A novel mechanical cleavage method for synthesizing few-layer graphenes

A novel method to synthesize few layer graphene from bulk graphite by mechanical cleavage is presented here. The method involves the use of an ultrasharp single crystal diamond wedge to cleave a highly ordered pyrolytic graphite sample to generate the graphene layers. Cleaving is aided by the use of ultrasonic oscillations along the wedge. Characterization of the obtained layers shows that the process is able to synthesize graphene layers with an area of a few micrometers. Application of oscillation enhances the quality of the layers produced with the layers having a reduced crystallite size as determined from the Raman spectrum. Interesting edge structures are observed that needs further investigation

Finite Element Study on The Effect of Substrate Properties in Micro-cutting Thin Workpiece Materials

The cutting mechanism and residual stress profile of the micro-cutting thin workpiece are affected by the interaction of the thin workpiece and the fixture (substrate) underneath it similar to that observed in the nano-indentation and nano-scratching of thin film. The appropriate substrate properties are necessary especially to avoid detachment during machining and to minimize deformation and warping of the machined thin workpiece. Thus, the investigations of the influence of substrate properties on micro-cutting thin workpiece are essentially to be conducted. The finite element study of orthogonal micro-cutting of thin Al6061-T6 is presented here. The simulations were conducted to study the residual stress profile across the thickness of the machined thin workpiece at various workpiece thicknesses and various substrate (adhesive) elastic properties. Simulations results show that as the machined workpiece become thinner, the stress is more significant not only on the machined surface but also it can reach the bottom of the workpiece. The stiffer substrate produces less variation of the stress across the workpiece thickness while more compliant substrate produces broader stress variation as the workpiece become thinner. The results show the significant effect of the workpiece thickness and the substrate properties on the stress profiles in the micro-cutting of thin workpiece

Burr Reduction of Micro-milled Microfluidic Channels Mould Using a Tapered Tool

Moulds with micro-sizes features needed for many applications, such as for hot embossing, can be manufactured using micro-milling process. However, the burrs formed in the micro-milling process are a challenge that needs to be addressed. The burr sizes are comparable to the micro-milled feature sizes and the common types of burr seen being the top/side and exit burrs. The use of a tapered geometry micro-milling tool is investigated in this paper that enables reduction in both the top and exit burrs. The straight and tapered micro-milling tools of various angles are used and the burrs formed are observed. Micro-milling experiments are conducted on an aluminium alloy by producing common positive features seen in the mould for the production of polymer microfluidic devices. The results show that the burr reduction can be attributed due to the increase of the taper angle. It is seen that the tapered tool not only substantially reduces the top burrs, but also leaves behind inclined walls which further help in reducing exit burrs formed during the subsequent finish face milling. Furthermore, embossing trials performed with the micro-milled tapered geometry moulds show improved performance not only because burrs are reduced and also because the taper eases mould release

Some Investigations of Scaling Effects in Micro-Cutting

The scaling of specific cutting energy is studied when micro-cutting ductile metals. A unified framework for understanding the scaling in specific cutting energy is first presented by viewing the cutting force as a combination of constant, increasing, and decreasing force components, the independent variable being the uncut chip thickness. Then, an attempt is made to isolate the constant force component by performing high rake angle orthogonal cutting experiments on OFHC Copper. The data shows a trend towards a constant cutting force component as the rake angle is increased. In order to understand the source of this constant force component the chip-root is investigated. By quickly stopping the spindle at low cutting speeds, the chip is frozen and the chip-workpiece interface is examined in a scanning electron microscope. Evidence of ductile tearing ahead of the cutting tool is seen at low and high rake angles. At higher cutting speeds a quick-stop device is used to obtain chip-roots. These experiments also clearly indicate evidence of ductile fracture ahead of the cutting tool in both OFHC Copper and Al-2024 T3. To model the cutting process with ductile fracture leading to material separation the finite element method is used. The model is implemented in a commercial finite element software using the explicit formulation. Material separation is modeled via element failure. The model is then validated using the measured cutting and thrust forces and used to study the energy consumed in cutting. As the thickness of layer removed is reduced the energy consumed in material separation becomes important. Simulations also show that the stress state ahead of the tool is favorable for ductile fracture to occur. Ductile fracture in three locations in an interface zone at the chip root is seen while cutting with edge radius tool. A hypothesis is advanced wherein an element gets wrapped around the tool edge and is stretched in two directions leading to fracture. The numerical model is then used to study the difference in stress state and energy consumption between a sharp tool and a tool with a non-zero edge radius.Ph.D.Committee Chair: Melkote, Shreyes; Committee Member: Danyluk, Steven; Committee Member: Koehler, Stephan; Committee Member: McDowell, David; Committee Member: Zhou, Che

