No abstract

Thermal barrier coatings (TBCs) are known to spall as a result of the residual stresses that develop during thermal cycling. TBC's are multi-layered coatings comprised of a metallic bond coat, thermally grown oxide and the ceramic top coat, all on top of a Ni-base superalloy substrate. The development of residual stresses is related to the generation of thermal, elastic and plastic strains in each of the layers. The focus of the current study is the development of a finite element analysis (FEA) that will model the development of residual stresses in these layers. Both interfacial roughness and material parameters (e.g., modulus of elasticity, coefficient of thermal expansion and stress relaxation of the bond coat) play a significant role in the development of residual stresses. The FEA developed in this work incorporates both of these effects and will be used to study the consequence of interface roughness, as measured in SEM micrographs, and material properties, that are being measured in a parallel project, on the development of these stresses. In this paper, the effect of an idealized three-dimensional surface roughness is compared to residual stresses resulting from a grooved surface formed by revolving a sinusoidal wave about an axis of symmetry. It is shown that cylindrical and flat button models give similar results, while the 3-D model results in stresses that are significantly larger than the stresses predicted in 2-D.

Thermal barrier coatings (TBCs) provide thermal insulation and oxidation protection of Ni-base superalloys in elevated temperature turbine applications. Thermal barrier coating failure is caused by spallation, which is related to the development of internal stresses during thermal cycling. Recent microstructural observations have highlighted the occurrence of a martensitic bond coat transformation, and this finiteelement analysis was conducted to clarify the influence of the martensite on the development of stresses and strains in the multilayered system during thermal cycling. Simulations incorporating the volume change associated with the transformation and experimentally measured coating properties indicate that out-of-plane top coat stresses are greatly influenced by the presence of the martensitic transformation, the temperature at which it occurs relative to the strength of the bond coat and attendant bond coat plasticity. Intermediate values of bond coat strength and transformation temperatures are shown to result in the highest top coat stresses.

The potential of lasercom could often be much more attractive to system designers if the terminals could be made very small. In particular, in systems where one end of the link is allowed to be somewhat more capable than the other, the lesser of the two terminals could take advantage of the asymmetry and shrink as much as possible. We have investigated how such asymmetry factors into the requirements for a small terminal and have designed a terminal with a very small aperture (35-75 mm) and an inertial stabilization scheme. The space-worthy terminal has applicability to Moon-to-Earth as well as near-Earth lasercom missions.

Microstructural evolution of bond coat with thermal cycling was characterized with transmission electron microscopy (TEM) and high temperature X-ray diffraction (HT-XRD) analysis. Before thermal cycling, the structure of asfabricated bond coat was confirmed to be a long-range ordered B2 β-phase. After thermal cycling to ∼28% of the cyclic life, the bond coat was found to transform into a Nirich L10 martensite (M) from its original B2 structure. The transformations, M ↔ B2, were demonstrated to be reversible and to occur on heating and cooling in each cycle. Quantitative high temperature XRD measurements verified the phase transformations produce about 0.7 % transformation strain. Finite element calculations incorporating the transformation strain indicate that the mertensitic transformation significantly influences the development of stresses and strains in TBC systems.