Papers by Author: Hui Ma

Abstract: The effects of multiple ultrasonic frequencies on grinding surface quality of Al2O3-ZrO2 nano-composite ceramics are researched to provide optimization laws for ultrasonic grinding parameters. Some SEM photographs of ultrasonic grinding surface with different frequencies are provided to research the surface quality. The enhanced surface quality of ceramics with ultrasonic grinding is explained as the transition of single abrasive particle removal mode from brittle to ductile, which is resulted from the reduction of equivalent rigidity on surface under ultrasonic vibration. And this phenomenon becomes more evident as frequency increases. Through the XRD experiment, the phase transition from tetragonal phase t-ZrO2 to monoclinic phase m-ZrO2 becomes increasingly obvious by the action of ultrasonic vibration. The compressive stress generated during this phase transition can suppress the initiation and expansion of microcracks on the processing surface. The research shows that, the grinding surface quality with ultrasonic vibration is superior to that without ultrasonic vibration, and increasing ultrasonic frequency can contribute to the improvement of the surface quality.

Abstract: This paper presents a research on the effects of ultrasonic frequencies of 19.9 kHz, 23.6 kHz and 29.3 kHz on both of cyclic amplitude and fatigue life of Al2O3-ZrO2 nano-composite ceramics. According to the high-cyclic-life tests at low load level, some prohibition influence on the process of crack initiation and propagation under ultrasonic vibration is revealed as frequencies are raised from 19.9 kHz to 29.3 kHz. During the low load level tests, the toughness increment induced by the transcrystalline fracture is found in the scanning electronic micrographs of the fractures, which shows the existence of the inhibition effect on the crack initiation and propagation. At the same time, the fracture XRD indicates that t-m phase transition of ZrO2 under 29.3 kHz is higher than that under 19.9 kHz, which means that both the required cyclic amplitude and the fatigue life can be improved if the ultrasonic frequency is increased to 23.6 kHz. All the test results show that high frequencies exhibit stronger toughening improvements than the lower ones do. This paper is a pre-research for the ultrasonic ceramic grinding and the conclusions obtained are meaningful for the study of the crack propagation mechanism during the grinding.

Abstract: This paper presents a research on the attenuation of cyclic stress amplitude and the variation of microstructure under the action of ultrasonic vibration in crack propagation for Al2O3-ZrO2 composite nano-ceramics. To interpret this phenomenon, the nonlocal theory is introduced to establish the constitutive model under the ultrasonic condition based on the specified 2D attenuating function. The mechanics behavior of the ceramics under the action of ultrasonic vibration is analyzed according to the relationship between the internal character length and the external character length (i.e. ultrasonic wave length). In addition, the variation law of the cyclic stress amplitude and microstructure with different ultrasonic frequencies is also studied in this paper. It is shown that the cyclic stress amplitude is attenuated under the action of ultrasonic vibration, and this attenuation phenomenon becomes more and more evident by increasing the ultrasonic frequency. The results of experiment are in accordance with the theoretical conclusions drawn from the nonlocal theory, which verifies that the action of ultrasonic vibration can actually accelerate the ceramic crack propagation.