Effects of HF treatments on tensile strength of Hi-Nicalon fibres

Tensile strengths of as-received Hi-Nicalon fibres and those having a dual BN-SiC surface coating, deposited by chemical vapour deposition, have been measured at room temperature. These fibres were also treated with HF for 24 h followed by tensile strength measurements. Strengths of uncoated and BN-SiC coated Hi-Nicalon fibres extracted from celsian matrix composites, by dissolving away the matrix in HF for 24 h, were also determined. 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 fibres showed an average strength of 3.04+/-0.53 GPa and Weibull modulus of 6.66. After HF treatment, the average strengths of the uncoated and BN-SiC coated Hi-Nicalon fibres were 2.69+/-0.67 and 2.80+/-0.53 GPa and the Weibull moduli were 4.93 and 5.96, respectively. The BN-SiC coated fibres extracted from the celsian matrix composite exhibited a strength of 2.38+/-0.40 GPa and a Weibull modulus of 7.15. The strength of the uncoated Hi-Nicalon fibres in the composite was so severely degraded that they disintegrated into small fragments during extraction with HF. The uncoated fibres probably undergo mechanical surface damage during hot pressing of the composites. Also, the BN layer on the coated fibres acts as a compliant layer, which protects the fibres from mechanical damage during composite processing. The elemental composition and thickness of the fibre coatings were determined using scanning Auger analysis. Microstructural analyses of the fibres and the coatings were done by scanning electron microscopy and transmission electron microscopy. Stengths of fibres calculated using ave rage a nd measured fibre diameters we re in good agreement. Th us, the strengths of fibres can be evaluated using an average fibre diameter instead of the measured diameter of each filament. (C) 1998 Kluwer Academic Publishers.