ROLE OF ADDITIVES IN THE SINTERING OF SILICON-NITRIDE - A SI-29, AL-27, MG-25 AND Y-89 MAS NMR AND X-RAY-DIFFRACTION STUDY

Multinuclear MAS NMR in conjunction with X-ray diffraction (XRD) has b een used to study the role of Al2O3, Y2O3 and MgO, both singly and in combination, in the sintering of silicon nitride at 1500-1800-degrees- C. Under the present experimental conditions, Al2O3 enters the silicon nitride to form a low-z beta'-SiAlON, whereas MgO reacts both with th e oxidised surface SiO2 layer to form forsterite (Mg2SiO4), and with t he Si3N4 to form an X-ray amorphous Mg-Si-O-N phase characterised by a broad Mg-25 signal at about -50 ppm and a fast Si-29 relaxation time. Y2O3 forms an yttrium-rich Y-Si-O-N phase at 1500-degrees-C which pro gressively becomes silicon-rich at higher temperatures. The Y-89 spect ra of these phases are broad and could be detected only in samples con taining added Yb2O3 to shorten the relaxation time. When used in combi nation, the Al2O3/MgO and Y2O3/MgO pairs behave similarly to the separ ate components, in terms of intergranular phase formation, but Al2O3/Y 2O3 forms Y10Al2Si3O18N4, for which the Si-29 and Al-27 MAS NMR spectr a are reported.