Tensile strength and microstructure of Hi-Nicalon fibers extracted from celsian matrix composites

Tensile strengths of uncoated and BN/SiC coated Hi-Nicalon fibers, extracted from celsian matrix composites by dissolving away the matrix in HF, were determined. Strengths of uncoated and BN/SiC coated Hi-Nicalon fibers before and after HF treatment were also measured. The elemental composition and thickness of the fiber coatings were determined using scanning Auger analysis. Microstructural analyses of the fibers and the coatings were done by SEM and TEM. The average tensile strength of uncoated Hi-Nicalon was 3.19+/-0.73 GPa with a Weibull modulus of 5.41. The Hi-Nicalon/BN/SiC fibers showed an average strength of 3.04+/-0.53 Cpa and Weibull modulus of 6.66. After HF treatments, the average strengths of the uncoated and BN/SiC coated Hi-Nicalon fibers were 2.69+/-0.67 Cpa and 2.80+/-0.53 Cpa and the Weibull moduli were 4.93 and 5.96, respectively. The BN/SiC coated fibers extracted from the celsian matrix composite exhibited a strength of 2.38+/-0.40 GPa and a Weibull modulus of 7.15. Strength of the uncoated Hi-Nicalon fibers in the composite was so severely degraded that they disintegrated into small fragments during extraction with HF. The uncoated fibers probably undergo mechanical surface damage during hot pressing of the composites. The BN layer acts as a compliant layer which protects the coated fibers from mechanical damage during composite processing.