On thermal mismatch and thermal gradients and the failure of thermal barrier coatings.

摘要

Thermal barrier coatings (TBCs) are widely used in the hot sections of gas turbine engines to protect the underlying structure from the damaging high temperatures. Unfortunately, premature failures prevent the design engineers from taking full advantage of the coatings. The lack of understanding of the failures is due to the complex nature of the coating, which consists of three major layers: (i) a metallic bond coat deposited on the superalloy, (ii) a ceramic top coat deposited on the bond coat, and (iii) a thermally grown oxide (TGO) that forms between the bond coat and top coat as the coating is exposed to elevated temperatures. This work investigates one dominate mode of failures seen in TBCs, "morphological instabilities", in an effort to better understand the failures, potentially leading to improvement of the reliability of the coatings.; The development of morphological instability (sometimes referred to as "ratcheting") occurs in TBCs subjected to cyclic loading and is characterized by an amplitude growth of initial small imperfections in the TGO. In particular, we are interested in how thermo-mechanically induced stresses in the coating, along with eigen-strains such as phase transformations, influence the development of the instabilities.; Based on experimental observations (conducted by our collaborators), numerical models are developed, spanning a range of properties to establish criteria for morphological instabilities. A key feature of the numerical models is to accurately capture the oxidation mechanisms of the TGO formation. Material changes in a Pt-modified aluminide bond coat, such as martensitic transformation and the change from beta- to gamma'-grains, are also important to incorporate. In addition, thermo-mechanical testing of systems containing NiCoCrAlY bond coats resulted in the development of morphological instabilities depending on the combination of thermal gradient over the structure. Thus, it is important to correctly simulate thermal gradients over the coating if gradients were present in the experiments.; The key results from the numerical simulations show that when the thermal mismatch is large enough to cause overall yielding in the bond coat, the thermal expansion of the substrate (superalloy) will govern the system response. It is furthermore seen that eigen-strain introduced by phase transformations may enhance or suppress the instability growth, depending on the class of phase transformation. Lastly, the numerical simulations capture the morphological instabilities seen in the experimental investigations of the NiCoCrAlY In this case, the morphological instabilities develop during thermal cycling with a thermal gradient over the cylinder wall, whereas the surface remains smooth for thermal cyclic conditions without a gradient. If an axial force is applied, the morphological instabilities become aligned with the axial direction.