Precise strain profile measurement as a function of depth in thermal barrier coatings using high energy synchrotron X-rays

C Li,S.D.M Jacques,Y Chen,P Xiao,A.M Beale,M Di Michiel,N Markossan,P Nylen,R.J Cernik

doi:10.1016/j.scriptamat.2015.10.032

Abstract

We have developed a method of directly measuring the strain gradient as a function of depth in plasma sprayed Thermal Barrier Coatings (TBCs). A 92.8keV monochromatic synchrotron X-ray beam was used to penetrate the 10×10×8mm samples in transmission geometry. The samples had been heated to 1150°C and held at that temperature for 190h. The diffraction patterns were collected using a DECTRIS pilatus3 X CdTe 300K area detector. The patterns were analyzed by partial circular integration followed by full Rietveld refinement to obtain the lattice parameters of the TBC top coat at 25μm intervals as function of depth. The coatings surviving the heat treatment process without significant damage were found to exhibit a variable compressive stress state inside the top coat. This was found to be about −600MPa at the bond coat interface decreasing in a non-linear fashion towards the surface. By refinement of the data collected from sectors of whole Debye Scherrer rings we were able to estimate both the in-plane and out-of-plane strain.

Highlights

Residual stress is a serious problem that is implicated in the failure mechanisms of ceramic/metal layered structures such as Thermal Barrier Coatings (TBCs) [1,2] and brazed joints [3,4]
The driving force for most failures of APS (Atmosphere Plasma Spray) TBCs is the residual stress generated by thermal mismatch between the top coat and substrate
Li et al / Scripta Materialia 113 (2015) 122–126 the geometry of which is too small to represent the real operational case. To overcome these difficulties we have developed a method to precisely measure the strain distribution in a TBC top coat as a function of depth in 25 μm steps

Summary

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We have developed a method of directly measuring the strain gradient as a function of depth in plasma sprayed Thermal Barrier Coatings (TBCs). The patterns were analyzed by partial circular integration followed by full Rietveld refinement to obtain the lattice parameters of the TBC top coat at 25 μm intervals as function of depth. The coatings surviving the heat treatment process without significant damage were found to exhibit a variable compressive stress state inside the top coat. This was found to be about −600 MPa at the bond coat interface decreasing in a non-linear fashion towards the surface. By refinement of the data collected from sectors of whole Debye Scherrer rings we were able to estimate both the in-plane and out-of-plane strain

Introduction

The penetration depth of the beam can be calculated by

Findings

Rwp R exp