N. M. Yanar

The influence of platinum on the failure of EBPVD YSZ TBCs on NiCoCrAlY bond coats

Abstract The failure of electron beam physical vapor deposition yttria stabilized zirconia thermal barrier coatings (TBCs) on NiCoCrAlY bond coats is described. It is shown that defects in the as-processed bond coats are responsible for TBC failures. By using a thin platinum layer deposited upon the NiCoCrAlY bond coat, it is shown that the lives of TBCs on such bond coats can be significantly extended since the as processed defects are removed.

The effect of the type of thermal exposure on the durability of thermal barrier coatings

Abstract The effect of cycle frequency on the spallation failure of thermal barrier coatings has been investigated. The exposure conditions affect the lifetimes of the coatings and can even change the relative performance of different bond coats. The very strong effect of exposure temperature is consistent with thermally grown oxide growth being a first order variable in scale failure.

Microstructural characterization of the failures of thermal barrier coatings on Ni-base superalloys

Abstract Typical thermal barrier coating (TBC) systems consist of a nickel-base superalloy substrate coated with a MCrAlY or diffusion aluminide bond coat, onto which is deposited a yttria-stabilized zirconia (YSZ) TBC. The bond coats are usually deposited via diffusion aluminizing processes or low pressure plasma spray processes (LPPS). The YSZ can be deposited by air plasma spraying (APS) or electron beam physical vapor deposition (EBPVD). A layer of thermally-grown oxide (TGO), which is usually alumina, forms between the bond coat and YSZ during TBC deposition and subsequent high-temperature exposure. The conventional wisdom is that APS coatings tend to fail in the YSZ and that EBPVD coatings tend to fail at the interface between the TGO and bond coat. However, current research has shown that the situation is much more complex and that the actual fracture path can be a function of the type of bond coat, the type of high-temperature exposure, and coating process parameters. This paper describes the results of a study of the failure of state-of-the-art EBPVD TBCs deposited on NiCoCrAlY and platinum-modified diffusion aluminide bond coats. The failure times and fracture morphology are described as a function of bond coat type. The failure times were found to be a strong function of temperature for both bond coats. The failure for NiCoCrAlY bond coats was found to initiate at defects in the coating, particularly at the TGO/YSZ interface, but the fracture propagated primarily along the TGO–bond coat interface. The failure times and morphologies for platinum-modified diffusion aluminide bond coats depended strongly on bond coat surface preparation. The mechanisms for failure of the two bond coats are described. Also, the effects of modifications to the bond coats and variations in processing parameters on these mechanisms are presented.

The effect of yttrium content on the oxidation-induced degradation of NiCoCrAlY coatings and the influence of surface roughness on the oxidation of NiCoCrAlY and platinum aluminide coatings

Abstract It has been found that the yttrium content of NiCoCrAlY coatings affects the useful lives of such coatings during cyclic oxidation. In particular, NiCoCrAlY coatings with 0.1wt% yttrium have more than twice the lifetime at 1100°C compared to NiCoCrAlY coatings with 0.5wt% yttrium. The mechanism by which the yttrium concentration influences the degradation of NiCoCrAlY coatings will be described. It has also been observed that the adverse effect of yttrium can be inhibited by reducing the roughness of the coating surface. The influence of surface condition on the oxidation of yttrium in NiCoCrAlY coatings will be examined in detail and the effects of surface roughness on the oxidation of NiCoCrAlY and platinum aluminide coatings will be compared. Finally the effects of yttrium in the substrate alloy on the oxidation of platinum aluminide coatings will be discussed.

The effect of superalloy substrate on the behaviour of high-purity low-density air plasma sprayed thermal barrier coatings

Abstract Several superalloy-bond coat couples were prepared without ceramic topcoat layers to better understand the effects of superalloy substrate on the oxidation behaviour of NiCoCrAlY bond coats. The same composition NiCoCrAlY bond coats were deposited on three superalloy substrates (Inconel 718, Haynes 188 and Rene’ N5) via argon-shrouded plasma spraying. The specimens were exposed to cyclic oxidation in laboratory air at 1100°C in a bottom loading furnace. Scaling behaviour and rate of aluminum depletion were compared between the various specimens. The bond coats on all three superalloys experienced some form of chemical failure after an extended number of cycles. The number of cycles until chemical failure was shortest for the IN718 specimen followed by the HA188 specimen, both of which experienced complete bond coat chemical failure, and then the Rene’ N5 specimen, which experienced localized chemical failure. The cycles to chemical failure coincide with the cycles to thermal barrier coating (TBC) spallation from previous work, indicating chemical failure of the bond coat is a critical event in the lifetime of TBCs. The effect of bond coat surface finish and porosity on the scaling behaviour has been investigated using specimens with the same superalloy substrate but with different bond coat surface finishes and porosity levels which were produced by utilizing two separate sized starting bond coat metallic powders. Bond coats with minimal porosity and smooth surface finishes did not experience chemical failure, at least in the time frame they were tested; however, oxide scale spallation was more apparent in the smooth bond coats as compared to the specimens with the rough surface finishes and high levels of porosity.