CYCLIC OXIDATION AND HOT CORROSION BEHAVIORS OF A GRADIENT NiCoCrAlYSi COATING

MCrAlY (M=Ni and/or Co) coatings are widely used as overlays or bond coats for thermal barrier coatings due to their good performance against high temperature oxidation and hot corrosion. Usually, high Al content in the MCrAlY coatings can benefit the performance and lifetimes of the coatings. However, MCrAIY coatings usually contain only restricted Al content because high Al content might lead to brittleness and potential crack. Design of gradient coating can be used to solve the problem, since it can provide a balance between high Al content and high stress bearing ability. Therefore, much attention has been paid to coatings with gradient structures, and these coatings show good oxidation and corrosion resistance. In this work, a gradient and a conventional NiCoCrAlYSi coating were prepared by arc ion plating technique and subsequent annealing treatment. Cyclic oxidation tests of the two coatings were carried out between room temperature and 1000 degrees C. The hot corrosion tests of the coatings were performed in two different mixed salts of 75%Na2SO4+25%K2SO4 and 75%Na2SO4+25%NaCl (mass fraction) at 900 degrees C. The results indicated that the gradient coating possessed a graded distribution of Al-rich outer layer and Cr-rich inner layer after annealing treatment, and it showed better performance of re-healing alumina scale due to its possession of more beta phase as Al reservoir during the cyclic oxidation. The degradation process of the gradient coating was favorably retarded by the formation of Cr(W, Re)-rich precipitates in the interdiffusion zone. In sulphates, the two coatings showed good corrosion resistance. The presence of NaCl aggravated the corrosion extent of the two coatings. Compared with the conventional coating, the gradient coating postponed the formation of internal oxidation and sulfidation, resulting from the gradient distribution of Al-enriched outer layer and Cr-enriched inner layer.