Development of multi-layered thermal protection system (TPS) for aerospace applications

Multi-layered UHTC composites for use in an extreme oxidation environment were fabricated by co-sintering. The multi-layered composite system consists of three layers and was designed with (i) an outer surface with the capacity to withstand heat fluxes in excess of 25 MW m−2, (ii) an intermediate layer with the ability to curtail diffusion of O2 and (iii) a bottom layer with better performance at high temperatures. One design has MeB2 (where Me = Zr or Hf) as the outer or top layer, MeCxOy as the intermediate layer and MeB2/SiC as the bottom layer. Since little is known about MeCxOy ceramics, a detailed study has been carried out to synthesise and characterise them focussed on controlling MeCxOy stoichiometry. During co-sintering, the outer MeB2 layer thickness played a crucial role in providing a crack-free component. Indicative calculations of the residual stresses confirm that a compressive stress due to bending of the disks can stop crack growth through the MeB2 layer when its thickness is increased to 3 mm, and that the tensile stress in the bottom layer is also reduced. The cross-section microstructure of the optimised multi-layered UHTC composite shows a crack-free seamless interface.