Cold spray for production of in-situ nanocrystalline MCrAlY coatings – Part II: Isothermal oxidation performance

Highlights

• An α-Al₂O₃ scale established on the coating surface after 1-hour exposure at 1100 °C.

• Subsurface Al concentration remained above 3.9 wt.% after 500-hour oxidation.

• TGO growth kinetics deviated from the parabolic rate law.

• The instantaneous rate constants decreased rapidly during the short-term oxidation.

• Coating characteristics produced by cold spray could potentially benefit its performance.

Abstract

Cold spray (CS) MCrAlY coatings have been widely explored as potential bond coats for thermal barrier coatings used on the hot-section components of modern gas turbines. In this study, NiCoCrAlTaY coatings were applied on CMSX-4 single-crystal Ni-base superalloy substrates using the CS technique. Nitrogen was used as process gas with low spray parameters to allow the production of in-situ nanocrystalline NiCoCrAlTaY coatings while using conventional powders of large grain size, as demonstrated in Part I of the study using scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM). In this part of the study (Part II), the coated samples were examined under isothermal oxidation conditions at 1100 °C for 1 h to 500 h. The as-deposited NiCoCrAlTaY coatings, characterized by dense microstructure, low surface roughness, nanoscale grains, and a large number of crystallographic defects such as grain boundaries and dislocations, promoted the formation of a protective α-Al₂O₃ scale during short-term oxidation. Fast diffusion via these crystallographic defects facilitated Al diffusion in the coatings. This provides sufficient Al replenishment to the coating subsurface region, sustaining the growth of the α-Al₂O₃ scale during prolonged oxidation. The relatively low scale growth kinetics found in the study demonstrated enhanced oxidation performance of the CS nanocrystalline NiCoCrAlTaY coatings.

Introduction

High-pressure turbine blades and nozzle guide vanes in modern aero-engines are manufactured from single-crystal Ni-base superalloys as their operating environment requires the use of materials with high creep and thermo-mechanical fatigue strength at elevated temperatures [[1], [2], [3]]. These components have hollow airfoils with internal cooling and are coated with a thermal barrier coating (TBC) consisting of a ceramic topcoat and a metallic bond coat [[4], [5], [6]]. Combined with air cooling, the ceramic topcoat, typically 7–8 wt.% yttria-stabilized zirconia (7YSZ) produced by electron beam physical vapor deposition (EB-PVD), provides effective heat shielding of the component surface from hot gases, allowing the turbine to operate at higher temperatures with enhanced efficiency and performance [[7], [8], [9]]. The metallic bond coat, typically a diffusion platinum aluminide or low-pressure plasma spray (LPPS) MCrAlY coating, minimizes the mismatch of coefficient of thermal expansion (CTE) between the 7YSZ topcoat and the turbine components and protects the latter from oxidation degradation [6]. This oxidation resistance function is achieved by forming a thermally grown oxide (TGO) layer between the bond coat and the topcoat. Once established, the TGO scale can effectively hinder the diffusion of oxidation reactants through the scale and hence reduce the oxidation rate of the coated components [10]. While these types of commercial bond coats meet the blade design requirements, they are costly to fabricate as platinum is an expensive precious metal and the LPPS process requires the use of sophisticated vacuum equipment for coating deposition. Therefore, it is desirable to develop alternative manufacturing techniques to substitute diffusion platinum aluminide or LPPS MCrAlY overlay bond coat without compromising oxidation performance.

