Isothermal and Cyclic Testing of An Oxidation-Corrosion Protective Coating for Turbine Components

The evolution of gas turbines has pushed the operating limit of their internal components in the quest for greater levels of performence. In order to maintain oxidation and corrosion resistance for an alloy designed for mechanical strength; coatings are applied to give multifuctionality to the component.

The present standard in oxidation protection is an aluminide based difusion coating for structural parts. However demands for increases in temperature, lifetime and the higher importance of corrosion has led to overlay coatings becoming an interesting alternative. However, in order to replace the existing process; comparative studies need to prove the suitability of the overlay coatings.

In this study, substrate materials of Haynes 230, Mar M247 and Mar M509 were coated with a commercially available NiCoCrAlY powder using the high velocity oxy-fuel process. Reference samples were produced with a diffusion aluminide and plasma sprayed overlay.

The oxidation behaviour and performance of the coating systems were tested using a range of high temperature experiments. Isothermal oxidation behaviour was observed in the lab environment. Cyclic thermal exposure was tested using a thermal shock rig. The influence of water vapour on the oxidation behaviour was studied using controlled environment testing. Finally tests were carried out using a turbine combustor to replicate real engine conditions and study the effect on oxidation. Samples were characterised using SEM, image analysis and F-SECT beta phase analsis.

The results conclude that HVOF coatings performed superiorly against oxidation under all conditions compared to the alternatives. From the three candidate materials some different behaviour was observed in tests; however, Mar M509 was found to perform best.