Research Progress on Phase Transformation of Al2O3 Grown on High Temperature Protective Coatings

DING Hang; LIU Qin; JIN Lei; XIE Yun; PENG Xiao

doi:10.16490/j.cnki.issn.1001-3660.2026.03.002

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Surface Technology ›› 2026, Vol. 55 ›› Issue (3) : 19-32. DOI: 10.16490/j.cnki.issn.1001-3660.2026.03.002

Special Topic—High-temperature Protective Coatings for Advanced Engines

Research Progress on Phase Transformation of Al₂O₃ Grown on High Temperature Protective Coatings

DING Hang¹, LIU Qin¹, JIN Lei², XIE Yun^1,*, PENG Xiao¹

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Abstract

With the development of advanced aircraft engines towards a high thrust-to-weight ratio, turbine inlet temperature rises continuously, which induces a continuing challenge for high temperature performance of the construction materials for the critical hot-section components (such as turbine blades) in aero-engines. As the currently used Ni-based superalloys reach their performance limit, high temperature protective coatings are indispensable to enhance their environmental resistance. The coatings primarily rely on the formation of a dense, continuous and slowly growing alumina (Al₂O₃) scale to resist high temperature oxidation and corrosion. Al₂O₃ exhibits multiple polymorphs, and accelerating the transformation of thermodynamically metastable phases to stable α phases is crucial for enhancing the protective property of these coatings.
This work aims to systematically review the recent progress on phase transformation of Al₂O₃ grown on the high temperature protective coatings, and a better understanding of this issue is anticipated to benefit the optimization of the coatings. As for the principal mechanism of the phase transformation, a two-step process is generally accepted, and increasing temperature is conducive to accelerating the transformation rate. Then, the effect of adding reactive elements on the phase transformation is analyzed, and the relationship between the transformation rate and the cationic radius of the corresponding reactive elements is discussed. In addition, the focus is also put on the microstructure of the coatings, especially grain boundaries, since the coatings with refined grains have been reported to contribute to the nucleation of thermodynamically stable α phases. Based on comprehensively examining the influence of dispersing specific types of oxide nanoparticles within or on the surface of aluminide coatings on the Al₂O₃ phase transformation, an in-depth exploration into the intrinsic mechanism of the so-called "template" effect whereby hcp-structured oxide nanoparticles promote the nucleation and growth of α-Al₂O₃ is provided.
Although the currently available research concerning the Al₂O₃ phase transformation has made a significant progress, there are still some vital challenges to be addressed by future research. At present, the precise mechanism by which different doping reactive elements influence the phase transformation of Al₂O₃ remains unclear. The existing theoretical model, which is established based on the differences in the cationic radius of various reactive elements, have certain limitations, and the influence of co-doping with multiple reactive elements on the phase transformation of Al₂O₃ is not involved. Moreover, there is a shortage of atomic-scale, direct evidence revealing the promoted α-Al₂O₃ nucleation by lattice defects, and in-situ observation or online characterization of the θ-Al₂O₃ to α-Al₂O₃ transformation process is relatively scarce. The kinetics of this phase transformation are still primarily described by statistically analyzing the evolution of the volume fraction of α-Al₂O₃ with time. This inevitably introduces interference from external factors such as temperature variations, statistical errors, and empirical judgments. The application of in-situ or online phases and composition analysis techniques undoubtedly enable probation of the Al₂O₃ phase transformation at an atomic scale and accurate tracking of the whole transformation process. In addition, more attention should be paid to artificial intelligence which may provide an innovative solution to the challenges. This work is expected to provide theoretical guidance for the development of advanced high temperature protective coatings.

Key words

superalloys / high temperature protective coatings / alumina scale / high temperature oxidation / principal mechanism

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DING Hang, LIU Qin, JIN Lei, XIE Yun, PENG Xiao. Research Progress on Phase Transformation of Al₂O₃ Grown on High Temperature Protective Coatings[J]. Surface Technology. 2026, 55(3): 19-32

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