Interfacial toughness evolution under thermal cycling by laser shock and mechanical testing of an EB-PVD coating system

One of the major challenges for coatings on superalloys is to keep adherence during aging, where damage is mostly driven by thermal cycling. On the other hand, the methodology of the evaluation of the interfacial toughness should be consistent with in service loading. Recently, the use of LAser Shock Adhesion Test (LASAT) has shown its capability for both ranking different coating solutions and evaluating the evolution of a given coating as a function of aging [1-2]. The intent of this paper is to demonstrate the ability of LASAT to reproduce damage mechanisms observed under quasi-static in plane mechanical testing and to propose a general methodology to assess interfacial toughness evolution based on LASAT measurements. The material chosen in this study is a partially Y2O3 stabilized EB-PVD zirconia layer coating deposited by Electron Beam – Physical Vapor Deposition (EB-PVD) onto a first generation Ni base superalloy. Aging has been performed using thermal cycling under laboratory air. Degradation of the coating system due to ageing is quantitatively assessed by LASAT and accompanied by different microstructural analysis methods. For LASAT, if laser flux is below a threshold, no delamination occurs. When increasing laser flux above this threshold, a systematic sequence is observed: i) delamination without buckling of the ceramic layer, ii) delamination and buckling, iii) partial cracking of the ceramic layer, and iv) spallation [1-2]. These different states are also achieved in compressive quasi-static testing and assessed by means of local strain measurement using digital image correlation technique [3]. Aging is evaluated through the evolution of both the delamination and the buckling behavior induced by the LASAT method or critical strain at ceramic spallation under compressive static load. Please click Additional Files below to see the full abstract

Crack morphology in a columnar thermal barrier coating system

For high temperature application, EB-PVD ceramic layers are commonly used as thermal barrier coating. During thermal transients, the thermal expansion mismatch between coating and substrate drives failure of the TBC mainly by interfacial cracking. Laser Shock Adhesion Test (LASAT) provides stresses at the ceramic/metal interface enabling controlled interfacial cracking [1-2]. For achieving a clear understanding of the influence of local morphology on interfacial toughness, this study aims at characterizing the 3D morphology of a crack at the interface between metal and an EB-PVD TBC having a columnar structure. Please click Additional Files below to see the full abstract

Probing deeper: Determining the 3D gamma-gamma'-microstructure in a Ni-base superalloy by FIB-SEM tomography

Ni-based superalloys consist of gamma- and gamma'-phases which control the high-temperature mechanical properties of these alloys [1]. Their microstructure is typically investigated by 2D imaging methods. Quantitative investigation of the 3D microstructure using 2D image data relies on assumptions and statistical modelling to construct a virtual 3D model [2,3]. Serial sectioning of the material in a Scanning Electron Microscope (SEM) equipped with a Focused Ion Beam (FIB) offers an approach to reconstruct the actual microstructure of a given sample. The morphology is interpolated from a FIB tomography consisting of a large and therefore statistically relevant set of serially acquired images. Preprocessing methods such as selective etching yield contrast enhanced images (Figure 1 a) but are impractical for serial imaging. Sufficient image quality in serial imaging requires the SEM to be operated with retractable detectors between electron source and sample, such as a Circular BackScatter (CBS) detector (Figure 1 b). However, FIB ablation and subsequent SEM imaging require the sample to be tilted facing the ion column, which in turn inhibits retractable detectors to be used for image capturing. In this work, a FIB-SEM setup is implemented that allows for serial sectioning of a Ni-based superalloy with a CBS detector. Here, the sample faces the electron column instead of the ion column. This setup accommodates for a detector to be inserted between electron column and sample, resulting in an angular ablation of the sample. The thus obtained images have high resolution and little noise but are distorted (Figure 1 b). Postprocessing of the image series therefore requires proper alignment and virtual tilt correction. The corrected images (Figure 1 c) are then segmented, and the gamma-gamma'-microstructure can be visualized in 3D representation (Figure 1 d). The 3D microstructural data is then evaluated for morphological features such as phase volume fractions, precipitate sizes, and shape factors. The results can be used to estimate the reliability of microstructural data obtained from 3D stereographic reconstructions from 2D images. References [1] R. C. Reed The Superalloys, Cambridge University Press, 2009 [2] A. Brahme et al. Scripta Materialia, 2006, 55, 75-80. [3] C. Zhang et al. Metallurgical and Materials Transactions A, 2004, 35(7), 1927-193

Capturing the Competing Influence of Thermal and Mechanical Loads on the Strain of Turbine Blade Coatings via High Energy X-rays

This paper presents findings of synchrotron diffraction measurements on tubular specimens with a thermal barrier coating (TBC) system applied by electron beam physical vapor deposition (EB-PVD), having a thermally grown oxide (TGO) layer due to aging in hot air. The diffraction measurements were in situ while applying a thermal cycle with high temperature holds at 1000 °C and varying internal air cooling mass flow and mechanical load. It was observed that, during high temperature holds at 1000 °C, the TGO strain approached zero if no mechanical load or internal cooling was applied. When applying a mechanical load, the TGO in-plane strain (e22) changed to tensile and the out of plane TGO strain (e11) became compressive. The addition of internal cooling induced a thermal gradient, yielding a competing effect, driving the e22 strain to compressive and e11 strain to tensile. Quantifying TGO strain variations in response to competing factors will provide a path to controlling the TGO strain, and further improving the lifetime assessment and durability design strategies for TBC systems

Combining 2D and 3D characterization techniques for determining effects of HIP-rejuvenation after fatigue testing of SX microstructures

