Zirconium diboride based coatings for thermal protection systems of new re-entry vehicles

Development of new generation re-entry vehicles with improved aerodynamic efficiency, wider choice of re-entry trajectory and lower mission costs is closely connected to development of new thermal protection systems that can withstand the severe environment of atmospheric re-entry. Zirconium diboride based materials exhibit outstanding oxidation and thermal shock resistance, high emissivity and very high melting temperature (ZrB2 belongs to Ultra High Temperature Ceramics, i.e. UHTC): all these properties make ZrB2 based materials possible candidates in designing thermal protection systems for the hottest structures of re-entry vehicles such as wing leading edges and nose-tip. Experimental activities were focused on eight different ZrB2 based coatings deposited by plasma spray technique and containing SiC and MoSi2 in different ratios; four of the eight coatings were sintered after the deposition. The aim of the work was to evaluate how oxidation resistance and mechanical properties are affected by the different amounts of SiC and MoSi2 and by sintering after deposition. Knowing material response at high temperatures in oxidizing atmosphere is crucial for establishing the suitability of a material as a thermal barrier; therefore the oxidation kinetics of the ceramic coatings exposed at 1200 °C and 1800 °C for 3, 30 and 42 minutes were analyzed and modelled. The mechanical characterization (four-point bending tests for the evaluation of flexural strength and elastic modulus) of as sprayed and high temperature exposed samples (30 minutes at 1800°C) was carried out on the three compositions having the highest oxidation resistance, in order to select the optimal composition in terms of thermal behaviour and capability of retaining the mechanical properties of the as-sprayed materials. Experimental activities aimed at optimization of the coating/substrate adhesion are currently in progress.