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Introduction

High-temperature protective coatings used in gas turbine components must withstand extremely severe environmental conditions. There are two basic coating systems, which are currently being used in gas turbines to provide improved corrosion and oxidation resistance, and thereby extend the component life. These are called diffusion coatings and overlay coatings. The diffusion coatings provide a surface enrichment of aluminum, chromium, or silicon, and aluminizing behavior of nickel-based superalloys has been fully reported .As operating temperatures of gas turbines are increased, it becomes impossible to achieve the service lives using the diffusion coatings. On the other hand, the overlay coatings are specifically designed corrosion and oxidation resistant MCrAlY alloys, where M is iron, nickel, and/or cobalt. In recent years, a vacuum plasma spraying ~VPS! process has been used to apply the overlay coatings of MCrAlY alloys . This coating process has been found to be very effective for gas turbine components, such as blades and duct segments, which are exposed to aggressive gases at high temperature. As operating temperatures are increased, the excellent performance of diffusion coatings applied on top of MCrAlY overlay coatings has been realized. The over-aluminized MCrAlY coatings have been used for improving further the high-temperature oxidation resistance.

On the other hand, thermal barrier coatings, which consisted of an air-plasma sprayed yttria partially-stabilized zirconia layer applied directly over an air-plasma sprayed MCrAlY bond coat layer, provide to improve the thermal resistivity for air-cooled gas turbine combustors. Nowadays, thermal barrier coatings, which consisted of a vacuum-plasma sprayed MCrAlY bond coat layer, are developed for gas turbine blades and vanes . In addition to extending the life of thermal barrier coatings, the aluminized process to enrich the outer surface of MCrAlY bond coats in aluminum content has been developed, and thereby high-temperature oxidation resistance was highly improved owing to the formation of the alumina layer at the interface between the yttria partially stabilized zirconia layer and the bond coat layer . As mentioned above, the over-aluminized MCrAlY coatings were one of the important technologies for improving the coating life. However, the aluminizing behaviors of MCrAlY coatings have not been fully clarified.

In the present paper, five typical kinds of vacuum plasma sprayed MCrAlY CoCrAlY, CoNiCrAlY, CoNiCrAlY1Ta, NiCrAlY, NiCoCrAlY! coatings were selected for aluminizing experiments, and the vacuum-plasma sprayed and the heat-treated ~1393 K, 2 h, argon cooled and 1116 K, 24 h, argon cooled! MCrAlY coatings were pack-aluminized at 1173, 1223, and 1273 K for 5, 10, and 20 h, respectively. The aluminizing behaviors of these MCrAlY coatings were investigated. Further, the effect of nickel content on the aluminizing behaviors of MCrAlY coatings was discussed.