Effect of temperature on the growth of boron nitride interfacial coatings on SiC fibers by chemical vapor infiltration

In order to improve bonding property between SiC fibers and matrix of SiCf/SiC composites, boron nitride (BN) interfacial coatings were synthesized by chemical vapor infiltration. BN coatings were fabricated from BCl3-NH3 gaseous mixtures at four different temperatures (843 degrees C, 900 degrees C, 950 degrees C and 1050 degrees C) with different deposition times. Growth kinetics, nucleation and growth processes, microstructure and chemical composition of boron nitride coatings were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and Raman spectrometry. Results showed that deposition rate increased as the temperature increased from 843 degrees C to 950 degrees C. However, deposition rate decreased slightly from 23.10 +/- 0.85 nm/min (950 degrees C) to 21.39 +/- 0.67 nm/min when the temperature was increased further to 1050 degrees C. It could be due to the nucleation occurring in the gas and the consumption of a large amount of BCl3 and NH3. When deposition temperature was 843 degrees C, BN grains deposited on top layer of the coating could not completely cross Ehrlich-Schwoebel barrier and grew in island growth mode. On the other hand, the deposition pattern followed a layer-by-layer growth mode when deposition temperature was 1050 degrees C. Deposition temperature significantly affected the microstructure of as-deposited BN coatings. At 843 degrees C, 950 degrees C and 1050 degrees C, the coatings presented amorphous, polycrystalline and hexagonal structures, respectively.