Browsing by Author "Zhang, Xun"

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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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    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).