Controllable coating of boron nitride on ceramic fibers by CVD at low temperature

Abstract

Boron nitride (BN) is a promising interface material for preparing continuous fiber reinforced ceramic matrix composites (CFCCs or CMCs). However, the controllable coating of BN on ceramic fibers remains a challenge. We developed a low-temperature chemical vapor deposition (CVD) approach for producing controllable coating BN on alumina fibers using ammonia borane (BH3NH3) as single source precursor at atmospheric pressure. The optimized sublimation temperature for BH3NH3 is 130°C and at this temperature the precursor could produce monomeric aminoborane (BH2NH2) and borazine (B3N3H6) for the subsequent CVD growth of BN. BN coatings obtained at the deposition temperature of 700°C was of high purity as confirmed by fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. Scanning electron microscopy images of BN coated fibers indicated that the coatings produced under deposition temperature in the range of 700–900°C enwrapped on fibers continuously and tightly as the surface of the fiber was smooth and uniform. Annealing at a higher temperature of 1000 and 1300°C greatly improved the crystallinity of BN coatings deposited at 700°C as confirmed by grazing incidence X-ray diffraction and high resolution transmission electron microscopy characterization. Mechanism of this low-temperature CVD was investigated as well, and based on the understanding of deposition mechanism, the thickness and structure of the BN coatings could be controlled by turning the deposition temperature and duration. The tensile strength of alumina fibers before and after BN deposition at the temperature of 700, 800, and 900°C for 1.5h were investigated, and at the optimized deposition temperature of 700°C, the tensile strength retention was up to 94.9%, which demonstrates the great potential of this approach for fabricating high performance continues fiber reinforced ceramic matrix composites.