Synthesis, Microstructures And Properties Of Ti₂AlN And Ti₂AlN/TiN Composite

There are two ternary compounds in Ti-Al-N system: Ti₂AlN and Ti₄AlN₃, which belong to the class of M_n+1AX_n (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 Ti₄AlN₃. However, the fabrication of single phase, bulk dense samples of Ti₂AlN, has still remained difficult. In this work, the first objective was to fabricate high-purity bulk Ti₂AlN.Ti₂AlN has some weakness which is the obstacle of its application, just like low hardness and weak corrosion resistance. TiN is a ceramic with high strength and hardness; it is resistant to elevated temperatures and corrosion; it owns good electrical and heat conductivity, and the most important is that the thermal-expansion coefficients of TiN and Ti₂AlN are similar. Therefore, introduction of small amount of TiN particles into the Ti₂AlN matrix to form Ti₂AlN/TiN composite may help to get the merits of both materials. The second objective was to fabricate Ti₂AlN/TiN composite.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:Polycrystalline bulk Ti₂AlN materials with high-purity could be fabricated by hot-pressing (HP) at 1300℃and spark plasma sintering (SPS) at 1200℃the mixtures with the raw materials ingredients of n(Ti):n(Al):n(TiN)=1:1:1. Ti₂AlN phase was well-developed with a close and lamellated structure; the grains were plate-like having the sizes of 3-5μm and 8-10μm (HP) and 8-12μm and 20-30μm (SPS) in thickness and elongated dimension, respectively. The density of Ti₂AlN prepared by HP was 4.22 g/cm³, which reached 97.9% of its theory densities; the density of Ti₂AlN prepared by SPS was 4.15 g/cm³, which reached 96.2% of its theory densities. The reaction of Ti₂AlN could be divided into two stages: before 900 ℃, Ti reacts with Al to form TiAl intermetallics; above 900℃, TiAl reacts with TiN to produce dense Ti₂AlN. As Ti-Al bond weaker than the Ti-N bond, Ti₂AlN was easy to lose Al layers and change into Ti₄AlN₃ phase in high temperature.Introduction of 15vol% TiN as reinforcement into the Ti₂AlN matrix, Ti₂AlN/TiN composite could be fabricated by HP at 1300℃and SPS at 1200℃. Ti₂AlN phase was well-developed with a close and lamellated structure; the grains were plate-like having the sizes of 2-3μm and 5-8μm (HP) and 2-3μm and 8-12μm (SPS) in thickness and elongated dimension, respectively. TiN grains were small tetragonal particles having the sizes of 1-2μm. The density of Ti₂AlN/TiN composite prepared by HP was 4.30g/cm³, which reached 96.2% of its theory densities; the density of Ti₂AlN/TiN composite prepared by SPS was 4.23g/cm³, which reached 94.7% of its theory densities. Incorporation 15% volume ratio of TiN, the grain size of composite was smaller than that of single-phase Ti₂AlN, so the introduction of TiN could play the role of refined grains.The physical properties of single-phase Ti₂AlN and Ti₂AlN/TiN composite were measured and discussed in chapter 5. Compared with the mechanical properties of single-phase Ti₂AlN and Ti₂AlN/TiN composite sintered by HP and SPS, the flexural strength, fracture toughness and Vickers hardness of Ti₂AlN/TiN composite were better than those of single-phase Ti₂AlN. The introduction of TiN would help to enhance the mechanical properties. The electrical conductivity of Ti₂AlN was 3.20×10⁶S·m^-1; the electrical conductivity of Ti₂AlN/TiN composite was 3.72×10⁶S·m^-1 and higher than Ti₂AlN/TiN composite. The high temperature performance of Ti₂AlN was investigated by high temperature microscope. The softening temperature of Ti₂AlN was at the range of 1500℃—1580℃and its melting temperature was higher than 1700℃. Compared with the quantum chemical calculations of Ti₂AlN and Ti₂AlC, the ionic and covalent bonds of Ti₂AlN were weaker than those of Ti₂AlC, so the mechanical properties of Ti₂AlN were lower than those of Ti₂AlC. Analysis of the total density of states, the trend forming forbidden band of Ti₂AlC was more evident than that of Ti₂AlN, which made the conductivity of Ti₂AlN higher than Ti₂AlC.In chapter 6, the oxidation behavior of Ti₂AlN and Ti₂AlN/TiN composite was investigated. Constant temperature oxidation study showed that the oxidation kinetics of Ti₂AlN and Ti₂AlN/TiN composite in the temperature range of 800-1000℃followed the parabolic law. The liveweight gains of the two samples were similar at 800 and 1000℃; at 900℃, the liveweight gain of the composite was less than that of Ti₂AlN, which mean that oxidation resistance of Ti₂AlN/TiN composite was better than that of Ti₂AlN at this temperater. The activation energies of Ti₂AlN and Ti₂AlN/TiN composite were 614.93 KJ·mol^-1 and 541.35 KJ·mol^-1 respectively. Cyclic-oxidation study showed that the oxidation kinetics of Ti₂AlN and Ti₂AlN/TiN composite in the temperature range of 800-1000℃for 30 cycles followed the parabolic law, with good thermal shock resistance. The liveweight gain of the two samples were similar for cyclic-oxidation at 800 and 900℃; at 1000℃, the liveweight gain of the composite were less than that of Ti₂AlN, which mean that cyclic-oxidation resistance of Ti₂AlN/TiN composite was better than that of Ti₂AlN at this temperature.Chapter 7 deled with the corrosion behavior of Ti₂AlN and Ti₂AlN/TiN composite. The corrosion rates of Ti₂AlN/TiN composite were nearly half than that of Ti₂AlN immersion in acid, particularly in HNO₃: the corrosion rates of Ti₂AlN in concentrated and dilute HNO₃ were 95.68μm/yr and 14.65μm/yr, and the corrosion rates of Ti₂AlN/TiN composite were 5.81μm/yr and 1.93μm/yr. The resistance of acid corrosion of the two materials was much better than that of alkali corrosion, but etched in NaOH, the corrosion rate of composite was larger than Ti₂AlN, 0.58μm/yr and 0.074μm/yr respectively. TiN had good corrosion resistance and chemical stability, so the resistance of Ti₂AlN/TiN composite was much better than that of Ti₂AlN in acid corrosion. But TiN in NaOH may decompose and escape hydrogen and ammonia after prolonged immersion, which made the resistant to corrosion of materials in NaOH lower.In conclusion, the synthesis of Ti₂AlN and Ti₂AlN/TiN composite by HP and SPS, the microstructures, the physical properties, the oxidation and corrosion behavior were together investigated in this paper. The research results may be helpful to synthesis the Ti₂AlN bulk materials and the composites materials in Ti-Al-N system, 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.