Effects of microstructures and flaw evolution on the fracture strength of ZrB2-MoSi2 composites under high temperatures

Based on the existing fracture theories and a temperature dependent fracture surface energy model, a temperature dependent fracture strength model for ZrB2-MoSi2 composites is established. The excellent agreement is obtained between the model predictions and experimental data. The model can be used for qualitative even quantitative analysis of the effect of SiO2 size on the strength of hot pressed composites under high temperatures, and to predict the temperature dependent critical flaw size of composites which thus can help us to discover the control mechanisms of material strength under different temperatures. The effects of microstructures and flaw evolution on the strength of ZrB2-MoSi2 under high temperatures are studied in detail using the model. The study shows that the critical flaw causing the failure of the hot pressed composites under high temperatures is related to the largest SiO2 grains. The fracture strength of materials is controlled by the combined effects of temperature, grains and flaws. Both with the increases of temperature and initial flaw size the sensitivities of fracture strength to grain size and flaw size decrease. During preparation of the composites reducing the initial flaw size should be firstly considered. Improving the starting powder purity and sintering method which could reduce the amount, sizes and their dispersion of dispersed phases present in the microstructure, MoSi2 grain size and initial flaw size would result in the greater strength of materials under high temperatures.