Coatings are well established as an important underpinning technology for the manufacture of aeroengine and industrial turbines. Higher turbine combustion temperatures are desirable for increased engine efficiency and environmental reasons (reduction in pollutant emissions, particularly NOx), but place severe demands on the physical and chemical properties of the basic materials of fabrication.
In this context, MCrAlY coatings (where M = Co, Ni or Co/Ni) are widely applied to first and second stage turbine blades and nozzle guide vanes, where they may be used as corrosion resistant overlays or as bond-coats for use with thermal barrier coatings.
In the first and second stage of a gas turbine, metal temperatures may exceed 850°C, and two predominant corrosion mechanisms have been identified:
Accelerated high temperature oxidation (>950°C) where reactions between the coating and oxidants in the gaseous phase produce oxides on the coating surface as well as internal penetration of oxides/sulphides within the coating, depending on the level of gas phase contaminants
Type I hot corrosion (850 - 950°C) where corrosion occurs through reaction with salts deposited from the vapour phase (from impurities in the fuel). Molten sulphates flux the oxide scales, and the microstructure is characterised by non-protective scales, extensive internal suphidation and a depletion zone of scale-forming elements
The choice of base material (Co or Ni) is dependant on the primary corrosion mechanism, but as engine temperatures increase, the trend is towards CoNiCrAlY compositions. Cr and Al are present in the MCrAlY composition because they form highly tenacious protective oxide scales, whilst Y promotes formation of these stable oxides. MCrAlY coatings may be applied by a number of processes including:
- Physical vapour deposition (PVD)
- Low pressure (LPPS), vacuum plasma (VPS) or air plasma spraying (APS)
- High velocity oxyfuel (HVOF) spraying
PVD and VPS offer high quality in terms of minimal oxidation of the coating during the deposition process, but are the most expensive. Approvals have been granted for the use of APS and HVOF coatings on certain components with significant cost savings. It is common for MCrAlY coatings to be deposited onto components pre-coated with Al, PtAl or Cr, which have been produced by vapour deposition techniques or diffusion processes.
What is a thermal barrier coating (TBC)?
Surface engineering at TWI
Thermal spraying- high velocity oxyfuel (HVOF), arc and flame processes
See further information about Materials and Corrosion Management or please contact us.