Research On Performance Improvement Of MoSi₂ By Cooperation Of SiC/ZrO₂

MoSi2 intermetallics possess an unusual combination of the properties such as low density, high melting temperature and good oxidation resistance at high temperatures. These unusual combinations of the properties render them promising applications for friction materials at room temperature and high temperature structural materials widely used in the fields of aviation and auto. However, the high room-temperature brittleness and poor high-temperature strength are the main obstacles of its application. Based on nanoparticles modification technlolgy, the preparation of MoSi2 matrix nanocomosites was proposed compostied with SiC/ZrO2 nanoparticles or SiC whisker/ZrO2 nanoparticles in order to improve the combination properties of MoSi2 ceramics. Furthermore, systemic investigation were based on the preparation process, microstructure and mechanical properties, which provide foundation for the application of MoSi2 matrix nanocomosites as sturcture materials.The main work done and the results obtained in this thesis are as follows:1. SiC/ZrO2 -MoSi2 composite powders were prepared by multi-phase suspending solution mixture. In this experiment, polyethylene glycol acted as dispersant agent, water acted as dispersive medium, and dispersed the SiC whisker in ultrasonic bath and adjusted pH in the ethanol suspending solution. According to above mentioned method, all phases complete mixed SiC(w)/ZrO2-MoSi2 composites powders were obtained.2. SiC/ZrO2-MoSi2 ceramic nanocomposites were prepared by hot pressed sintering. The synergic effect of SiC/ZrO2 to improve bending strength, fracture toughness and grain refining were observed by microstructure observation and mechanical property analysis. Compared to MoSi2 ceramics, for 20%SiC(p) +10%ZrO2+MoSi2, the bending strength increased by 3.8 times, the fracture toughness improved by 2.4 times; Furthermore, for 15% SiC(w)+15%ZrO2+MoSi2, the bending strength increased by 2.6 times, the fracture toughness improved by 2.5 times.3. The high-temperature fracture toughness of ceramic nanocomposites were analyzed used Moiréinterferometry for the first time. It was found the SiC/ZrO2 synergism enhanced the high-temperature fracture toughness of ceramic nanocomposites. The effect of ZrO2 particle on high-temperature toughening were better than that of SiC particle, and the effect of SiC particle on high temperature toughening were better than that of SiC whisker.4. The heat-shocking resistance of SiC/ZrO2-MoSi2 ceramic nanocomposites were studied using indentation-quick cooling method in the temperature range from 100℃to 600℃. The results showed that the SiC particle or whisker combined with ZrO2 nanoparticles changed the expand path and shape of the cracks in MoSi2 ceramics, which enhanced the heat-shocking resistance. The effect of SiC particle on improving heat-shocking resistance of MoSi2 were better than that of SiC whisker. The"pesting"phenomena, oxidation resitance as well as the forming mechanism of the surface film of SiC/ZrO2-MoSi2 nanocomposites were discussed. The results showed that the"pesting"phenomenon decreased or did not exist, the amorphous film was easy to form on the surface, and the SiC/ZrO2 synergism was advantageous to improve the high and low temperature oxidation resistance.5. The wear characteristic of SiC/ZrO2-MoSi2 nanocomposites were analyzed through the wear test on room temperature. The results showed that the SiC/ZrO2 synergism could improve the wear resistance of MoSi2 ceramics obviously. The addition of ZrO2 nanoparticles caused the proportion of adhesive wear increase, and the addition of SiC nanoparticles caused the proportion of abrasive wear increase. Abrasive wear characteristic of SiC/ZrO2 nanoparticles composited MoSi2 was more obvious than that of SiC whisker/ZrO2 nanoparticles composited MoSi2.6. The microstrain of SiC/ZrO2-MoSi2 nanocomposites were investigated using X ray diffraction firstly. The dislocation relation between SiC/ZrO2 synergism and MoSi2 matrix were analyzed. The mechanism of toughening and strenghening were discussed as well. It was discovered that the microstrain of ZrO2 decreased in the fracture process, which caused by part of stress inducing ZrO2 transform to form microcrack. SiC nanoparticles distributed in the composite ceramics induced high microstrain as the result of difficulty alleviating the stress that substrate around them bring or pass. ZrO2 produced dislocation in the substrate depending on volume effect caused by its own phase transitions. The pining effect of intracrystalline type SiC and ZrO2 particles on the dislocation in the composite ceramics was obvious, and the SiC whisker block dislocation movement which caused the dislocation tangle, intersect and formed the dislocation network knot. Moreover, the twin crystals appeared around the second particles as well as the SiC whisker caused stacking faults. The toughening meachanism of composites concludes the phase transitions toughening and microcrack forming of ZrO2 particles, the crack deflection and bridge union of SiC whisker or SiC and ZrO2 particles, the grains refined as well as"intracrystalline type"structure in the composites and so on. The strenghening meachanism of composites was mainly for the fine-grain strengthening and dispersion strengthening.