On the Rumpling Instability in Thermal Barrier Systems

113 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2004.Thermal barrier coatings (TBCs) are protective multi-layered metal-ceramic coatings used in hot sections of jet engines and gas turbines. The TBCs are composed of a superalloy substrate, an intermediate metallic bond coat (BC) and a ceramic topcoat. The TBCs are beset by reliability problems arising from delamination of the ceramic topcoat due to various instabilities in the system. The present work examines one such instability of "rumpling", or progressive roughening of the BC surface in the BC-superalloy systems upon high temperature exposure. A combined experimental and analytical approach is taken to study the rumpling phenomenon. Thermal cycling and isothermal experiments are carried out in air and in vacuum to identify the driving force and the kinetics governing rumpling. The experiments show that a nominally flat BC surface rumples to a wavelength of about 60--100 mum, and an amplitude of about 4--8 mum. The rumpling is seen to be relatively insensitive to the initial BC surface morphology. Significant initial flaws are not necessary for rumpling to occur. Further, rumpling occurs even in absence of thermal cycling. To explain BC rumpling, we develop a linear stability model for surface evolution of BCs under a remote stress. The driving force for this process is the in-plane stress in the BC due to its thermal mismatch with the substrate as indicated by the experimental results. The BC volume and BC surface diffusion governs the deformation kinetics. A governing equation is derived that gives the amplitude evolution of BC surface perturbations as a function of time. The analysis establishes a range of wavelengths for which the perturbation amplitude increases at a significantly higher rate as compared with other wavelengths. At the dominant instability wavelength, under low-stress and high-temperature conditions, the model shows that the roughening is caused only by volume diffusion, while smoothing is caused only by surface diffusion. The results from this thermodynamic model agree with the experimental observations quite well. Particular BC material properties and testing conditions are identified that control the BC rumpling and hence an important TBC failure mode. Guidelines to improve TBC performance are presented.Ope

Experimental investigation of the bond coat rumpling instability under isothermal and cyclic thermal histories in thermal barrier systems

Reliable life prediction models for the durability of thermal barrier coatings require the identification of the relative importance of various mechanisms responsible for the failure of the coatings at high temperatures. Studies of these mechanisms in sub-systems of thermal barrier coatings can provide valuable information. In the present work, we undertake an experimental study of "rumpling", or progressive roughening of the bond coat surface in the bond coat-superalloy systems upon high temperature exposure. Thermal cycling and isothermal experiments are carried out on a platinum-aluminide bond coat and on a NiCoCrAlY bond coat deposited on a Ni-based superalloy in air and in vacuum. The cyclic experiments are conducted in air from 200°C to 1200°C for different levels of initial roughness of the bond coat surfaces. Isothermal experiments are carried out at various temperatures, ranging from 960°C to 1200°C. The bond coat surfaces in cyclic experiments rumple to a similar characteristic wavelength of about 60-100 µm and an amplitude varying from 2 µm to 5 µm. Additional small scale fluctuations are seen to develop between the thermally grown oxide (TGO) and the bond coat surface with a wavelength of about 3-5 µm. Smooth initial bond coat surfaces (fluctuations in tens of nanometers) are seen to have rumpled, indicating that significant initial flaws are not required for rumpling to occur. Observations of the rumpled bond coat edges are shown to indicate that bond coat stresses play a dominant role during the rumpling process. On comparing the experimental observations with existing rumpling models in literature, it is concluded that the TGO and the microstructural changes in the bond coat have a rather limited role in inducing rumpling. Diffusion driven by thermal mismatch stress in the bond coat is likely to be the dominant mechanism during rumpling.published or submitted for publicationis peer reviewe

Influence of surface morphology on the adhesive strength of aluminum/epoxy interfaces

Adhesively bonded aluminum joints have been increasingly used in automotive industry because of their structural and functional advantages. Interfacial debonding in these joints has become a major concern limiting their performance. The present work is focused on experimental investigation of the influence of surface morphology on the interfacial fracture behavior of aluminum/epoxy interface. The specimens used in this experimental study were made of an aluminum/epoxy bimaterial stripe in the form of a layered double cantilever beam (LDCB). The LDCB specimens were debonded by peeling off the epoxy layer from the aluminum substrate using a steel wedge. Interfacial fracture energy was extracted from the debonding length by using a solution for the specimen geometry based on a model of a beam on an elastic foundation. This model was validated by direct finite element analysis. The experimental observations establish a direct correlation between the surface roughness of aluminum substrate and the fracture resistance of the aluminum/epoxy interface. The results emphasize the importance of choosing surface features at an appropriate length scale in studying their effects on interfacial fracture resistance.published or submitted for publicationis peer reviewe

Rumpling instability in thermal barrier systems under isothermal conditions in vacuum

Bond coat (BC) surface rumpling has been identified as one of the important mechanisms that can lead to failure of the thermal barrier coatings. The driving force behind rumpling—whether the stresses in the thermally grown oxide over the BC or the stresses in the BC—remains to be clarified. Meanwhile, the mass transport mechanisms in the BC leading to rumpling are not clearly identified. In the present investigation, we subject two types of BC-superalloy systems, nickel aluminide and platinum aluminide BCs on a Ni-based superalloy, to isothermal exposure at temperatures ranging from 1150 to 1200°C in vacuum. The results show that the nickel aluminide BC rumples at 1200°C and at 1175°C in absence of significant oxidation. The wavelength of the rumpled surfaces was 60–100 µm, with an amplitude of 5–8 µm. The rumpling was insensitive to the initial BC surface morphology. At 1150°C, no clear rumpling was observed, but some surface undulations could be seen related to the BC grains. ...[more]...published or submitted for publicationis peer reviewe