Monitoring Delamination Progression in Thermal Barrier Coatings by Mid‐Infrared Reflectance Imaging

Abstract

Mid‐infrared (MIR) reflectance imaging is shown to be a reliable diagnostic tool for monitoring delamination progression in thermal barrier coatings (TBCs). MIR reflectance imaging utilizes the maximum transparency of TBCs in the 3–6 μm wavelength region to probe below‐surface delamination crack propagation that is typically hidden from visible wavelength inspection. The image contrast that identifies delamination progression arises from the increased reflectance produced by a large component of total internal reflection at the TBC/buried‐crack interface. Imaging was performed at a wavelength of 4 μm to take advantage of the relatively high transmittance of plasma‐sprayed 8 wt% yttria‐stabilized zirconia (8YSZ) TBCs along with a desirable relative insensitivity to potentially interfering absorptions by atmospheric constituents at that wavelength. A key advantage of MIR reflectance imaging over competing techniques is that it is sensitive to delamination progression even at very early stages before delamination cracks start linking together; therefore, TBC health assessment can be achieved throughout the life of the TBC well before TBC failure is imminent. Examples are presented to demonstrate monitoring delamination progression by MIR reflectance imaging in 8YSZ TBC‐coated specimens subjected to furnace cycling to 1163°C. The experimental results were in good agreement with reflectance values predicted by a four‐flux Kulbelka–Munk approximation applied to the extreme cases of a completely adherent and a completely detached TBC. Practical considerations, including potential interfering effects from surface contamination, sintering, and erosion are discussed.

Normal spectral emittance of Inconel 718 aeronautical alloy coated with yttria stabilized zirconia films

“…In the case of the YSZ films a transmittance maximum is found around 4-6 m for both PS [11,26] and EB-PVD [3] coatings.…”

Monitoring delamination of plasma-sprayed thermal barrier coatings by reflectance-enhanced luminescence

“…The reflectivity at the TBC/air gap interface at the back of a delaminated (unbacked) TBC is very high because a large fraction of the incident light has an angle of incidence above the critical angle for total internal reflection due to the large difference in index of refraction, n, between the TBC (n = 2.2 at 500 nm wavelength for 8YSZ) and air (n = 1.0). This difference in index of refraction can be used to predict [7] the diffuse reflectivity at the back side of the respectively. This shows that just a few backside reflections, caused by TBC internal scattering, will greatly accentuate the contrast between regions of attached and delaminated TBC.…”

“…However, these methods are much more successful for probing through the thickness of electron-beam physical vapor deposited (EB-PVD) TBCs than for probing through the much more highly scattering plasma-sprayed TBCs. [5] While midinfrared (MIR) reflectance [6][7] was shown to detect the increased reflectance at 4 μm wavelength that accompanies subsurface crack propagation in plasma-sprayed TBCs, the relative increase in overall reflectance associated with TBC delamination was modest due to the presence of strong background scattering from the pre-existing porosity and cracks present throughout the TBC. This background volume scattering cannot be distinguished from scattering by subsurface delamination cracks, making delamination monitoring difficult for thicker plasma-sprayed TBCs.…”

Microstructure, oxidation behaviour and mechanical properties of Fe2O3 doped yttria-partially-stabilized zirconia coatings produced on metallic substrates by electrophoretic deposition

“…The impurities such as Ti, Cr and Fe elements from Fecralloy substrates mainly exist in the equiaxed and columnar zone in TGO scales [42][43][44][45][46][47]. Quadakkers et al [48] demonstrated that the incorporation of Zr or Y promoted the alumina growth with columnar grains.…”