Investigation of the bond coats for thermal barrier coatings on Mg alloy
Highlights
▸ Bond coats of Al@Ni, MCrAlY(M = Co, Ni) and Ni@Al for Mg alloy were investigated. ▸ The bond coats had moderate TECs (13–15 × 10−6 K−1). ▸ The structure of original powders affected the properties of the sprayed bond coats. ▸ MCrAlY bond coat had excellent oxidation resistance at 400–500 °C. ▸ MCrAlY bond coat had excellent corrosion resistance in 3.5% NaCl solution. ▸ MCrAlY was demonstrated as the most appropriate bond coat for TBCs on Mg alloy.
Introduction
Mg alloys are the lightest materials among those metals used for structural or mechanical applications [1]. Besides, Mg alloys have many excellent properties such as high specific tensile strength, good stiffness and vibration absorption [2]. Thus Mg alloys have potential applications in automobiles, electronic products, aerospace, etc. [3], [4]. Unfortunately, Mg alloys have low melting point, poor wear resistance and high chemical activity, which have limited their widespread applications [5]. Several researches reported that various coatings prepared by thermal spraying can provide desired properties of corrosion and wear resistance for the Mg alloy substrate [6], [7].
Thermal barrier coatings (TBCs), typically consisting of MCrAlY (M = Ni, Co) bond coat and yttria stabilized zirconia (YSZ) top coat, have low thermal conductivity and excellent surface properties [8], [9], [10]. They are generally used for gas turbines and diesel engines to increase the operation temperature. In a previous study [11], TBCs were designed for Mg alloy with an attempt to improve its thermal and corrosion resistance. The bond coat and the 8 wt% yttria stabilized zirconia (8YSZ) top coat were successfully deposited on rare earth-magnesium alloy MB26 by atmospheric plasma spraying (APS). The TBCs have an effective protection for Mg alloy.
Compared with the Ni-based superalloy, Mg alloy has large thermal expansion coefficient and vulnerable surface. The oxidation of Mg alloy during APS and the mismatch of the thermal expansion coefficient between the coatings and substrate have an unfavorable effect on the stability of the coating. In order to improve the stability of the sprayed coating on Mg alloy substrate, one available method is to protect the substrate against oxidation during APS and improve the compatibility between the coating and the substrate [12]. The other effective method is to adopt an appropriate bond coat. Taking into account the thermal expansion coefficient and chemical stability, MCrAlY and Ni–Al alloys (Ni@Al and Al@Ni) were chosen as the candidate bond coats.
The oxidation and corrosion of the metallic bond coat could easily lead to some defects in the coatings during APS and application. The defects together with stress concentration could finally cause the failure of the coatings [13]. There have been some studies about the oxidation of the bond coat of typical TBCs (MCrAlY bond coat, Ni-based super-alloy substrate and YSZ top coat) under high temperature 800–1300 °C [14], [15], [16], [17]. However, there is no study about the oxidation of the bond coat on Mg alloy during spraying and under the applied temperature below 600 °C. Besides, the coated specimens are easy to be exposed in the humid corrosive environment during storage and transport. Therefore, the oxidation and corrosion of bond coat will be regarded as important assessment references to determine the optimal bond coat of TBCs on Mg alloy.
This study was carried out to characterize properties of the candidate bond coats via studying the thermal expansion behavior, oxidation and corrosion of the sprayed coats in correlation with a similar application environment, from which the optimal bond coat of TBCs on Mg alloy was determined. It might provide some useful information for potential application of TBCs on more reactive metallic substrate.
Section snippets
Experimental
TBCs comprising of bond coat and top coat on Mg alloy were fabricated by APS. Candidate metal powders (Ni@Al, Al@Ni and MCrAlY) were sprayed onto substrate to prepare coatings with Unicoat Spraying System (F4 gun, Sulzer Metco, Switzerland) with Ar-H2 as plasma gases. The spray distance was 100 mm with a spray current of 600 A. The original powders were purchased from Beijing General Research Institute of Mining and Metallurgy. In this study, the molten particles from the outer part of plasma
The candidate bond coat materials
The nickel-aluminum alloys attract significant attention due to a promising combination of properties such as high melting temperature, low density, high corrosion resistance and high temperature strength [18]. The metal could be prepared as coatings to improve their low room temperature ductility. Therefore, three types of Ni–Al metal (Ni@Al, Al@Ni and MCrAlY) were sprayed as bond coat on Mg alloy in this study.
The original powders were processed into compact blocks to determine the content of
Conclusions
Bond coats of Al@Ni, MCrAlY(M = Co, Ni) and Ni@Al on Mg alloy were prepared by APS. The spraying behavior, oxidation and corrosion resistances were investigated. The main conclusions are following:
- (1)
The composition and structure of original powders affected the structure and properties of the sprayed bond coats. The outer layer of the original spraying powders was ablated during APS. Al@Ni powder was ablated severely, and the formed bond coat was compact. Although Ni@Al had higher Al content than
Acknowledgements
Financial support from projects of NSFC-50825204, NSFC-51101143 and Lotus Scholars Program of Hunan are gratefully acknowledged.
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