The microstructure of a silicon nitride (Si3N4)-based ceramic, prepared by a process combining direct nitridation and reactive liquid phase sintering of silicon/ceramic oxide powder compacts, has been characterised using analytical transmission electron microscopy. The presence of the reactive liquid phase, promoted by the addition of oxides from the CaO-Al2O3-SiO2 ternary system, resulted in an as-fired microstructure containing a mixture of crystalline phases based on α-Si3N4, β-Si3N4 and Si2 N2O, and distinct amorphous regions rich in Si, Al and Ca. X-ray microanalysis revealed the calcium to be wholly partitioned to the glassy phase, while significant concentrations of aluminium were detected in both β-Si3N4 and Si2N2O. The observed compositions of these phases, together with measured lattice parameters systematically in excess of those of the pure compounds, imply that they are in fact β- and O-sialons respectively. Semi-quantitative energy dispersive X-ray spectroscopy, using an ultra-thin window detector, is demonstrated to be capable of distinguishing clearly between these phases according to their oxygen content and of determining the aluminium content of both phases to within ± 1 equ.%, even at concentration levels of <5 equ.%.
- analytical transmission electron microscopy
- energy dispersive X-ray spectroscopy
- reaction-bonded ceramics
- Silicon nitride (SiN) ceramics