Preparation And Characterization Of SiC/MoSi₂ Composites By Mechanical-Assistant Combustion Synthesis

MoSi₂ is a special material which behaves both as a ceramic and as an intermetallic compound. MoSi₂ has attracted great research interest due to its rather low density, high melting point, good thermal conductivity, good compatibility with many reinforcements and very good anti-oxidation properties at high temperature, even in very aggressive environments. In this work, MoSi₂ and SiC/MoSi₂ composites have been successfully prepared by pressureless sintering from mechanical-assistant combustion synthesized powders. Then the mechanical properties at room-temperature were investigated. The phase composition, microstructure and surface micrograph were observed by means of SEM and XRD. The cyclic oxidation behaviors of SiC/MoSi₂ composites at the temperature of 500℃and 1200℃were studied, and the anti-oxidation mechanism was discussed.1. 20vol%SiC/MoSi₂ composites have been successfully prepared from mechanical-assistant combustion synthesized powders. XRD analysis results showed that the as-milled powder's phase composition consisted of Mo, Si and C, the as-synthesized powder's main phases were MoSi₂ and SiC. Mechanical milling refined the sizes of powders and improved their activity. The composites obtained best mechanical properties when they were prepared with the milling rotation speed and time being 400r/min and 2h, 0.9% Zinc stearate being added in the raw powders, Y₂O₃ being added in the as-synthesized powders and sintering at the temperature of 1500℃for 3h. The relative density, flexural strength, Vickers hardness and fracture toughness of the sintered samples were up to 93.9%,414.5MPa, 13.1GPa, 8.58MPa·m1/2, respectively. As a dispersant, Zinc stearate promoted the refinement of the powders. Y₂O₃ refined the grain size of the composite, so promoted the activated sintering of SiC/MoSi₂.2. The mechanical properties of SiC/MoSi₂ composites have a greater increase than that of MoSi₂. The mechanical properties of SiC/MoSi₂ composites increased quickly accompany with the increase of SiC firstly, and then decreased when the fraction of SiC exceeded 15%. The flexural strength, Vickers hardness and fracture toughness of 15vol%SiC/MoSi₂ were up to 493.6MPa, 16.8GPa, 9.35MPa·m1/2, increased by 36.5%, 73.9%, 71.6% as compared to MoSi₂ respectively. The strengthen mechanisms of the composites were fine-grain strengthening and particle strengthening. The toughening mechanisms were fine-grain toughening and rack deflection toughening.3. SiC/MoSi₂ composites showed good low-temperature and high-temperature anti-oxidation properties. At 500℃, the cyclic oxidation curves of SiC/MoSi₂ composites were similar. During the initial 12 hours of the oxidation process, weight increased rapidly. After approximately 160 hours, oxidation weight gain was basically stable. After 240 hours, the oxide layers of SiC/MoSi₂ composites were smooth and dense with no"Pesting"phenomenon. At 1200℃, the cyclic oxidation curves of MoSi₂ and SiC/MoSi₂ composites were similar. The weights were increased firstly, then decreased and then increased again with the increase of oxidation time. After 60 hours, oxidation weight gain was basically stable. After 124 hours, the oxide layers of SiC/MoSi₂ composites and MoSi₂ were smooth and dense with no"Pesting"phenomenon. At the temperature of 500℃and 1200℃, the oxidation weight gain of 20vol%SiC/MoSi₂-Y₂O₃ was higher than that of 20vol%SiC/MoSi₂.