Understanding the Failure Mechanism of Thermal Barrier Coatings Considering the Local Bulge at the Interface between YSZ Ceramic and Bond Layer

The TC/BC interface morphology in APS TBC is one of the important factors leading to crack propagation and coating failure. Long cracks are found near the bulge on the TC/BC interface. In this study, the TBC model with the bulge on the interface is developed to explore the influence of the bulge on the coating failure. Dynamic TGO growth and crack propagation are considered in the model. The effects of the bulge on the stress state and crack propagation in the ceramic layer are examined. Moreover, the effects of the distribution and number of bulges are also investigated. The results show that the bulge on the interface results in the redistribution of local stress. The early cracking of the ceramic layer occurs near the top of the bulge. One bulge near the peak or valley of the interface leads to a coating life reduction of about 75% compared with that without a bulge. The increase in the number of bulges further decreases the coating life, which is independent of the bulge location. The results in this work indicate that a smooth TC/BC interface obtained by some possible surface treatments may be an optional scenario for improving coating life.

Numerical Study on Effect of Non-uniform CMAS Penetration on TGO Growth and Interface Stress Behavior of APS TBCs

The penetration of CaO–MgO–Al2O3–SiO2 (CMAS) is one of the most significant factors that induce the failure of air-plasma-sprayed thermal barrier coatings (APS TBCs). The direct penetration of CMAS changes the thermal/mechanical properties of the top coat (TC) layer, which affects the thermal mismatch stress behavior and the growth of thermally grown oxide (TGO) at the TC/bond coat (BC) interface, thereby resulting in a more complicated interface stress state. In the present study, a two-dimensional global model of APS TBCs with half of the TC layer penetrated by CMAS is established to investigate the effect of non-uniform CMAS penetration on the interface stress behavior. Subsequently, a local model extracted from the global model is established to investigate the effects of interface morphologies and CMAS penetration depth. The results show that non-uniform CMAS penetration causes non-uniform TGO growth in APS TBCs, which consequently causes the stress behavior to vary along the interface. Furthermore, the CMAS penetration depth imposes a significant effect on the TC/TGO interface stress behavior, whereas the interface roughness exerts a prominent effect on the stress level at the BC/TGO interface under CMAS penetration. This study reveals the mechanism associated with the effect of non-uniform CMAS penetration on the interface stress behavior in APS TBCSs.

Oxidation Behavior of LPPS MCrAlY Coatings at High Temperature: Part I TGO Growth and β Phase Depletion

MCrAlY can be used as bond coats for thermal barrier coatings (TBCs) with good ductility and excellent resistance against high temperature oxidation and hot corrosion. The behavior of the thermally grown oxide (TGO) scale formed at the MCrAlY coatings plays a key role on the oxidation resistance. In this paper, the oxidation kinetic curves of a MCrAlY coating at 900~1000 °C were obtained by measuring the thickness of the TGO scales. The curves basically conveyed parabolic laws, indicating a diffusion-controlled mechanism of the TGO growth. The thickness of TGO was positively correlated with the consumption of β phase during the early stage of the oxidation processes. After about the half-life of the β phase consumption, the depletion of the β phase significantly accelerated, which was caused by coating-substrate interdiffusion. In addition, the microstructure of the TGO was analyzed

A Numerical Study about the Effect of Non-uniform CMAS Penetration on the TGO Growth and Interface Stress Behavior of APS TBCs

The penetration of calcium-magnesium-alumino-silicate (CMAS) is one of the most vital factors inducing the failure of air plasma sprayed thermal barrier coatings (APS TBCs). The CMAS penetration into the porous microstructures of TBCs changes the thermal/mechanical properties of top coat (TC) material, this brings about considerable thermal mismatch stress at the TC/bond coat (BC) interface, and accelerates the growth of the thermally grown oxide (TGO), finally leading to more complicated stress state at the interface. In present study, a two-dimensional global model of APS TBCs with half of the TC penetrated by CMAS is built to study effect of non-uniform CMAS penetration. Then, a local model extracted from the global are built to investigate the effect of interface morphologies and CMAS penetration depth. The results showed that non-uniform CMAS penetration in APS TBCs causes non-uniform TGO growth, which further leads to more complicated interface stress distribution. The CMAS penetration depth had a greater effect on the TC/TGO interface stress behavior, while the interface roughness had an more obvious influence on the stress level at BC/TGO interface under CMAS penetration.