Reaction Behaviors And Mechanism Of Combustion Synthesis In The Al-Ti-BN System

Under the development of modern technology, fine engineering ceramic material are widely used in national defence and industry and some other advanced areas. And the demand for its comprehensive properties is growing fast. TiB₂ ceramics has so high melting point, high hardness, good specific conductance, chemical stability and wearing resistance property, that has a large space to be used as hyperthermia structural material, wearing resistance and corrosion-resistant material, electric engineering material and so on. AlN has good properties such as high heat conductivity, low linear expansibility, high melting point and high strength at room temperature. And TiN has a series of advantages, such as high-intensity, high-hardness, high-temperature resistant, acid and alkali resistant, wearing-resistance, good electric and heat conductivity. So AlN and TiN are the ideal choices to product TiB₂ composite ceramics.In recent years, by adopting many new means people have tried to product high-performance TiB₂ ceramic in a large scale and reduce cost. A large number of researches and surveys have been done for its manufacturing and technology ameliorating. One of the synthesis techniques is self-propagation high-temperature synthesis (SHS) technique, besides pure products, small dimension and homogeneous distribution, the simple technology and low cost are the reasons for its famous position in material science.SHS reaction of Ti-B system reacts so violent that releases large quantity of heat, the final product has low density and it is difficult to sinter. When researching in SHS reactions of Ti-BN system, people find that certain nitrogen pressure is required to produce compact results and to make sure the reaction carried out entirely. But if the nitrogen pressure is much high over enough, some micro-cracks may appear. For the above reasons, people choose many methods and techniques, such as adding some dilution or bonding agent or raise the chamber pressure, for full densification and improving the physical and mechanical properties of its products. About metal bonding agents, such as Al, Cu, Fe, Ni, etc., people have already done many researches on them. There are lots of reports that ideal metal-matrix composite materials have been produced successfully. However, as a result of metal existence in the materials, this kind of composite material cannot be used in high-temperature services. To keep good mechanical property of ceramics, AlN and TiN are the ideal choices to product TiB₂ composite ceramics with good properties.According to the researches on Ti-B system, Ti-BN system and Al-Ti-BN system, the paper takes thermodynamics calculation and analysis as the basis, it mainly studies the SHS reaction of Al-Ti-BN system. By means of differential thermal analysis and X-ray micro-diffraction results of the quenched sample, it primarily discusses on the reaction mechanism of Al-Ti-BN system. And it also analyses different effects on the reaction behavior and product with different Al content, BN granularity and Ti granularity. The mainly findings of the present study are following points:a) According to the calculation of thermodynamics, it is feasible that every two components in Al-Ti-BN system take place reaction; and there is a growing tendency for the possible inter-phase to change into TiB₂ and AlN. From the Mezhanov experience, when Ti:BN is equal to 1:2 and Al-Ti-BN system is not preheated, the Al content should be higher than 7.4wt.% and lower than 58wt.% to maintain the spontaneous SHS reaction.b) Differential Thermal Analysis (DTA) shows that, in Al-Ti-BN system every two components take place reaction. Al content and Ti granularity have some impacts on the ignition temperature of Al-Ti-BN system under DTA situation. When Ti granularity is low, the reaction is in advance.c) Al content has a very large impact on phase composition of SHS product and reaction behavior of Al-Ti-BN system. When Al content is about 35.6wt.%, combustion temperature reaches its maximum, approximately 2480K. When Al content is between 35.6wt.% and 55wt.%, as it rising, the combustion temperature of the system falls down, ignition requires long time and combustion wave velocity turns down.d) Ti granularity has a little impact on the phase composition of SHS product of Al-Ti-BN system. Ti granularity has a little impact on the SHS reaction behavior and the product size of Al-Ti-BN system. But 150μm Ti power has great impact on ignition time and combustion wave velocity of Al-Ti-BN system. It also makes the reaction proceed imperfectly.e) BN granularity has certain impact on the results of the system. It greatly inflences SHS reaction behavior of Al-Ti-BN system. As the BN granularity rising, combustion temperature falls down, ignition time be prolonged and the combustion wave velocity slowing down.f) The similarity for the phase evolution in the 25wt.%Al and 45wt.% Al–Ti–BN samples is that the TiAl3 phase forms prior to any other new phases, then AlN and TiB₂ forms, then TiN forms, and finally Al or Ti is exhausted. However, the reaction in the samples with 25%Al content is different from that in 45%Al-Ti-BN in final stage. TiN phase exists in the final products with 25%Al contents, but TiN phase formed in the middle stage finally disappears owing to excessive Al in 45wt.%Al-Ti-BN.