Si3N4 Phase Research Articles

Phase relationships in the system Si3N4-Y2O3-SiO2

Examination of compositions in the system Si3N4-Y2O3-SiO2 using sintered samples revealed the existence of two regions of melting and three silicon yttrium oxynitride phases. The regions of melting occur at ∼1600° C at high SiO2 concentrations (∼13 mol% Si3N4 + 19 mol% Y2O3 + 68 mol% SiO2) and at 1650° C at high Y2O3 concentrations (25 mol % Si3N4 + 75 mol % Y2O3). Two ternary phases 4Y2O3 ·SiO2 ·Si3N4 and 10Y2O3 ·9SiO2 ·Si3N4 and one binary phase Si3N4 ·Y2O3 were observed. The 4Y2O3 ·SiO2 ·Si3N4 phase has a monoclinic structure (a= 11.038 A, b=10.076 A, c=7.552 A, β=108° 40′) and appears to be isostructural with silicates of the wohlerite cuspidine series. The 10Y2O3 ·9SiO2 ·Si3N4 phase has a hexagonal unit cell (a=7.598 A c=4.908 A). Features of the Si3N4-Y2O3-SiO2 systems are discussed in terms of the role of Y2O3 in the hot-pressing of Si3N4, and it is suggested that Y2O3 promotes a liquid-phase sintering process which incorporates dissolution and precipitation of Si3N4 at the solid-liquid interface.

Multipurpose vacuum furnace with induction electron beam and resistance heating

The mechanical and thermal properties, before and after thermal shock testing, of reaction bonded silicon nitride (RBSN) produced by slip casting, were studied. Slips with solids contents of 27 wt% and 50 wt% silicon powder were used to produce bend bars. Metals casting ladles, with wall thicknesses of 8 mm and 24 mm, were cast from the 50 wt% slip. Nitridation was conducted at 1500 °C for 4 h. X-ray diffraction (XRD) showed α and β phases of Si3N4 were formed. It was found that increasing the solids content increased the density and flexural strength of the bars. After thermal shock testing the flexural strength decreased. The 24 mm ladle which was exposed to 700 °C aluminum fractured after one exposure, while the 8 mm ladle withstood five exposures before failure. This was attributed to the difference in thermal strain.