Pressure-less joining of SiCf/SiC composites by Y2O3-Al2O3-SiO2 glass: Microstructure and properties

In this study, Y2O3-Al2O3-SiO2 (YAS) glass was prepared from Y2O3, Al2O3, and SiO2 micron powders. Thermal expansion coefficient of as-obtained YAS glass was about 3.9 x 10(-6), matching-well with that of SiCf/SiC composites. SiCf/SiC composites were then brazed under pressure-less state by YAS glass and effects of brazing temperature on microstructures and properties of resulting joints were investigated. The results showed that glass powder in brazed seam sintered and precipitated yttrium disilicate, cristobalite, and mullite crystals after heat treatment. With the increase in temperature, joint layer gradually densified and got tightly bonded to SiCf/SiC composite. The optimal brazing parameter was recorded as 1400 degrees C/30 min and shear strength of the joint was 51.7 MPa. Formation mechanism of glass-ceramic joints was proposed based on combined analysis of microstructure and fracture morphology of joints brazed at different temperatures. Thermal shock resistance testing of joints was also carried out, which depicted decline in shear strength with the increase of thermal shock times. The strength of the joint after three successive thermal shock cycles at 1200 degrees C was 35.6 MPa, equivalent to 69% of that without thermal shock.