TENSILE-STRENGTH AND MICROSTRUCTURAL CHARACTERIZATION OF HPZ CERAMIC FIBERS

Tensile strengths of as-received HPZ fiber and those surface coated wi th BN, BN/SiC, and BN/Si3N4 have been determined at room temperature u sing a two-parameter Weibull distribution. Nominally similar to 0.4 mu m BN and 0.2 mu m SiC or Si3N4 coatings were deposited on the fibers by chemical vapor deposition (CVD) using a continuous reactor. The ave rage tensile strength of uncoated HPZ fiber was 2.0 +/- 0.56 GPa (290 +/- 81 ksi) with a Weibull modulus of 4.1. For the BN coated fibers, t he average strength and the Weibull modulus increased to 2.39 +/- 0.44 GPa (346 +/- 64 ksi) and 6.5, respectively. The HPZ/BN/SiC fibers sho wed an average strength of 2.0 +/- 0.32 GPa (290 +/- 47 ksi) and Weibu ll modulus of 7.3. Average strength of the fibers having a dual BN/Si3 N4 surface coating degraded to 1.15 +/- 0.26 GPa (166 +/- 38 ksi) with a Weibull modulus of 5.3. The chemical composition and thickness of t he fiber coatings were determined using scanning Auger analysis. Micro structural analysis of the fibers and the coatings was carried out by scanning electron microscopy (SEM) and transmission electron microscop y (TEM). A microporous silica-rich layer similar to 200 nm thick is pr esent on the as-received HPZ fiber surface. The BN coatings on the fib ers are amorphous to partly turbostratic and contaminated with carbon and oxygen. The silicon carbide coating was crystalline whereas the si licon nitride coating was amorphous. The silicon carbide and silicon n itride coatings are non-stoichiometric, non-uniform, and granular. Wit hin a fiber tow, the fibers on the outside had thicker and more granul ar coatings than those on the inside.