Land-based gas turbines components are subjected to high mechanical loads at elevated temperatures, whilst having to contend with hot gas corrosion and erosion from aggressive air and fuel contaminates. For protection from these damaging environments, special thermal sprayed coatings have been developed. These coating extend operating life, and allow increased turbine entry temperatures (TET), leading to an improvement in thermal efficiency.
Turbine blades and guide vanes are generally engineered from either high-temperature resistant nickel-based super-alloys, such as IN738, which have good oxidation-resistant properties but are not capable of withstanding high-temperature corrosion and erosion, or cobalt-based alloys that are excellent against corrosion but have a low resistance to oxidation. To alleviate these problems components are coated with an MCrAlY overlay, the composition of which, and especially the metal component M (Ni, Co, Fe or a combination thereof), depends on the application and the environment in which it is used. MCrAlY alloys form internal oxide structures and spinels, which renders them with excellent resistance to high-temperature oxidation and hot corrosion. The life of the coating is closely related to the oxide content, porosity and the bonding of the coating to the substrate. The addition of yttrium (Y) to the coating improves the overall oxide structure and tenacity of the oxide films.
Thermal Barrier Protection (TBC)
Some components operate in temperature regimes exceeding the maximum allowed for either the overlay coating or the component material, which leads to a requirement for thermal protection. Zirconia based ceramics are applied over the MCrAlY coating to provide a temperature gradient between the gas stream and substrate. With the aid of cooling gases, these components can withstand the elevated gas stream temperatures. The thickness of the zirconia required depends upon the gas stream temperature and the amount of cooling available. These coatings can be applied to a thickness of 2 mm, although typical thickness’ is in the range of 0,2 mm to 0,5 mm.
Thermal spray coatings for industrial gas turbines produce substantial benefits. These include:
- Oxidation and corrosion resistance
- Low thermal conductivity
- Enhanced erosion resistance
- Longer duty cycle and increased part life
- Reduction of substrate operating temperature, resulting in lower thermal and creep stresses
- Protection against hot spots
- Enhanced operating temperature, resulting in increased thermal efficiency