Processing of Nb-coatings on ZrB2 and C-ZrB2/SiC composites

Abstract

Ultra-high temperature ceramics (UHTCs) are a highly investigated group of materials, due to its combination of a high melting point and a high electrical/thermal conductivity. Therefore, these materials have a high potential for the space- and aerospace industry as thermal protective systems (TPS) and leading edges of hypersonic vehicles. Their mechanical properties were largely improved by introducing C fibers in to the matrix. It was also known that the addition of certain amount of SiC in to the ZrB2 matrix will also help in improving their oxidation resistance at elevated temperatures. The formation of a borosilicate layer at the surface of C-ZrB2/SiC composites provides reduced oxidation kinetics up to 1650°C. Unfortunately, the evaporation of B2O3 from the borosilicate leaves a less protective layer of ZrO2/SiO2 behind for longer exposure times/elevated temperature regimes. Previous results on the functionality of metallic Nb-coatings on baseline ZrB2 have proven that a dense mixed oxide scale of Nb2Zr6O17 formation on the surface and a stable B2O3-based liquid solution has improved the overall oxidation behavior. This work focuses on the application of thin metallic Nb-coatings on ZrB2 and C-ZrB2/SiC composites by means of magnetron sputtering. Firstly, the requirements for coatings on top of ZrB2-based UHTCs and UHTCMCs will be presented. Secondly, all process steps for the application of reliable Nb-coatings will be discussed.