Study On The Mechanism And Process Of Combustion Synthesis Of Hexagonal BN-Based Ceramics

Hexagonal boron nitride (h-BN) is a high-performance structural ceramic with excellent properties. The development of technology raises requirements for advanced ceramics with increasing properties and low cost. In this paper, h-BN-based ceramic composites were prepared by combustion synthesis and instantaneously hot isostatic pressing (SHS-HIP) ignited under high nitrogen pressure from relative cheap raw materials of B₄C and Si or Ti, in which B₄C served as B source and Si or Ti acted as reducing agents. Based on the low elastic modulus, coefficient of thermal expansion and high thermal conductivity, h-BN-based ceramic composites have excellent corrosion resistance and thermal shock resistance.Firstly, thermodynamic and kinetic parameters of the B₄C-Si-N₂ system were calculated theoretically, and the combustion synthetic mechanism was analyzed. Process parameters such as compacts porosity, nitrogen pressure and diluent content have significant effect on the microstructure and properties of the products. The research results show that higher relative density and mechanical properties of the products were obtained with the increasing of nitrogen pressure. However, Si3N4 phase formed under higher nitrogen pressure over 120MPa; The relative density of the products has a maximum value at compacts relative density of 52.3%. h-BN-SiC ceramic composites with bending strength and fracture toughness were 83.3MPa and 1.96MPa·m1/2, respectively, were prepared under 100MPa nitrogen pressure from the compacts with relative density of 52.3%.Thermodynamic calculations of the B₄C-Ti-N₂ system were analyzed. The effect of Ti content in reactant on reaction, structure and mechanical properties of the products were studied by experiments. With the B₄C/Ti ratio changed from 1/1 to 1/4, bending strength and fracture toughness of the h-BN-TiCN ceramics increased from 42MPa and 0.7MPa·m1/2 to 67MPa and 1.1 MPa·m1/2, respectively. The reason for that is the increasing Ti(C,N) phase content in products. The N content in Ti(C,N) phase on the product surface is higher than that of interior product because of lower nitrogen penetration resistance during combustion reaction. In comparison with the experimental results of these two systems, B₄C-Si-N₂ system shows more intensive liquid-phase sintering, higher relative density and mechanical properties because of lower melting point of Si (1410oC) than that of Ti (1668oC). Relative density and mechanical properties of h-BN-AlN-based ceramic composites improved notably through introducing Al-TiB₂ combustion system into B4C-Si system because of good liquid-phase sintering of Al during combustion process. However, the thermal decomposition of TiB₂ phase under high nitrogen pressure and temperature during the combustion synthesis of B4C-Si-Al-TiB₂-N2 system leads to the non-homogeneous structure in resulting products. In order to overcome this drawback, we obtain homogeneous h-BN-AlN-based products by replacement of TiB₂ with TiN because of higher stability of TiN under high nitrogen pressure and temperature in experiments. Volume content of AlN-TiN has a critical effect on the mechanical properties of h-BN-AlN-based composites. When volume content of AlN-TiN was lower than 50%, h-BN was the main phase and determined the mechanical properties. Fracture characteristics show intergranular type. When volume content of AlN-TiN exceeded 50%, AlN was the main phase and dominated the mechanical properties and fracture characteristics exhibit transgranular type. Bending strength and fracture toughness improve to 274MPa and 5.1MPa·m1/2, respectively.Thermal shock resistance test on products with AlN-TiN content of 30, 50 and 70vol% were carried out. The results show that h-BN-AlN-based ceramic has excellent thermal shock resistance. h-BN was the matrix when h-BN phase was higher than 50vol%, and the excellent thermal shock resistance resulted from the low elastic modulus, coefficient of thermal expansion and high thermal conductivity of h-BN phase. The residual strength was 67% and 32.5% of original strength, and thermal shock temperature difference ((?)T) reached 1000oC and 700oC when AlN-TiN content was 30vol% and 50vol%, respectively. Microcracks bring by weak h-BN phase and dispersion strengthening of TiN grains make the thermal shock temperature difference ((?)T) reached 800oC when AlN-TiN content was 70vol%.h-BN-MgO and h-BN-Al₂O₃ composites were prepared by combustion synthesis under high nitrogen pressure from powder compacts of B₂O₃, acting as B source, and Mg and Al, acting as reducing agent, respectively. The reaction mechanisms of these two combustion systems were replacement reaction of thermite. 3MgO·B₂O₃ and 9Al₂O₃·2B₂O₃ phase were observed because of high reaction temperature and velocity, as well volatilization of Mg and Al. So the relative density was not high.