Heat Treatment Effect on the Phase Composition of the Silica Electrochemical Coating and the Carbon Fiber Strength

Abstract

This work is devoted to the study of the chemical and phase composition of a carbon fiber coating obtained by the electrochemical sol-gel method. The experimental data obtained using several independent complementary methods, including X-ray phase analysis, thermogravimetric and differential thermal analysis, scanning electron microscopy and elemental analysis, and X-ray photoelectron spectroscopy, are in good agreement with each other. It was found that the resulting coating consists of amorphous silicon oxide and crystalline potassium carbonate. Heating above 870 °C leads to the crystallization of cristobalite from amorphous silicon dioxide. At a temperature of about 870 °C, the coating acquires a smooth surface, and heating above 1170 °C leads to its destruction. Thus, the optimum temperature for the heat treatment of the coating is about 870 °C. The loss of strength of carbon fiber at each stage of coating was estimated. A full coating cycle, including thermal cleaning from the sizing, coating, and heat treatment, results in a loss of fiber strength by only 11% compared to the initial state.