Nanometric Cutting Mechanism And High-efficient Machining Technique Of Brittle Materials

Surface integrity and tool wear are two important research topics in ultra-precision diamond cutting of brittle materials.To achieve an optical grade surface,material to be removed has been approached to nanometric scale.However,the theory of nano-cutting has not been well established.It is also difficult to simulate the fracture damage using numerical method at atomic level.In addition,there exist two kinds of auxiliary methods for improving the machining efficiency,i.e.material surface modification by ion implantation(Ni IM)and ultrasonic vibration assisted cutting.The former is seriously restricted by the high cost due to the large ion dosage and processing time,and the latter has not been applied on brittle materials as well as ferrous metals.Based on these issues,fundamental studies are conducted to obtain further understanding of nano-cutting,improve the simulation approach and optimize the processes.The results would enhance the controllability in the high-efficient machining of brittle materials.The main contents include:1.Based on the concept of subtractive manufacturing,atomic bond breakage is analyzed to reveal the mechanism of material removal during nano-cutting.The amorphization by contact and strain energy release is critical for the plastic deformation.Differences and transition of two cutting mechanisms,extrusion and shear,are clarified,and the experimental observation of polycrystalline chip is successfully interpreted.2.Aiming at the extremely small scale of the cutting model in molecular dynamics simulation,a novel method based on the dynamic adjustment of workpiece is proposed to improve the computation efficiency.A customized code is developed and its performance is systematically tested.Processes on brittle crystals with large cutting depths are simulated using a tool model which has an edge radius as a real one.It reveals the dislocation pile-up mechanism of cutting induced fracture in monocrystalline germanium,as well as the close relation between the cutting force vibration and material behaviors.The simulation result is verified by experimental observations.3.Multi-implantation technique is developed to reduce the irradiation dosage in surface modification.A modified layer with micron thickness is fabricated on silicon,which achieves a significant improvement in the machinability and the brittle-to-ductile transition depth.Micro-structure array is realized by diamond cutting in ductile regime.The chip analysis by scanning electron microscope provides evidence of the cutting mechanism transition from shear to extrusion.Multi-implantation has been used in the production of optical surface and micro structure on brittle materials.It verifies that material property can be also modified by the ionization effect during ion implantation.Influence of amorphous layer configuration on the nano-cutting is also investigated.4.Surface generation and material removal in the ultrasonic vibration assisted diamond turning are studied.A roughness model is built and a program is developed for process parameters design.The thermo-chemical tool wear in the cutting of K9 optical glass is significantly reduced,accompanied with the realization of a machined surface with nanometric roughness.For tungsten carbide,the material deformation,stress field and intergranular fracture are analyzed by molecular dynamics.A comprehensive comparison between ultrasonic vibration and surface modification is conducted,which shows the Ni IM technique helps to achieve better results.Experiments on different types of WC indicate that the material strength has a strong influence on the ultrasonic vibration cutting of super-hard material.