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Previous work has demonstrated that the lifetime of atmospheric plasma sprayed thermal barrier coating (TBC) systems incorporating cryomilled NiCrAlY bond coats show superior reliability with up to 3 times longer lifetimes compared to conventional ones. These conventional and cryomilled NiCrAlY bond coats at end-of-life (after thermal cycle failure) were studied in detail in the present work with a focus on the mechanical behavior in the temperature range from room temperature to 1273 K (1000 °C). The investigations were carried out using microtensile samples and the DIC technique. It turns out that the low-temperature strength of the cryomilled NiCrAlY bond coat is inferior to that of conventional ones, which might be due to a more pronounced porosity. At higher temperatures (between 1173 K and 1273 K (900 °C and 1000 °C)), the cryomilled bond coat shows almost twice the strength of the conventional bond coat, despite having been exposed to almost 3 times as many thermal cycles. The thermal stability of the nitride dispersoids appears to compensate for the gamma prime dissolution that typically occurs at these elevated temperatures, allowing for strength retention. [ABSTRACT FROM AUTHOR]

Abstract: The mechanical behavior of a ceramic coating and the evolution of this behavior in-service play crucial roles in governing the performance and lifetime of these materials. A protocol is presented that allows for characterization of the in-plane elastic modulus and strain to failure of fragile ceramic coatings. The protocol employs digital image correlation (DIC) to measure time-resolved, full-field strain maps of bilayer microbeams, and material properties are extracted through direct comparison with finite element simulations of microbending experiments. The efficacy of the method is demonstrated by the measurement of the in-plane Young’s modulus (ETBC =15–30GPa) and the strain to fracture (3.5–5×10−3) of electron beam physical vapor deposited 7% yttria-stabilized zirconia thermal barrier coatings (TBCs). The results from this study also indicate that the in-plane TBC modulus has no strain dependence when measured in tension. [Copyright &y& Elsevier]

In this article authors present similarities between an article by J.G. Zhu and colleagues regarding measurement of the residual stress in thermal barrier coatings (TBCs) using a self developed deformation sensor and the article regarding the same by M. Sebastiani and colleagues published in the previous issue. Topics discussed include similarity in experimental implementation, use of scanning electron microscope images, and use of micro-ring core method for thermal spray coatings.

Abstract: The performance and reliability of thermal barrier systems are greatly affected by the behavior of their bond coats. Microsample tensile experiments conducted on commercial two-phase NiCoCrAlY bond coats have unveiled impressive room temperature strength and tensile ductility; in the as-processed state yield strengths exceeding 1200MPa and elongations to failure of up to 0.75% plastic strain have been measured. Thermally cycled specimens show a lower yield strength of 660MPa and up to 1.5% plastic strain. Elevated temperature microsample experiments revealed less impressive properties. Transmission electron microscopy points to the importance of nano-scale γ′ precipitates as a contributor to room temperature strength. Thermal cycling has been related to a general coarsening of the microstructure and a modest reduction in room temperature strength. Thermal cycling also results in the transformation of β-grains from an ordered B2 structure to L10 martensite, but the significance of this transformation remains to be elucidated. [Copyright &y& Elsevier]