The durability of TBCs is closely related to the characteristics of the TGO scale developed during thermal exposure [[11], [12], [13], [14], [15]]. A continuous, uniform, dense, chemically stable, and adherent TGO scale is required for a durable TBC [12]. Aluminum oxide, particularly α-Al₂O₃, is known for its high melting point, excellent stability, slow growth rate, and low oxygen diffusivity at high temperatures [16], and thus satisfies most of the requirements for a protective TGO scale. MCrAlY coatings usually contain a sufficient amount of Al, and their compositions are specifically designed such that selective oxidation of Al occurs and the growth of a protective α-Al₂O₃ TGO scale is promoted at temperatures relevant to gas turbines [6]. The oxidation of MCrAlY coatings is a dynamic process, in which the coating composition, microstructure, and TGO characteristics continuously evolve. Under certain circumstances, the TGO scale may contain oxides other than α-Al₂O₃, such as mixed oxides (NiO, CoO, and Cr₂O₃) and spinel-type oxides (Ni, Co)(Al, Cr)₂O₄ [17,18]. These oxides are characterized as porous and fast-growing, and thus considered non-protective and should be avoided or delayed from being formed. Moreover, the TGO scale is often subjected to mechanical stresses, e.g., externally applied stress, growth stress, and residual stress due to thermal expansion misfit between the scale and the underlying coatings [19]. These stresses, when exceeding certain material and geometry-dependent critical values, may provoke scale spallation resulting in premature TGO failure, even before the formation of undesirable oxides in the scale (or known as chemical failure) [20,21]. The protectiveness of MCrAlY coatings is generally considered to come to an end either (i) chemically, when substantial amounts of non-protective oxides are formed, or (ii) mechanically, when excessive TGO scale spallation occurs prior to the chemical failure. It was found that the oxidation behavior and failure mechanisms of MCrAlY coatings strongly depend on the coating and substrate compositions [[22], [23], [24]], coating microstructure and surface morphology [[25], [26], [27], [28], [29], [30]], as well as thermal exposure conditions [31,32].

To improve the performance and durability of MCrAlY coatings, extensive studies have been carried out focusing on composition optimization, microstructure control, and surface modification. One of the topics that have attracted research attention is the fabrication of nanocrystalline MCrAlY coatings (grain size <100 nm). The presence of enhanced grain boundaries associated with the nanostructure promotes atomic diffusion at elevated temperatures and facilitates the formation and growth of a protective α-Al₂O₃ scale [17,[33], [34], [35], [36]]. However, other coating characteristics, e.g., internal oxide content and surface finish, also play a critical role in the performance of nanocrystalline MCrAlY coatings. Mercier, et al. [37] found that grain refinement by cryomilling contributed to the establishment of a single α-Al₂O₃ TGO layer on a cryomilled CoNiCrAlY powder during isothermal oxidation at 1000 °C. In contrast, high velocity oxygen fuel (HVOF) CoNiCrAlY coatings produced from cryomilled powders yielded a duplex TGO scale consisting of α-Al₂O₃ and mixed oxides at the same temperature [25,37]. This change in coating oxidation behavior was attributed to in-process powder oxidation during the HVOF sprays, which led to early nucleation and subsequent growth of mixed oxides [25,37]. Zhang, et al. [38] observed that the surface morphology of nanocrystalline NiCrAlY coatings had a significant impact on the TGO constitution. After 200 h of oxidation at 1000 °C, a TGO scale containing spinel and mixed oxides was formed on the coatings without post-deposition surface treatment (Ra = 5.6 ± 1.2 μm), whereas a continuous and uniform α-Al₂O₃ TGO scale was observed on their shot-peened counterparts (Ra = 3.1 ± 0.6 μm). These findings demonstrated that low oxide content and reduced surface roughness are beneficial to the oxidation performance of MCrAlY coatings.

Cold gas dynamic spray or simply cold spray (CS) has been widely investigated as a promising technique for manufacturing MCrAlY bond coats [30,[38], [39], [40], [41], [42], [43], [44], [45], [46], [47]]. Its low process temperature minimizes in-process feedstock powder oxidation, and the high-velocity particle impacts induce severe plastic deformation leading to adherent and dense coatings. It has been demonstrated in a previous study that NiCoCrAlTaY coatings can be deposited by CS on CMSX-4 single-crystal Ni-base superalloy substrates using either nitrogen or helium as the process gas [48]. The use of nitrogen with relatively low spray parameters was found to produce extensive plastic deformation in CS NiCoCrAlTaY coatings due to enhanced in-process particle impingements [48]. This allowed the production of in-situ nanocrystalline NiCoCrAlTaY coatings while using conventional powders of large grain size, as demonstrated in the first part of the current study (Part I) [49]. SEM and STEM characterization revealed that the NiCoCrAlTaY coatings produced under such CS conditions were characterized by dense microstructure, low oxide content and surface roughness, and nanoscale grains, offering potentially improved oxidation performance. In this part of the study (Part II), the isothermal oxidation performance of the CS NiCoCrAlTaY coatings was examined at 1100 °C for durations of 1 h to 500 h. Coating morphology, microstructure, and composition were characterized after different oxidation durations. The TGO growth kinetics on the CS NiCoCrAlTaY coatings was evaluated and compared to similar MCrAlY coatings produced using other coating techniques. The beneficial effect of the CS NiCoCrAlTaY coatings characteristics on the oxidation performance is discussed.