Synthesis of high-purity Ti3SiC2 and Ti3AlC2 by hot-pressing (HP)

Abstract

Recently, both Ti3SiC2 and Ti3AlC2 had gained interest due to their unique combination of properties that make them candidates for many high temperature applications. They combine the merits of both metals and ceramics. Like metals they are thermally and electrically conductive, easy to machine with conventional tools, and resistant to thermal shock; like ceramics they have high strength, high melting point and thermal stability. Moreover, Ti3SiC2 and Ti3AlC2 are also damagetolerant materials [1]. The fabrication of single phase, bulk dense samples of both Ti3SiC2 and Ti3AlC2, has been proved to be very difficult. Recently, Barsoum and his co-workers have fabricated high-purity Ti3SiC2 and Ti3AlC2 polycrystals by hot isostatically pressing (HIP) a mixture of Ti, graphite and SiC powders, and Ti graphite and Al4C3 respectively [2, 3], however the process was very complex. Most recently, our early research revealed that appropriate addition of aluminum improved the synthesis of Ti3SiC2, And polycrystalline bulk Ti3SiC2 material with high-purity could be fabricated by spark plasma sintering (SPS) an elemental powder mixture with a starting composition of Ti: Si: Al: C = 3: 1: 0.2: 2 in molar ratio [4]. The objective of this work was to fabricate high purity Ti3SiC2 and Ti3AlC2 by hot pressing. The effect of raw materials ingredients on their synthesis was especially investigated. All of the work was conducted using powder mixtures of TiC (99.2% pure, 8.4 μm), Ti (99.0% pure, 10.6μm), Si (99.5% pure, 9.5μm), and Al (99.8% pure, 12.8 μm) (all from Institute of Non-Ferrous Metals, Beijing, China). In brief, the mixture with a designed composition was first mixed in ethanol for 24 h, then placed in a graphite die, 20 mm in diameter, and finally sintered in a hot pressing system. The samples were heated at a rate of 50 ◦C/min until the requisite temperature was reached; the soaking times were 2 h, and the pressure was 30 MPa. Shown in Fig. 1 are the X-ray diffraction patterns of samples obtained from the mixture of raw materials ingredients of 2TiC + 1Ti + 1.2Si (in molar). Sintered at the temperature range of 1200–1400 ◦C, the main phases were Ti3SiC2 and TiC. At 1200 ◦C, the peaks of Ti3SiC2 were very weak in contrast with those of TiC. The strengths of peaks of Ti3SiC2 reached the maximum levels at the sintering temperature of 1300 ◦C. Sintered at 1400 ◦C, peaks of Ti3SiC2 became weakened, while those of TiC became strong. The results indicated that it is difficult to synthesize high purity Ti3SiC2 from the mixture with the above materials ingredients. Fig. 2 shows the X-ray diffraction patterns of samples obtained from the mixture of raw materials ingredients of 2TiC + 1Ti + 1Si + 0.2Al (in molar), which means 0.2 molar of Si was substituted by the same amount of Al. Judging from the X-ray diffraction patterns, for sample sintered at 1200 ◦C, the product reached a high purity, only a very weak peak of TiC (2θ = 41.82 ◦) was identified by X-ray diffraction. For samples sintered at both 1300 ◦C and 1400 ◦C. The products were of pure Ti3SiC2, no phase but Ti3SiC2 was identified by X-ray diffraction. Shown in Fig. 3 are the X-ray diffraction patterns of samples obtained from the mixture of raw materials ingredients of 2TiC + 1Ti + 1Al + 0.2Si (in molar). The results revealed that high purity Ti3AlC2 could be synthesized by hot pressing at the temperature range of 1300–1500 ◦C from the mixture of the mentioned above raw materials ingredients. Fig. 4 shows the scanning electron micrographs of the fracture surfaces of Ti3SiC2 and Ti3AlC2 materials synthesized at 1400 ◦C. The grains of both Ti3SiC2 and Ti3AlC2 were plate-like. Ti3SiC2 grains have a size of 4–10 μm in elongated dimension, while Ti3AlC2 developed to larger grains with a size of 10–25 μm in the same direction. The densities of Ti3SiC2 and Ti3AlC2 materials prepared at 1400 ◦C were measured to be 4.43 and 4.19 g/cm3, which reached 97.8%, and 98.6% of their theory densities, respectively. It is concluded that polycrystalline bulk Ti3SiC2 and Ti3AlC2 materials with high purity and density