Effect of aging on the onset of cracks due to redistribution of residual stresses in functionally graded environmental barrier coatings of mullite/ZrO2

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Abstract

Environmental barrier coatings (EBCs) are proposed as an option to reduce the high temperature water vapour corrosion in gas turbines ceramic components made of Si3N4 or SiC/SiCf, which are projected to achieve further energy efficient gas turbines. These coating are commonly designed as multilayer systems firmly attached to the ceramic substrate with the aim of retarding or avoiding its degradation after exposure to environmental conditions close to those in gas turbines. Therefore, to fulfil this function crack formation/propagation in the coatings must be controlled. In present work, three types of environmental barrier coatings fabricated by air plasma spray and containing a Si layer attached to SiC substrate plus 2 to 5 layers of different mullite/Y2O3 stabilized–ZrO2 mixtures are examined. To determine the level of residual stresses in the as-sprayed coating/substrate systems a three dimensional finite element model is developed and also tested for same coatings but aged under, high temperature and rich water vapour atmosphere. The model calculates the zones of maximum tensile stresses in the coatings which agree with experimental observation identifying the type, number and location of cracks. This model could be extended to similar EBC systems, and more importantly, could be use as a powerful designing tool for these complex structures.

Introduction

The seek for higher efficiency and energy saving in the future gas turbine engines, in addition to the requirements of environmental friendly systems, has greatly focused on using high performance non-oxide ceramics (SiC/SiCf and in situ toughened Si3N4) for the hot-sections [1]. During turbine operation the extreme temperature and water vapour pressure conditions can corrode these materials [1], and this effect is retarded or minimized by depositing the so-called environmental barrier coatings (EBCs). In particular, different multilayer coatings have been designed to serve as operative barrier against diffusion of hot gases towards the substrate and to suppress the deleterious crack formation as well [2], [3], [4]. Several studies have evidenced that the layer arrangement in a multilayer system has a crucial influence over the crack growth resistance and the mechanical reliability of the whole system [5], [6]. Therefore, determining the crucial parameters for the design of these complex multilayer EBCs presents a clear interest and furthermore, can help to assess the behaviour of tentative systems after subjected to aging conditions.

In a recent work by some of the authors, graded mullite/YSZ (Y2O3 stabilized ZrO2) EBC systems consisting of a YSZ top layer, a Si coated SiC substrate and diverse mullite + YSZ intermediate functional layers aimed for reducing thermal expansion mismatch were investigated under hot water vapour environments simulating engine conditions [7]. In general, the mullite/YSZ layered systems showed good water vapour corrosion under cyclic and static conditions, where some layer combinations seemed more successful for reducing crack formation/propagation. Hence, it became clear that the build-up of residual stresses in these complex systems is decisive, and find out the distribution of residual stresses in the coatings can help not only to the design of more promising systems but also to understand the behaviour of these coatings after being subjected to corrosion conditions. Presently, some of the more successful EBC systems in terms of crack suppression consist on multilayer coatings of the type Si/Mullite/BSAS (BaO–SrO–Al2O3–SiO2 compound), which clearly benefit from the low elastic modulus of BSAS phases [8]. Our previous work [7] also raised the question on the beneficial effect of a confined compliant mullite porous layer for reducing the number of cracks. Consequently, porosity was induced as a way to decrease the layer stiffness without compromising much the diffusion hindrance of hot gases towards the substrate.

In the present work, the distribution and the effect of residual stresses are studied using a three-dimensional finite element (3D FE) model on three mullite/YSZ multilayer coatings over SiC substrates. The coatings, deposited by air plasma spraying, are formed by different number of layers of variable composition and two of the systems enclose a relatively porous compliant mullite layer, to quantify the effects of these modifications in the overall residuals stress distributions. The changes in elastic modulus caused by long term thermal aging in rich water atmosphere, simulating combustion conditions, are also used to model redistribution of stresses in the coatings after aging. Location of maximum tensile stresses predicted by the model is supported by onset observation of “tunnelling“ and “edge“ cracks in the different coatings. The present 3D FE model can serve as a powerful tool to tailor architectures beforehand not only for present coatings but in alike complex multilayer systems as well.

Section snippets

Characteristics and properties of the multilayer EBC systems

Coatings were created by sequentially plasma spraying mullite/YSZ blends, always ending with a bimodal ZrO2 topmost layer and having a Si layer attached to the SiC substrate of 25 × 25 mm size. The formation of amorphous mullite that could induce extensive cracking upon crystallization was essentially prevented as stated in a previous paper [9]. The composition of each layer was: Si, mullite (M100), 75 vol.% mullite +25 vol.% YSZ (M75), 50 vol.% mullite +50 vol.% YSZ (M50) and YSZ. Coatings were aged

FE model for simulation of stress distribution in the EBC systems

The stress distribution after sintering was estimated in three EBC systems by means of a 3D FE analysis. For the FE simulations a parametric symmetric 3D model, meshed using the “SOLID186” (Ansys 20-node brick) elements, was developed. By taking advantage of the structure symmetry, only one fourth of the whole multilayer structure was modelled and on the areas in planes XZ and YZ (of the global CS – see Fig. 2) the symmetry conditions were prescribed. The parametric model allows the creation of

Results and discussion

Fig. 3 shows the stress distribution across the substrate and coating both in the centre and at the edge (in y-direction and z-direction) for the three systems EBC1, EBC2 and EBC3, in the as-sprayed (Fig 3a) and aged (Fig. 3b) conditions. In general, the substrate (SiC) remains under compressive biaxial stresses, whereas the layers on top of it are under tensile stresses. The magnitude of biaxial stresses in the different layers is associated with the coefficient of thermal expansion of each

Summary

Three layered systems consisting of a SiC substrate with a Si bond coat and three types of multilayer coatings formed by combining (mullite/YSZ) layers of different composition and designed as environmental barrier coatings were analyzed in terms of residual stresses built up. 3D Finite Element model of these multilayer systems was developed to estimate the stress distribution through the layers at three different locations (i.e. centre, edge and corner). The model helped to identify locations

Acknowledgements

This work has been funded by the project IPT-2012-0800-420000 (Spain). Dr. Eugenio Garcia acknowledges the financial support of the Ramón y Cajal Program (MINECO, Spain) and Dr. Oldrich Sevecek acknowledges financial support through the project CZ.1.07/2.3.00/30.0005 of Brno University of Technology. The contribution of Dr. R. Lima (from NRC, Canada) in developing these coatings and performing aging tests under a preceding joint project (CNRC-CSIC) is deeply acknowledged.

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    Present address: Department of Materials and Ceramics Engineering, Aveiro University, Aveiro, Portugal.

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