Degradation behaviour of a composite material for thermal protection systems Part III Char characterization

In the former parts of this work (J. Mat. Sci.33 (1998) pp. 3137 and 3145) the kinetic characterisation and the mathematical modelling of the behaviour of an ablative material subjected to high heat fluxes were analysed. In this work the structure and morphology of the char produced from the degradation of the same ablative material, occurring as a consequence of plasma torch exposure, are evaluated in order to improve the ablative behaviour of an active protection. Usually the char produced from the degradation of polymeric ablators is weak and brittle, and therefore is susceptible of rapid removal by the mechanical stresses due to the high vehicle speed, this phenomena provokes a reduction of the thermal insulation. Reinforcing fibres and other inorganic fillers must be therefore included in the ablative formulation in order to improve the char stability. One of the roles of the char, which is very important, is to limit oxygen diffusion from the boundary layer to the bulk to prevent the exothermic degradation reactions of the polymeric matrix. Therefore, the mechanical characteristics and the physic-chemical properties of the char are of fundamental importance to ensure good quality of an ablative material. In this paper Scanning Electron Microscopy (SEM) elemental microanalysis (EDAX) and wide angle X-ray diffraction (WAXD) are used to provide respectively the morphological characteristics of the char and further insight on the crystalline structure resulting after quenching at room temperature.