Characterizing the Thermally Grown Oxide in Thermal Barrier Coating by Terahertz Time Domain Spectroscopy

Abstract

This paper presents a method to simultaneously measure the thickness and refractive index of the thermally grown oxide (TGO) in thermal barrier coating (TBC) by using a reflective terahertz time-domain spectroscopy (THz-TDS) system. First, an optical transmission model of THz radiation in the multilayer structure of TBC is established. Owing to the different structures of TBC before and after forming the TGO layer, two different transmission models are established, respectively. Then, the experimental signals from the samples after different thermal cycles are obtained by the THz-TDS system. By fitting the experimentally measured reflected THz signals from the TBC samples to the proposed optical model using an optimization algorithm, the thickness and refractive index of the TGO are determined. In this work, four samples with different thicknesses of TGO layers are analyzed. The results of thickness of TGO layer are verified by SEM observation, and a reasonable agreement is obtained.

The Influence of Microstructure Characteristics on Thickness Measurement of TBCs Using Terahertz Time-Domain Spectroscopy

“…The NDT of the thickness of TBCs on aeroengine blades is crucial to the evaluation of thermal insulation and thermal shock performances [24]. As an emerging frontier in science and technology, the research work of THz-TDS in the field of thickness measurement of TBCs is currently focused on the deduction and calculation of thickness measurement formulas [25][26][27]. Moreover, combined with experimental data, the characterization of microstructure features such as porosity by using model formulas and machine learning methods [28][29][30] is also researched.…”

Effects of High Al Content on the Phase Constituents and Thermal Properties in NiCoCrAlY Alloys

“…A standard TBC system consists of a two-layer structure, including an yttria-stabilized zirconia (YSZ) ceramic top coating and a MCrAlY (M = Ni and/or Co) metallic bonding coating [ 4 ]. The MCrAlY bond coating consists of an Al-poor γ-Ni solid solution phase and an Al-rich β-(Ni, Co)Al phase, which is designed to protect the substrate from corrosion and oxidation by forming the protective thermally grown oxide (TGO) layer [ 5 , 6 , 7 ]. The γ-Ni phase serves as the matrix phase in MCrAlY coatings, showing good thermal stability.…”