Papers by Author: Y. Zhao

Abstract: Ultrasonic testing porosity of TBC (thermal barrier coating) has been investigated by numerical simulation based on the implementation of FDTD (Finite Difference Time Domain) method for the RPM (random pores model). Numerical simulations of measuring ultrasonic longitudinal and transverse velocities were carried out for the plasma sprayed ZrO2 coatings with porosities ranging from 0.5 % to 4 %. The results show both longitudinal and transverse velocity decrease with the increase of porosity, which is similar to the experimental results in the reference (J. Thermal Spray Technol 12 (2003) 530-535). The investigation proves that the combination of the RPM and FDTD method is available for simulating ultrasonic testing of TBC porosity.

Abstract: This paper provides a nondestructive method for determination of the density, thickness and velocity of homogeneous coating from an inversion of experimental ultrasonic reflection coefficient amplitude spectrum (URCAS). The detailed analyses of the inverse algorithm and sensitivity of URCAS to the density, thickness and velocity are presented, and the sensitivity is identified as a basis for selecting the experimental data used in inversion. An example of the method and experimental measurement for comparison was given for an epoxy coating with the thickness of 0.277 mm. The relative errors between the inversion and reference value of the density, thickness and velocity were found to be 0.5 %, 3.97 %, and 0.97 %, respectively.

Abstract: Ultrasonic phase spectrum of reflection coefficient has been used to nondestructively characterize the density, porosity, and microcracks of plasma sprayed Cr2O3 coatings irradiated by high-intensity pulsed ion beam (HIPIB). The ultrasonic measurement was investigated using immersion focusing pulse echo method with a 25 MHz transducer and the phase spectrum of reflection coefficient has been experimentally obtained. The relationship among velocity, density, attenuation coefficient and the phase spectrum was analyzed based on an acoustic transmission model in a multi-layered structure. The velocity relates to the harmonic frequency of the phase spectrum, and the density and attenuation coefficient play roles on the amplitude of the maximum and the shape of the spectrum. The effects of the three parameters (velocity, density and attenuation coefficient) on the amplitude of the maximum of the phase are similar. Some critical value exists which change the relationship between the individual parameter and the maximum of the phase spectrum. When one of the three parameters exceeds the corresponded critical value, the amplitude of the maximum decreases in the form of exp( ) n y = A Bx . For the as-sprayed and irradiated coatings with the thickness of 50 0m, the velocity calculated from the phase spectrum is 2522 and 2772 m/s, harmonic frequency corresponding to 12.61 and 13.86 MHz, respectively. These ultrasonic determinations demonstrated that the coating becomes more densified after irradiation by HIPIB, in agreement with SEM observations. Ultrasonic phase spectrum itself is a good indicator to reveal differences between density and microstructures of coating prepared with various conditions as well as to follow their evolution after HIPIB irradiation.

Abstract: High-intensity pulsed ion beam (HIPIB) irradiation at 300 A/cm2 with a shot number of 1, and 5 was performed on the coatings and caused the modification of properties. Porosity and rough surface of EB-PVD (Electron Beam-Physical Vapor Deposition) deposited ZrO2-7%Y2O3 coatings with the thickness of 150 μm on heat-resistant steel have been characterized using the ultrasonic reflection coefficient phase spectrum. With increasing the shot number, the surface remelting and ablating filled gaps and caves between columns, and induced more uniform and compact structure. The ultrasonic measurement was investigated using immersion focusing pulse echo method with a 10 MHz transducer. The ultrasonic reflection coefficient related to frequency, velocity and attenuation coefficient were analyzed based on the acoustic transmission model in a multi-layered structure. For the as-deposited coating and coatings irradiated by HIPIB with the shot number of 1 and 5, the ultrasonic velocity changed from 2950 to 3170, and 3255 m/s respectively. The relationship between the attenuation coefficient and the frequency has been deduced based on the numerical fitting of the phase spectrum. The corresponded expressions are 1.35 α = 0.105 f , 1.2 α = 0.045 f and 1.14 α = 0.035 f , which displays that the attenuation coefficient decreases with the increasing of shot number. The ultrasonic results are in agreement with SEM observations, which have indicated that the coatings became denser and uniform with increasing the shot number. From the velocity and attenuation coefficient, the density, porosity, and microcracks of the coatings can be nondestructively evaluated utilizing the method of this paper.