Protective Ultra-High Temperature Coatings/ Ceramics (UHTCs) for Ceramic Matrix Composites in Extreme Environments

This thesis is focused on the development of a protective coating system for Cf/SiC SiCARBONTM (Airbus trademark) materials against very high temperatures in extreme environment. Here, we concentrate on the application of this technology in combustion chambers, for example in orbital thrusters. During combustion, the composite material needs to be protected against oxidation caused by the extreme conditions. With the aim to increase the combustion performance using higher temperatures (up to 1850 °C), this thesis deals with the replacement of the current Environmental Barrier Coating (EBC) solution (CVD-SiC coating, Chemical Vapor Deposition) by an Ultra High Temperature Ceramic (UHTC) based coating system. Different challenges of this approach are, for instance, the CTE mismatch between Cf/SiC and UHTC materials and the feasibility to create a dense, thick and adherent UHTC based coating on the hot gas wall (inner wall) of a small combustion chamber. In this work, a suitable coating process (High Performance Plasma Coating process, HPPC) for inner wall coatings is selected and further developed to create ZrB2 based coatings on Cf/SiC based substrate materials. Based on a parameter study, the coating quality of HPPC based ZrB2 coatings is optimized depending on plasma current, chamber pressure, powder flow rate, preheating and cooling rate. HPPC coatings with different material combinations (ZrB2, ZrB2-SiC, ZrB2-TaC, ZrB2-LaB6) are investigated regarding coating adhesion, voids, composition and thermo-chemical behavior within a combustion chamber-like environment. To decrease the CTE mismatch between Cf/SiC substrate and a ZrB2 based coating and to increase the thermo-chemical resistance of the composite, the SiC matrix material is modified by ZrB2 and Ta additions. Cf/SiC-ZrB2-TaC composites with different SiC/ZrB2-TaC ratios are fabricated and investigated regarding microstructure, chemical composition and material properties (physical, thermo-physical, mechanical and thermo-chemical). The adhesion of HPPC based ZrB2 coatings on Cf/SiC composites is enhanced by a ZrB2 and TaC matrix modification. Based on the results, interactions between process parameters, coating composition and substrate material are analyzed and provide the base for ZrB2 based EBCs of the inner wall coatings on Cf/SiC based components. By means of the obtained findings, the potential of several material systems is derived in order to develop a protective coating for long-term applications in combustion chamber environments.