Combustion Synthesis And Properties Of Al(Cr)₂O₃-Cr(Mo) Cermets

The thermodynamic calculations are firstly carried out, and the effects of diluents (Al₂O₃ & Cr₂O₃) and high exothermic reaction systems (Al-CrO₃ & Al-MoO₃) on the Al-Cr₂O₃ combustion synthesis temperature (starting temperature T₀ & adiabatic temperature T_ad) under ideal hypotheses are investigated. The results show it is possible to change the products composition in a rather large scale, adjust combustion temperature (T₀ & T_ad), and so as to improve the microstructures and properties of products by means of adding diluents (Al₂O₃ & Cr₂O₃) and high exothermic reaction system components (Al-CrO₃ & Al-MoO₃) into Al-Cr₂O₃ reaction system without affecting the sustainable ability of combustion synthesis reaction. Base on the results of thermodynamic calculation, the effects of reactant ratio on the combustion synthesis of Al-Cr₂O₃ system have been investigated respectively under self-propagating mode and thermal explosion mode.Under thermal explosion mode, the effects of heating rate, relative density of reactant compact and some other processing parameters on the reaction are investigated. A rather large heating rate can make reaction excessively fury. When the heating rate is much slower, the reaction may be difficult to happen. A relative low density of reactant compact may cause the thermal explosion reaction disabled. Based on the foregoing works, the polymer is selected as reaction active component and some diluents are added into Al-Cr₂O₃ system. By means of thermal explosion plus hot-pressing process, the cermet products with relative density of 90 percent, certain shape and size are fabricated successfully. With the pressure increasing on the high temperature reaction product, the density of product can be improved efficiently. Furthermore, cermets fabricated by this means have certain shape and size, which provide us a new method to solve the problem of hard machinability of cermets.By using X-Ray diffraction instrument, SEM, energy spectrum analysis, and metallographic microscope, the phase composition and microstructure morphology are investigated and analyzed. The results show that the ceramic phases of combustion synthesis product are consisted of (Al,Cr)₂O₃ solid solutions with different solid solubility, and Al₂O₃ is in the majority. There are fine, nano-size metal particles in the ceramic matrix. In the microstructure of products, ceramic phase has three typical morphoses, block grains with round edges, small slag grains crossing with different orientations and bigger slag grains. Metal particles distribute transgranular or intergranular in ceramic crystal grains, or form eutectic mixture with ceramic phase. The eutectic mixture of products has a nearly regular structure. Fine rod metal phase disperses in the continued ceramic matrix. The distribution of eutectic mixture is regionally. Basing on the studies of reaction process and microstructure, the reaction micro-mode is proposed.Finally, some properties of products including micro-hardness, wear resistance and high temperature oxidation resistance are tested and contrasted with some materials. The test results show that Al(Cr)2O3-Cr cermets fabricated by thermal explosion plus hot-pressing process have superior wear resistance than hot-pressed and sintered Al₂O₃-base ceramic material. The high temperature oxidation of Al(Cr)₂O₃-Cr cermet follows a parabolic law, and it is controlled by a diffusion mechanism.