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 R_a 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.

Splat morphology is an important factor that influences the mechanical properties and durability of thermal barrier coatings (TBCs). In this study, yttria-stabilized zirconia (YSZ) coatings with different lamellar interface morphologies were deposited by atmospheric plasma spraying (APS) using feedstocks with different particle sizes. The influence of lamellar interface roughness on the cracking resistance of the coatings was investigated. Furthermore, the thermal shock and erosion resistance of coatings deposited by two different powders was evaluated. It was found that the particle size of the feedstock powder affects the stacking morphology of the splat that forms the coating. Coatings fabricated from coarse YSZ powders (45–60 μm) show a relatively rough inter-lamellar surface, with a roughness about 3 times greater than those faricated from fine powders (15–25 μm). Coatings prepared with fine powders tend to form large cracks parallel to the substrate direction under indentation, while no cracking phenomena were found in coatings prepared with coarse powders. Due to the higher cracking resistance, coatings prepared with coarse powders show better thermal shock and erosion resistances than those with fine powders. The results of this study provide a reference for the design and optimization of the microstructure of TBCs.

Microstructures of yttria-stabilized zirconia (YSZ) atmospher-ic-plasma-sprayed (APS) thermal barrier coatings (TBCs) and their terahertz (THz) dielectric properties were investigated to quantitatively determine porosities by effective medium theory (EMT). Various samples with different porosities ranged from 9.15% to 21.58% were prepared by different particle size powders and spray power. Both the theoretical and experimental results showed that the interferential oscillation period of reflectance spectra of APS YSZ TBCs samples depended on optical thickness of ceramic coatings, and optical thickness depended on permittivity for known geometric thickness. The real part of permittivity ϵ decreased as the porosity increased, the depolarization factor δ of APS YSZ TBCs pores which was estimated by compared the experimental value with the simulated value obtained by the Maxwell-Garnett EMT model and Bruggeman EMT model ap-proached to a fixed value, owing to the typical lamellar structure of APS-TBCs, and was 0.7 and 0.6 in respective model. Samples of 185—459 μm thickness verified that the technique varied from scanning electron microscope statistics results by <15%. Finally, a novel, convenient, online, nondestructive, and noncontact porosity evaluation method is proposed that can potentially be utilized on the health monitoring of TBCs in gas turbine blades.

In this study, yttria-stabilized zirconia (YSZ) coatings were deposited by atmospheric plasma spraying (APS) using feedstocks with two different particle sizes. The effect of particle size on the pore structure and failure mechanism of the coatings was investigated. The evolution of the pore structure of the two kinds of coatings during cyclic thermal shock test was described by quantitative metallography. The influence of pore orientation on the thermal stress of the coating system was analyzed by the finite element method. It was found that the coatings deposited using coarse particles show a high thermal shock life time. The orientation of the pores in the coatings prepared by different particle sizes was different. A structural parameter was proposed to effectively characterize the pore orientation of the coatings. Coatings prepared by coarse YSZ powder tend to form almost the same number of horizontal and vertical pores, while coatings prepared by fine powder tend to form horizontal ones parallel to the direction of the substrate. The simulation results revealed that the vertical pores can reduce the thermal stress in the coating. The results of this investigation are a benefit to the design and integrity of TBCs.

Thermal barrier coatings (TBCs) are usually subjected to the combined action of compressive stress, tensile stress, and bending shear stress, resulting in the interfacial delamination of TBCs, and finally causing the ceramic top coat to peel off. Hence, it is vital to detect the early-stage subcritical delamination cracks. In this study, a novel hybrid artificial neural network combined with the terahertz nondestructive technology was presented to predict the thickness of interface delamination in the early stage. The finite difference time domain (FDTD) algorithm was used to obtain the raw terahertz time-domain signals of 32 TBCs samples with various thicknesses of interface delamination, not only that, the influence of roughness and the thickness of the ceramic top layer were considered comprehensively when modeling. The stationary wavelet transform (SWT) and principal component analysis (PCA) methods were employed to extract the signal features and reduce the data dimensions before modeling, to make the cumulative contribution rate reach 100%, the first 31 components of the SWT detail data was used as the input data during modeling. Finally, a back propagation (BP) neural network method optimized by the genetic algorithm (GA-BP) was proposed to set up the interface delamination thickness prediction model. As a result, the root correlation coefficient R² reached over 0.95, the various errors—including the mean square error, mean squared percentage error, and mean absolute percentage error—were less than or equal to 0.53. All these indicators proved that the trained hybrid SWT-PCA-GA-BP model had excellent prediction performance and high accuracy. Finally, this work proposed a novel and convenient interface delamination evaluation method that could also be potentially utilized to evaluate the structural integrity of TBCs.

In this study, a new pore structure control method for plasma-sprayed thermal barrier coatings (TBCs) through the addition of unmelted, porous yttria-stabilized zirconia (YSZ) particles was investigated. Through a unique way of feeding powder, two powder feeders were used simultaneously at different positions of the plasma flame to deposit a composite structure coating in which a conventional plasma-sprayed coating was used as a matrix and unmelted micro-agglomerated YSZ particles were dispersed in the dense conventional coating matrix as second-phase particles. The effects of the distribution and content of second-phase particles on the microstructure, mechanical properties, and lifetime were explored in a furnace cyclic test (24 h) of the composite coating. The mechanical properties and lifetime of the composite coating depend on the content and morphology of the particles embedded in the coating. The lifetime of the composite structure coatings is significantly higher than that of the conventional coatings. By adjusting the spraying parameters, the lifetime of the composite coating prepared under the optimum process is up to 145 days, which is about three times that of the conventional coating. The results of this study provide guidance for the preparation of high-performance composite structure TBCs.