Compound To Add Si <sub> 3 </ Sub> N <sub> 4 </ Sub> Particles, Sic Whiskers Of Mosi <sub> 2 </ Sub>-based Composite Materials Research

MoSi₂ is becoming a research hotspot of high temperature structural material due to its high melting point,moderate density,excellent high temperature oxidation resistance,good high-temperature plasticity and high thermal conductivity.However, the application of MoSi₂ is limited by the low ductility at room temperature and the low strength at high temperature.Compounding is an important method to improve the properties of MoSi₂,therefor,this paper prepared MoSi₂-based composites reforced and toughened by Si₃N₄ particles and SiC whiskers using hot-pressing sintering,the microstructure,mechanical properties,strengthing-toughening mechanism and oxidation behavior in high and low temperature were investigated by Metallographic Microscope,X-ray Diffraction,Scanning Electron Microscope, Transmission Electron Microscope,and mechanical analysis.Research results were obtained as follow:(1) The relative densities of composites prepared by hot-pressing sintering at 1700℃were higher than 93%,with the increase of strengthening and toughening phase,the relative density decreased.And the impact of SiC whiskers on relative density was more than that of Si₃N₄ particles because the sintering temperature of SiC whisker was much higher than 1700℃.The XRD analysis showed that there were good compatibility among MoSi₂,Si₃N₄ and SiC.(2)Adding Si₃N₄ particles and SiC whiskers could evidently improve the hardness,bend strength,fracture toughness and high temperature strength of MoSi₂. Micron SiC whiskers was more effective to the improvement of mechanical properties than Si₃N₄ particles and nano SiC whisker.Adding Si₃N₄ and SiC at the same time could make synergistic strengthening and toughening effect.The room temperature bend strength and fracture toughness of MoSi₂-20%Si₃N₄/20%nanoSiC_w were 427MPa and 8.38MPa·m^1/2 respectively,which were 69.4%and 189%higher than MoSi₂.The flow stress of MoSi₂-20%Si₃N₄/20%micronSiC_w was evidently higher than MoSi₂ at 800℃,1000℃and 1200℃.The peak stress of composites at 1200℃increased as the content of strengthening phase increased.(3)The main strengthening mechanism of composites was fine grain strengthening at room temperature.Adding Si₃N₄ particles and SiC whiskers at same time had the most obvious effect of grain refinement and resulted in high flexural strength.The main strengthening mechanisms of composites were fine grain and dispersion strengthening at high temperature.The SiO₂ glass phase and impurity phase in the grain boundary decreased the strength of MoSi₂-20%Si₃N₄/20%nanoSiCw.The main toughening mechanisms of composites were fine grain,crack deflection,whisker pullout and residual stress toughening. MoSi₂-20%micronSiC whisker had better fracture toughening effect because of obvious whisker pullout phenomenon,but increasing the content of micron SiC whisker to 40%caused fracture toughness decreasing.Adding Si₃N₄ particles and SiC whiskers to MoSi₂ at same time could regulate residual stress,so the fracture toughness of MoSi₂-20%Si₃N₄/20%nanoSiC_w and MoSi₂-20%Si₃N₄/20%micronSiC_w were very high,exhibiting excellent synergistic and toughening effect.(4)During oxidation at 500℃,the surface of composites containing nano SiC whiskers generated Na₂MoO₄ crystal which made the composites excellent oxidation resistance.Adding 20vol.%Si₃N₄ could not form a Si₂N₂O-SiO₂ protective layer on composites surface to prevent "Pesting".While increasing the content of Si₃N₄ to 40%,the surface of the composites formed a Si₂N₂O-SiO₂ protective layer,enhancing the oxidation resistance of the composites.Micron SiC whiskers were oxidized to SiO₂,making composites show good oxidation resistance at low temperature.During oxidation at high temperature,composite materials were able to generate dense SiO₂ membrane,but the mismatch of oxidation products and reactant thermal volume,as well as the mismatch of matrix and SiO₂ thermal expansion coefficient in thermal cycling resulted in cracks.