TWI Industrial Member Report Summary 649/1998
A J Sturgeon
Protective overlay coatings based on aluminide intermetallics are being considered for components operating in aggressive high temperature and corrosive environments. The high velocity oxyfuel (HVOF) thermal spraying process is believed to represent a competitive process for depositing good quality aluminide coatings.
The excellent oxidation resistance of the aluminide intermetallics has been employed extensively in diffusion coating systems used to protect nickel based superalloy components in aero engine, marine and industrial gas turbines for the past 30 years.
Recently there has been renewed interest in protective overlay coatings of aluminide intermetallics for improved corrosion protection in applications with corrosive environments that combine high temperatures ( ?-Fe in solid solution with about 5-8wt% Al and a phase closer to stoichiometric FeAl. The presence of the ?-Fe phase is of possible concern because it may be less resistant to oxidation and corrosion.
A similar situation was observed for the coating prepared with reaction sintered Ni 3Al powder. Here the starting powder was found to be inhomogeneous, containing Ni and NiAl in addition to Ni 3Al. These phases are retained in the deposited coating. The presence of a Ni phase in the coating is again of concern as it may also degrade oxidation and corrosion performance.
Only NiAl was deposited as an intermetallic compound, with an ordered B2 structure similar to that observed for the initial powder. This aluminide compound forms congruently and does not disorder below its melting point.
For Ni 3Al and Fe 3Al ordering present in the initial powders was lost in the deposited coatings.
The HVOF process may offer a more flexible alternative to diffusion coating processes and controlled atmosphere plasma spraying for the preparation of high quality aluminide coatings. Potential applications exist for the protection and repair of larger sized components found in land based turbines, coal gasifier and waste incinerator plants.
- To deposit nickel aluminide and iron aluminide coatings using the HVOF spraying process
- To determine the microstructure and quality of the prepared coatings
- To identify industrial applications that could benefit from using aluminide coatings deposited by the HVOF process