X. Montero , M.C. Galetz, M. Schütze
Slurry aluminide coatings are widely applied to protect metallic surfaces from oxidation and corrosion. They are frequently used in gas turbine engine nozzles because of economical advantages and a straight-forward manufacturing route. A variety of commercial slurries are available to aluminize the surfaces of nickel-based superalloys, however, they have two main disadvantages. First, the phosphates and chromates or halides used as binders and to activate the diffusion species are environmentally harmful; second, the conventional systems have to be heat-treated in an inert atmosphere. As an outcome of the PARTICOAT project the variety of slurry derived coatings has been extended by tailoring the particle size of the metallic source. By doing that, environmentally friendly water-based slurries were developed to produce in a one-step process und atmospheric conditions, a thermal barrier system based on an aluminum diffusion layer and an alumina foam layer which serves as bond coat as well as top coat (TC). CM 247 nickel base superalloy was coated and heat-treated in air using newly developed Al and Al–Si slurries. The oxidation behavior was investigated at 1,000 °C and then compared to pack-cemented aluminide coatings. The sulphidation behavior was investigated at 1,000 °C in an atmosphere of 1.5 vol% SO2 in synthetic air for Al and Al–Si slurry coated samples with and without the alumina foam TC layer. PARTICOAT Al-based slurries,, after the initial stabilization of the TC, showed similar oxidation kinetics as pack cemented aluminides when exposed to air. When the coatings were exposed to sulphide-containing atmospheres, their oxidation rates increased, producing typical type I corrosion damage. Coatings without TC produced more protective oxide scales. The weight gain and coating area affected by corrosion were slightly lower for the Al-based slurries after 1,000 h of exposure than for the Al–Si based ones. The new coating presented here offers unique advantages in comparison to state-of-the-art slurry and pack cemented coatings by opening a potential way to manufacture a complete thermal barrier coating system by a simple, inexpensive and environmentally safe deposition and heat-treatment in air.