Mechanical degradation of coating systems in high-temperature cyclic oxidation

Abstract

Cyclic oxidation tests were performed on a large variety of commercially available overlay coatings. The results confirmed that the composition of the coating as well as the processing method of the coating can affect the life of the system. Coating life was determined by the time of coating penetration and initiation of substrate attack. As this work was part of an effort to evaluate the relative protectiveness of available coating systems for Industrial Gas Turbine applications at relatively high temperature, an extensive testing programme was undertaken. The different coatings were tested to visual failure. The results presented in this paper concentrate on the surface instability of the coating. The success of a coating in a high-temperature application is measured by its ability to remain in place, to resist oxidation, and to avoid cracking. In general, overlay coatings developed for intermediate temperature hot corrosion are often limited by oxidation behaviour, while diffusion coatings are more susceptible to thermal fatigue cracking in cyclic application. Apart from usual oxidation of the coating an additional degradation mechanism was observed. A mechanical effect caused by the instability of the coating as a result of the difference in the thermal coefficient of expansion and mechanical properties between the substrate and coating. This effect, known as the “rumpling” effect, resulted in a significant reduction in time of penetration and was in some systems the life controlling factor.