Preparation, Structure And Properties Of Cf/BN-SiC Composites

The boron nitride is an advanced ceramic with many advantageous properties owing to its excellent properties such as high temperature stability, high oxidation resistance, self-lubricating capability, low density, etc. It has great potential in the applications such as thermal protection materials, advanced bearing materials, high-temperature wave-transparent materials, etc. In this dissertation, we have studied the preparation of carbn fiber reinforced boron nitride and silicon carbide(Cf/BN-SiC) composites through the precursor infiltration and pyrolysis(PIP) combined with gaseous silicon infiltration(GSI) process.On the basis of GSI route that introduces silicon, the BN matrix of Cf/BN-SiC composites was prepared through the PIP route. The amount of BN matrix can be controlled by adjusting the concentration of borazine in the precursor solution. Owing to the poor wetting ability between BN and Si in the GSI process, the composites have low density and high porosity. The effects of BN inclusions on mechanical, friction and oxidative properties of Cf/BN-SiC composites are investigated as follows.Fristly, the effects of BN content on mechanical properties of Cf/BN-SiC composites has been studied. Cf/BN-SiC composites that are pyrolyzed by three cycles of PyC have the best mechanical properties and that the mechanical properties decrease with increase of BN inclusion. That is, the flextural strenghs are 126.1MPa, 97.9MPa, 71.4MPa and 52.4MPa; whereas the elastic moduli are 19.1GPa, 17.6GPa, 16.5GPa and 12.7GPa, for the Cf/BN-SiC composites with an inclusion of BN at 0%, 1.6vol.%, 6.6vol.% and 11.8vol.% respectively. This is attributed to that BN in the composites forms a porous and discontinuous interphase between carbon fiber and matrix.Secondly, the effect of BN content on friction and wear properties of Cf/BN-SiC composites has been studied. It is found that the Cf/BN-SiC composite has a lower mass wear rate than the Cf/SiC counterpart owing to the inclusion of BN enabling the adjusting of the friction coefficient of the composites. The friction coefficient of the composites gradually decreases from 0.70 to 0.56 with an increase of the BN content. This is because that hexagonal-BN has a good self-lubrication property as a result of a layered crystal structure and that it tends to form a complete and continuous friction film that reduces the shear wear among fibers during the wear process.Lastly, the oxidation behavior of Cf/BN-SiC composites has been studied. In a sharp contrast to the poor oxidative performance of Cf/SiC composites, the introduction of BN has greatly improved the oxidation-resistance behavior. That is, the composites with 11.8vol.% of BN introduced, it shows almost no weight loss up to 800℃ in air and still keeps 82.2% of its initial weight at 1100℃. This excellent oxidation-resistant behavior is attributed to that BN oxidizes to B2O3 that has a good fluidibility and self-healing properties. The B2O3 forms a dense protective layer on the fiber surface and thus isolates them from the attack of oxygen. In addition, owing to the good fluidibility of B2O3, it tends to fill and block the holes formed from the oxidation of fibers.