Preparation And Properties Of Boron Nitride Fiber Reinforced Wave-Transparent Ceramic Matrix Composites

Due to the excellent high temperature mechanical properties and thermal stability, nitride fibers are potential to be the desirable reinforcement materials of high-Mach-number aircrafts. Based on the radomes of missiles with high-Mach numbers, the unidirectional boron nitride fiber reinforced boron nitride matrix (BN_f/BN) and silica matrix (BN_f/SiO₂) composites were prepared via precursor infiltration and pyrolysis (PIP) route and sol-gel method using boron nitride fiber, respectively. The properties of boron nitride fiber were studied, the relations among process, microstructure and properties were investigated, and the mechanical, thermophysical and dielectric properties of BN_f/BN composites were characterized.The effects of heat treatment at different temperatures in nitrogen and air atmosphere as well as infiltration in water on the composition, structure and morphology of BN fiber were discussed. The original BN fiber mainly consists of BN (including h-BN, a-BN and t-BN) with a little B₂O₃ due to the weak oxidation. The B₂O₃, which is decomposed from (B₂O₃)n·NH₃, (BN)x(B₂O₃)y·(NH₃)z and (B₂O₃)n·H2O during synthesis process of BN fiber, forms and diffuses on the surfaces of the fiber, and volatilizes at above 1100oC, leading to the pores on the fiber. With heat treatment in nitrogen at 1400oC, the fiber displays rough surfaces with little change in mass. Higher crystallinity can be obtained with the increasing temperature. From the thermodynamics analysis, BN fiber will suffer oxidation just at room temperature (R.T.) in dry air or oxygen. The kinetics shows that the essence of oxidation is the gas (O₂)/solid (BN) diffusion and surface chemical reactions. The pores on the surfaces of the fiber are not so deep even at 1400oC, which can be attributed to the protection of boron oxide glaze for the fiber from further oxidation. The mechanism of moisture involves physical adsorption, hydrolyzation of B-N bond and strong water sensibility of B₂O₃. The essential reason for this is the loose structure of amorphous BN which has a larger layer spacing and higher chemical activity. As is shown by the results of FT-IR, XRD and SEM, temperature plays a key role in the moisture property of BN fiber.The effects of different PIP parameters on the properties of BN_f/BN composites were analyzed, then BN_f/BN composites were prepared with optimized process, and the mechanical, thermophysical and dielectric properties were characterized. As the fiber volume fraction increases, the density of the composites after four PIP cycles increases. The fiber volume fraction of 40% is beneficial to the toughness, while the excessive infiltration times, hold time, temperature, heating and cooling rate are detrimental to the mechanical properties of the composites. The optimized parameters are: fiber volume fraction 40%, infiltration time 120 min, heating rate 5oC·min^-1, temperature 1000oC, hold time 60 min, cooling rate 10oC·min^-1 and four PIP cycles. The density of the composites obtained is 1.60 g·cm-3 with an open porosity of 4.66%. The composites display good mechanical properties with the average flexural strength, elastic modulus and fracture toughness being 53.8 MPa, 20.8 GPa and 6.88 MPa·m1/2, respectively. Lots of long fibers pulled out from the fracture surface, suggesting a good fiber/matrix interface. As temperature increases, both of the flexural strength and elastic modulus exhibit a decreasing trend, with the lowest values of 36.2 MPa and 8.6 GPa at 1000oC, respectively. The desirable residual ratios of the flexural strength and elastic modulus at 1000oC are 67.3% and 41.3%, respectively. The CTE of the composites at R.T. is 3.81×10^-6 K^-1, which declines with the increasing temperature. The coefficient of thermal conductivity and specific heat of the composites at R.T. are 1.016 W·m^-1·K^-1 and 0.618 J·g^-1·K^-1, respectively, and they increase as the temperature grows up below 300oC, while the coefficient of thermal conductivity at 300oC is only 1.654 W·m^-1·K^-1. The composites have excellent dielectric properties, with the average dielectric constant and loss tangent being 3.07 and 0.0044 at 2¹8 GHz, respectively.The effects of heat treatment on the properties of the composites were investigated. After heat treatment at 1300oC in air, the content of oxygen soars to 46.26%. The flexural strength, elastic modulus and fracture toughness fall to 46.4%, 41.3% and 31.2% of the asreceived ones, and the fracture mode is evidently the brittle one. The CTE increases significantly to 7.25×10^-6 K^-1 at R.T. and 3.44×10^-6 K^-1 at 900oC. The coefficient of thermal conductivity and specific heat of the composites at R.T. increase slightly to 0.989 W·m^-1·K^-1 and 0.630 J·g^-1·K^-1, respectively. The dielectric constant and loss tangent is 3.32 and 0.0035 (Ku band), respectively, with the former increasing a little and the latter decreasing.The unidirectional BN_f/SiO₂ composites were prepared via sol-gel method, and the composition, structure and mechanical properties were studied. The results show that the interface phases of the composites consist ofα-Si3N4 and B₂O₃. The composites have a density of 1.70 g·cm^-3 and an open porosity of 20.8%. The average flexural strength, elastic modulus and fracture toughness at room temperature are 51.2 MPa, 23.2 GPa and 1.46 MPa·m^1/2, respectively. The composites show a very plane fracture surface with practically no fiber pull-outs, which is a brittle fracture mode. The mechanical properties of BN_f/SiO₂ composites at 300¹000oC are desirable, with the maximum flexural strength and residual ratio being 80.2 MPa and 156.8% at 500oC, respectively. The high thermal stability of BN fibers and self-healing properties caused by the fused SiO₂ and B₂O₃ enable the composites fine high-temperature mechanical properties.