Orthogonal microcutting of thin workpieces

With a broader intention of producing thin sheet embossing molds, orthogonal cutting experiments of thin workpieces are conducted. Challenges in machining thin workpieces are many: machining induced stress and deformation, fixturing challenges, and substrate effects. A setup involving continuous orthogonal cutting with a single crystal diamond toolof an aluminum alloy (Al6061-T6) workpiece fixtured using an adhesive to reduce its thickness is used to study trends in forces, chip thickness, and to understand to what level of thickness we can machine the workpiece down to and in what form the adhesive fails. There are no significant changes observed in the forces and chip thickness between thick and thin workpieces during the experiments, meaning that the cutting energy required is the same in cutting thick or thin workpieces. The limitation to achieve thinner workpiece is attributed mainly due to the detachment of the thin workpiece by peel-off induced by adhesive failure mode, which occurs during initial chip formation as the tool initially engages with the workpiece. We use a finite element model to understand the stresses in the workpiece during this initial tool engagement when it is thick and when it is thin, as well as the effect of the adhesive itself and the effect of adhesive thickness. Simulation results show that the tensile stress induced by the tool at the workpiece-adhesive interface is higher for a thinner workpiece (45 µm) than a thicker workpiece (150 µm) and higher at the entrance. As such, a thinner workpiece is more susceptible to peel-off. The peeling of thin workpiece is induced when the high tensile stress at the interface exceeds the tensile-at-break value of the adhesive

Microstructural changes during precision machining of thin substrates

This paper reports investigations in machining of thin substrates with thickness less than 100μm. The machining process induces severe plastic deformation through the thickness of the machined thin workpiece due to the high ratio of the depth of cut to workpiece thickness. The diamond face turning is used to machine thin workpieces down to a thickness less than 100μm. The microstructure of the machined sample is studied and x-ray diffraction used to observe the crystallographic orientation / texture. The microstructures of the thin machined workpieces are seen to become more random, denser, and finer with the shape of the grains less elongated as compare to the bulk and thick machined sample. The x-ray diffraction analyses indicate that machining of thin substrates changes the texture or orientation. Different deformation mechanisms may occur when machining thin workpiece especially at thicknesses below 100μm

Orthogonal cutting study of the micro-cutting thin workpiece

With a broader intention of producing thin sheet embossing molds, results from investigations in orthogonal cutting of thin workpieces are presented here. Challenges in machining thin workpieces are many: residual stress effects, fixturing challenges, and substrate effects. Aluminum alloy Al6061-T6 workpiece fixture using an adhesive is orthogonally cut with a single crystal diamond tool. We study trends in cutting forces, understand to what level of thickness we can machine the workpiece down to and in what the form the adhesive fails. Two types of workpiese-adhesive anomalies were noticed. One is the detachment of the thin workpiece by peel-off and the other one is where the workpiece did not get detached but the final width of the workpiece was non-uniform. We then use a validated finite element machining model to understand the stresses in the workpiece when it is thick and when machined to thin condition, effect of the adhesive itself and also the effect of adhesive thickness. Simulations show that the stress induced by the cutting process at the bottom of the workpiece is higher for the thinner workpiece (40 μm) compare to a thicker workpiece (400 μm) especially at the tool entrance region for adhesive thicknesses of 30 μm and 100 μm. Hence a thinner workpiece is more susceptible to failure by adhesive peeling