Comprehensive damage evaluation of localized spallation of thermal barrier coatings

Journal of Advanced Ceramics, Sep 2017

Thermal barrier coatings (TBCs) enable the hot section part to work at high temperatures owing to their thermal barrier effect on the base metal components. However, localized spallation in the ceramic top-coat might occur after long duration of thermal exposure or thermal cycling. To comprehensively understand the damage of the top-coat on the overall hot section part, effects of diameter and tilt angle of the spallation on the temperature redistribution of the substrate and the top-coat were investigated. The results show that the spallation diameter and tilt angle both have a significant effect on the temperature redistribution of the top-coat and the substrate. In the case of the substrate, the maximum temperature increment is located at the spallation center. Meanwhile, the surface (depth) maximum temperature increment, having nothing to do with the tilt angle, increases with the increase of the spallation diameter. In contrast, in the case of the top-coat, the maximum temperature increment was located at the sharp corner of the spallation area, and the surface (depth) maximum temperature increment increases with the increase of both the spallation diameter and the tilt angle. Based on the temperature redistribution of the substrate and the top-coat affected by the partial spallation, it is possible to evaluate the damage effect of spalled areas on the thermal capability of TBCs.

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Comprehensive damage evaluation of localized spallation of thermal barrier coatings

Journal of Advanced Ceramics 2226-4108 Comprehensive damage evaluation of localized spallation of thermal barrier coatings Wei-Wei ZHANG 1 2 3 Guang-Rong LI 3 Qiang ZHANG 0 3 Guan-Jun YANG 3 0 AECC Beijing Institute of Aeronautical Material , Beijing 100095 , China 1 Institute of Publication Science, Chang'an University , Xi'an 710064 , China 2 School of Materials Science and Engineering, Chang'an University , Xi'an 710064 , China 3 State Key Laboratory for Mechanical Behavior of Materials, School of Materials Science and Engineering, Xi'an Jiaotong University , Xi'an 710049 , China Thermal barrier coatings (TBCs) enable the hot section part to work at high temperatures owing to their thermal barrier effect on the base metal components. However, localized spallation in the ceramic top-coat might occur after long duration of thermal exposure or thermal cycling. To comprehensively understand the damage of the top-coat on the overall hot section part, effects of diameter and tilt angle of the spallation on the temperature redistribution of the substrate and the top-coat were investigated. The results show that the spallation diameter and tilt angle both have a significant effect on the temperature redistribution of the top-coat and the substrate. In the case of the substrate, the maximum temperature increment is located at the spallation center. Meanwhile, the surface (depth) maximum temperature increment, having nothing to do with the tilt angle, increases with the increase of the spallation diameter. In contrast, in the case of the top-coat, the maximum temperature increment was located at the sharp corner of the spallation area, and the surface (depth) maximum temperature increment increases with the increase of both the spallation diameter and the tilt angle. Based on the temperature redistribution of the substrate and the top-coat affected by the partial spallation, it is possible to evaluate the damage effect of spalled areas on the thermal capability of TBCs. thermal barrier coatings (TBCs); thermal property; localized spallation; damage evaluation - During past decades, the operating temperature of gas turbine engines has been elevated significantly, with the aim to increase their efficiency. Correspondingly, high temperature durability of the engine components has increased as well [ 1–4 ]. Significant advances in the high temperature capability have been achieved through the  * Corresponding author. E-mail: application of nickel-based and cobalt-based superalloys [ 5–7 ]. Nonetheless, these monolithicformed alloys are often susceptible to damage by oxidation and hot corrosion, so it is impossible to retain adequate mechanical strength. Moreover, these alloys are unable to bear high service temperature (e.g., above 1000 ℃ [ 8,9 ]). Therefore, thermal barrier coatings (TBCs) are deposited on the hot section components to enhance the temperature capability of the underlying metal substrate. For instance, in the turbine components with suitable internal cooling, temperature drops of more than 200 K can be realized by the TBCs with thicknesses from 200 to 500 µm [ 8,10 ]. In order to work effectively, the TBCs show excellent thermal barrier effect and high resistance to spallation when exposing to high temperature environment. A typical TBC system often exhibits a multi-layer structure: a metallic bond layer deposited preferentially on the component surface, followed by an adherent ceramic layer providing the thermal insulation effect. Commonly, the ceramic top layer of a TBC deposited by plasma spraying (PS), makes for a lamellar microstructure composing of splats lying parallel to the substrate surface [ 11 ]. Moreover, the quantity of intersplat pores is connected with intrasplat cracks, making for a continuous pore network. Consequently, the unique porous structure of the PS top-coat contributes to a low thermal conductivity in the through-thickness direction, as well as high strain tolerance in the in-plane direction. However, during the service, the TBC system may fail by spallation of the ceramic top-coat, which originates from the extension of those existing microcracks. On one hand, during thermal exposure at high temperatures, a thermally grown oxide (TGO) layer is predominantly formed by alumina [ 12 ]. The formation of the TGO plays a crucial role on the failure of the TBCs. The associated failure mechanism often results from the spallation at or close to the TGO layer within either the yttria stabilized zirconia (YSZ) or the bond-coat [ 13–15 ]. To begin with, small cracks and separations nucleate at imperfections near the TGO. Once nucleated, the small cracks extend and coalesce, while the TBC may remain attached by remnant ligaments. Finally, spallation occurs when the ligaments are detached over a sufficient area and eventually spalls from the substrate. On the other hand, sintering may lead to the stiffening of the top-coat. Consequently, the strain energy r (...truncated)


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Wei-Wei Zhang, Guang-Rong Li, Qiang Zhang, Guan-Jun Yang. Comprehensive damage evaluation of localized spallation of thermal barrier coatings, Journal of Advanced Ceramics, 2017, pp. 230-239, Volume 6, Issue 3, DOI: 10.1007/s40145-017-0234-4