Preparation And Microstructure Evolution Mechanism Of Porous TiC-TiB₂-NiAl

TiC-TiB₂possesses the advantages of high melting point, high hardness, excellentthermal conductivity, low coefficient of thermal expansion, high abrasive resistance andexcellent electrical conductivity; it has promising application as high temperaturefiltration materials. NiAl intermetallic compound has high strength, high temperatureresistance, excellent thermal and electrical conductivity, and good high temperatureoxidation resistance, which make it an ideal purifier carrier and filter materials forexhaust purifier. Therefore, this study focuses on the development of TiC-TiB₂-NiAlcomposites with aiming for carriers and filters. Self-propagating High-temperatureSynthesis technology was employed to obtain TiC-TiB₂and TiC-TiB₂-NiAl compositeswith porous structures. The phase composition, microstructure, pore structure and itsevolution mechanism were systematically studied. Results are expected to provide thebasis of theory and experiment for further research and applications of the composites.(1) Porous TiC-TiB₂ceramics were prepared by SHS with Ti and B₄C as rawmaterials. The influences of various parameters on porous TiC-TiB₂were analyzed,including the grain size and ratio of raw materials, the content of C particles and theblocking pressure.The results show that the phases consist of TiB₂and TiC, however, phases insamples with coarse particle B₄C(75μm) is Ti(C, N), TiB₂and little reactants of B₄C.The reaction of Ti-B₄C is uncompleted. Then by increasing Ti content, the ratio of C/Nchanges from7/3to3/7and finally without B₄C was detected. When adding C particles,the relative content of TiC increases. The blocking pressure has no effect on phasecomposition.The TiC-TiB₂has a complicated porous structure with open pores plusclosed pores. The open pores distribute uniformly and interconnected with each other. Alarge number of closed pores exist in the inner of skeletons. The morphology of closedpores shows high similarity to the initial B₄C grains. With the B₄C particle size decrease,the diameters of closed pores decreases and the porosity of samples increase. Byincreasing the Ti content in3Ti-B₄C(75μm) system, the sizes of closed pores decrease,the pore wall thicknesses increase, and the porosity decreases. When finer Ti particlesused, the diameter of open pores decreases, the number of holes decreases first, and thenincreases and then decreases again, while the skeleton keeps dense. Increase the Ccontent, a part of C plays the role of the pore-forming agents and the porosity increases. (2) Porous TiC-TiB₂-NiAl composites were prepared by SHS from3Ti-B₄C systemby adding Ni-Al (atomic ratio1:1). Reaction products consist of TiC, TiB₂and NiAlthree phases. The content of NiAl has no influences in the phase composition. Theinitiate manners of SHS have no effect on product phases either. TiC-TiB₂-NiAl hascomposite structure of open pores plus closed pores, too. With the NiAl content increase,the Shape and distribution of open pores gets unevenly, and the number of closed poresdecreases, i.e. the porosity of reaction products reduce, however, the compressionstrength are enhanced.(3) The influences of powder particle size of Ti and B₄C, the technology ofpreparation, different Ti sources and the content of Ti and NiAl on microstructure andthe formation of TiC-TiB₂were studied by analyzing the liquid content, reaction heatrelease and reaction process, which impact the reaction rate. Combined with XRDanalysis of vacuum sintering products,â…¡ the reaction process of Ti-B₄C is as follows:Ti+B₄C→TiB+TiC→Ti3B₄+TiC→TiB₂+TiC.(4) The formation mechanisms of the pore structure were analysed in the reactionprocess of Ti-B₄C and Ti-B₄C-Ni-Al. The formation mechanism of closed pore wasproposed based on the decomposition of B₄C and the reaction process. Atomic diffusionlayer was formed between Ti and B₄C at the initial reaction stage. The liquid phase Tiappeared with the temperature rise, which can promote the decomposition of the B₄C,and C, B atomic spread rapidly that combined with Ti, thus TiC and TiB₂were generated.Interstitial vacancy was migrated towards B₄C particles and to form closed pore with thedecomposition and consumption of B₄C as well as the reaction of Ti-C/B. By addingNi-Al, liquid Al will form firstly and spread. Due to the exothermic reaction of Ni-Al,the melting of Ti and the decomposition of B₄C, the Ni-Al-Ti-B-C liquid was formed.TiC and TiB₂would nucleate and precipitate from the liquid. Finally, closed pore wereformed by the migration and growth of interstitial vacancies.