Terahertz optical thickness and birefringence measurement for thermal barrier coating defect location

Abstract

We present a normal incidence terahertz reflectivity technique to determine the optical thickness and birefringence of yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBCs). Initial verification of the method was achieved by measurement of a set of fused silica calibration samples with known thicknesses and showed excellent agreement (<1% of refractive index) with the literature. The THz-measured optical thickness and its variation through the depth profile of the YSZ coating are shown to be in good agreement (<4%) with scanning electron microscope cross-sectional thickness measurements. In addition, the position of discontinuities in both the optical thickness and birefringence appear to be correlated to coating failure points observed during accelerated aging trials.

Non-destructive thickness measurement of thermal barrier coatings using terahertz radiation

“…THz frequency radiation has been proposed as an NDT technique to reduce or eliminate the need for destructive testing of high-value components because it can penetrate non-metallic dielectric materials, such as the topcoat and TGO, while reflecting off metals, such as the bondcoat. THz radiation illumination offers several advantages as it is non-ionising and can provide non-destructive, non-contact, real-time evaluation of the topcoat thickness and TGO growth [7,8]. The thickness values obtained are precise, with most studies reporting error values of 1-5% [7][8][9].…”

“…THz radiation illumination offers several advantages as it is non-ionising and can provide non-destructive, non-contact, real-time evaluation of the topcoat thickness and TGO growth [7,8]. The thickness values obtained are precise, with most studies reporting error values of 1-5% [7][8][9]. An additional advantage is that THz techniques can be employed for in-service monitoring to track aging due to thermal-cycling (TGO growth and void formation) [10] and the effect of erosion [7].…”

“…This paper describes the use of an alternative normalincidence THz reflectivity technique [8] to measure the thickness of a number of 7 wt.% YSZ topcoat TBCs, produced from the same batch, using a single refractive index value. The THz measurements were taken by evaluating the reflectivity of the TBC to a high-power (5 mW) tuneable laser.…”

“…The proposed method measures the optical thickness, the combined effect of refractive index (n) and the thickness of the coating (t), returning n*t values across the sample surface, and relies on the determination of a single n value to then calculate the thickness. This method has been described in detail on our previous publication [8]. In this paper, the study of this technique is expanded to analyse the effect of defects within the ceramic topcoat, and to test the effectiveness of using a single n value across different samples coated in the same deposition run.…”

Modelling evaporation in electron-beam physical vapour deposition of thermal barrier coatings

“…The material to be evaporated is in the form of an ingot of 40 mm in diameter and up to 300 mm long, which rests in a water-cooled hearth, so it is not greatly affected by the pre-heat temperature. Further detail on deposition parameters and coting structure can be found in our previous work such as [14][15][16][17]. The arrangement is described in Fig.…”