Browsing by Author "Nicholls, John"

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    A core-shell thermal barrier coating with strong resistance to molten silicate attack and fracture
    (Elsevier, 2025-04-15) Li, Zichen; Brewster, Gyaneshwara; Isern, Luis; Chalk, Christine; Nicholls, John; Xiao, Ping; Zhang, Xun; Chen, Ying
    We report a new thermal barrier coating (TBC) with strong resistance to calcia–magnesia–alumina–silicate (CMAS) attack and fracture. The design is based on a core-shell microstructure where each building block of the TBC comprises a tough yttria stabilised zirconia (YSZ) core and a CMAS-resistant shell. To demonstrate the feasibility of the design, we select alumina, an established CMAS-resistant ceramic, as the shell material and manufacture core-shell TBCs by first synthesising “YSZ core-alumina shell” powder using the sol-gel method and then thermally spraying the powder to form core-shell TBCs. Microstructural characterisations confirm that a core-shell coating structure is successfully manufactured. However, the melting and mixing of alumina and YSZ in thermal spray result in the formation of a microstructure composed of alumina-alloyed zirconia grains and intergranular alumina, with the overall alumina concentration increasing from the core to the shell. The CMAS penetration depth through the core-shell TBC is over an order of magnitude lower than that through the benchmark YSZ TBC, which is attributed to the multiscale protection of the core-shell microstructure against CMAS infiltration through cracks and grain boundaries. The core-shell TBC has a similar stiffness to the YSZ TBC but exhibits a lower erosion rate and higher fracture toughness, indicating enhanced fracture resistance without compromising strain tolerance. The improved fracture resistance of the core-shell TBC is attributed to its less defective intersplat structure and greater ferroelastic toughening strain. Compared to the YSZ TBC, the core-shell TBC shows lower stiffness and nearly identical fracture toughness after annealing.
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    Chlorine-induced stress corrosion cracking of single crystal superalloys at 550 °C
    (Springer, 2024-08-05) Duarte Martinez, Fabian; Dawson, Karl; Tatlock, Gordon; Leggett, J.; Gibson, G.; Mason-Flucke, J. C.; Nicholls, John; Syed, Adnan; Morar, N.; Gray, Simon
    This study has investigated the effect of NaCl and different gaseous environments on the stress corrosion cracking susceptibility of CMSX-4 at 550 °C. The presence of SOx leads to the rapid dissociation of NaCl into Na2SO4 and the release Cl2 and HCl, which then trigger an active oxidation mechanism and stress corrosion cracking. The incubation time for crack initiation at 690 MPa and in the presence of a sulphur containing environment is 10 min. A working hypothesis is that stress corrosion cracking occurs due to the hydrogen released at the oxide/alloy interface when metal chlorides are formed; however, this hypothesis needs to be further explored.
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    Data for the paper titled Analysis of Combined Static Load and Low Temperature Hot Corrosion Induced Cracking in CMSX-4 at 550°C
    (Cranfield University, 2020-02-03 08:09) Gray, Simon; Brooking, Laurie; Nicholls, John; Sumner, Joy; Simms, Nigel; J. Tatlock, Gordon; Dawson, Karl
    Presentation containing figures within the paper
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    Environmentally assisted cracking of a single crystal nickel-based superalloy
    (Taylor and Francis, 2023-03-03) Dawson, Karl; Duarte Martinez, Fabian; Gray, Simon; Nicholls, John; Gibson, G.; Leggett, J.; Tatlock, G. J.
    Single crystal material, of CMSX-4® alloy composition, was cast and secondary orientation was controlled at the machining stage, to produce c-ring cross-section tubes with known crystallographic orientations. The c-ring tubes were coated with NaCl before being subject to loading up to 700MPa and heated for durations of up to 2 hrs at 550°C in flowing environments containing air and SO2. No cracking was observed in short term tests that were run in the absence of either NaCl, or SO2, indicating a symbiotic interaction is required to initiate cracking. Experiments confirm the presence of oxygen, chlorine and sulphur at the crack tips, formed along {001} crystallographic planes, however, they were distributed discretely, with several oxide and sulphide phases observed. In this work, we image, analyse and identify the phases formed during the cracking and corrosion of CMSX-4® superalloy and hypothesise on the complex chemical interactions that take place during crack initiation.
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    High temperature corrosion of HVOF coatings in laboratory-simulated biomass combustion superheater environments
    (Springer, 2022-12-26) Pidcock, Andy; Mori, Stefano; Sumner, Joy; Simms, Nigel; Nicholls, John; Oakey, John
    This study examines the fireside corrosion of FeCrAl, NiCr, NiCrAlY and A625 coatings applied by ‘high velocity oxy fuel’(HVOF) and exposed to simulated biomass firing conditions (gas composition CO2, N2, SO2 and HCl). The coatings and a typical base steel alloy (T92) were exposed to simulated conditions at 600 °C for 1000 h in a laboratory scale furnace. Samples were coated with a potassium chloride deposit. Samples were then cold mounted in a low-shrinkage epoxy resin and then cross-sectioned. Corrosion was assessed by dimensional metrology comparing the coating thickness change of the samples. The cross-sections of the ‘worst’ and ‘best’ coatings were examined. Results show that all but one coating (HVOF NiCr) outperformed the T92 alloy. No coating composition or method was conclusively better. Evidence of Cr depletion as well as the formation of a sulphidation layer have been found in the exposed samples with coatings. The formation of a K2SO4 layer has also been observed on all coated specimens.
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    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, John
    Pseudo-code for EBPVD ingot evaporation computational model
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    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, Ping
    The 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).
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    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, John
    Normalized (apparent) thermal diffusivity values for yttria-stabilised zirconia (7YSZ) Thermal Barrier Coatings undergoing isothermal oxidation ageing with data captured at 16-hour intervals.