Over the past two decades there has been considerable interest in the development of coatings with finer microstructures approaching nanometer scale because these coatings are more resistant to high-temperature oxidation and corrosion than their counterpart conventional coatings. Long-term cyclic oxidation behavior of nanocrystalline FeCrNiAl and NiCrAl coatings were evaluated at different temperatures and the results showed that ultra-fine grain structure promoted selective oxidation of Al during thermal exposure. The protective Al2O3 scale formed on these coatings with Al content as low as 3 wt.% and exhibited excellent spallation resistance during thermal cycling. The nanocrystalline NiCrAl coating showed significantly higher oxidation resistance compared to the conventional plasma sprayed NiCoCrAlY and PWA 286 coatings. However, the Al content in the nanocrystalline coatings was consumed in relatively short time due to inward and outward diffusion of Al. Variation of oxide-scale spallation resistance during thermal cycling and the rate of Al consumption between the nanocrystalline and plasma sprayed coatings are compared.

High temperature protection requires full coating density, high adhesion, minor oxide inclusions, and preferably fine grains, which is not achievable in most thermal spray processes. High velocity oxygen-fuel (HVOF) thermal spray process has been applied extensively for making such coatings with the highest density and adhesion strength, but the existence of not or partially melted particles are usually observed in HVOF-formed coatings because of relative low flame temperature and short particle resident time in the process. This work has investigated the development of an innovative HVOF process using a liquid-state suspension/slurry containing small alloy powders. The advantages of using small particles in a HVOF process include uniform coating, less defective microstructure, higher cohesion and adhesion, full density, lower internal stress and higher deposition efficiency. Process investigations have proven the benefits for making alloy coatings with full density and high bond strength attributing to increased melting of the small particles and the very high kinetic energy of particles striking on the substrate. High temperature oxidation and hot corrosion tests at 800°C have demonstrated that the alloy coatings made by the novel process have superior properties to conventional counterpart coatings in terms of oxidation rates and corrosion penetration depths.

High velocity oxygen-fuel (HVOF) thermal spray processes are used in applications requiring the highest density and adhesion strength, which is not achievable in most other thermal spray processes. Like other thermal spray processes, however, a normal HVOF process is not able to apply fine powders less than 10 µm via a conventional powder feeder. The advantages of using smaller and even nano-sized particles in a HVOF process include uniform coating, less defective microstructure, higher cohesion and adhesion, full density, lower internal stress and higher deposition efficiency. A new process has been developed to realize HVOF forming of fine-grained alloy layers by using liquid precursors containing fine metallic particles. Process investigations have shown the benefits for making single and duplex layered coatings with full density and high bond strength attributing to the very high kinetic energy of particles striking on the substrate surface and the better melting of the small particles. One of the targeted applications is for the water walls of a fossil-fired boiler that operate in a high temperature and corrosive environment. The new coating system is based on material selection, structure design, process innovation and diagnostics, microstructure, and property evaluation. It is promising to provide better protection of the boilers against various types of degradations like corrosion, oxidation, erosion and interfacial failure.