The Research Of Strengthening, Toughening, Ablation And Oxidation Of Molybdenum Disilicide Based Composite

Intermetallic compounds MoSi₂ is a hot candidate material for next generation of aero-engine because of its excellent oxidation resistance at high temperature. However, MoSi₂ use is limited by the brittleness at room temperature and the low strength at high temperature. The components of aero-engine need to withstand the dramatic changes in temperature and the high-temperature gas flow erosion. So the thermal shock resistance and dynamic oxidation resistance of MoSi₂ must to be researched.SiC whiskers (SiCw) has excellent mechanical properties for its little defects and high aspect ratio. Its tensile strength can be up to 14 GPa and elastic modulus can reach to 700 GPa. Therefore using the SiCw as the toughening agent to improve the intrinsic brittleness of MoSi₂ and enhance its thermal shock resistance is of great practical significance.MoSi₂ powder was wet milled to 1.80μm for enhancing the intrinsic driving force of sintering. Polyethylene glycol and sodium pyrophosphate were used as the dispersant of SiCw. And ethanol was choosed as the dispersion medium. Ultrasonic vibration and milling mixed approach were used to scatter the SiCw. Experimental results showed that SiCw were well dispersed in MoSi₂ matrix. MoSi₂\SiCw and MoSi₂\SiCp composites material were prepared by the hot pressing sintering. The sintering temperature was 1600℃and the holding pressure was 30 MPa. Densification rate curve of MoSi₂\30vol% SiCw is significantly different from MoSi₂\₂0vol%SiCw and MoSi₂\10vol%SiCw by studying the sintering kinetics of the system. The content of the SiCw affected on the densification significantly. And the shape of whisker is not the main factor affecting sintering.The mechanical properties improvement of MoSi₂ by the adding of SiCw was significant. The fracture toughness of the composite materials rose with the increasing of SiCw content. As the bamboo-shaped outer surface of SiCw and the residual compressive stress results from thermal expansion mismatch between SiCw and MoSi₂, interface debonding is very difficult. Two important whisker toughening mechanisms of whisker bridging and pullout were both limited to playing their roles. The main toughening mechanisms are crack deflection, grain refinement and residual stress toughening.The oxygen-acetylene device was set up for the thermal shock and oxidation dynamic testing. Calculations shows the rise of strength is the key factor for the increasing of thermal shock fracture resistance in MoSi₂\SiCw composite. And lower thermal expansion coefficient was account for the rising of thermal shock damage resistance in MoSi₂\SiCw. Although theoretical calculations indicated that the SiCw was able to improve the material thermal shock resistance, all of the samples were damaged in the actual thermal shock tests. It was infered that MoSi₂\30vol%SiCw has a higher thermal shock resistance than MoSi₂\₂0vol%SiCw and MoSi₂\10vol%SiCw by studying the surface and the fracture macrograph of the thermal shocked samples with different content of SiCw. MoSi₂\SiCw generated compact surface SiO₂ protection layer at 1710℃dynamic oxidation conditions. So there was no ablation in 4 min. And the silica oxide layer began to severely degrade up to 1817℃.