Sathyan Subbiah

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.

Evidence of Ductile Tearing Ahead of the Cutting Tool and Modeling the Energy Consumed in Material Separation in Micro-Cutting

Orthogonal cutting experiments using a quick-stop device are performed on A12024-T3 and OFHC copper to study the chip-workpiece interface in a scanning electron microscope. Evidence of ductile tearing ahead of the tool at cutting speeds of 150 m/min has been found. A numerical finite element model is then developed to study the energy consumed in material separation in micro-cutting. The ductile fracture of A12024-T3 in a complex stress state ahead of the tool is captured using a damage model. Chip formation is simulated via the use of a sacrificial layer and sequential elemental deletion in this layer Element deletion is enforced when the accumulated damage exceeds a predetermined value. A Johnson-Cook damage model that is load history dependent and with strain-to-fracture dependent on stress, strain rate, and temperature is used to model the damage. The finite element model is validated using the cutting forces obtained from orthogonal micro-cutting experiments. Simulations are performed over a range of uncut chip thickness values. It is found that at lower uncut chip thickness values, the percentage of energy expended in material separation is higher than at higher uncut chip thicknesses. This work highlights the importance of the energy associated with material separation in the nonlinear scaling effect of specific cutting energy in micro-cutting.

Separation, folding and shearing of graphene layers during wedge-based mechanical exfoliation

We report, using molecular dynamics simulation studies, how and under what conditions graphene layers separate, fold and shear during a wedge-based mechanical exfoliation machining technique to produce few-layer graphene. Our previously reported experimental results using this novel technique have shown clear evidence of few-layer graphene being subjected to such phenomena. Molecular simulations of initial wedge engagement show that the entry location of the wedge tip vis-á-vis the nearest graphene layer plays a key role in determining whether layers separate or fold and which layers and how many of them fold. We also show that depending on this entry location several successive layers beneath the wedge undergo significant elastic bending, consuming energies requiring large vertical forces to be imposed by the moving wedge. The layer separation force itself is seen to be minimal and consistent with breaking up of van der Waals interactions. In addition, shearing of layers occurs mainly during wedge exit and depends largely on the wedge speed and also its depth of insertion. Understanding the conditions at which this separation, folding and shearing of the graphene layers takes place, one can control or tune the wedge-based exfoliation technique for particular kinds of graphene layers.

The Constant Force Component due to Material Separation and Its Contribution to the Size Effect in Specific Cutting Energy

The contribution of material separation in cutting ductile metals to the constant force component, and, hence, to the size effect in specific cutting energy is explored in this paper. A force-decomposition-based framework is proposed to reconcile the varied reasons given in literature for the size effect. In this framework, the cutting force is broken down into three components: one that is decreasing, another that is increasing, and the third that remains constant, with decreasing uncut chip thickness. The last component is investigated by performing orthogonal cutting experiments on OFHC copper at high rake angles of up to 70 deg in an attempt to isolate it. As the rake angle is increased, the resulting experimental data show a trend toward a constant cutting-force component independent of the uncut chip thickness. Visual evidence of ductile tearing ahead of the tool associated with material separation leading to chip formation is shown. The measured constant force and the force needed for ductile crack extension are then compared.

On the Size-Effect in Micro-Cutting at Low and High Rake Angles

A partial explanation of the size-effect in the specific cutting energy in micro-cutting is provided in this paper. For a simple orthogonal cutting condition, with constant width of cut, the specific cutting energy is viewed as a ratio of the cutting force and the uncut chip thickness. Size-effect, i.e., an unbounded increase in the specific cutting energy with decrease in uncut chip thickness, will occur under two conditions: one, if a component of the cutting force remains constant with uncut chip thickness and two, if some component of the cutting force increases with the uncut chip thickness. Experiments have been performed at high rake angles in an attempt to isolate and detect the presence of the constant component of the cutting force. The trend confirming the presence of this component is reported and explained.© 2004 ASME

Study of Pressure Distribution in Compliant Coated Abrasive Tools for Robotic Polishing

Robotic polishing applications involve the use of coated abrasive tools along with a compliant backing pad. The compliance helps in conforming to curved surfaces and also in blending with unpolished areas. This compliance and its effect are currently controlled only by empirical choice of various backing pad designs and materials. A better understanding of this important characteristic of these tools will lead to better process control. One of the effects of the compliance or stiffness of the backing pad under a certain applied load (robotic force control) is on the contact area and pressure distribution applied on the abrasive grains in that area. This pressure distribution in turn dictates how material is removed in the area of contact. Here, we report preliminary results of the pressure distribution exerted by an abrasive tool mounted on a robot using pressure film sensors and compare results with a simple finite element model.Copyright

A mechanical route to carbon nanoscrolls

Recent developments in isolating two dimensional materials has triggered interests in different forms of these materials such as carbon nanoscrolls (CNS). Here we report formation of nanoscrolls using our previously reported technique of wedge based mechanical exfoliation. TEM images reveal the presence of nanoscrolls in many forms. Scroll formation was more prominent when the cleaving wedge was oscillated along its sharp edge at ultrasonic frequencies. Among several possible hypotheses, we present a simple one and show using molecular simulations that previously formed randomly scrolled edges roll by the external energy assistance of the moving penetrating wedge to form scrolls upon cleaving. Our mechanical exfoliation method has potential to synthesize CNS in a controlled manner.

Wedge Radius Effects in Mechanical Exfoliation of HOPG: A Molecular Simulation Study

We study the effects of wedge bluntness in mechanically exfoliating graphene layers from highly ordered pyrolytic graphite (HOPG), a layered material. Molecular dynamics simulations show that the layer initiation modes strongly depend on the wedge radius. Force and specific energy signatures are also markedly affected by the radius. Cleaving with a larger wedge radius causes buckling ahead of the wedge; larger the radius more the buckling. A critical depth of insertion of 1.6 A° is seen necessary to cleave a single layer; this is also found to be independent of wedge radius. Hence, with accurate positioning on an atomically flat HOPG surface it is possible to mechanically cleave, using a wedge, a single sheet of graphene even with a blunt wedge.© 2014 ASME

Analysis of 1D Abrasive Vibratory Finishing Using Acoustic Emission

Acoustic emission (AE) is a widely used non-destructive method for monitoring and control of machining processes. Vibratory finishing is a surface modification process used for polishing, deburring and finishing of components (aerospace, automotive and other industries). The polishing action takes place due to the action of abrasive particles called media on the components subjected to finishing. The media motion is complex and involves a combination of normal and oblique impacts, scratching and rolling. This work deals with the characterization of basic types of media contact occurring in the vibratory finishing process using acoustic emission signals. A novel one dimensional vibratory simulator was developed for this purpose using a tribometer setup. The one dimensional simulator was used to differentiate between the normal and scratching types of media contact and corresponding AE signals were measured. The preliminary results shows that the AE signals obtained for normal and scratching type of contacts are different. In addition to this, AE signals were used to characterize the amount of media.Copyright

Finite Element Study of 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.