Residual stresses in strongly-textured EB-PVD Thermal Barrier Coatings: X-ray Diffraction and Raman Spectroscopy Study

Abstract

Thermal barrier coatings (TBCs) are widely employed to protect the turbine blades located in the hot sections of aero engines and stationary gas turbines. Their performance and lifetime are strongly affected by the presence of residual stresses, but only limited experimental data for the state and amount of residual stresses in TBCs are available. TBCs were deposited in the present study by electron-beam physical vapour deposition (EB-PVD). The ceramic top coat (TC) of these TBCs consists of Y2O3-stabilised tetragonal ZrO2 (YSZ) and has strongly-textured columnar microstructure [1,2]. The residual stresses on the surface of our TC were investigated by X-ray diffraction (XRD) and Raman spectroscopy. For cubic materials a linear relationship between sin2() and the lattice spacings of the (hhh) and (h00) planes exists, even in the presence of non-random texture [3]. The tetragonal distortion of the YSZ phase is rather small. That is why, we have used the (220) reflection, corresponding to the (400) reflection in cubic setting, for the determination of the residual stresses by the sin2() method. Measurements were done at two different polar angles  =0o and =90o and show sin2()–splitting for <0 and >0, indicative for non-zero shear stresses. From these measurements, the main components of the stress tensor, 11, 22 and 33, as well as the shear stresses 13 and 23 were determined using a procedure based on the work of Jo and Hendricks [4], taking into account the effect of small  misalignment errors. The influence of the crystallographic texture on the accuracy of the stress tensor components is discussed. The residual stress state is tri-axial. In the as-deposited state the normal stresses 11, 22 and 33 are compressive, 13 is negative and 23 is positive. The shear stresses are attributed to the inhomogeneous microstructure and the presence of stress gradients in the surface layer of the TC. In TC samples annealed for different times at 1000oC in air, stress relaxation and development of in-plane tensile stresses is observed. The behaviour with annealing time of the average stress <> = kk, determined from the XRD measurements, is in qualitative agreement with independent Raman spectroscopy measurements. The relaxation of the residual stresses in the TC can be tentatively explained by widening of the gaps between the columns and reorientation of the grains near the TC surface, as observed by XRD texture measurements.