Synthesis of SiC/Ti3SiC2 composites with high SiC contents: sintering and thermodynamics
ROGER J. 1, CHAHHOU B. 1
1 Laboratoire des Composites ThermoStructuraux, UMR 5801, Université de Bordeaux, CNRS, CEA, SAFRAN, 33600 Pessac, France , Pessac, France
Recent works have shown the interest of strengthening the MAX Ti3SiC2 phase with SiC grains within SiC/Ti3SiC2 composites with SiC contents reaching up to 40% in volume. MAX phases are a family of ternary compounds with exceptional properties and whose general formula is Mn+1AXn, n = 1, 2 or 3, where M is a transition metal, A is an element belonging to groups IIIA and IVA, and X represents carbon and/or nitrogen. Among the MAX phases, titanium silicon carbide, Ti3SiC2, has gained much interest due to its combination of mechanical, electrical, and thermal properties which make it a serious candidate for high-temperature applications. Ti3SiC2 has also a low density, a good thermal conductivity and an elevated electrical conductivity, a high elastic modulus, a high compressive strength and a high flexural strength. Additionally, Ti3SiC2 has a good machinability and a thermal-shock resistance, and a high temperature oxidation-corrosion resistance. In order to improve the mechanical properties and/or increase the oxidation resistance, combinations of Ti3SiC2 with another phase such as SiC were previously studied in the literature. SiC is particularly interesting as reinforcement phase because it has high oxidation resistance, wear and hardness. This study concerns the synthesis of SiC/Ti3SiC2 composites with SiC volume fractions between 0 and 70%. These Ti3SiC2+nSiC composites were obtained from mixtures based on the reactions of 2TiC+1Ti+1.2Si+0.2Al+xSiC and 3TiC+2.2Si+0.2Al+xSiC mixtures by natural sintering or under load (SPS) at 1500°C for 15 minutes. Four final volume percentages of SiC were targeted: 0, 21, 50, 60 and 70% by adding the appropriate amounts of SiC powder to the reactive mixtures. Thermodynamic calculations in the Ti-Si-C-O-Al system were used to support this work using the ThermoCalc software. The results of this work showed that the starting composition and the application of a load during the sintering played a crucial role. During the rise in temperature, the presence of oxygen in the powder mixture caused the volatilization of silicon and consequently created a shift in the composition. Titanium carbide, which was an initial precursor, was then present in large quantities due to a silicon defect. The loss of silicon was amplified by SPS sintering due to the applied load which caused the ejection of the liquid silicon from the mold. Adding aluminum to the starting composition made it possible to obtain high purity Ti3SiC2+nSiC composites. The effect of aluminum was linked to the formation of a liquid at low temperature which reacted with the oxygen present at the surface of the gains of the reagents by forming grains of alumina (Tmelting(Al)=660°C and Tmelting(Si)=1410°C). This deoxidation of the grains then promoted their reaction. In addition, the dissolution of silicon in the molten aluminum accentuated its diffusion and promoted the initiation of the reaction. Elaboration by SPS ensured significant densification and limits the porosity. However, the results showed that the content of added SiC had a negative effect. Indeed, for the highest targeted SiC contents, the residual porosity was greater in the materials and the reaction of formation of Ti3SiC2 was found less favourable.