IMPROVEMENT OF MCRALY COATINGS BY ADDITION OF RHENIUM

In order to improve the efficiency and power output of gas turbines, turbine inlet temperatures are being rapidly increased. Since high strength blade materials and sophisticated processes such as single-crystal solidification are nowadays available also for large parts, it is possible to increase metal surface temperatures to save cooling air. The increase in surface temperature causes a more severe oxidation attack on the blade coating, which is usually of the MCrAlY type in stationary gas turbines. Additionally, coating degradation by interdiffusion with the substrate and by thermal mechanical stress is increased. Considerable research has been carried out to improve the high temperature properties of MCrAlY (with M=Co, Ni or combinations thereof) coatings by additions of minor alloying elements such as Si, Hf, Ta, Zr, etc. However, up to this time no work has been published on the properties of MCrAlY coatings with rhenium additions for stationary gas turbines. In this paper we report on the properties of MCrAlY coatings containing 1.5-10 wt.% Re. The coatings were applied to INCO 738 LC material by low pressure plasma spraying. Static and cyclic oxidation tests were carried out for up to 5000 h and 1500 cycles at 950 and 1000 degrees C. Thermal fatigue tests under near-service stresses for up to 3000 cycles were performed. The microstructural stability and the interdiffusion behaviour were studied after long-time exposure. Rhenium additions can considerably improve the oxidation resistance of ''classical'' MCrAlY systems. Even more important, however, is the improvement in thermal cycle fatigue as properties as reflected by the number of cycles until crack initiation. Thus several of these coatings can be recommended for high temperature-high thermal load applications.