Preparation and characterization of boron nitride coating on silicon carbide fibers based on low-temperature chemical vapor deposition

Abstract

Boron nitride (BN) has been considered as promising material owing to its unique properties, such as high thermal stability, high chemical stability, high thermal conductivity and thermal shock resistance. Chemical vapor deposition (CVD) is the most popular method for the preparation of BN. However, the side-products (like HF, HCl and HBr) are corrosive and the crystallinity of BN is poor if the deposition temperature is 1300°C. Herein we reported a new method for preparing BN by low-temperature CVD using ammonia borane as precursor on silicon carbide fibers. Ammonia borane was sublimated at 110–130°C using a heating belt, and the sublimated gaseous substance was transported to the deposition region where the silicon carbide fibers were placed at the deposition temperature of 900°C. During the sublimation, a certain quality of ammonia borane was uniformly distributed in the quartz boat with a certain length, and the heating belt moved along the quartz tube with a fixed speed to the end of the boat. Thus the sublimation gas substances could be supplied continually and steadily. Scanning electronic microscopy (SEM), auger electron spectroscopy (AES), Fourier translation infrared spectroscopy (FTIR), and grazing incidence angle X-ray diffraction (GIAXRD) were used to characterize the properties of the coatings, and single fiber electron tensile machine was used to test the tensile strength of silicon carbide fibers without and with different thickness of coatings. The results illustrated that BN coating enwrapped well with the fibers with no defects like voids or cleavages. Meanwhile, the surface of the coating was uniform and dense. And the ratio of B and N was approximate 1: 1. Pure BN coating could be acquired at a relatively low deposition temperature, and microstructure of the BN coating was more ideal with the increasing of deposition temperature. Considering the balance between the coating crystallinity and strength retention of the fiber, 800–1000°C was the optimum deposition temperature range of BN coating on silicon carbide fibers. Compared with previous reports, this method reduced the preparation temperature effectively and could acquire pure BN on relatively low temperature. In this work, we aimed at preparing the ideal interphase of ceramic composite, and reducing the deposition temperature to elevate the fiber strength retention. This work could reduce the deposition temperature by at least 100°C and even 500°C compared with some cases. It would solve the reduction of mechanical properties during long time and high temperature deposition. At 900°C, the strength retention of the fibers retained 92.7%, 83.6% and 77.7%, as the coating thickness was 0.28, 0.51 and 0.82 μ m, respectively. The strength retention of the silicon carbide fiber can meet the requirement of ceramic composite.