Ultra-precision Machining Of Brittle Materials Research Articles

Investigation on residual scratch depth and material removal rate of scratching machining single crystal silicon with Berkovich indenter

Single crystal silicon is the basic material for the fabrication of micro-devices and micro-structures. It exhibits plastic behavior in micro/nano-machining. The key to obtain high-quality machined surfaces in ultra-precision machining of brittle materials is the material removal in ductile region. This paper focuses on single crystal silicon scratching machining in ductile region. The contact area and contact force of Berkovich indenter along two typical scratching machining directions (Φ=0° and Φ=60°) are analyzed. The residual scratch depth of typical scratching direction of Berkovich indenter is calculated theoretically. The experiments of scratching machining on (001) crystal plane of single crystal silicon using nanoindenter and Berkovich indenter are conducted. Based on experiments of nanoindenter scratching machining, the effects of normal load and scratching direction on the residual scratch depth and material removal rate in ductile region machining are investigated. Between the theoretical calculation results and the experimental results, the maximum deviation of residual scratch depth and material removal rate of Berkovich indenter scratching machining single crystal silicon is less than 3.5%. The residual scratch depth and material removal rate of Berkovich indenter scratching machining single crystal silicon along directions Φ=0° and Φ=60° are quite different. When the normal load is 10 mN and 20 mN, the material removal rate of single crystal silicon scratching machining along direction Φ=60° is more than 2 times that of along direction Φ=0°. The residual scratch depth of scratching machining single crystal silicon with Berkovich indenter varies nonlinearly with normal load, and the material removal rate is obviously affected by the direction of the scratching machining. The results in this paper are useful for the determination of process parameters for ultra-precision machining of single crystal silicon.

Ductile-regime machining model for ultrasonic elliptical vibration cutting of brittle materials

Ultrasonic elliptical vibration cutting (UEVC) is attracting much attention in ultra-precision machining of brittle materials as it was found be able to effectively increase the critical depth of cut (dc) of brittle materials. However, the mechanisms for the increased dc are still not fully understood. In this study, a ductile-regime machining model was developed for the UEVC of brittle materials to achieve maximum dc and thus the maximum machining efficiency. It shows that the reasons for the increase of dc are: (1) The actual undeformed chip thickness (UCT) is smaller than the nominal depth of cut (DOC); (2) The distance from transient surface to target surface (DTSTS) can endure the crack propagation within a certain range when UCT exceeds the critical UCT (tc). Based on the ductile-regime machining model, a predictive model was developed for the determination of dc in microgroove plunge-cutting with respect to several determining factors, including DTSTS, UCT, tc and length of the crack (Cm). A series of experiments on plunge-cutting of KDP crystal were conducted. Results showed that the predicted dc match well with the experimental ones. In addition, it was found that dc increases as the amplitude of vibration along the cutting direction increases or the cutting speed decreases, and dc increases first and then decreases as the amplitude of vibration along the thrust direction increases.

A Study of Mechanics in Brittle⁻Ductile Cutting Mode Transition.

This paper presents an investigation of the mechanism of the brittle–ductile cutting mode transition from the perspective of the mechanics. A mechanistic model is proposed to analyze the relationship between undeformed chip thickness, deformation, and stress levels in the elastic stage of the periodic chip formation process, regarding whether brittle or ductile mode deformation is to follow the elastic stage. It is revealed that, the distance of tool advancement required to induce the same level of compressive stress decreases with undeformed chip thickness, and thereby the tensile stress below and behind the tool decreases with undeformed chip thickness. As a result, the tensile stress becomes lower than the critical tensile stress for brittle fracture when the undeformed chip thickness becomes sufficiently small, enabling the brittle–ductile cutting mode transition. The finite element method is employed to verify the analysis of the mechanics on a typical brittle material 6H silicon carbide, and confirmed that the distance of the tool advancement required to induce the same level of compressive stress becomes smaller when the undeformed chip thickness decreases, and consequently smaller tensile stress is induced below and behind the tool. The critical undeformed chip thicknesses for brittle–ductile cutting mode transition are estimated according to the proposed mechanics, and are verified by plunge cutting experiments in a few crystal directions. This study should contribute to better understanding of the mechanism of brittle–ductile cutting mode transition and the ultra-precision machining of brittle materials.

Application potential of coarse-grained diamond grinding wheels for precision grinding of optical materials

Fine-grained resin bonded diamond tools are often used for ultra-precision machining of brittle materials to achieve optical surfaces. A well-known drawback is the high tool wear. Therefore, grinding processes need to be developed exhibiting less wear and higher profitability. Consequently, the presented work focuses on conditioning a mono-layered, coarse-grained diamond grinding wheel with a spherical profile and an average grain size of 301 µm by combining a thermo-chemical and a mechanical-abrasive dressing technique. This processing leads to a run-out error of the grinding wheel in a low-micrometer range. Additionally, the thermo-chemical dressing leads to flattened grains, which supports the generation of hydrostatic pressure in the cutting zone and enables ductile-mode grinding of hard and brittle materials. After dressing, the application characteristics of coarse-grained diamond grinding wheels were examined by grinding optical glasses, fused silica and glass–ceramics in two different kinematics, plunge-cut surface grinding and cross grinding. For plunge-cut surface grinding, a critical depth of cut and surface roughness were determined and for cross-grinding experiments the subsurface damage was analyzed additionally. Finally, the identified parameters for ductile-machining with coarse-grained diamond grinding wheels were used for grinding a surface of 2000 mm2 in glass–ceramics.

