13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Effect of pre-existing microcrack on the cyclic displacement instability in thermal barrier coating systems Luochuan Su, Weixu Zhang, T.J. Wang* State Key Laboratory for Strength and Vibration of Mechanical Structures, School of Aerospace Engineering, Xi’an Jiaotong University, Xi’an 710049, China * Corresponding author: wangtj@mail.xjtu.edu.cn Abstract The displacement instability of the thermally grown oxide (TGO) layer in thermal barrier coating systems is a critical factor inducing the failure of the system by causing large scale delamination and spallation. In this paper, the effect of pre-existing microcracks within the ceramic top-coat (TC) on the cyclic displacement instability under thermal cycling is investigated by a finite element analysis. Stress and the interface morphology are obtained after thermal cycling when the microcracks pre-exist within the TC. The influence of various parameters including crack location, crack length and crack direction on the interface morphology are discussed in detail. It is demonstrated that the existing of microcracks within TC can aggravate the displacement instability and affect the stress state in TC significantly. However, when the crack is perpendicular to the flat interface, the aggravation of it on the instability is weakest. Keywords Microcracks, Displacement instability, Thermal cycling , Thermal barrier coating 1. Introduction Thermal barrier coating (TBC) system is widely used in the blades of gas turbines or aircraft engines as the thermal insulation component, which can protect the base material against the extreme temperature due to its low heat conductivity. Usually, a typical TBC system consists of the following multilayer (Fig. 1a): (i) a thick superalloy substrate, (ii) a bond-coat (BC) next to the substrate protecting the substrate from oxidation, (iii) a ceramic top-coat (TC) for providing thermal insulation, and (iv) a thermally grown oxide (TGO) layer forming between TC and BC due to the high temperature oxidation[1-4]. The durability of TBC focuses world attentions in recent years. The coating performance usually begins to degrade associated with the large scale delamination and spallation of TC. Studies have confirmed that the displacement instability of TGO (Fig. 1) under cyclic oxidation is a critical factor inducing the fracture and delamination in TBC made by electron bean-physical vapor deposition (EB-PVD) [2, 5-9]. The most vivid manifestation of the instability is the downward displacement of TGO into soft BC (Fig. 1a), which is motivated by the large residual compressive stress caused by thermal expansion misfit and the thermal growth of TGO [6, 10, 11]. The instability usually initiates at the region near the imperfection of BC-TGO interface and the amplitude of it extends as the extensive thermal cycling. Following factors have confirmed to have a strong influence on the instability [2, 3, 5, 6, 12-19]. (i) The thermal cycling: It is a necessary condition to cause large instability; (ii) The growth strain resulting from the volume increase in the process of BC transforming to new TGO by high temperature oxidation; (iii) The initial morphology imperfection in the BC-TGO interface; (iv) The accumulation of plastic strain in BC near the initial imperfection, which is the most relevant factor controlling the evolution of displacement instability, and (v) The creep behavior of component materials. Due to the need of enhancing the strain tolerance of the ceramic coating, the microcracks are widely introduced in the process of coating deposition [20, 21]. Many experimental and analytical results have confirmed that pre-existing vertical surface cracks in TC can enhance the resistance to the initiation of interface crack [22-26]. Therefore, when these cracks exist at the surface of the TC, they can provide an additional capability to prevent the interface cracks developing besides enhancing the coating strain tolerance. However, as shown in Fig. 1a, these introduced microcracks
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