Multifunctional coatings of ultra-high temperature ceramics consisting of transition metal based borides (ZrB2, HfB2, TaB2), carbides (TiC, TaC, ZrC, HfC), and nitrides (ZrN, HfN, TaN) exhibit high melting temperature, high hardness, outstanding oxidation and corrosion resistance, thermal shock resistance and high thermal stability. Such coatings have high potential for a much wider industry-specific applications, such as, surface protection of high-speed cutting tools, turbine blades and vanes, hypersonic vehicles (to construct sharp wing leading edges and nose tips for thermal protection), atmospheric re-entry and rocket propulsion systems. The coatings with such desirable properties possess unique microstructures. In this talk, we present transmission electron microscopy (TEM) of selected multifunctional hard coatings with unique structures: (1) Si-B-C-N and Hf-B-Si-C-N coatings with extraordinary high temperature stability, high hardness and high oxidation resistance. The coatings remain amorphous when annealed > 1500 °C in air and exhibit an extraordinary oxidation resistance due to formation of finely distributed h-BN or HfO2 nano-crystals embedded in an amorphous SiO2 matrix at very high temperatures. (2) Zr-B-C-N coatings with a high hardness of 37 GPa and a modulus of 317 GPa, possess an unique nano-needle structure with ZrN and/or Zr(B,N) nano-domain structures (~2 nm) that are semi-coherently joined via Zr-N monolayer interfaces. (3) Hf-B-Si-C coatings which with only a slight variation in Si content exhibit entirely different microstructures and thus, mechanical and oxidation resistance properties.
This work was supported by the U.S. NSF under Award NSF/CMMI DMREF-1335502.