Nondestructive Interface Morphology Characterization of Thermal Barrier Coatings Using Terahertz Time-Domain Spectroscopy

Abstract

In this work, a terahertz time-domain spectroscopy (THz-TDS) system was used to measure the thickness of thermal barrier coatings (TBCs) and characterize the interface morphology of TBCs after erosion. Reflection mode, with an angle of incidence of 0, was used for inspection before and after erosion. The refractive index, thickness, and internal structure evolution tendency of the yttria-stabilized zirconia (YSZ) top coat were estimated under consideration of the interaction between the pulsed THz waves and the TBCs. The surface roughness of the top coat surface was considered for the errors analysis in the refractive index and thickness measurement. To reduce the errors introduced by the refractive index change after erosion, two mathematical models were built to assess the thickness loss. Then, the thickness loss was compared with results estimated by the micrometer inspection method. Finally, the basic erosion sample profile with Ra roughness was obtained, and the broadening of THz pulses were suggested as a possible measure for the top coat porosity change, showing that THz waves can be a novel online non-destructive and non-contact evaluation method that can be widely utilized to evaluate the integrity of TBCs applied to gas turbine blades.

Non-Contact, Non-Destructive Testing in Various Industrial Sectors with Terahertz Technology

“…In [49], the authors demonstrated the use of THz pulse reflection to measure the topcoat thickness; the results were verified with microscopic imaging to be within measurement error. In [50], a similar measurement was carried out with a custom erosion platform to simulate topcoat erosion in the real operating environment. The measurement was validated with the micrometer (see Figure 5 and Table 1).…”

“…Photograph of the thermal barrier coating sample after artificial erosion. Spots A, B, and C has undergone increasing degrees of erosion and were scanned using THz reflection spectroscopy [50].…”

“…Loss of thickness (∆d) calculated from time delay (∆T) spot positions A, B, C compared to digital micrometer measurements[50].…”

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

“…The applicability of contact methods is limited by the need for good physical contact with the coated surface and therefore the development of non-contact optical methods has also been necessary. THz time-domain spectroscopy (TDS) [6][7][8][9][10] enables either the thickness or the refractive index of the coating to be determined by measuring the time delay of an ultrashort pulse (∼500 fs -2 ps duration) through the sample. THz illumination is preferred because TBCs are opaque at visible optical frequencies and GHz illumination does not provide adequate resolution for the thin coatings.…”

Contribution of High Mechanical Fatigue to Gas Turbine Blade Lifetime during Steady-State Operation

“…Significant technological advances have led to increasing operating temperatures and pressures in recent decades, enhancing the efficiency of gas turbines to over 40%. These technological advances include coatings, heat treatments, a new superalloy permitting high operating temperatures, and a new cooling system [4][5][6][7][8][9][10][11]. Specifically, an F-class gas turbine, which operates at approximately 1300 • C and a pressure ratio of 16 during full-load conditions, was developed in the late 1990s and deployed to many thermal power plants.…”