Investigation and modeling of mechanical properties for thermal barrier coatings under bending

Abstract

This paper is aimed at developing analytical models for predicting bond coat properties during crack propagation studies under bending on thermal barrier coatings (TBCs) and to reveal the stress distribution pattern along the directions of global axes and the equivalent stress for all types of specimens used in the present investigation. Crack propagation studies were performed with plasma spray coated zirconia, bonded by a MCrAlY layer to Ni-base superalloys (Inconel 617 and CMSX-4). Such thermal barrier composites are currently considered as candidate materials for advanced stationary gas turbine components. Coating as a protective layer improves the surface properties of the substrate. At a temperature of 800 degreesC, the crack propagation was found to be confined to the TBC (ceramic layer), as the ductile bond coat offers an attractive sink for stress relaxation. The stress-strain behaviour is a function of the elastic modulus of coating, bond coat as well as that of substrate. Thus, having known the elastic modulus of TBC and that of substrate, the elastic modulus of the bond coat needs to be evaluated which is a basic parameter for characterising coating performance. In this paper, the elastic modulus of the bond coat has been determined by modifying the existing model for two-layered composite beam specimen to a three layered composite beam specimen. FEM (finite element) analysis for all types of tested specimens was conducted with ANSYS software using a 2-D isoparametric solid element, defined by four nodal points having two degrees of freedom at each node. Simulations were carried out all specimens with selected load conditions in the elastic region of the stress-strain curves.. The outputs of the stresses along the direction of global axes i.e. sigma (x), sigma (y) (component stresses), the shear stress sigma (xy), the principal stresses sigma (1), sigma (2), sigma (3) and the equivalent stress, sigma (eq) have been presented and discussed.