Thermal barrier coatings (TBCs) are applied to metallic components and are used to protect such metallic components from large and/or prolonged heat exposure. Various such coated metallic components can be used in aircraft engines, such as gas turbine engines. These thermal barrier coatings can promote heat protection by utilizing materials that can sustain an appreciable temperature difference between the coating surface and the underlying metallic component. Thermal barrier coatings can permit the coated metallic components to operate at higher temperatures than uncoated equivalent metallic component can be operated.
Use of these thermal barrier coatings can change the thermal diffusivity of the metallic component. The thermal diffusivity is defined as the thermal conductivity divided by density and specific heat capacity at constant pressure. Thermal diffusivity is a metric indicative of rate of heat transfer through a component from a hot surface to a cold surface. A high value of thermal diffusivity indicates that heat is transferred more readily through the component, whereas a low value of thermal diffusivity indicates that heat is transferred less readily through the component.
Low values of thermal diffusivity permit a higher thermal gradient to be maintained across the metallic component. Such high thermal gradients thereby permit a low-temperature surface of the metallic component to operate at a relatively low-temperature, even while the high-temperature surface of the metallic component is being exposed to very-high temperatures. Such control of component temperature profile can result in extended part life by reducing oxidation and thermal fatigue, for example.
Thermal barrier coatings can separate from a bond coating and/or from the underlying metallic component upon which it has been formed. Various mechanisms can cause such separation of the coating from its component. For example, compression waves can cause the coating to separate from the component via spallation. When the coating separates from the underlying component, control of the temperature profile across the metallic component is reduced. Furthermore, the coating can be entirely separated from the component, break off, and be ingested by an aircraft engine, for example. Such ingestion of spalled coating materials can cause damage to the equipment ingesting such materials. There is a need to better predict when coating spallation is imminent.