Improvements in refractoriness and properties of Nicalon fibres by high-temperature heat-treatment

Nicalon SiC fibres were heat-treated in various atmospheres and at various pressures. Initially CO, nitrogen and air were used as the heat-treatment e nvironment at one atmosphere pressure. Microstructural changes and any rela ted strength degradation or improvement were measured for the heat-treated fibres. After heat-treatment in the temperature range 1000 degreesC-1600 de greesC, each sample showed different weight changes. Thus, in air, a weight gain was observed with increasing temperature, whereas in CO and N-2, weig ht losses were observed but with a smaller weight loss observed for CO. Mor eover, carbon monoxide had a significant effect on the strength retention o f the fibres. Since the lowest weight loss was observed after heat-treatmen t in CO at one atmosphere, high pressure CO gas was used to heat-treat Nica lon fibres between 1000 degreesC and 1700 degreesC and the resulting fibres were analysed by single-filament strength testing, scanning electron micro scopy, and X-ray diffraction. The results were completely different compare d with those in one atmosphere of CO. As the temperature increased, weight and strength increased whereas at one atmosphere pressure, both weight and strength had decreased. The weight increase was because of surface reaction between the CO atmosphere and the SiC fibre and/or because of deposition o f carbon from the pressurised CO gas, giving the fibre a surface carbon coa ting. Carbon coating of a fibre is a beneficial property for CMCs since it provides a weak interface which facilitates pull-out during fracture. (C) 2 001 Kluwer Academic Publishers.