The failure of thermal barrier coatings (TBCs) during operation depends mainly on the thermal mismatch between the ceramic top coat (TC) and the metal bond coat (BC). The thermal mismatch at the interface is influenced by the dynamic changes in the composition and morphology of the thermally grown oxide (TGO) between TC and BC during thermal cycling. This work focuses on the establishment of a TGO dynamic growth model, which considers the changes in TGO composition and morphology for investigating the effect of dynamic growth of TGO on local mismatch stresses during thermal cycling. The results show that the sharp locations at the TGO/BC interface are more prone to high tensile stresses during thermal cycling due to the uneven growth behavior of TGO, leading to crack initiation. The valley region of the interface is in a state of compressive stress σ_xx during the early stages of thermal exposure. The peak region preferentially forms a concentration of tensile stress σ_yy. Once large-scale “layer” (Ni, Co)Al₂O₄-based spinel-like mixed oxides(MO) growth occurs in TGO, the stress σ_xx changes from compressive stress to tensile stress in the valley region, eventually forming high tensile stress (Max: +158 MPa). The maximum tensile stress σ_yy in the peak region is increased to 256 MPa, which is more than two times larger than the early period of thermal exposure. As a result, the dramatic changes in local stresses seriously affect the time and location of microcracks.

It is well known that coating spalling often occurs between the top ceramic coating (TC) and the metal bonding coating (BC) during the working process where the thermal mismatch stress between the layers plays a key role. Dynamic changes in the thermally grown oxide (TGO) between the facing and bonding layers during thermal cycling increase the thermal mismatch at the interface. The effect of oxide thickening on localized mismatch stresses under thermal cycling with different interfacial curvatures is quantitatively investigated using numerical methods in this paper. A dynamic growth model of the oxide was developed based on the consideration of the composition and morphology of the thermally grown oxide. The results show that TGO growth behavior, local stress evolution, crack initiation location, and crack propagation length are influenced by the interface curvature at the same aspect ratio. The interface between the oxide layer and the bonding layer gradually developed significant tensile to dangerous stresses during thermal cycling. These tensile stresses are predominantly distributed at the crest during the early stages of thermal exposure and gradually transferred to the near-peak (the region near the peak) and ramp regions (the region in between the peak and the valley) as the oxidation process progressed. The crack initiation and propagation phenomenon can be observed at various regions during thermal exposure. Increasing interface curvature leads to an earlier dangerous stress moment in the ramp area due to higher maximum tensile stress. Therefore, the small changes in the interface curvature severely affected the location and time of crack initiation and the crack length.

To address the softening and limited oxidation and water-vapor corrosion resistance of Si bond coats at ultra-high temperatures, Si and Si–15 mol% HfO₂ (denoted 15HS) composite bond coats were deposited on pressureless sintered SiC substrates by air plasma spraying (APS). Their phase evolution, cross-sectional microstructure, and failure mechanisms were examined after 20–200 h at 1623 K in dry oxygen and in a 90% H₂O–10% O₂ environment. In dry oxygen, a SiO₂ thermally grown oxide (TGO) layer formed on the Si coating surface, cracked and spalled with time, and the coating thinned progressively. In the 15HS coating, HfO₂ reacted in situ with SiO₂ to form HfSiO₄; the combined volume expansion and spallation resistance of the dense layer caused the coating to thicken rather than thinning, and the TGO remained dense and crack-free. Under water-vapor corrosion, the Si coating was continuously consumed through gas-phase volatilization of SiO₂ as Si(OH)₄, accompanied by porosity and through-thickness cracks; after 200 h the coating thickness was only ~70% of the initial value. The 15HS coating instead underwent a three-stage evolution—homogeneous oxidation, layered construction, and barrier stabilization—after which a dense double-layer barrier formed, with an outer HfSiO₄ layer and an inner SiO₂ band, giving a thickness retention of ~90%, well above that of the Si coating. Mechanistic analysis showed that HfO₂ particles remained in place while Si⁴⁺ diffused outward and converted to HfSiO₄ at the HfO₂ surface. HfSiO₄ protected the coating through two mechanisms: blocking OH⁻/H₂O diffusion pathways and suppressing Si(OH)₄ volatilization. These findings inform the compositional optimization and structural design of Si-based bond coats for ultra-high-temperature water-vapor service.

As turbine inlet temperatures increase, the long-term stability of conventional yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBCs) becomes difficult to maintain. Molten calcium–magnesium–alumino-silicate (CMAS) rapidly infiltrates atmospheric plasma-sprayed (APS) coatings through interconnected pores and cracks. In this work, an AlCl₃-derived precursor sol prepared from AlCl₃·6H₂O in a deionized water–ethanol mixture (1:1, v/v) was introduced into the defects of high-entropy rare-earth zirconate (HE-REZ) coatings by vacuum-assisted drop-casting and subsequently converted into Al₂O₃ in situ through heat treatment. The influence of this treatment on the microstructure, surface wettability, and CMAS corrosion resistance of the coatings was examined. The newly formed Al₂O₃ was present on the coating surface and within pores and microcracks, where it narrowed or sealed the infiltration paths. After modification, surface roughness decreased by 8.96%, and the mean apparent contact angle measured on cooled CMAS/coating cross-sections increased by approximately 28°. Defect filling reduced the effective size of the infiltration channels, while dissolution of Al₂O₃ into the melt increased its viscosity and slowed capillary infiltration. Filling retained the dominant corrosion mechanism but shifted CMAS attack from rapid, localized penetration to a more uniform advance of the reaction front, reducing the corrosion rate and structural damage. Alumina filling therefore improves the CMAS corrosion resistance of APS HE-REZ TBCs by restricting infiltration channels and increasing melt viscosity.