Characteristics and formation mechanism of compact/porous structures in char layers of EPDM insulation materials

A char layer is the first barrier against the ablation of insulation materials because it is the first to be subjected to thermal-chemical ablation and particle and gas-mechanical erosion. To study the ablation mechanism of the char layer in depth and to develop accurate models of the ablation phenomenon, the structural characteristics of the char layer must be understood. In this work, the structural characteristics of the char layer were studied using an ablation motor and several other analytical techniques. It was found that the char layer usually has a compact/porous structure with a compact surface. But in alumina deposition mode, the middle portion remains relatively compact. A mechanism whereby the compact layer is formed by re-deposition of the pyrolysis gas in the char layer skeleton, which is initially relatively porous, is proposed to explain the formation of the compact/porous structure. It was found that a liquid silica film contributes to the formation of the compact layer; this explains the formation of the porous/compact/porous structure. The gas-pyrolysis-based deposition which forms the surface compact layer can be regarded as being a self-reinforcing mechanism and can be exploited to improve the ablation resistance of insulation materials. (C) 2017 Elsevier Ltd. All rights reserved.