Oxidation resistance of a gradient self-healing coating for carbon/carbon composites

A gradient self-healing coating consisting of three layers, SiC-B₄C/SiC/SiO₂, was examined as a multilayer protection for carbon/carbon composites. The inner layer was made of B₄C and β-SiC, the middle layer was a SiC based layer, and the outer layer was SiO₂ as an airproof layer. Both inner and middle layers were produced to be diphase structure by a pack cementation technique, and the outer airproof layer was prepared by hydrolyzing tetraethylorthosilicate. SEM and EDS investigations showed that the coating had a compositional gradient between B₄C and SiC. The coating showed great self-healing properties from 500 °C to 1500 °C. The weight loss rate of the coated composites was less than 1.3% after 50 h at 1500 °C, and coating represented excellent thermal shock resistance at 1500 °C. The oxidation kinetics of coated carbon/carbon composites showed that the Arrhenius curve consisted of three parts with two broken points at about 700 °C and 1100 °C, and the three parts corresponded to three different self-healing mechanisms in different temperature regions.