JOURNAL OF MATERIALS SCIENCE 33 (1 9 9 8 ) 5277 – 5289 Chemical, mechanical and microstructural characterization of low-oxygen containing silicon carbide fibers with ceramic coatings N. P. BANSAL National Aeronautics and Space Administration, Lewis Research Center, Cleveland, OH 44135, USA E-mail: narottam.p.bansal@lerc.nasa.gov Y. L. CHEN Dynacs, Inc., Lewis Research Center Group, Brookpark, OH 44142, USA Room temperature tensile strengths of as-received Hi-Nicalon fibers and those having BN/SiC, p-BN/SiC, and p-B(Si)N/SiC surface coatings, deposited by chemical vapor deposition, were measured using an average fiber diameter of 13.5 μm. The Weibull statistical parameters were determined for each fiber. The average tensile strength of uncoated Hi-Nicalon was 3.19 ± 0.73 GPa with a Weibull modulus of 5.41. Strength of fibers coated with BN/SiC did not change. However, fibers coated with p-BN/SiC and p-B(Si)N/SiC surface layers showed strength loss of ∼10% and 35%, respectively, compared with the as-received fibers. The elemental compositions of the fibers and the coatings were analyzed using scanning Auger microprobe and energy dispersive X-ray spectroscopy. The BN coating was contaminated with a large concentration of carbon and some oxygen. In contrast, p-BN, p-B(Si)N, and SiC coatings did not show any contamination. Microstructural analyses of the fibers and the coatings were done by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and selected area electron diffraction. Hi-Nicalon fiber consists of fine β -SiC nanocrystals ranging in size from 1 to 30 nm embedded in an amorphous matrix. TEM analysis of the BN coating revealed four distinct layers with turbostratic structure. The p-BN layer was turbostratic and showed considerable preferred orientation. The p-B(Si)N was glassy and the silicon and boron were uniformly distributed. The silicon carbide coating was polycrystalline with a columnar structure along the growth direction. The p-B(Si)N/SiC coatings were more uniform, less defective and of better quality than the BN/SiC or the p-BN/SiC coatings. C  1998 Kluwer Academic Publishers 1. Introduction Polymer derived, small diameter Hi-Nicalon silicon carbide fibers [1], because of their high strength and excellent strength retention after high temperature ex- posures, are currently of great technical interest as a reinforcement in high performance structural compos- ites with polymer, metal, and ceramic matrices. How- ever, these fibers need to be protected against chemi- cal reactions with metal and ceramic matrices during composite processing and use, and also against oxidiz- ing and other harsh environments. The protective ce- ramic surface coating(s) on the fiber surface also pro- vide a weak fiber/matrix interface [2–3] for toughened ceramic matrix composites. The BN/SiC dual layer is the state-of-the-art interface coating for ceramic matrix composites. The pyrolytic boron nitride (p-BN) coat- ing is more stable than BN. Also, silicon containing py- rolytic boron nitride (p-B(Si)N) shows [4] 1–3 orders of magnitude greater oxidation resistance at elevated temperatures than p-BN and is more resistant to re- action with moisture. It is necessary to know if the application of these coatings results in any strength degradation or microstructural changes in Hi-Nicalon fibers. The primary objective of the present study was to investigate the effects of various BN/SiC type surface coatings on the tensile strength of Hi-Nicalon fibers. Another objective was to carry out chemical and mi- crostructural analyses of the fibers and the coatings. Microstructural analyses of the fiber and the coatings were done by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Elemental compositions and thicknesses of the fiber and the coat- ings were determined by scanning Auger microprobe, energy dispersive analysis of X-rays (EDAX), and elec- tron microscopy. Room temperature tensile strength was measured and the Weibull statistical parameters were determined for fibers with different coatings. 0022–2461 C  1998 Kluwer Academic Publishers 5277