Failure Modes, Mechanisms and Solutions of Thermal Barrier Coatings

       In recent decades, with the development of science and technology, the application of thermal barrier coatings has become more and more demanding. Although TBCs have been improved to a certain extent, premature failure and spalling of TBCs during service is still a big problem. There are many factors involved in the failure and spalling of TBCs, mainly in the following categories:



(1) The thermal expansion mismatch causes stress to cause the coating to fail;



(2) The coating fails due to the volume change of the coating caused by the phase transition;



(3) Failure due to the thermally grown oxide (Thermally Grown Oxide, TGO) layer at the interface between the intermediate layer and the outer layer destroying the interface between the metal substrates;



(4) Coating degradation caused by thermal corrosion.



1 Thermal expansion mismatch causes stress to fail the coating



In general, TBCs consist of a thermally insulating ceramic layer and a metallic underlayer, often referred to as the ceramic surface and the bonding underlayer, respectively. Due to the difference in physical properties between ceramics and metals, the ceramic surface layer is hard and brittle, and has poor thermal shock resistance; while the hardness of the metal or alloy bonding layer is much lower than that of ceramics, with better plasticity and thermal shock resistance. The thermal expansion coefficient of the two is very different. The thermal expansion coefficient of ceramics is usually 7x10 6K-1 ~12x10 6K-1, while the thermal expansion coefficient of metal is usually 18x10 6k-1 ~20x10 6K-1. This difference in physical properties causes the thermal deformation of the ceramic surface layer and the bonding layer to be different when the TBCs experience temperature changes, resulting in deformation thermal stress, which will cause cracking and cracking when the ceramic coating works for a long time in this harsh condition. peeling off, causing the coating to fail. Therefore, a bonding layer is usually added between the ceramic surface layer and the substrate to improve the physical compatibility between the ceramic and the alloy substrate. When thermal stress occurs, this intermediate coating can act as a buffer, so that the ceramic and the substrate do not prematurely fall off and fail.



2 Coating failure due to coating volume change due to YSZ phase transformation



The ceramic layer material is generally selected from ZrO2 with a mass fraction of 6% to 8% Y2O3, that is, ZrO2 partially stabilized by Y2O3. It is widely used because of its high melting point, low thermal conductivity, high thermal expansion coefficient, good thermal shock resistance, excellent high temperature chemical stability against high temperature oxidation, excellent comprehensive mechanical properties, and high flexural strength. Zirconia is a high temperature resistant oxide with a melting point of 2680°C. ZrO2 has three crystal forms: monoclinic, tetragonal and cubic. Under normal temperature conditions, the stable phase is monoclinic, and under high temperature conditions, the stable phase is cubic. When the temperature increases to 950℃~1220℃, ZrO2 changes from monoclinic phase to square phase. The transformation between the monoclinic phase and the square phase is reversible. During cooling, when the temperature is lower than 600 ℃, ZrO2 transforms from the square phase to the monoclinic phase, with a volume expansion of 3% to 5%, and absorbs heat 11.8 kJ/mol, enough stress builds up inside the ZrO2 coating to cause cracking or chipping, which leads to the failure of the coating. In the high temperature stage, when the temperature exceeds 2370 °C, ZrO2 transforms from the square phase to the cubic phase, the transformation between the square phase and the cubic phase is irreversible, and the cubic phase is stable at room temperature and high temperature. In order to avoid the failure of the coating due to phase change during thermal cycling, Y2O3 must be added as a stabilizer to ZrO2, and 17% Y2O3 is required to completely stabilize the square phase. However, the thermal cycling of ZrO2 stabilized with different contents of Y2O3 shows that the thermal cycling resistance of completely stabilized ZrO2 is not the best, and the ceramic layer has the best coating life at 6% to 8%.



3 Coating failure caused by TGO growth



In recent years, through a lot of analysis and research on TBCs serving in high temperature oxidizing atmosphere, it is found that there is an extra layer in the original TBCs system, which is formed by the growth of oxides of the ceramic layer and the intermediate transition layer. , thermally grown oxides between the MCrAIY bonding layer and TBCs are considered to be one of the root causes of stress and lead to coating failure. As a result, TGO has attracted great attention from researchers all over the world. The research results show that TGO is a non-negligible factor for coating failure caused by TBCs serving in a long-term high-temperature oxidizing environment. As long as 3 μm ~ 4 μm of oxide growth between the MCrAlY/YSZ interface is enough to cause the spalling of the ceramic surface layer, the failure often occurs at the metal bonding layer/ceramic layer interface. The oxidation process of MCrAlY bonding layer is divided into two stages: in the first stage, Al in the bonding layer is selectively oxidized to form an Al2O3 layer, and the mass increment depends on the coating thickness and the content of Al element; in the second stage, Al After consumption, other elements are oxidized, and the oxidation rate is proportional to the first stage. Due to the large consumption of Al elements due to the selective oxidation, the elements such as Cr, Co, and Ni are enriched there, and then the selective oxidation disappears, thereby generating oxides of elements such as Cr, Co, and Ni. When the Al content is less than 10%, Cr2O3 and NiO phases are formed. In the oxidized products, the Cr/Ni-enriched regions, such as Ni(Al, Cr)2O4, are more prone to crack generation and propagation than Al2O3. The formation of NiCr2O4, Ni(Al, Cr)2O4, Cr2O3, NiO, and CoO will greatly accelerate the failure of thermal barrier coatings.



4 Coating degradation caused by hot corrosion



The fuel used in aviation gas turbine engines contains impurities such as Na and S, which are deposited on high-temperature components in the form of Na2SO4. Therefore, the thermal barrier coating often encounters the corrosion problem of Na2SO4. For engines using inferior fuels, V, P, etc. The effect of thermal barrier coatings cannot be ignored either. The Y2O3 of the stable component is susceptible to corrosion and reacts in the above atmosphere, and precipitates from ZrO2, resulting in the transformation of ZrO2 from tetragonal or cubic phase to monoclinic phase, which causes volume change and then leads to coating failure.