Modelling and experimental investigation on the ductile machining mechanism in ultra-precision fly cutting of brittle materials for functional surface

Abstract

The manufacturing of functional surfaces on brittle materials has attracted widespread attention in an extensive range of applications, such as photoelectronic products, semiconductors and advanced optics. However, the inherent hard-and-brittle nature of brittle materials imposes great challenges in obtaining functional surfaces with ultra-smooth roughness and high form accuracy. To avoid brittle fractures, very small machining parameters, i.e., feed rate and depth of cut, are necessarily selected in the processing of brittle materials, which inevitably leads to low machining efficiency, rapid tool wear and limited azimuthal height variation (AHV) of the generated functional surfaces. Especially, for the complicated hybrid surfaces with secondary micro/nanostructures, complex machining technologies that combine multi-step machining processes are generally required to generate hybrid structures on brittle materials, having very low-efficiency and difficulty in controlling form accuracy. Ultra-precision fly cutting (UPFC) is widely regarded as a very promising technique for achieving deterministic generation of functional surfaces on a wide range of engineering materials. In UPFC, the diamond tool rotates on the spindle with a large swing radius. Nevertheless, the unique machining strategy of UPFC leads to totally different ductile machining mechanisms from those of turning and ball end milling. It is worth noting that the ductile machining models of micro-grooves, freeform surfaces and hybrid functional surfaces are different for UPFC owing to the different machining processes. This research study provides an experimental and theoretical investigation on the ductile machining mechanisms in UPFC of brittle materials for different functional surfaces. Specifically, the thesis includes four independent parts as follows: (1) The first part experimentally investigates the brittle-to-ductile transition characteristics in UPFC of brittle materials, through machining taper micro-grooves on single-crystal silicon. The influence of the intermittent cutting operation of fly cutting on the brittle-to-ductile transition of brittle materials is determined by observing the depth of ductile-cut region. It is observed that a much deeper ductile-cut region can be acquired by UPFC compared with the ordinary sculpturing method. The unique surface topography and cutting force signals in UPFC of brittle materials are also studied by machining a flat surface on a silicon wafer and comparing the experimental results with those in diamond turning. (2) The second part theoretically and experimentally investigates the ductile machining mechanism in fly grooving of brittle materials. Through modelling the chip formation process, it is demonstrated that very small chip thicknesses can be generated by UPFC, even under large feed rates and cutting depths, which indicates that very deep micro-grooves can be machined by UPFC without brittle fractures. To validate the proposed model, micro-grooves with different cutting depths are fabricated on single-crystal silicon, and the surface topographies are compared with those of conventional diamond sculpturing. In addition, to further study the unique material removal mechanism of UPFC, the chip morphology and material phase transformation are also investigated. Finally, the tool wear patterns of UPFC and the influence on the finished surface topography are also studied. (3) The third part proposes a novel ductile machining model in UPFC of brittle materials for freeform surfaces with large AHV. Through modelling the chip formation process considering the step movement of the diamond tool, the effect of the swing radius of the diamond tool and the step distance on the chip thickness are studied. It is demonstrated that by configuring a large swing radius of the diamond tool, a deep ductile cut region can be achieved by UPFC. To validate the proposed model, an F-theta lens surface with an AHV over tens of micrometers is fabricated on a silicon wafer. The efficiency of UPFC is also compared with that of Fast and slow tool servo. (4) In the final part, UPFC technology is applied to fabricate hybrid functional surfaces on brittle materials. The generation mechanism of the hybrid surfaces is introduced, and the optimal tool path generation strategy is detailed considering the compensation of the swing radius and tilting geometry of the diamond tool. Besides, the selection criteria of the machining parameters are also discussed to ensure the ductile machining of brittle materials. Two types of hybrid micro-optic components, namely hybrid micro-aspheric arrays and sinusoid grid surface with high-frequent secondary unidirectional phase gratings, were fabricated on a single-crystal silicon wafer. In this research, the unique ductile machining mechanisms in UPFC of brittle materials for different functional surfaces, including micro-grooves, freeform surfaces and hybrid structures, are studied, and the main contributions of the thesis are: (1) Providing a comprehensive understanding of the unique ductile machining phenomena in UPFC of brittle materials, including brittle-to-ductile transition, surface topography, chip formation, material phase transformation and tool wear patterns. (2) Providing a theoretical explanation why UPFC can achieve a much deeper ductile-cut region compared with the ordinary diamond sculpturing and turning method, and the proposed ductile machining models are meaningful in achieving ductile machining of brittle materials for functional surfaces with large AHV and complicated structures, as well as with optically qualified form accuracy. (3) Providing an efficient and flexible machining strategy based on UPFC to achieve one-step generation of hybrid structures on brittle materials with high form accuracy.