Browsing by Author "Chalk, Christine"
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Item Open Access Estimation of thermal barrier coating fracture toughness using integrated computational materials engineering(Elsevier, 2023-05-15) Geng, Xibo; Wellman, Richard; Arrom, Luis Isern; Chalk, Christine; Castelluccio, Gustavo M.The fracture toughness of thermal barrier coatings (TBC) is a critical mechanical property that governs damage resistance. Catastrophic delamination of TBC under erosion conditions occurs in TBC with low toughness. Prior research has explored indirect and complex experiments to measure TBC toughness, but the miniaturized nature of the multi-layered coating makes it difficult to quantify its intrinsic toughness. This paper integrates computational modeling and experimental approaches to estimate the TBC toughness and the substrate delamination strength. The results show that a typical newly fabricated yttrium stabilized zirconia coating under service conditions has a toughness estimated in the range of 0.1–0.5 MPa m1/2 and a toughness of thermally grown oxide layer in between 1.5 and 1.7 MPa m1/2. The analysis also determined that a thermally grown oxide with a fracture toughness above 2.0 MPa m1/2 would not delaminate under the service conditions. Overall, the approach demonstrates the value of integrated computational material approaches, which can save time and enhance predictive power.Item Open Access Modelling evaporation in electron‑beam physical vapour deposition of thermal barrier coatings(Springer, 2021-08-18) Chevallier, Julie; Isern, Luis; Almandoz Forcen, Koldo; Chalk, Christine; Nicholls, John R.This work presents computational models of ingot evaporation for electron-beam physical vapour deposition (EB-PVD) that can be applied to the deposition and development of thermal barrier coatings (TBCs). TBCs are insulating coatings that protect aero-engine components from high temperatures, which can be above the component’s melting point. The development of advanced TBCs is fuelled by the need to improve engine efficiency by increasing the engine operating temperature. Rare-earth zirconates (REZ) have been proposed as the next-generation TBCs due to their low coefficient of thermal conductivity and resistance to molten calcium-magnesium alumina-silicates (CMAS). However, the evaporation of REZ has proven to be challenging, with some coatings displaying compositional segregation across their thickness. The computational models form part of a larger analytical model that spans the whole EB-PVD process. The computational models focus on ingot evaporation, have been implemented in MATLAB and include data from 6 oxides: ZrO2, Y2O3, Gd2O3, Er2O3, La2O3 and Yb2O3. Two models (2D and 3D) successfully evaluate the evaporation rates of constituent oxides from multiple-REZ ingots, which can be used to highlight incompatibilities and preferential evaporation of some of these oxides. A third model (local composition activated, LCA) successfully predicts the evaporation rate of the whole ingot and replicates the cyclic change in composition of the evaporated plume, which is manifested as changes in compositional segregation across the coating’s thickness. The models have been validated with experimental data from Cranfield University’s EB-PVD coaters, published vapour pressure calculations and evaporation rate formulas described in the literature.Item Open Access Non-destructive thickness measurement of thermal barrier coatings using terahertz radiation(Springer, 2021-08-06) Isern, Luis; Waddie, Andrew J.; Chalk, Christine; Moore, Andrew J.; Nicholls, John R.A non-destructive thickness measurement technique based on terahertz (THz) reflectivity was successfully deployed to interrogate 7 wt.% yttria-stabilised zirconia thermal barrier coatings (TBCs) produced by electron-beam physical vapour deposition (EB-PVD). The THz technique was shown to produce accurate thickness maps for different samples with a resolution of 1 × 1 mm over a surface of 65 × 20 mm that were compared with direct examination of key cross-sections. All thickness measurements on different samples were calculated using a single value of refractive index. Small defects characteristic of EB-PVD, such as “carrot growths” and variations on column inclination, were evaluated and did not produce significant variations in the refractive index of the TBC. Moreover, the thickness maps correctly display thickness variations that are a consequence of the point-source nature of EB-PVD evaporation. In summary, this paper demonstrates the technique can be successfully deployed on large surfaces, and across different coatings of the same material produced under the same deposition conditions. It is shown that a single n value is required to map the thickness distribution for all samples. This combination of qualities indicates the potential of the technique for in-line control of TBC manufacture.Item Open Access Pseudo-code: Modelling evaporation in electron-beam physical vapour deposition of thermal barrier coatings(Cranfield University, 2023-08-11 15:26) Chevalier, Julie; Isern arrom, Luis; Almandoz Forcen, Koldo; Chalk, Christine; Nicholls, JohnPseudo-code for EBPVD ingot evaporation computational modelItem Open Access Randomised nano-/micro- impact testing – A novel experimental test method to simulate erosive damage caused by solid particle impacts(Elsevier, 2024-04-09) Beake, Ben D.; Goodes, Stephen R.; Zhang, Hannah; Isern, Luis; Chalk, Christine; Nicholls, John R.; Gee, Mark G.A novel randomised nano-/micro-scale impact test method has been developed to experimentally simulate particulate erosion where statistically distributed impacts with defined energy occur sequentially within the test area. Tests have been performed on two brittle glasses (fused silica and BK7) to