The Effect of the Linear Coefficient of Thermal Expansion of Thermal Barrier Coatings

Key Takeaways

• The linear coefficient of thermal expansion of TBCs is crucial in preventing cracks in the layers. TBC materials with a high coefficient of thermal expansion are preferred for crack-free coatings. 

• The difference in the linear coefficient of thermal expansion on TBC layers is one of the factors causing interlayer mechanical stress. Thermal expansion mismatch in the layers allows delamination or cracking in the TBC. 

• The most important step towards reducing thermal stress in TBCs is to minimize the thermal expansion mismatch between the layers of TBC. The general trend is to modify the composition of the TBC layers, which is effective at minimizing thermal stress. 

Thermal barrier coatings, shown above, protect equipment from heat flux

Thermal Barrier Coatings (TBCs) are used to protect metallic equipment from heat flux when working under elevated temperatures. TBCs consist of four layers: a ceramic thermal barrier, a bond coat, a thermally grown oxide layer, and a topcoat. TBCs are carefully built in this order on the metal substrate. 

The mechanical and thermal properties of TBC materials play a significant role in improving the reliability and durability of TBCs. Almost all materials expand when heated and shrink when cooled. The linear coefficient of thermal expansion expresses the change in length due to the change in temperature. The linear coefficient of thermal expansion of TBC materials is crucial in bringing down the TBC fracture rates. TBC materials with a higher linear coefficient of thermal expansion than the metal substrate are preferred for crack-free coatings. 

The Linear Coefficient of Thermal Expansion

Consider TBC coating on a gas turbine, where the ceramic TBC layer is made of yttria-stabilized zirconia (YSZ). An alloy of nickel, chromium, aluminum, and silicon can be used as a bond coat. A compound of aluminum oxide holds good to be grown thermally over the bond coat and the finishing coat can be made from YSZ. From the above example, we can see that the TBC materials used for each layer are different in their chemical, mechanical, and thermal properties. The different materials of different atomic bonding used as TBC layers increases the complexity of the coating system and generates thermal stress. 

The difference in the linear coefficient of thermal expansion on TBC layers is one of the factors causing interlayer mechanical stress. Thermal expansion properties change over time and affect the performance and reliability of TBCs. In ceramic TBCs,  the thermal coefficient mismatch, along with the temperature gradient, creates residual thermal stress during thermal cycling. The adhesive failure from thermal stress appears as delaminations, and cohesive failures appear as spalling or micro-cracking. Interlayer thermal stress also limits the thickness of TBC layers. The thermal expansion mismatch in the layers gives rise to delamination or cracking in the TBC. 

In TBC layers, the failure rate is high for the thermally grown layer and they are most often susceptible to spallation due to the thermal expansion mismatch. The thermal mismatch between the substrate and the TBC topcoat affects the elastic strain energy stored in TBC layers during cooling from high temperatures. The high amount of elastic strain leads to premature failures in TBCs, such as removal or breaking of the coating due to thermal cycling. Therefore, the thermal expansion coefficient match is important to resist thermal cycling in TBCs. 

TBC Materials

When selecting materials for TBCs, properties such as the thermal coefficient of expansion, melting point, density, thermal conductivity, thermal shock resistance, and surface emissivity are critical parameters. Ceramics, polymers, and composite materials are commonly used TBC materials since high thermal shock resistance is a desirable property for TBC materials. The thermal shock resistance is dependent on thermal expansion, as the physical length of the material varies with thermal expansion. Thermal expansion mismatch not only produces thermal stress but also varies the thermal shock resistance.  

The thermal expansion mismatch in TBCs needs to be addressed to enhance the success and life-time of thermal insulation. The most important step to reducing thermal stress in TBCs is to minimize the thermal expansion mismatch between the layers of the TBC. The general trend is to modify the composition of the TBC layers which minimizes thermal stress. 

Thermal Expansion Considerations for TBC Layers

While it is important to match the thermal coefficient of expansion of TBC layers, it is also important to understand the thermal expansion considerations for each layer. The current trend is to use superalloy substrate materials. In an aggressively hot environment, superalloys are the best substrate material. The TBC layer that comes in direct contact with the substrate is the ceramic layer. The thermal expansion coefficient of the ceramic layer should be higher than the substrate material to prevent cracking. 

The linear coefficient of thermal expansion of the bond coat is selected in such a way that it keeps the thermal expansion and shrinkage of the material during heating and cooling at a minimum. With this consideration, the thermal stress between the topcoat and substrate is also reduced to the minimum value. Usually, materials with a high thermal expansion coefficient are used for bond coats. The topcoat material is chosen in accordance with the thermal expansion coefficient of the underlying superalloy. Conventionally, the bond coat is made of metal alloys of high thermal expansion and top coats with ceramics of low thermal expansion.  

The TBC is vulnerable to thermal stress, cracks, and delaminations when there is a thermal expansion mismatch between TBC layers. The linear coefficient of thermal expansion of TBC materials is an important property for enhancing the reliability and durability of TBCs. One of the best approaches to optimizing a TBC structure is to minimize the thermal mismatch between the TBC layers. If the probability of a thermal expansion mismatch between TBC layers is low, then the TBC materials selected are the best suitable ones.