Ceramic coatings, characterized by their tailored microstructures, exceptional high temperature stability, and engineered functionalities, have been widely explored for extreme environment applications. To protect TiAl alloy from oxidation attack, a variety of silicon-based coatings have been developed by actively or passively forming SiO2, such as pure SiO2 coatings, enamel coatings, silicide coatings. Conventional SiO2 coatings obtained by spraying, spinning, and immersion can improve the oxidation resistance of TiAl alloys to some extent, they still suffer from inadequate adhesion, insufficient densification, and crack sensitivity. Recently, electrodeposition has been proposed to fabricate SiO2 coatings on TiAl alloys. The electrodeposited SiO2 coating is well adherent with the TiAl substrate and exhibits excellent densification ability, resulting in significant enhancements to the high temperature oxidation resistance. Due to the use of KNO3 in the precursor electrolyte, a certain amount of K+ ions is contained in the SiO2 coating, promoting the sintering and densification of the coating. However, the preferential accumulation of alkali metal cations can lead to the precipitation of a large amount of cristobalite. The substantial volume changes of cristobalite during the heating and cooling stages may induce crack formation, adversely impacting the high temperature protective performance of the coating. Thus, controlling the precipitates of cristobalite and promoting the stability of the SiO2 coating are crucial.
Recently, a team of material scientists led by Lian-Kui Wu from Sun Yat-sen University, China employed Al nanoparticles with high surface activity to modify SiO2 coatings. Incorporating Al nanoparticles significantly inhibits and optimizes the generation of cristobalite, suppressing the formation of cracks, leading to the improved oxidation resistance of the SiO2 coating. Moreover, the stabilizing effect of Al on the K⁺ ion migration and coating structure is revealed by this study.
The team published their work in Journal of Advanced Ceramics on May 13, 2025.
"In this study, we develop Al-modified SiO2 coatings (Al-SiO2 coatings) via co-electrodeposition. Fig. 1a displays the formation mechanism of Al-modified SiO2 coatings. Through the application of cathodic potential, water molecules and dissolved oxygen at the electrode interface are electrochemically reduced to catalytically generate OH⁻ ions in situ. These OH⁻ ions facilitate the condensation reaction of TEOS, leading to the formation of a SiO2 coating on the TiAl alloy surface. During this process, the hydroxylated surface of Al nanoparticles adsorbs protons, inducing their cathodic migration, which enables their encapsulation within the SiO2 coating to form an Al-SiO2 composite coating." said Lian-Kui Wu, professor at School of Materials at Sun Yat-sen University (China), a senior expert whose research interests focus on the field of high temperature oxidation.
Compared with TiAl alloy coated with other ceramic coatings in the literature, the Al-SiO2 ceramic coating developed in this study offers superior performance in high temperature oxidation environment. The researchers explained that the good oxidation resistance of Al-SiO2 coating comes from two aspects. On the one hand, the reduced proportion of cristobalite reduces the residual stress after 100 h oxidation and inhibits the occurrence of cracks and other defects. "After thermal treatment, the microstructure of SiO2 coating is regulated by the incorporated Al nanoparticles. The incorporation of Al nanoparticles inhibits the precipitation of cristobalite, so as to reduce the crack initiation and improve the overall compactness of the coating. Therefore, the Al-SiO2 coating exhibits better oxidation resistance compared with the SiO2 coating." said Liankui Wu.
