Effect Of Kinetics Factors On Products Of Self-propagating High-temperature Synthesis Reaction Of Cr-Ti-C System

With the progress and development of the scientific and technology, the demands towards combination properties of the materials tend to be stricter and stricter, especially in the aerospace field, there is a much higher requirement of the high-temperature performance of the material. In order to meet the needs of the development of the high-tech aerospace, it is absolutely necessary to exploit and develop the materials that combined with good insulation properties and mechanical properties. Genearlly, these materials include ceramics and intermetallic compounds and so on. Among them, the TiC ceramic has attraced widespread attention owing to its excellent combination of various properties, such as low density (4.93g/cm3), high melting point (3523K), high hardness (microhardness 28.5~32GPa) and high modulus of elasticity (350GPa). However, due to its characteristics of ceramic, such as great brittleness and low toughness, the structural applicantion of TiC ceramic is rather limited. Therefore, it has been reported that a third metal element (Me) addition to Ti-C system might be an alternative way to improve the properties of TiC ceramic, since the metal element can act as a bonding agent when the reaction is completed. In the previous study, the third metal elements mainly include Al, Cu, Fe and Ni and so on. However, the melting points of these matals are reletively low, and which significantly limits the serving temperature of the composite synthesized. In this paper, a third metal element Cr which has much higher melting point is added to Ti-C system. Besides of serving as a bouding agent, also Cr can replace Ti forming substitutional solid solution, consequntely, the present study is of great theoretical investagation and practical application value.The traditional technology of preparing high-temperature refractory materials is melting or sintering at a high temperature; however, this process needs a very high temperature and long time, resulting in greater consumption of energy and time. A new method developed recently for preparing of materials is used in this paper, which is called self-propagating high-temperature synthesis (SHS). Compared with other traditional methods, SHS has its unique advantages, such as high purity of products, low processing cost, time and energy efficiency, etc. Because of these advantages, SHS has been paid more and more attention by the materials science field.Based on thermodynamic calculation and analysis, the effects of the kinetics factors, namely, the particle size (Ti, C and Cr) and the composition (Cr content, Ti/C molar ratio and TiC/Cr7C3 volume ratio) of the reactant, on the products of SHS reaction of Cr-Ti-C system are studied. The principle findings of the present study are as follows:(1) The thermodynamic calculation and analysis show that TiC is the most stable phase in the products synthesized by the Cr-Ti-C system, and the reaction of synthesizing TiC has the highest exothermicity. Furthermore, the adiabatic combustion temperature decreases with the increase of Cr content expect for the phase transition ranges. In addition, according to the experiential criterion proposed by Mezhanov, when the reaction is ingnited at the room temperature, the SHS reaction can be self-sustaining only when the Cr content blow 64.74 wt.%.(2) It is found that the size of Ti has little effect on the phase composition of the SHS products of Cr-Ti-C system, which mainly consist of Cr7C3, Cr23C6 and (Ti,Cr)C. When the coarse Ti is used (~149μm), the morphology of (Ti,Cr)C systhesized is irregular. Moreover, with the size of Ti futher decreasing, the (Ti,Cr)C becomes more and more spherical, and the average size is ~2μm. However, the size of C has obvious effect on phase composition of the SHS products of Cr-Ti-C system. When the coarse C is used (~149μm), the reaction is incomplete; besides the (Ti,Cr)C is systhesized, the metastable phase Cr2Ti is also formed and lots of C remains in the final products. With the decrease of C particle size from ~38μm to ~1μm, the SHS reaction becomes more and more complete, and the CrxCy formed changes from low-Cr to high-Cr compounds. Nevertheless, the size of C has little effect on the size and morphology of (Ti,Cr)C, namely, the size of (Ti,Cr)C is maintain ~2μm and the morphology is nearly spherical. In addition, the size of Cr has little effect on the phase composition of the products as well as the morphology and size of the (Ti,Cr)C.(3) When the Cr content increases from 10 wt.% to 50 wt.%, the type of SHS products changes significantly, namely, a higher Cr content corresponds to higher chromium carbide. The average size of (Ti,Cr)C particulates decreases from ~8μm to ~2μm due to the decrease of combustion temperature. Besides, the solubility of Cr in TiC reaches the maximum when the Cr content is 20 wt.%.(4) The proportion experiments show that when the Ti/C molar ratio increases from 0.6 to 1.2, the lattice parameter of (Ti,Cr)C increases gradually, suggesting that the solubility of Cr in TiC decreases. Furthermore, the (Ti,Cr)C particulate size increases from ~1μm to ~5μm because of the increase of the combustion temperature, and the morphology of (Ti,Cr)C particulate becomes more and more spherical.(5) With the TiC/Cr7C3 volume ratio increasing from 1:3 to 3:1, the reactions take place more and more easily both in the argon protection and in the steel liquid condion. The quantity of (Ti,Cr)C systhesized increases gradually, and the morphology becomes gradually spherical.(6) The test of the particulate locally reinforced steel matrix show that the hardness of the materials becomes higher, and the wear resistance becomes better with the TiC/Cr7C3 volume ratio increasing from 1:3 to 3:1. Moreover, the wear resistance of the composites futher improves after the heat treatment.In a word, TiC ceramics have a briliant future for application and a great development space owing to the excellent combination of properties. Consequently, it is expected that the present study could establish some experimental and theoreticalfoundation for the in situ synthesis of TiC ceramics.