Formation and properties of YAlSiON glasses in the grain boundaries of Si 3N 4 ceramics

Abstract

A series of oxynitride glasses were prepared from mixtures of Y 2O 3,O 3, SiO 2 and Si 3n 4. The glass forming region was determined quantitatively under conditions similar to those for the sintering of Si 3N 4 ceramics. In the Y 2O 3Al 2O 3 SiO 2 system with 5 wt% Si 3N 4, glass formation is limited due to the occurence of frothing on the high SiO 2 side and due to the tendency to devitrify on the low SiO 2 side. Glass transition temperature, Vickers hardness, flexural strength and fracture toughness increase with nitrogen content. A sample with a high strength (187 MPa) and a fracture toughness of 2.07 MN/m 3/2 was obtained. Infrared spectral analysis indicates the presence of SiN bonds in the glasses. The crystalization behaviour of oxynitride glasses after annealing at 1200 and 1400°C was characterized by powder X-ray diffraction. This shows that when the amount of SiO 2 is less than 25wt%, yttrium aluminum garnet (YAG) crystallizes out and when the SiO 2 content is more than 30 wt%, no YAG crystallizes. In addition, according to thermodynamical calculations, the reaction between Si 3N 4 and SiO 2 to give gaseous decomposition products takes place easily and the reaction between Si 3N 4 and Y 2O 3 or Al 2O 3 shows little tendency to occur in practice.