Fabrication And Performance Of SiNO Fiber Reinforced Boron Nitride Wave-Transparent Ceramic Matrix Composites

As the key component of hypersonic aircraft, the radome must possess multifunction such as long life, thermal protection, electromagnetic tranparence and load bearing, especially the former two requirements. In this dissertation, aiming at requirements of radome, based on the review of wave-transparent materials, the disadvantages of existing material systems were analyzed, the SiNO fiber reinforced boron nitride ceramic matrix (SiNO_f/BN) wave-transparent composites were prepared by precursor infiltration and pyrolysis (PIP) process with Borazine as the precursor. The mechanical, dielectric and thermophysical properties of the composites were measured and evaluated. The following research was designed and conceived.The discipline of the pyrolytic product characteristics with heating ratio and pyrolysising temperature was investigated. The ceramic yield was decreased when the heating ratio elevated. With the increasing of pyrolysis temperature, the density, crystallinity and stability of pyrolytic product inceased. In N2 atmosphere, with 10℃/min, at 1000℃, the ceramic yield was about 88.95wt%.The effects of the calefactive velocity, treating time, temperature and times on the mechanical properties of SiNO fiber were studied via XRD, SEM, TEM to examine its surface performances and structures. It showed that the strength of SiNO fiber was sensitive to the treating temperature and times. The strength decreased while the heating temperature increased. After heated at 1100℃in air, the residual strength ratio of the fiber is only 40%. With the increase of treating time, the strength of SiNO fiber also decreased with more than 20% after the first time and smoothly at 5% level after the following times. But the calefactive velocity and treating time had low influence to the strength of SiNO fiber.SiNO_f/BN composites were prepared from Borazine by PIP process and the key factors that effect the mechanical properties of it were investigated and analyzed. Because the precursor Borazine had good wet-ability to SiNO fibers and the preformed unit of unidirectional fibers had comparatively holes, the density of the composites increased fast with the PIP cycle doing. With the increase of PIP cycles, the porosity of the composites decreased, the density increased, hence the mechanical properties enhanced. With the increase of pyrolysis temperature from 800℃to 1000℃, the flexural strength and elastic modulus of the composites increased from 128.9MPa and 23.5GPa to 148.2MPa and 26.2GPa respectively. But at different pyrolysis temperature, the densification behavior was consistent. The mechanical properties of the composites varied little with different pyrolysis time. After heat treated at 1600℃, the SiNO fibers in the composites were crystallized and partial decomposed led to lost the enhancing function, hence the mechanical properties of the composites decreased seriously. The dielectric and thermo physical properties of SiNO_f/BN composites were tested and studied. When the pyrolysis temperature increased from 800℃to 1000℃, the dielectric constant increased from 3.45 to 3.83, while the loss tangent decreased from 7.0x10^-3 to 4.6x10^-3. Composites pyrolyzed at different temperature all had good dielectric properties that could meet the operating requirements. The specific heat capacity and thermal conductivity (from R.T. to 100℃and R.T. to 200℃) was 0.91kJ/kg·℃⁰.97kJ/kg·℃and 1.2W/m·K¹.3W/m·K respectively. From R.T. to 800℃, the average CTE was 2.20².75x10^-6/K and varied less than 10%.Various performances of SiNO_f/BN were studied contrast to that of SiO_2f/BN. They were differed tiny in dielectric and thermo physical properties. But the former could maintain most of the mechanical properties at high temperature better than that of the latter.