Research On The In-situ Synthesis And Sintering Properties Of Nano-BN/micron-TiB₂ Composite Powder

Titanium diboride (TiB2) is a new engineering ceramic material, with excellent physical and chemical properties that can be used to manufacture anti-oxidation coatings, wear-resistant plastic and high-performance tool materials. It also has excellent impact resistance and can be used as a candidate of high-performance bulletproof materials.But simultaneously the TiB2 ceramic material has high brittleness, poor thermal shock resistance and difficult machining. These poor problems limit its further application. Adding soft ceramics into the hard ceramic, such as h-BN of low intensity can effectively improve the machinability of hard ceramics. At present a chemical solution process is usually used to in-situ lead into nano-BN particles and it achieves the dispersion in the matrix. In this paper, TiB2 were dispersed into the solution of H3BO3 and CO(NH2)2 via a chemical solution process.Three main parts were involved in this study:research on the suitable technology of synthesizing BN;fabrication of BN/TiB2 composite powder; TiB2/BN composite powders was sintered by spark plasma sintering. The microstructure and properties of BN/TiB2 composite ceramic were studied.It was investigated that the impact on the crystal phase and morphology of BN synthesized by changing the process parameters such as raw material ratio, temperature,atmosphere, holding time. The optimum conditions of synthesizing BN were obtained. The result indicated BN could be prepared with the molar ratio of 3:1 for urea and boric acid at 1100℃for 8h under the flowing hydrogen/argon mixed atmosphere.The amorphous BN obtained had a higher yield and less residual condensation product of urea decomposition.According to the optimum condition of BN synthesis, the mixture of TiB2, H3BO3 and CO(NH2)2 were heated to fabricate BN/TiB2 composite powder. The research showed that TiB2 particles were coated by amorphous BN uniformly.After BN/TiB2 composite powder was heated at 1500℃,amorphous BN transformed into h-BN.After ball-milling, the average diameter of TiB2 was less than lum. A part of TiB2 was nitrided into TiN due to the higher surface activity, and in the atmosphere of high nitrogen, when the minor TiB2 was used to fabricate BN/TiB2 composite powder. BN/TiB2 composite powder was sintered by spark plasma sintering. The results showed that the microstructure of BN/TiB2 composite after sintering was uniform and dense. Amorphous BN transformed into h-BN and dispersed in the grain boundary, a small amount embedded into TiB2 grains.With the addition of BN content increasing, the electrical resistivity of BN/TiB2 composite ceramics gradually increased as Vickers hardness decreased. Compared with TiB2/BN composites by directly mechanical mixing, the studies indicated that nano-BN of BN/TiB2 composite powder inhibit the generation of TiB2 particles to a certain extent. The traditional ways of material mixing can not make BN evenly dispersed in the TiB2 matrix and in turn affect the uniformity of microstructure of sintered bodies.