Abstract
The residual strain distribution has been measured as a function of depth in both top coat and bond coat in as-received and heat-treated air plasma sprayed thermal barrier coating samples. High-energy synchrotron X-ray beams were used in transmission to produce full Debye-Scherrer rings whose non-circular aspect ratio gave the in-plane and out-of-plane strains far more efficiently than the sin2ψ method. The residual strain in the bond coat is found to be tensile and the strain in the β phase of the as-received sample was measured. The residual strains observed in the top coat were generally compressive (increasing towards the interface), with two kinds of nonlinear trend. These was a 'jump' feature near the interface, and in some cases there was another 'jump' feature near the surface. It is shown how these trend differences can be correlated to cracks in the coating.
Highlights
Thermal barrier coatings (TBCs) (Zhao & Xiao, 2009; Padture et al, 2002; Clarke & Levi, 2003) are a type of ceramic coating applied to the hottest parts of turbine blades in jet engines to increase the operation temperature of the engines and improve their efficiency
The failure of air plasma sprayed (APS) TBCs usually occurs at the interface region between the top and the bond coat, and it is generally believed that residual stresses in the coating are the driving forces for the failure (Evans et al, 2001)
Most reported research on the residual stress distribution in TBCs has been by modelling, which allows the stress distribution as a function of depth in the coating to be simulated
Summary
Thermal barrier coatings (TBCs) (Zhao & Xiao, 2009; Padture et al, 2002; Clarke & Levi, 2003) are a type of ceramic coating applied to the hottest parts of turbine blades in jet engines to increase the operation temperature of the engines and improve their efficiency. Some research has been carried out to measure the residual stress distribution in TBCs as a function of depth by synchrotron XRD (Thornton et al, 2005, 1999; Weyant et al, 2010). Most of the residual stress measurements of TBCs were carried out by the well known sin method This method requires azimuthal diffraction data, which are usually obtained by integrating sectors of the Debye–Scherrer rings. Weyant et al (2010) measured the residual stresses in an NiCoCrAlY bond coat as a function of depth by synchrotron XRD. Even though some work has been carried out on measuring the residual stress in NiCoCrAlY bond coats, either the samples used were too small to represent the real case or the measurement was only limited to the surface. The microstructure of the coating was observed by scanning electron microscopy (SEM) (FEI, QUANTA)
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