ANALYTICAL ELECTRON-MICROSCOPY OF A REACTION-BONDED SI3N4-BASED CERAMIC CONTAINING OXIDE ADDITIONS

The microstructure of a silicon nitride (Si3N4)-based ceramic, prepare d by a process combining direct nitridation and reactive liquid phase sintering of silicon/ceramic oxide powder compacts, has been character ised using analytical transmission electron microscopy. The presence o f the reactive liquid phase, promoted by the addition of oxides from t he CaO-Al2O3-SiO2 ternary system, resulted in an as-fired microstructu re containing a mixture of crystalline phases based on alpha-Si3N4, be ta-S3N4 and Si2N2O, 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 wer e detected in both beta-Si3N4 and Si2N2O. The observed compositions of these phases, together with measured lattice parameters systematicall y in excess of those of the pure compounds, imply that they are in fac t beta- and O-sialons respectively. Semi-quantitative energy dispersiv e X-ray spectroscopy, using an ultra-thin window detector, is demonstr ated to be capable of distinguishing clearly between these phases acco rding to their oxygen content and of determining the aluminium content of both phases to within +/-1 equ.%, even at concentration levels of <5 equ.%.