Thermochemical Analyses of Interface Reactions in Carbon-Fiber Reinforced Glass Matrix Composites

Abstract

A thermochemical analysis of localized equilibria was used to model the interface reactions which may occur during the processing of carbon-fiber reinforced glass matrix composites. The influence of glass composition upon the course of the reactions and the interface reaction products was of primary interest. In the case of simple sodium-borosilicate glass, the calculations show that initial contact between the glass and carbon fibers leads to the generation of CO-gas and SiC at the interface. This reaction is limited by mass transport of CO and SiO gaseous reaction products away from the interface. The oxygen activity at the interface must be reduced to 10–20% of its initial value before the formation of B₄C and Na(metal) can occur. The effects of adding fractional concentrations of aluminum oxide, molybdenum oxide and niobium oxide to the base sodium-borosilicate glass were also evaluated. The reduction of MoO₃ to MoC is favored over the formation of SiC — even in the initial stages of the reaction when the interfacial oxygen activity is high. In the case of Nb₂O₅, the formation of NbC is also favored over the reduction of SiO₂ to SiC, but is limited to some extent by the equilibrium pressure of CO-gas. In contrast, the formation of interfacial A1₄C₃ is not predicted even when all of the SiO₂, B₂O₃ Na₂O have been reduced. These data are being used to design new matrix compositions where interface reactions and/or chemical bonding at the interface can be tailored to enhance strength retention in glass matrix composites.