目的 研究带Al2O3防护层的NiCoCrAlYHf涂层的氧化及腐蚀行为机制。方法 MCrAlY金属涂层作为一种成熟的高温防护涂层体系,广泛应用于航空发动机涡轮叶片表面,承受高温服役环境下的氧化和腐蚀,针对MCrAlY涂层体系的材料改性可一定程度上提升涂层的抗氧化腐蚀性能,但无法从根本上阻隔熔盐等外来物与涂层的直接接触,针对上述挑战,本研究选用热化学稳定性优异的Al2O3为原材料,采用EB-PVD工艺在NiCoCrAlYHf金属涂层表面制备了Al2O3防护层,对Al2O3防护层体系进行了高温氧化与熔盐腐蚀性能测试,并通过显微组织结构演变与成分变化,阐述了高温氧化及腐蚀行为机制。结果 Al2O3涂层为单一α相结构,呈现细小柱状晶排列,形成微观间隙,NiCoCrAlYHf涂层与NiCoCrAlYHf/Al2O3涂层的平均氧化速率分别为0.043 mg/(cm2·h)与0.041 mg/(cm2·h),Al2O3涂层对抗氧化性能提升不显著,但Al2O3涂层显著提升了涂层体系的抗熔盐腐蚀能力。结论 在高温氧化性能过程中,Al2O3防护层中的微观间隙为氧原子提供扩散路径,促使NiCoCrAlYHf涂层发生选择性氧化,形成TGO层,TGO层生长以及Al2O3防护层高温烧结引起的纵向开裂,加剧了氧原子向内扩散。高温熔盐腐蚀过程中,熔盐向NiCoCrAlYHf金属涂层内部渗透,与涂层中的Cr元素发生反应,破坏了涂层的内部结构,而Al2O3在熔盐中化学稳定性优异,防护层结构有效阻隔了熔盐向内扩散,显著提升了涂层抗熔盐腐蚀性能。
Abstract
In the high-temperature service environment of aircraft engine turbine blades, which are continuously subject to intense impact from high-temperature gases, exceptional resistance to both elevated temperature and corrosion is required. MCrAlY metallic coatings, as a well-established high-temperature protective coating system, are widely applied on the surface of turbine blades to withstand oxidation and corrosion under such demanding conditions. While material modifications to the MCrAlY coating system, such as adjustments in composition or the incorporation of reactive elements like Hf, can enhance its oxidation and corrosion resistance to some extent. A primary limitation is their inherent inability to completely block direct contact between the coating and external corrosive agents, such as molten sulfate salts. Alumina (Al2O3) coatings have long been recognized as a promising candidate for high-temperature protection due to their high melting point, excellent thermochemical stability, and superior resistance to high-temperature oxidation. However, its low coefficient of thermal expansion and low fracture toughness often lead to stress concentration and spallation under high temperature service condition. Electron beam physical vapor deposition (EB-PVD) is characterized by its ability to generate a unique columnar microstructure. This specific architecture can provide exceptional strain tolerance, allowing the coating to accommodate thermal expansion mismatch and thereby enhancing the coatings durability. The application of EB-PVD to the as-deposited Al2O3 coating presented a strategic approach to mitigate its brittleness. To address the aforementioned challenges, the EB-PVD technique is employed to deposit an Al2O3 protective layer onto a NiCoCrAlYHf metallic coating. The high-temperature performance of this Al2O3/NiCoCrAlYHf coating system is systematically evaluated through oxidation tests and molten salt corrosion experiments. The degradation mechanisms are elucidated via detailed analysis of microstructural evolution and compositional changes. The as-deposited Al2O3 coating is found to consist of a single α-phase structure, exhibiting morphology of fine columnar grains with distinct microscopic intercolumnar gaps and a clear, sharp interface with the underlying NiCoCrAlYHf coating, without observable elemental interdiffusion. During high-temperature oxidation testing, the microscopic gaps within the Al2O3 layer act as diffusion pathways for oxygen atoms. This ingress of oxygen prompts the selective oxidation of the NiCoCrAlYHf coating, leading to the formation of a continuous TGO layer at the interface. The subsequent growth of this TGO layer, combined with the internal stresses generated within the Al2O3 overlay due to sintering at high temperature, induces the formation of vertical cracks within the Al2O3 layer. These cracks, in turn, create additional and more direct pathways, further accelerating the inward diffusion of oxygen. Consequently, under the tested oxidation conditions, the EB-PVD Al2O3 coating is unable to effectively block oxygen penetration and thus does not confer a significant improvement in oxidation resistance. In contrast, the behavior during high-temperature molten salt corrosion is markedly different. In the corrosion tests, the molten salt is observed to penetrate through the Al2O3 layer via its inherent gaps and reach the NiCoCrAlYHf coating. There, it reacts predominantly with the Cr content within the metallic coating, leading to the dissolution of protective oxides and degradation of the coating's internal structure. Al2O3 material itself demonstrates excellent chemical stability and inertness in the aggressive molten salt environment. The columnar structure of the Al2O3 overlay effectively acts as a substantial barrier, significantly hindering the adherence and inward diffusion of the molten salt to the metal surface. This barrier effect reduces the kinetics of the corrosion reaction, thereby significantly enhancing the overall resistance of the coating system to hot corrosion attack.
关键词
Al2O3 /
电子束物理气相沉积 /
NiCoCrAlYHf /
高温氧化 /
熔盐腐蚀
Key words
Al2O3 /
EB-PVD /
NiCoCrAlYHf /
high temperature oxidation /
molten salt corrosion
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基金
国家科技重大专项项目（J2019-Ⅶ-0010-0150）; 国家自然科学基金项目（52202073）; 超高温耐蚀热障涂层技术项目（30204）
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