead to the formation of celsian (BaAl2Si2O8) at grain boundaries when densification proceeds. Unfortunately, the presence of residual hexacelsian induces microcracking due to the transformation of the alpha <----> beta type at 300 degrees C, associated with large volume expansion. This thermal accident can explain tow mechanical properties obtained for this material.