a Study of Interface Coatings for Fiber Reinforced Ceramic Matrix Composites.

Amorphous boron nitride thin coatings were formed on individual fibers in several different ceramic yarns via dip or wick coating in boric acid solutions followed by reaction with ammonia gas at 1000^circ C. The ceramic yarns used in this study are Nicalon (SiC), C-Nicalon (SiC), Tyranno (SiC), Sumitoma rm (Al_2O_3 type), Carbon(T300) and Nextel 440 (Mullite type). Control of the crystallization of rm H_3BO_3 on the fibers plays an important role in the coating evenness and in the prevention of bridging between fibers. X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) analyses indicated the formation of a boron rich boron nitride layer with the N/B and O/B ratios ranging from 0.6-0.8 and 0.1-0.25, respectively. Tensile strength measurements revealed that the same dip coating process applied on different yarns resulted in significant fiber degradation except for C-Nicalon fibers. It was found that a pre-coated carbon layer plays an important role in protecting fibers. Therefore, CVD carbon coatings were deposited onto the fibers of those ceramic yarns using rm C_3H_6 mixed with N_2 and H_2 prior to BN dip coatings. The dual carbon/BN (dip) coated Nicalon yarns showed a dramatic increase in tensile strength. It appears that the pre-coated carbon layer was sacrificially oxidized during the nitridation process protecting the fibers. BN coatings were also applied on Nicalon and C -Nicalon fibers via a CVD method using BCl_3 and NH_3 as precursors. The properties of the fibers with dip coated boron nitride were compared with those of the CVD BN coated ones. SiC matrices were introduced into the coated yarns via a CVI process using methyltricholorosilane (MTS) as the precursor. The mechanical properties of the SiC/BN/SiC composites with dip coated BN and CVD BN interfaces were compared. The properties of coatings and composites were characterized by XPS, AES, SEM-EDX, XRD, and tensile strength measurements.