Oxidation and hot corrosion behavior of gradient NiCoCrAlYSiB coatings deposited by a combination of arc ion plating and magnetron sputtering techniques
Introduction
The efficiency of all types of gas turbine engines, aircraft, terrestrial and marine, is proportional to firing or turbine inlet temperature, increases in temperature are facilitated by improved structural design and airfoil cooling technology, and coating with steadily improved protection systems [1], [2]. In fact, coatings are playing a significant role in today’s aero and industrial turbine engines to extend the life or enhance the performance of components. About 75% of all the components in jet engines are coated [3], [4]. The most common metallic coatings used in the hot section of turbine engines are MCrAlY type overlay coatings, which can be specifically designed and produced to meet the particular environmental operating conditions independent of substrates.
Many studies have been conducted in which the composition and microstructure of MCrAlY coatings are modified in order to form a dense, continuous α-alumina, which protects the MCrAlY coatings from rapid oxidation [5], [6], [7], [8]. The Al content in the coating alloys is important, because selective oxidation of Al occurs only on the surface of alloys with adequate Al content [9]. During the oxidation, Al is depleted not only by oxidation on the surface of coatings, but also by diffusion into the substrates. To warrant a high durability life, coatings should be designed that the aluminum concentration and activity in the coating is as high as possible, together with a high chromium concentration [10], [11]. If aluminum and chromium contents are too high, however, the melting point of the coating will decrease and the brittleness of the coatings will increase. Gradient coatings can resolve this problem properly [12], [13].
By using multiple sources and a shutter system, coatings which progressively changing composition and characteristics can be manufactured. Recently, arc ion plating (AIP) shows many advantages in the preparation of MCrAlY coatings due to the ion bombardment of the substrate during deposition which can improve both the adhesion and the structure of the coating [14], [15]. Magnetron sputtering (MS) produces very good thickness uniformity and high density of the films, and the process has good controllability and long term stability.
In this study, a combination of arc ion plating and magnetron sputtering was used to prepare a gradient NiCoCrAlYSiB coating with Cr and Al gradient distribution. The oxidation and hot corrosion resistance of the coatings was studied.
Section snippets
Experiments
The AIP target material studied in this work was Ni32Co20Cr8Al0.5Y1Si0.03B (wt.%), and the magnetron sputtering target was Cr–15Al (wt.%). Directionally solidified Ni-base superalloy DSM 11 (mainly of Ni with 3 wt.% Al, 14 wt.% Cr, 10 wt.% Co, 5 wt.% Ti, and some W, Mo, Ta, C) was used as the substrate. Rectangular specimens of dimensions (in mm) 15 × 10 × 2 were ground to 800-mesh, peened in a wet atmosphere (200-mesh glass ball), and then ultrasonically rinsed in acetone. Before deposition, the
Coatings structure
Fig. 1 shows the cross-section SEM images of the NiCoCrAlYSiB coating and the gradient NiCoCrAlYSiB coating. The thickness of each coating was about 30–50 μm. From the figure, we can see that the annealed coatings are composed of two types of zone, the bright and the dark zones. The dark zones are rich in Al and Cr, and the bright zones rich in Ni and Co. In Fig. 2, the quantitative electron probe microanalysis (EPMA) of the elemental concentration in the cross-section of both the annealed
Oxidation mechanism
MCrAlY coatings have been used extensively in aircraft, utility and marine propulsion due to the flexibility of design, good oxidation and hot corrosion properties and ductility which can be varied over wide ranges according to the various requirements. At high temperature, the MCrAlY coatings form a protective alumina or chromia scale by interaction with the oxidizing atmosphere. The microstructure of the oxide scale depends on the composition of the coating, the manufacturing process and the
Conclusions
- (1)
On a directionally solidified Ni-based superalloy a NiCoCrAlYSiB coating was prepared by AIP and an Al and Cr gradient NiCoCrAlYSiB coating was prepared by a two-step co-deposition. The results of EPMA show that the Al and Cr are uniformly distributed in the NiCoCrAlYSiB coatings but have a gradient distribution in the surface layer of the gradient coatings.
- (2)
After 100 h oxidation at 1000 °C, both the NiCoCrAlYSiB coating and the gradient coating improve the oxidation resistance of the DSM 11 alloy
Acknowledgements
This work was supported by the Chinese Academy of Sciences-KGCX2-212-02. The authors wish to acknowledge Dr. H. Li for supplying the DSM 11 samples and Dr. Y.J. Tang for checking the paper.
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