Effect of carbon monoxide partial pressure on the high-temperature decomposition of Nicalon fibre

The high-temperature equilibrium partial pressures of the predominant gaseous species over Nicalon were determined thermochemically. It was calculated that the most prevalent gaseous species in equilibrium with Nicalon at 1300 °C is carbon monoxide. Subsequently, fibres of Nicalon (NLM 202) were heat treated at 1300 °C in various partial pressures of carbon monoxide gas and analysed via single filament strength testing, scanning electron microscopy, X-ray diffraction, and scanning Auger microscopy. The heat treatments in carbon monoxide had a significant effect on the strength retention and composition of the fibres (∼75% retained) compared to the treatments in argon where only 25% of the initial strength was retained. The Auger analysis revealed that the treatment in argon evolved carbon and oxygen from the fibre while in carbon monoxide atmospheres a carbon layer was deposited on the fibre surface. X-ray diffraction showed that grain growth had not occurred in any of the heat treatments. This study shows the important role of thermochemical reactions in the strength degradation of Nicalon, and its possible relationship to the formation of carbon surface/interface layers.