A synchrotron X-ray diffraction deconvolution method for the measurement of residual stress in thermal barrier coatings as a function of depth

Evaluation of extremely steep residual stress gradients based on a combined approach using laboratory-scale equipment

Surface treatments characterized by rapid heating and cooling (e.g. laser hardening) can induce very steep residual stress gradients in the direct vicinity of the area being treated. These gradients cannot be characterized with sufficient accuracy by means of the classical sin2Ψ approach applying angle-dispersive X-ray diffraction. This can be mainly attributed to limitations of the material removal method. In order to resolve residual stress gradients in these regions without affecting the residual stress equilibrium, another angle-dispersive approach, i.e. the universal plot method, can be used. A novel combination of the two approaches (sin2Ψ and universal plot) is introduced in the present work. Prevailing limits with respect to profiles as a function of depth can be overcome and, thus, high-resolution surface layer characterization is enabled. The data obtained are discussed comprehensively in comparison with results elaborated by energy-dispersive X-ray diffraction measurements.

Measurement of the Elastic Modulus and Residual Stress of Thermal Barrier Coatings Using a Digital Image Correlation Technique

This paper presents an experimental study on simultaneously measuring the elastic modulus and residual stress of a thermal barrier coating (TBC) after different isothermal heat treatments. The elastic modulus and residual stress of TBCs were theoretically analyzed based on composite beam bending theory. Thereafter, an experimental setup was established combining the 3D digital image correlation method with the bending test to obtain the curvature changes in the TBC sample. Finally, the elastic modulus and residual stress of the ceramic layer with different isothermal heat treatments were obtained. The results show that the elastic modulus of the ceramic layer measured under compression is greater than that under tension, and the elastic modulus of the ceramic layer increases first and then tends to be stable as the heat treatment time increases. In addition, the residual stress of the TBCs ceramic layer quickly changes from compressive stress to tensile stress with heat treatment, and the tensile stress increases with the increase in thermal exposure time. Furthermore, the reasons for the change tendency were analyzed according to the variation in porosity and microstructures by processing the scanning electron microscope (SEM) figures. The results demonstrate that simultaneously determining the elastic modulus and residual stress of TBC based on combining the 3D digital image correlation method with the bending test is effective and reliable.

The nondestructive measurement of strain distributions in air plasma sprayed thermal barrier coatings as a function of depth from entire Debye-Scherrer rings

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.

Understanding the Effect of Surface Machining on the YSZ/Ti6Al4V Joint via Image Based Modelling

A method to improve the brazing between YSZ and Ti6Al4V by femtosecond laser surface machining is introduced. The highest strength of ~150 MPa (which is 95.2% higher than that of the flat YSZ/Ti6Al4V joint) is achieved when the processing speed is 200 μm/s. To understand the strengthen mechanism of the surface machining on the joint strength, image based models, based on the observed microstructure, have been used to probe the stress distribution in the joint. It is found that through surface machining on the ceramic, the residual stress distribution in ceramic becomes nonlinear. Upon shear testing, for the joint with a flat interface, the failure happens in the reaction layer and the out of plane stress in this layer is found to be tensile, which acts as the driving force for the crack generation and propagation. But for the joint with a rumpled interface, the compressive out of plane stress at the boundary of the grooves in the reaction layer could inhibit the propagation of the cracks. Finally, by surface machining on the ceramic, the maximum shear stress in the reaction layer is decreased, which could also help to improve the reliability of the joint.

Residual stress distribution analysis of heat treated APS TBC using image based modelling

We carried out a residual stress distribution analysis in a APS TBC throughout the depth of the coatings. The samples were heat treated at 1150 °C for 190 h and the data analysis used image based modelling based on the real 3D images measured by Computed Tomography (CT). The stress distribution in several 2D slices from the 3D model is included in this paper as well as the stress distribution along several paths shown on the slices. Our analysis can explain the occurrence of the “jump” features near the interface between the top coat and the bond coat. These features in the residual stress distribution trend were measured (as a function of depth) by high-energy synchrotron XRD (as shown in our related research article entitled ‘Understanding the Residual Stress Distribution through the Thickness of Atmosphere Plasma Sprayed (APS) Thermal Barrier Coatings (TBCs) by high energy Synchrotron XRD; Digital Image Correlation (DIC) and Image Based Modelling’) (Li et al., 2017) [1].