Microstructure and Thermophysical Properties of CVD BN Treated at Temperature up to 1550°C

ABSTRACT

Boron nitride (BN), as an interphase, plays an important role in thermophysical and mechanical properties of continuous fiber‐reinforced ceramic matrix composite. In this work, BN sheets were fabricated by chemical vapor deposition method. To simulate the service temperature, the BN sheets were treated at high temperatures up to 1550°C, and then thermophysical properties at elevated temperatures were investigated. The results exhibited that the high‐temperature treatment promoted the crystallization and structural rearrangement of BN. Particularly, BN treated at 1550°C showed the compacted layer structure. BN treated at 1550°C exhibited the highest thermal conductivities in direction parallel to the deposition direction, with 1.21 W/(m·K) at RT and 1.50 W/(m·K) at 300°C (peak), which were 49.6% and 29.7% higher than those of as‐fabricated BN, respectively. The increase of thermal conductivity was mainly attributed to the improvement of crystallization and the diminishment of defects. Meanwhile, BN treated at 1550°C exhibited the lowest coefficients of thermal expansion in direction perpendicular to the deposition direction, which were 1.13 × 10 ⁻⁶ /K at 100°C and 1.02 × 10 ⁻⁶ /K at 900°C, about 29.4% and 26.3% of that as‐fabricated BN. The decrease in coefficient of thermal expansion was primarily dominated by the improvement of texture degree.