Study On Surface/subsurface Damage Mechanism Of Nano-ZrO₂ Ceramics In The Multi-frequency Ultrasonic Vibration Assisted Grinding

With the increasing demand of hard brittle materials in national defense and other cutting-edge fields, requirements for part quality become increasingly harsh, but the surface and subsurface damage generated in machining process is the bottleneck hindering its widespread applications. Therefore, it is very significant to seek the high quality and efficiency precision machining method for hard brittle materials.According to the unique superiority of ultrasonic vibration for hard brittle materials and special phase transition property of ZrO2 ceramics, nano-ZrO2 ceramics was machined by combination of ultrasonic vibration and grinding in this paper, and the damage mechanism and characteristics of nano-ZrO2 ceramics in multi-frequency ultrasonic vibration grinding were mainly studied, so as to reveal the precision and high efficiency essence of ultrasonic vibration ductile grinding hard brittle materials. The main contents of this study are as follows:The acoustic systems were calculated and designed on the basis of the wave theory,and optimized by finite element method. The ultrasonic vibration grinding devices with different frequencies were developed, influence rules of structural parameters on system vibration characteristics were got, and vibration performances of devices were obtained by experiments.From the analysis of effects of vibration frequency, amplitude and mode on abrasive trace, the ultrasonic ground surface generation process was discussed. The results show that: 1) Ultrasonic vibration can change the straight line trace of abrasive grains, and enhance the interference between abrasive grains, thus, abrasive grains can play a reciprocating extrusion and lapping role on the ground surface. 2) The energy superposition of two-dimensional ultrasonic vibration reinforces the ability of ultrasonic vibration intervening to grinding process. The bigger of ultrasonic vibration amplitude, the wider interference width of abrasive grains to material, and the higher of frequency, the denser of abrasive trace, so the interference degree of abrasive grain to material is increased. 3) In ultrasonic grinding, periodic vibration extrusion force is generated by rake faces of abrasive grains, and the effect of this extrusion force is equivalent to the indenters'loading and unloading process on action surface of material during indentation, therefore fine transverse cracks generated and propagated to surface and make the material removal in fine crushing and pulverizing.In the present paper, the general grinding force model and stress field model of single-grit in ultrasonic grinding were built, which broaden harsh conditions of previous studies, such as single clamping angle of acoustic system and the same ultrasonic frequency in two directions, in addition, unify the grinding force model and stress field model of single grit with diverse clamping angles and different vibration frequencies.Based on indentation fracture mechanics, the critical grinding depth model of radial crack nucleation was built, and the effect of ultrasonic vibration on the critical grinding depth was qualitatively analyzed through the experiment. The results show that under high frequency ultrasonic vibration, the material equivalent hardness is decreased, and its fracture toughness is increased. Ultrasonic vibration can widen the critical grinding depth of crack nucleation, and contribute to the ductile grinding of material.The experimental devices of ultrasonic assisted single-grit scratching were developed, which can effectively simulate the grinding process of diamond wheel. From the study of ceramics damage on the condition of one dimensional and two dimensional ultrasonic vibration assisted scratching with multi-frequency, the rules of material damage with ultrasonic dimension and frequency variation were obtained. The research shows the damage in two dimensional ultrasonic assisted scratching is better than that in one dimensional and the scratch in one dimensional ultrasonic is better than that without ultrasonic; the material damage shows parabola with open side up changing along with frequency, in other words, the damage decreases with the growth of frequency under 35KHz, and the damage degree is the lowest at 35KHz, later the damage increase with frequency increasing. The above rules were validated in grinding experiment.From multi-frequency ultrasonic grinding test, the rules of the surface compressive residual stress changing with ultrasonic dimension and frequency can be drawn, that is, the surface residual stress in two dimensional ultrasonic grinding is bigger than that in one dimensional with the same frequency, and it is minimum in common grinding; the surface residual compressive stress shows parabola with open side down changing along with frequency, that is, the residual stress increases with the growth of frequency under 35KHz, and the residual compressive stress is the biggest at 35KHz, later the residual stress decreases with frequency increasing.The results show that ultrasonic vibration assisted grinding is an effective method for hard brittle materials. Especially in two-dimensional ultrasonic vibration grinding with 35kHz frequency, the interference ability of the ultrasonic vibration to machined material was greatly enhanced, and the scope of material ductile regime removal was expanded, so that the surface and subsurface damage was significantly alleviated. The relatively large compressive residual stress which generated on the ground surface can close surface micro-cracks effectively and improve the material strength in a certain range.