Research On The Preparation And Properties Of Reaction-bonded Al₂O₃/SiC Composites

Silicon carbide(Si C) materials possess excellent physical and mechanical properties such as high mechanical performance at high temperature, high thermal conductivity, low coefficient of thermal expansion and good thermal shock resistance. However, the widespread application of Si C composites has been restricted because of the poor sinterability due to a low self-diffusion coefficient and low oxidation resistance as non-oxide materials. The optimum methods to modify the performance are using special sintering methods or adding sintering aids or second phases to Si C composites. Therefore, based on the principle and process of reaction sintered nitrides bonded silicon carbide materials, we find that preparing reaction-bonded Al2O3/Si C composites is a good idea. The main purpose of this article is to prepare reaction-bonded Al2O3/Si C composites with good physical and mechanical performance at low sintering temperature. It will have a very important significance to develop the application of Si C composites.Firstly in this experiment, α-Al2O3﹣ Si C composite ceramics were synthesized successfully buried in graphite with silica, ultra-fine aluminum powder and various carbons as raw materials. The effect of process parameters on chemical reaction of Si O2﹣Al﹣C system was investigated through DSC, XRD and SEM. The experimental results were shown as follows. The pure and uniformly distributed Si C whiskers and Al2O3 particles can be obtained at 1450 ℃ for soaking 3 hours when the stoichiometric ratio of Si O2﹣Al﹣C system was 3:4:4, molding pressure was equal or greater than 10 Mpa and carbon black was appropriate. The open porosity, volume density and bending strength were 13.39 %, 3.12 g/cm3 and 172.4 MPa.Then Al2O3/Si C composite has been successfully fabricated at 1450 ℃ holding 3 hours with different sizes Si C, silica, ultra-fine aluminum powder and carbon black as raw materials. XRD analysis showed shat the phases of this composite consisted of Si C and α-Al2O3. Si C small particles and α-Al2O3 filled in the continuous gaps among Si C coarse and middle grains. The effect of partical size distribution and the content of Al2O3 on Al2O3/Si C composite has been studied through XRD and SEM. Based on the analysis of orthogonal experiments, it could be concluded that when the particle size distribution was 6:2:3:1, the Al2O3/Si C composite showed the optimum properties. The open porosity, volume density and bending strength were 15.39%, 2.71 g/cm3 and 70 MPa. The bending strength after once thermal cycle was 93.1 MPa at 1200 ℃. The oxidation gain in weight was 0.153 % at 1350 ℃ for holding 2 hours. It was found that the reaction-bonded Al2O3/Si C composite showed inter/transgranular fracture mode.Finally, the thermal shock resistance and antioxidant properties of the optimal sample were tested. The flexural strength increased initially, then decreased and trended to balance ultimatlly after six thermal cycles at 1200 ℃. The relationship between oxidation time and gain in weight at 1350 ℃ in air followed a parabolic law. The gain in weight of composite was 0.72 mg/cm2 after 20 h.