Papers by Author: M.K. Lei

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: Surface treatment of hard nitride film with high-intensity pulsed ion beam (HIPIB) was investigated in the present research. On considering the high energy density and short pulse duration of HIPIB source, a one-dimension physical model was built according to the structure feature of film-base sample. It was found that the irradiation of HIPIB lead to a very fast thermal recycle of heating rate 1011K/s and cooling rate up to 1010K/s. The highest temperature located at the surface of film irradiated. When using the HIPIB parameters of accelerating voltage 350kV, pulse duration 70ns and current density 60A/cm2, the surface layer of film would be melt with depth of about 0.35mm, that was verified by the experimental result along with the grain refinement effect due to the fast solidification process.

Abstract: An innovative fixed abrasive grinding process of chemo-mechanical grinding (CMG) by using soft abrasive grinding wheel (SAGW) has been recently proposed to achieve a damage-free ground workpiece surface. The basic principle, ideas and characteristics of CMG with SAGW are briefly introduced in this paper. The CMG experiments using newly developed SAGW for Si wafer are conducted at the condition of dry grinding. The grinding performances are evaluated and analyzed in terms of surface roughness, surface topography and surface/subsurface damage of ground wafer by use of Zygo interferometer, Scan Introduction ning Electron Microscope (SEM) and Cross-section Transmission Electron Microscope (Cross-section TEM). The component of product of ground Si surface is studied by X-ray Photoelectron Spectroscopy (XPS) to verify chemical reaction between the abrasive / additives of grinding wheel and Si wafer. The CMG process model by using SAGW is developed to understand the material removal mechanism and generation principle of damage-free surface. The study results show that the material removal mechanism of CMG by using SAGW can be explained as a hybrid process of chemical and mechanical action.

Abstract: 1 mol% Er3+- and 0–10 mol% Y3+-codoped Al2O3 powders is prepared in a non-aqueous sol–gel method. Two crystalline types of doped Al2O3,  and θ, are obtained for the 0–10 mol% Y3+-codoped Al2O3 powders sintered at the sintering temperature of 1000 °C by x-ray diffraction analysis. The green and red up-conversion emissions centered at about 550 and 670 nm increase with Y3+ codoping concentration, and the maximal intensity of both the green and red up-conversion emissions is obtained as about 100 and 10 times higher than that of the 1 mol% Er3+-doped Al2O3 powders, respectively. The enhancement in the green and red up-conversion emissions is ascribed to the improved dispersion of Er3+ in Er3+–Y3+-codoped Al2O3 powders.

Abstract: The microarc oxidation (MAO) films on AZ31 magnesium alloy were modified by high-intensity pulsed ion beam (HIPIB) at an ion current density of 200 A/cm2 with 1-5 shots. The modified MAO films presented a corrosion resistance superior to that of the original films. Scanning electron microscopy (SEM) observation revealed that a sealing layer was formed on the MAO films by HIPIB irradiation. The corrosion behaviors of the MAO films in 3.5 % NaCl solution were characterized by using electrochemical impedance spectroscopy (EIS). The noticeable improvement in the corrosion resistance of MAO films is attributed to the blocking effect of the sealing layer that hinders the process of electrolyte penetrating the MAO films to the magnesium alloy.

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.

Abstract: Silicone rubber (SIR) samples are exposed to CF4 capacitively coupled plasma (CCP) and inductively coupled plasma (ICP) at radio frequency (RF) power of 60–200 W for a treatment time up to 20 min, respectively. Static contact angle is employed to estimate the change of hydrophobicity of the silicone rubber modified by the two coupled types of CF4 RF plasma. A milder enlargement of static contact angle of SIR samples modified by ICP treatment is observed compared with that by CCP treatment. The hydrophobicity of the modified SIR surface by CCP treatment increases to a maximum, and further decreases toward the hydropholicity. The higher self-bias on the SIR samples being modified by CCP treatment than that by ICP treatment leads to the more dramatic physical and/or chemical reaction on the SIR surface, resulting in the competition between fluorination and ablation or etching.

Abstract: Effects of stacking faults in a high nitrogen face-centered-cubic phase (γΝ) formed on plasma source ion nitrided 1Cr18Ni9Ti (18-8 type) austenitic stainless steel on peak shift and peak asymmetry of x-ray diffraction were investigated based on Warren’s theory and Wagner’s method, respectively. The peak shift from peak position of the γΝ phase is ascribed to the deformation faults density α, while the peak asymmetry of the γΝ phase is characterized by deviation of the center of gravity of a peak from the peak maximum (Δ C.G.) due to the twin faults density β. The calculated peak positions of x-ray diffraction patterns are consistent with that measured for plasma source ion nitrided 1Cr18Ni9Ti stainless steel.