Novel Cr-Si Slurry Coatings for High Temperature Materials

TiAl- and Ni-based superalloys are commonly used in aerospace engines due to their outstanding high temperature strength and creep properties. To also ensure sufficient oxidation and corrosion resistance, protective coatings are commonly applied.

Within the temperature range of 650-950 °C, where oxidation and Na₂SO₄-induced hot corrosion occurs, Cr-based coatings are advantageous. Cr-based diffusion coatings are applied by pack cementation, where the components are usually fully embedded into a powder mixture which is energy- and labor-intensive. Slurry-based coatings are significantly more economical and standard for aluminization. Recently, novel Cr-Si slurry coatings, applied by spray deposition, have been successfully developed and demonstrated at the industrial scale. Here, the TiAl alloy GE 48-8-7 and the Ni-based superalloy Rene 80 were coated. During the oxidation exposures carried out up to 900°C, the Cr/Si-coatings showed a significantly increased oxidation resistance and correspondingly lower weight gains compared to uncoated and Cr-pack cementation coated material. This can be attributed to a highly protective multi oxide scale consisting of an outer Cr₂O₃ and slow growing SiO₂. Consequently, the oxygen partial pressure can be effectively reduced through the scales and a dense Al₂O₃ subscale forms underneath the outer coating instead of detrimental internal oxidation of Al. Hot corrosion resistance was tested by the deposition of 4 mg/cm² Na₂SO₄ salt and following exposure at 700°C and 900°C for 300 h in a gas mixture of synthetic air containing 0.1 % SO₂. Compared to the uncoated pack cemented references, the Cr-Si coatings showed higher corrosion resistance.

Besides their superior corrosion resistance and longer part lifetimes through improved hot corrosion resistance, this sustainable slurry process offers several advantages over the pack cementation process: reduced waste, local recoating for repairs, avoidance of hazardous substances such as halide activators.