A hot isostatic pressing rejuvenation heat treatment is applied to a CMSX-4 type SX superalloy after it has been subjected to a low cycle fatigue test to rupture. The evolution of microstructural defects, such as pores and cracks which are present after fatigue, has been tracked in 3D by X-ray tomography before and after rejuvenation. From the rejuvenated specimen series of metallographic cross sections were prepared and investigated by scanning electron microscopy for getting complementary 2D information at high resolution. The micrographs were stitched to a panorama which was then matched into the 3D representation of the specimen volume. By combining 3D and 2D data, statistical volume related quantities were achieved while detailed characteristics have been assigned to individual defects present in the 2D panorama micrograph. This technique is in general appropriate for length-scale bridging microstructural investigations. Results of the performed investigations concerning the rejuvenation effect on the microstructure are presented and discussed

Strain response of thermal barrier coatings captured under extreme engine environments through synchrotron X-ray diffraction

The mechanical behaviour of thermal barrier coatings in operation holds the key to under-standing durability of jet engine turbine blades. Here we report the results from experiments that monitor strains in the layers of a coating subjected to thermal gradients and mechanical loads representing extreme engine environments. Hollow cylindrical specimens, with electron beam physical vapour deposited coatings, were tested with internal cooling and external heating under various controlled conditions. High-energy synchrotron X-ray measurements captured the in situ strain response through the depth of each layer, revealing the link between these conditions and the evolution of local strains. Results of this study demonstrate that variations in these conditions create corresponding trends in depth-resolved strains with the largest effects displayed at or near the interface with the bond coat. With larger temperature drops across the coating, significant strain gradients are seen, which can contribute to failure modes occurring within the layer adjacent to the interface

Whole body and hematopoietic ADAM8 deficiency does not influence advanced atherosclerotic lesion development, despite its association with human plaque progression

Although A Disintegrin And Metalloproteinase 8 (ADAM8) is not crucial for tissue development and homeostasis, it has been implicated in various inflammatory diseases by regulating processes like immune cell recruitment and activation. ADAM8 expression has been associated with human atherosclerosis development and myocardial infarction, however a causal role of ADAM8 in atherosclerosis has not been investigated thus far. In this study, we examined the expression of ADAM8 in early and progressed human atherosclerotic lesions, in which ADAM8 was significantly upregulated in vulnerable lesions. In addition, ADAM8 expression was most prominent in the shoulder region of human atherosclerotic lesions, characterized by the abundance of foam cells. In mice, Adam8 was highly expressed in circulating neutrophils and in macrophages. Moreover, ADAM8 deficient mouse macrophages displayed reduced secretion of inflammatory mediators. Remarkably, however, neither hematopoietic nor whole-body ADAM8 deficiency in mice affected atherosclerotic lesion size. Additionally, except for an increase in granulocyte content in plaques of ADAM8 deficient mice, lesion morphology was unaffected. Taken together, whole body and hematopoietic ADAM8 does not contribute to advanced atherosclerotic plaque development, at least in female mice, although its expression might still be valuable as a diagnostic/ prognostic biomarker to distinguish between stable and unstable lesions

Harmonizing DTI measurements across scanners to examine the development of white matter microstructure in 803 adolescents of the NCANDA study

Neurodevelopment continues through adolescence, with notable maturation of white matter tracts comprising regional fiber systems progressing at different rates. To identify factors that could contribute to regional differences in white matter microstructure development, large samples of youth spanning adolescence to young adulthood are essential to parse these factors. Recruitment of adequate samples generally relies on multi-site consortia but comes with the challenge of merging data acquired on different platforms. In the current study, diffusion tensor imaging (DTI) data were acquired on GE and Siemens systems through the National Consortium on Alcohol and NeuroDevelopment in Adolescence (NCANDA), a multi-site study designed to track the trajectories of regional brain development during a time of high risk for initiating alcohol consumption. This cross-sectional analysis reports baseline Tract-Based Spatial Statistic (TBSS) of regional fractional anisotropy (FA), mean diffusivity (MD), axial diffusivity (L1), and radial diffusivity (LT) from the five consortium sites on 671 adolescents who met no/low alcohol or drug consumption criteria and 132 adolescents with a history of exceeding consumption criteria. Harmonization of DTI metrics across manufacturers entailed the use of human-phantom data, acquired multiple times on each of three non-NCANDA participants at each site’s MR system, to determine a manufacturer-specific correction factor. Application of the correction factor derived from human phantom data measured on MR systems from different manufacturers reduced the standard deviation of the DTI metrics for FA by almost a half, enabling harmonization of data that would have otherwise carried systematic error. Permutation testing supported the hypothesis of higher FA and lower diffusivity measures in older adolescents and indicated that, overall, the FA, MD, and L1 of the boys was higher than that of the girls, suggesting continued microstructural development notable in the boys. The contribution of demographic and clinical differences to DTI metrics was assessed with General Additive Models (GAM) testing for age, sex, and ethnicity differences in regional skeleton mean values. The results supported the primary study hypothesis that FA skeleton mean values in the no/low-drinking group were highest at different ages. When differences in intracranial volume were covaried, FA skeleton mean reached a maximum at younger ages in girls than boys and varied in magnitude with ethnicity. Our results, however, did not support the hypothesis that youth who exceeded exposure criteria would have lower FA or higher diffusivity measures than the no/low-drinking group; detecting the effects of excessive alcohol consumption during adolescence on DTI metrics may require longitudinal study