| Thermal barrier coatings, commonly used in modern gas turbines and jet engines, are dynamic, multilayered structures consisting of a superalloy substrate, an Al-rich bond coat, a thermally grown oxide, and a ceramic top coat. Knowledge of the disparate material properties for each of the constituents of a thermal barrier coating is crucial to both better understanding and improving the performance of the system. The efforts of this dissertation quantify fundamental aspects of two intrinsic strain mechanisms that arise during thermal cycling. This includes measurement of the thermal expansion behavior for bond coats and superalloys as well as establishing specific ternary compositions associated with a strain-inducing martensitic phase transformation, which is known to occur in Ni-rich bond coat alloys.;In order to quantify the coefficient of thermal expansion for a number of actual alloys extracted from contemporary thermal barrier coating systems, this work employs a noncontact high temperature digital image correlation technique to nearly 1100°C. The examined materials include: two commercial superalloys, two as-deposited commercial bond coat alloys, and three experimental bond coat alloys. The as-deposited specimens were created using a diffusion aluminizing and a low pressure plasma spray procedure to thicknesses on the order of 50 and 100 mum, respectively. For the plasma sprayed bond coat, a comparison with a bulk counterpart of identical composition indicated that deposition procedures have little effect on thermal expansion. An analytical model of oxide rumpling is used to show that the importance of thermal expansion mismatch between a commercial bond coat and its superalloy substrate is relatively small. Considerably higher expansion values are noted for a Ni-rich bond coat alloy, however, and modeling which includes this layer suggests that it may have a substantial influence on rumpling.;Combinatorial methods based on diffusion multiples are also applied in this study to determine the effect that nine ternary elements (Pt, Ta, Cr, Mo, Re, W, Zr, Si, and Co) have on the martensitic transformation in beta-NiAl. The outcomes of this endeavor demonstrate that martensite formation is highly sensitive to composition, temperature, and kinetics, and the use of lower heat treatment temperatures and slower cooling rates lessens the presence of both the binary and ternary martensitic phase. Electron microprobe analysis was implemented to establish the elemental composition of the binary and ternary martensitic regions on the diffusion multiples. With the exception of Pt, Cr, and Co, the solubility limits of the ternary additions in beta-NiAl are low (less than ∼ 2 at. %) and the elements with the highest solubilities, Pt and Co, appear to substitute predominantly with Ni. Finite element modeling of the ternary regions was performed in order to calculate the residual stress states, which are generated due to the thermal mismatch that arises during the cooling stage of sample processing. These results verify that prior to the transformation, variations in stress from one pre-martensitic area to the next are small and therefore should not locally assist or mitigate the creation of martensite. |