A predictive model of the critical undeformed chip thickness for ductile–brittle transition in nano-machining of brittle materials

There is a distinct transition in the mode of material removal in machining of brittle materials if the undeformed chip thickness is below a critical threshold of submicron scale. It is believed that at such small scale of material removal, the energy required to extend pre-existing flaws in the microstructure of brittle material exceeds the energy required to mobilize the micro-structural dislocations and hence plastic deformation serves as the dominant mode of material removal. It is postulated that a transition in the mode of material removal in machining of brittle materials is accompanied by a corresponding shift in the representative mode of energy expenditure. Hence, machining energy is a viable parameter to characterize the modes of material removal in machining of a brittle material. This paper presents a specific cutting-energy based model to predict the ductile–brittle transition point in ultra-precision machining of brittle materials. The energy expended in brittle and ductile modes of machining is modeled as a function of work-material intrinsic properties, tool geometry and process parameters. The transition point is identified in terms of undeformed chip thickness at which the mode of energy undergoes a transition from the plastic deformation based one to the fracture based one. The validity of the proposed model is verified by single-edge cutting tests on single-crystal silicon and BK7 glass. The experimental results are found in good agreement with model results.

Fabrication of Diamond Micro Tool Array (DMTA) for Ultra-Precision Machining of Brittle Materials

A novel conditioning technique to precisely and effectively condition the nickel electroplated mono-layer coarse-grained diamond grinding wheel of 91m grain size was developed to fabricate a Diamond Micro Tool Array (DMTA) in ductile machining of brittle materials. During the fabricating process, a copper bonded diamond grinding wheels (91m grain size) dressed by ELID (electrolytic in-process dressing) was applied as a conditioner, a force transducer was used to monitor the conditioning force, and a coaxial optical distance measurement system was used to insitu monitor the modified wheel surface status. The experimental result indicates that the newly developed conditioning technique is applicable and feasible to generate required wheel topography of less than 2μm run-out error and grain geometries. The taper cutting test on BK7 proves the fabricated DMTA is capable of realizing ductile machining of brittle materials.

Analysis on Nano-machinability in Ultraprecision Machining of Brittle Materials(Analytical advancement of machining process)

Nano-machinability results from a complex interaction of both direct and indirect effects at the machining point, and hence it is difficult to evaluate nano-machinability. In this study, an analysis method of nano-machinability for brittle materials was proposed with multiple variable models. The proposed method is based on covariance structure analysis, which can evaluate complex interaction network models that consist of both the measured variables and the unmeasured latent variables. An analysis using the proposed method with actual machining data was carried out quantitatively, and the availability of the method has been confirmed.

Theoretical and Experimental Studies of Ultraprecision Machining of Brittle Materials with Ultrasonic Vibration

Ductile machining of brittle materials, even at a high critical depth-of-cut, has been realised by applying ultrasonic vibration to a diamond tool tip. A surface roughness Ra of 100 nm at a 2 μm depth-of-cut was achieved. A discussion on fundamental principles of ultrasonic machining, the material removal mechanism and the calculation of critical factors are presented in this paper.

Brittle-Ductile Transition and Its Control in Machining of Monocrystal Silicon

This paper describes the basic conditions for the brittle-ductile transition in the ultraprecision machining of brittle materials in order to explain the deformation and fracture mechanism of a sub-surface layer induced by the microinteraction between a tool and a material. The temperature and stress distribution were simulated by a finite element method using a indenter sliding contact model in diamond cutting of monocrystal silicon. From this analysis, the characteristics of high temperature and stress field during the microcutting of brittle materials, and also the deformation and fracture behavior were clearly understood. The stable ductile mode machining of brittle materials is achieved by controlling the depth of cut, cutting speed and lubrication.

Chemomechanical effects in ductile-regime machining of glass

Optical glass was subjected to controlled, low-force scratching, and ultraprecision, fixed-abrasive grinding in various environments, including a homologous series of n-alcohols and water. It was found that the liquid environment significantly affected the mechanism of deformation and the extent of subsurface damage. Minimum subsurface damage occurred in a heptanol environment. Observed chemomechanical effects are linked to surface charge and ion adsorption through measurements of zeta-potential between the glass and the various environments. By comparing the present results with previously reported studies of chemomechanical effects, it was found that environments that inhibit fracture in fine microgrinding enhanced fracture at higher machining rates. This finding can be used to improve efficiency and increase productivity in ultraprecision machining of brittle materials.

Ultraprecision Ductile Cutting of Glass by Applying Ultrasonic Vibration

Summary Ultraprecision ductile cutting of glass is realized by applying ultrasonic vibration to a single crystal diamond tool in the cutting direction. It is clarified by grooving experiments that the critical depth of cut for ductile cutting is increased and the surface profile of diamond cutting edge is better transferred to the cut groove as compared to the conventional Cutting. The transparent surface of soda-lime glass with surface roughness of 0.03 μm Rmax is obtained by face turning. The work is considered as a contribution to the ultraprecision machining of brittle materials.