Sintering temperature dependent micro and macro mechanical properties of Si3N4f/SiO2 composite materials

Wave-transparent materials for the high temperature use require highly reliable mechanical properties and excellent anti-oxidation performance. This study reports the densification, interfacial evolution, micro and macro mechanical properties of Si3N4f/SiO2 composites sintered at 800-1200 degrees C. The results show that significant shrinkage of silica matrix at 1100-1200 degrees C helps improve the density of Si3N4f/SiO2 composites, but also leads to an obvious reduction of mechanical properties. The bending strength of composites sintered at 800-1000 degrees C reaches 166.5-174.0 MPa, but linearly decreases to 102.5 MPa at 1100 degrees C and 19.8 MPa at 1200 degrees C. Similarly, fracture toughness also experiences a linear decline, decreasing from 7.0 to 7.2 MPa m(1/2) at 800-1000 degrees C to around 3.7 MPa m(1/2) at 1100 degrees C and 1.0 MPa m(1/2) at 1200 degrees C. As to the high-temperature performance, the in-situ bending strength at 1200 degrees C of the 800-1000 degrees C sintered composites is lower than 75Mpa. Interfacial diffusion cooperated with the uneven shrinkage of silica matrix is regarded as major reasons to the degradation of composites sintered or tested at over 1100 degrees C, for that they can lead to an increase of fiber-matrix interfacial bonding and trigger the generation of internal stresses and even visible cracks.