Research On The Synthesis And Properties Of Ti₄AlN₃ By Spark Plasma Sintering

There are two ternary compounds in Ti-Al-N system:Ti2AlN and Ti4AlN3, which belong to the class of Mn+1AXn (n=1,2,3). Bulk Ti2AlN and Ti4AlN3 combine unusual properties of both metals and ceramics. Like metals, they are good thermal and electrical conductors, relatively soft and can be easily machined with traditional drill without lubrication or cooling water. Like ceramics, they are elastically stiff; exhibit excellent high temperature mechanical properties. They are resistant to thermal shock and unusually damage tolerant. Recently, research work mainly concentrates on Ti2AlN thin films and bulk. However, the fabrication of single phase, bulk dense samples of Ti2AlN, has still remained difficult. In this work, the first objective was to fabricate bulk Ti4AlN3.In the previous research, there is no report about the mechanics of phase formation of Ti4AlN3. Anglicizing the phase formation will be beneficial to accomplish the controllable synthesis of Ti4AlN3 or other Ti4AlN3 composite, while will be conducive to improve the material's properties and enlarge the material's application range. The second objective was to research the mechanics of phase formation of Ti4AlN3.Thirdly, the mechanics of phase formation, microstructure and properties of Ti2AlN and Ti2AlN/TiN composite were especially investigated.In light of the three major objectives above and after a series of experiments based on an extensive survey of literatures, the following conclusions were drawn:The effect on preparing Ti4AlN3 material, which include different powder stock, mole ratio, spark plasma sintering temperature, spark plasma sintering processes, annealing temperature and holding time, was researched. The powder mixture of TiN, Ti and Al was used with a designed composition of Ti:Al:TiN=1:1.2:3 (in molar ratio). The mixture was firstly mixed in ethanol for 24h, and then spark plasma sintered at 1300℃for 8 min, and finally was annealed in argon ambient for soaking time of 4 h. The density of dense Ti4AlN3 samples with few TiN was measured to be 4.59 g/cm3, reaching 99.6% of the theoretical density. The grains were well developed, homogeneous, and exposure plate-like with the size of 2～5μm and 10～15μm in thickness and elongated dimension, respectively. And the grains present plane-plane contact. The grain boundary phase and porosity were much few. Ti4AlN3 grain presents obvious hexagonal structure.The the mechanics of phase formation of Ti4AlN3 was anglicized by X-ray diffraction, scanning electron microscopy analysis and energy-dispersive spectroscopy. The results indicate that Ti react with Al to form intermetallic TiAl3 during 643.9～ 911.6℃. TiAl3 react with TiN to form Ti2AlN during 911.6℃～1214℃. In the range of 1214～1320℃, TiN react with Ti2AlN to form Ti4AlN3. When sintered at 1300℃, the Ti4AlN3 grain was well developed and exposure plate-like with the size of 5μm and 12μm in thickness and elongated dimension, respectively. And when the sintered temperature was higher to 1300℃, Ti4AlN3 generated opposite direction at high temperature.The bulk Ti4AlN3 material prepared with the best mole ratio and technological condition is soft and high-intensitive. The Vickers hardness of sample was 2.7 GPa. The fracture toughness and flexural strengths at room temperature were 6.2 MPa·m1/2 and 350 MPa, respectively, which indicate this material to be a very promising candidate for application as structural material. The thermal conductivity and molar heat capacity at normal atmospheric temperature, were 12.3W/m-K and 145J/mol-K, respectively, while the average thermal expansion coefficient at 25～1100℃was 9.3×10-6K-1. That displays that the prepared Ti4AlN3 material possesses excellent thermal conductivity and thermal shock, which compatibly applies to high-intensitive and high- abrasion resistance high temperature component. The electrical conductivity at common temperature was 4.9×105Ω-1/m, which is similar to metal and is lower than other ternary layered nitride. Prepared with the Ti4AIN3 material prepared by hot isocratic pressing or hot pressing, the bulk Ti4AlN3 material synthesized by spark plasma sintering shows a big advantage. The prepared process is shot-time consuming, low cost and easy to realize industrialization. The prepared materials possess high machinability by conventional tools, high strength, excellent thermally and electrically conductivity.In conclusion, the synthesis of Ti4AlN3 by SPS and annealing, the microstructures, the sigma-phase formation and physical properties were together investigated in this paper. The research results may be helpful to synthesis the Ti4AlN3 bulk materials, and to develop a new technical approach of other high-performance ceramic materials. These will provide the application of theory and practical technical approach a useful method to layered ternary carbides or nitrides.