easily highlight the interaction between impacts, as well as on two ceramic thermal barrier coating systems (TBCs, yttria stabilised zirconia, 7YSZ, and gadolinium zirconate, GZO) that experience erosion in service. Differences in erosion resistance were reproduced in the randomised impact tests, with GZO less impact resistant than 7YSZ, and BK7 significantly worse than fused silica. The impact data show that erosion resistance is influenced by different factors for the glasses (crack morphology, longer-length interaction of radial-lateral cracks in BK7 vs cone-cracking in fused silica) and TBCs (fracture toughness).Item Open Access Strain tolerance evolution of EB-PVD TBCs after thermal exposure or CMAS attack(Elsevier, 2023-08-28) Gao, Zhaohe; Zhang, Xun; Chen, Ying; Chalk, Christine; Nicholls, John; Brewster, Gyaneshwara; Xiao, PingThe microstructural evolution and Young’s modulus evolution of EB-PVD TBCs upon thermal exposure and separately after CaO-MgO-Al2O3-SiO2 (CMAS) attack have been compared and investigated. Moduli measured by four methods all show an increase due to sintering whereas their rates of increase are different. On finer scale (i.e. nano indentation), modulus increases from 87.3 GPa in as-deposited coatings to 198 GPa after sintering at 1400 °C for 100 h. While on global scale, the modulus increases from below 10 GPa to153 GPa after identical exposure. For the CMAS attacked TBCs at 1300 °C for 0.5 h, modulus values acquired by different methods are much closer. The effect of sintering and CMAS infiltration on coating’s structural integrity is discussed in terms of elastic strain energy available for driving edge delamination. The energy release rate of CMAS attacked TBCs at 1300 °C for 0.5 h is ∼1200 J/m2, which is equivalent to that of TBCs exposed at 1400 °C for 250 h (no CMAS).Item Open Access A study of pulsed thermography for life assessment of thin EB-PVD TBCs undergoing oxidation ageing(Elsevier, 2017-08-03) Tinsley, Lawrence; Chalk, Christine; Nicholls, John R.; Mehnen, Jorn; Roy, RajkumarThis paper presents an assessment of ageing for thin Thermal Barrier Coatings (TBC) using active thermography. As TBCs undergo ageing during their service life, sintering changes the porosity, elements migrate from the substrate, and micro-cracks build up in the structure of the material, exhibiting a change in thermal conductivity and diffusion properties. As the material ages and these properties change over time, it is possible to exploit trends in this change for characterisation of coating ageing, which would provide a diagnostics tool to estimate remaining useful life. In this study, through-depth diffusivity measurement has been applied to thin EB-PVD coatings which are artificially aged via oxidation furnace cycles. In order to address the difficulties of capturing a fast thermal event in a thin coating, a novel parametric study approach has been carried out to optimise data capture and analysis, maximising available frames for the model fitting step. Through-depth diffusivities have been measured during ageing for six samples, yielding a repeatable trend in thermal diffusivity measurements, with three features, which can be exploited for ageing characterisation of thin EB-PVD TBCs, and used as an alarm of imminent failure.Item Open Access Terahertz optical thickness and birefringence measurement for thermal barrier coating defect location(Optical Society of America, 2020-10-06) Waddie, Andrew J.; Schemmel, Peter J.; Chalk, Christine; Isern, Luis; Nicholls, John R.; Moore, Andrew J.We present a normal incidence terahertz reflectivity technique to determine the optical thickness and birefringence of yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBCs). Initial verification of the method was achieved by measurement of a set of fused silica calibration samples with known thicknesses and showed excellent agreement (<1% of refractive index) with the literature. The THz-measured optical thickness and its variation through the depth profile of the YSZ coating are shown to be in good agreement (<4%) with scanning electron microscope cross-sectional thickness measurements. In addition, the position of discontinuities in both the optical thickness and birefringence appear to be correlated to coating failure points observed during accelerated aging trialsItem Open Access Thermal Diffusivity Changes of EB-PVD Thermal Barrier Coatings Undergoing Oxidation Ageing.(Cranfield University, 2020-03-13 09:59) Tinsley, Lawrence; Roy, Rajkumar; Mehnen, Jorn; Chalk, Christine; Nicholls, JohnNormalized (apparent) thermal diffusivity values for yttria-stabilised zirconia (7YSZ) Thermal Barrier Coatings undergoing isothermal oxidation ageing with data captured at 16-hour intervals.Item Open Access Thermal insulation of YSZ and erbia-doped yttria-stabilised zirconia EB-PVD thermal barrier coating systems after CMAS attack(MDPI, 2020-10-01) Boissonnet, Germain; Chalk, Christine; Nicholls, John R.; Bonnet, Gilles; Pedraza, FernandoThe impact of small deposits of calcium–magnesium–aluminium silicates (CMAS) on the top of thermal barrier coatings (TBCs) made of yttria-stabilised zirconia (YSZ) produced via electron-beam physical vapour deposition (EB-PVD) is shown to play a role in the microstructural and chemical stability of the coatings; hence, it also affects the thermal insulation potential of TBCs. Therefore, the present work investigates the degradation potential of minor CMAS deposits (from 0.25 to 5 mg·cm−2) annealed at 1250 °C for 1 h on a novel Er2O3-Y2O3 co-stabilised ZrO2 (ErYSZ) EB-PVD TBC, which is compared to the standard YSZ coating. Due to the higher reactivity of ErYSZ coatings with CMAS, its penetration is limited in comparison with the standard YSZ coatings, hence resulting in a better thermal insulation of the former after ageing.