The Study Of Hexagonal Boron Nitride Based Ceramics By Combustion Synthesis

In this paper, h-BN ceramic and h-BN based ceramics, including h-BN+SiC, h-BN+SiC+SiO2, were fabricated by combustion synthesis, which also called Self-propagating High-temperature Synthesis (SHS), in the super-high nitrogen pressure (100MPa). Combustion process, phase composition and microstructures of the products were studied with X ray diffraction analysis (XRD) and scanning electronic microscope (SEM). Mechanical properties of the ceramics were also measured and analyzed. Through theoretical calculation, effect of diluents contents on the adiabatic temperature (Tad) and effect of porosity on the reaction pattern were analyzed, and finding that the reactions can proceed in the direction we designed, yielding the products we anticipated. Results show that the influence of diluents contents on the relative density of products was rather important. Percent conversion of the green compact contenting higher diluents was higher than the lower one. With the increasing of diluents contents, the relative density of products also increased. But the relative density of products has a peak point (1.38g/cm3) at the diluents content of 67%. This is because of the volume gain (149%) when B converted into h-BN. But when the diluents contents is too high, as volume gaining can not fill up the pore, the density will decrease. The influence of diluents contents on the mechanical properties of products was also important. With the increasing of diluents contents, the percent conversion and mechanical properties also increased. Bending strength and fracture toughness of products have peak points (41MPa, 1.2MPa·m1/2) at the diluents contents of 67%, and fall down exceeding this value because of the decreasing of volume gain. Fracture toughness has the same trend as bending strength. In order to reduce the production cost and improve the mechanical properties of the pure h-BN ceramic, we adopt two methods: one is that B4C, which is less expensive, replace of B, the other way is that add in some addition agent, which is hard and refractory. Experiments shows that the porosity decreased by 10%～20%, bending strength increased 10MPa. The method has practical purpose. At last, combining the technology of injection molding and combustion synthesis, h-BN+SiC thermocouple-protection tube were fabricated.