Nondestructive evaluation of the oxidation stresses through thermal barrier coatings using Cr3+ piezospectroscopy

Abstract

The stresses in the aluminum oxide formed during high-temperature oxidation of a bond-coated superalloy are shown to be measurable through zirconia thermal barrier coatings. The basis for the measurements is the piezospectroscopic shift in the R-line fluorescence (photoluminescence) from Cr3+ impurities incorporated into the growing aluminum oxide scale. Measurements through the thermal barrier coating are feasible because (partially stabilized) zirconia coatings have some transparency at both the excitation and at fluorescence frequencies.

Spalling failure of a thermal barrier coating associated with aluminum depletion in the bond-coat

“…As oxidation proceeds, the compressive stress in the alumina under the TBC increases over short times and then remains essentially unchanged for about 100 h at 11218C. In this respect, the evolution in residual stress in the thermally grown oxide is similar to that seen on the oxidation of NiAl [13], Ni 3 Al [14] and of TBC coated N5 superalloy with a PtAl bond-coat [15]. However, in contrast to the behavior of those alloys, the apparent compressive stress, shown in Fig.…”

The role of beam dispersion in Raman and photo-stimulated luminescence piezo-spectroscopy of yttria-stabilized zirconia in multi-layered coatings

“…The TGO layer is mainly α-Al2O3 doped with chromium ions (Cr 3+ ) which produce the characteristic luminescence R1 and R2 lines (λ = 693 and 694.3 nm) when stimulated with the laser light used for PLPS [31]. The positions of these lines change with stress and have been calibrated on a ruby standard by Clarke et al [10] and [32] allowing PLPS to be used for the measurement of stress in various alumina-containing materials [33] and [34]. As a consequence, when undertaking PLPS measurements of stresses within the TGO, lateral spreading of the incident laser beam within the TBC will affect the measurement resolution, which is defined by the diameter of the laser at the TGO surface [35].…”

“…The growth of this oxide [3] and the sintering [5] of the YSZ layer during thermal ageing are the main sources of residual stresses in the TGO and YSZ and lead to the eventual failure of the coating system. The extensive use of the YSZ layer as a TBC has given rise to much research focused on how the degradation of this coating system depends upon different application methods [6], the microstructure of the coatings [7] and [8], and the residual stress distributions in the YSZ and TGO layers [9], [10] and [11]. Among the non-destructive approaches to material investigation, Raman and photostimulated luminescence piezeo-spectroscopy (PLPS) are the techniques favoured for the measurement of stresses in YSZ and TGO respectively.…”

Development of Oxide Scale at 1,100 °C on Fe20Cr5Al Alloy Non-Implanted and Yttrium-Implanted

“…1b, 2b, 3b, 4b, 5b, 6b, 7b and 8b, because of the strains resulted from stresses generated in the scale related to the volume contraction during the phase transformation of transient aluminium oxides (c, d, and/or h) into a-phase and compressive stresses in a-Al 2 O 3 during its further growth and cooling, as described elsewhere [9,10]; (iii) the lack of distinct doublets in PLS spectra may result from contribution of different grains to the signal. It happens if the probed size is larger than grain size of the target or if few grains appear across the scale.…”