High-temperature oxidation behavior of arc ion plated NiCoCrAlYSiB coatings on cobalt-based superalloy
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NiCoCrAlYSiB coatings were deposited on the Co-based superalloy K40 by arc ion plating (AIP). The oxidation behavior of the bare alloy and of the coated specimens was tested in static air for 200 h at 1000 °C and 100 h at 1050 °C. The results showed that the oxidation rate of the system was greatly reduced by the addition of the NiCoCrAlYSiB coatings. Thin and adherent ␣–Al2O3 scales that formed on the coated specimens protected the substrates from further oxidation attack while non-protective oxide scales, mainly of Cr2O3 and CoCr2O4, appeared on bare K40 alloy. Element profiles on metallographic cross sections indicated that apparent interdiffusion occurred between the coatings and the substrates. The interdiffusion behavior and the resulting microstructure were investigated. As compared to aluminide coatings, NiCoCrAlYSiB coatings have less influence on the substrate microstructure.
I. INTRODUCTION
Cobalt-based superalloys are currently used in many aircraft turbine engines for high-temperature structural components. Choice of this material in gas turbine applications is primarily based on a good combination of superior tensile strength, excellent fabricability and weldability, and good hot corrosion resistance under prolonged exposure.1 Degradation by high-temperature oxidation is one of the main failure modes of hot-section components in gas turbines. To protect cobalt-base superalloys from high temperature corrosion, aluminide coatings have been developed since the 1950s. Thereafter, a variety of modifications have been developed to improve the oxidation and hot corrosion resistance of the aluminide coatings.2 Nevertheless, both the common and the modified aluminide coatings have some serious problems. One of them is the undesirable structure.3,4 Because of the low solubility in cobalt aluminide (CoAl), chromium and tungsten are concentrated and form a continuous carbide layer between the cobalt aluminide and the substrate during inward coating growth. The cracking of the Cr and W carbide enriched layer coupled with oxidation inside the crack network undercuts the aluminide coating, resulting in massive coating delamination.3 Moreover, the high ductile–brittle transition temperature and the low average cracking strains5 of CoAl phase
could also lead to coating cracking problems and shorten the working life since the peak tensile surface strains on the turbine blades are likely to occur at relatively low temperature. As compared to aluminide coatings, MCrAlY (M ⳱ Ni or/and Co) overlay coatings possess an optimized balance of oxidation resistance, corrosion resistance, and coating ductility through composition design. MCrAlY coatings have been successfully used on Ni-base superalloys and Ni3Al-based superalloys.6–11 Therefore, some researchers3 suggest that MCrAlY overlay coatings may be a viable alternative to diffusion coatings for the protection of cobalt base superalloys in service. However, few efforts seem to have been involved in the case of the MCrAlY coatings on Co-based superalloys. The aim of this w
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