| As the third-generation major semiconductor crystal material,silicon carbide(SiC)monocrystalline has been widely used in space optics,microelectronics,information technology,optoelectronic integrated and other high-tech field,because of its excellent electrical and physical-chemical properties,such as unique large band gap,high breakdown field strength,high saturated electron drift,high thermal conductivity,high radiation resistance and low dielectric constants.On the other hand,unique covalent bond crystals structure determines that SiC single-crystal has ultra-high hardness and brittleness.And therefore SiC single-crystal is a typical difficult-to-machine materials.It is very difficult to meet the requirements for cutting SiC single-crystal materials in high efficiency,high quality,narrow kerf and low cost by traditional wire-saw cutting technology,especially for precisely slicing large diameter,ultra-thin SiC wafers.Based on the slicing technology of traditional diamond wire-saw cutting hard and brittle materials,the ultrasonic vibration was exerted on the axis moving wire-saw along transverse direction in current study.The integrated method of theoretical analysis and experimental research was adopted to research cutting mechanism of SiC single-crystal cut by diamond wire-saw exerted by transverse ultrasonic vibration.The researched results would provide the theoretical basis and technical support for large size and ultra-thin SiC wafer cut by wire-saw in high efficiency and precision.The transverse vibration model of axial moving wire-saw was established and the partial differential equations of wire-saw system was deduced based on coordinate transformation.Utilizing the separation-of-variables method,the expressions of transverse vibration displacement field in which the wire-saw is excited in different positions by ultrasonic wave were solved,and the expression of system natural frequency was obtained,too.The result indicates that the vibration displacement at middle cutting point on the wire-saw has nothing to do with the position where the transverse ultrasonic applied,and the frequency at middle cutting point of the wire-saw is equal to it of the ultrasonic wave.The conclusion would provide theory basis for feasibility of cutting hard-brittle material by reciprocating motion wire-saw excited by transverse ultrasonic wave.Based on ultrasonic vibration cutting characteristics of contact-separation,the cutting conditions for flexible wire-saw vibration cutting hard-brittle material was studied and analyzed,the conditions is that the transverse feeding of the work-piece in a vibration cycle should be less than the maximum cutting depth of diamond abrasive on wire-saw in a vibration cycle.The characteristic function was used to characterize the state of wire-saw vibration cutting SiC single-crystal.Based on the theory of grinding,the mean effective cutting force of SiC single crystal cut by wire-saw excited by transverse ultrasonic was researched and analyzed,and the formula for predicting critical cutting depth of SiC was deduced.In same machining condition,the critical cutting depth of SiC single-crystal cut by wire-saw excited by transverse ultrasonic vibration is proportional to the square root of the ratio of the vibration cycle and net cutting time of diamond abrasive.The result theoretically reveals the mechanism that the transverse ultrasonic vibration can improve critical cutting depth in wire-saw cutting SiC single-crystal.And the conclusion had been analyzed and verified by experiment.Based on the principle of impulse,the maximum normal pulse impact forces applied to SiC work-piece by diamond abrasive particle at different locations on wire-saw’s cross-section in forced vibration wire-saw cutting process was analysed and studied.Applying indentation fracture mechanics theory,the propagating length and depth of median/lateral crack beneath the diamond abrasive induced by normal and tangential load were analyzed quantitatively.By analyzing the influence of lateral crack induced by different location abrasive particle of wire-saw circular cross-section on wafer surface,the formula for predicting wafer’s surface roughness was deduced.The scratching effect of the diamond abrasive in plastic region helps to improve surface quality of wafer,and that in brittleness area helps to material removal.The result obtained above is verified by experiment.The machining of SiC single-crystal cut by wire-saw excited by transverse ultrasonic indeed is the process of which diamond grits on wire-saw continuously load and unload on work-piece’s surface in high frequency.The process forms a shielding effect which prompt lateral cracks priority extending and the median cracks being suppressed.The effect of median cracks being suppressed can ameliorate the quality of SiC wafer’s sub-surface,and the lateral cracks extending priority can improve the removal of material.In the process of SiC single-crystal cut by wire-saw excited by transverse ultrasonic wave,the average effective normal force is lower and the maximum pulse normal impact force is larger than that in traditional wire-saw cutting,owing to transverse ultrasonic vibration.Diminution of the average effective normal force induces the depth of lateral cracks beneath the abrasive particle to shoal,and thus the surface quality of the SiC wafer can be improved.On the other hand,there is a increment of maximum pulse normal impact force in process of vibration wire-saw cutting,the transverse crack extending length increases,which makes material removal rate to increase.The model of theoretical volume of hard-brittle material removed by vibrating diamond grits was established,and the formula for calculating the average material removal rate in vibration cutting was deduced.By changing the main cutting parameters,the contrast experimental study of material removal rate of SiC single-crystal cut by ordinary wire-saw and ultrasonic vibration wire-saw respectively,in order to verify the correctness of theoretical analysis. |
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