Ashraf F. Bastawros

Experimental analysis of deformation mechanisms in a closed-cell aluminum alloy foam

Abstract The evolution of plastic deformation in a cellular Al alloy upon axial compression is monitored through a digital image correlation procedure. Three stages in the deformation response have been identified. The first involves localized plastic straining at cell nodes. It occurs uniformly and leads to a nominal loading modulus appreciably lower than the stiffness. The second comprises discrete bands of concentrated strain containing cell membranes that experience plastic buckling, elastically constrained by surrounding cells. In this phase, as the loading increases, previously formed bands harden, giving rise to new bands in neighboring regions. The localized bands exhibit a long-range correlation with neighboring bands separated by 3–4 cells along the loading direction. This length scale characterizes the continuum limit. Thirdly, coincident with a stress peak, σo, one of the bands exhibits complete plastic collapse. As the strain increases, this process repeats, subject to small stress oscillations around σo.

Compressive deformation and yielding mechanisms in cellular Al alloys determined using X-ray tomography and surface strain mapping

The mechanisms of compressive deformation that occur in both closed and open cell Al alloys have been established. This has been achieved by using X-ray computed tomography (CT) and surface strain mapping to determine the deformation modes and the cell morphologies that control the onset of yielding. The deformation is found to localize in narrow bands having widths of order of a cell diameter. Outside the bands, the material remains elastic. The cells within the bands that experience large permanent strains are primarily elliptical. A group of cells work collectively to allow large localized deformation. Size does not appear to be the initiator of the deformation bands. Equiaxed cells remain elastic. The implications for manufacturing materials with superior mechanical properties are discussed.

Molecular dynamics simulation of nanoscale machining of copper

Molecular dynamics simulations of the nanometric cutting of single-crystal copper were performed with the embedded atom method. The nature of material removal, chip formation, material defects and frictional forces were simulated. Nanometric cutting was found to comprise two steps: material removal as the tool machines the top surface, followed by relaxation of the work material to a low defect configuration, after the tool or abrasive particle has passed over the machined region. During nanometric cutting there is a local region of higher temperature and stress below the tool, for large cutting speeds. Relaxation anneals this excess energy and leads to lower dislocation work material. At high cutting speeds (180 m s−1), the machined surface is rough but the work material is dislocation free after the large excess energy has annealed the work material. At lower cutting speeds (1.8– 18 m s−1), the machined surface is smooth, with dislocations remaining in the substrate, and there is only a small excess temperature in the work material after machining. The size of the chip grows with increasing cutting speed.

Superhard self-lubricating AlMgB14 films for microelectromechanical devices

Performance and reliability of microelectromechanical system (MEMS) components can be enhanced dramatically through the incorporation of protective thin-film coatings. Current-generation MEMS devices prepared by the lithographie-galvanoformung-abformung (LIGA) technique employ transition metals such as Ni, Cu, Fe, or alloys thereof, and hence lack stability in oxidizing, corrosive, and/or high-temperature environments. Fabrication of a superhard self-lubricating coating based on a ternary boride compound AlMgB14 described in this letter has great potential in protective coating technology for LIGA microdevices. Nanoindentation tests show that the hardness of AlMgB14 films prepared by pulsed laser deposition ranges from 45 GPa to 51 GPa, when deposited at room temperature and 573 K, respectively. Extremely low friction coefficients of 0.04–0.05, which are thought to result from a self-lubricating effect, have also been confirmed by nanoscratch tests on the AlMgB14 films. Transmission electron microscopy st...

A Scratch Intersection Model of Material Removal During Chemical Mechanical Planarization (CMP)

A scratch intersection based material removal mechanism for CMP processes is proposed in this paper The experimentally observed deformation pattern by SEM and the trends of the measured force profiles (Che et al., 2003) reveal that, for an isolated shallow scratch, the material is mainly plowed sideway along the track of the abrasive particle with no net material removal. However, it is observed that material is detached close to the intersection zone of two scratches. Motivated by this observation, it is speculated that the deformation mechanism changes from ploughing mode to shear-segmentation mode as the abrasive particle approaches the intersection of two scratches under small indentation depth for ductile metals. The proposed mechanistic material removal rate (MRR) model yields Preston constant similar to those observed experimentally for CMP processes. The proposed model also reveals that the nature of the slurry-pad interaction mechanism, and its associated force partitioning mechanism, is important for determining the variation of MRR with particle size and concentration. It is observed that under relatively soft pads, small particles and low particle concentration, the pad undergoes local deformation, yielding an increased MRR with increasing particle size and concentration. At the other extreme, the intact walls of the surface cells and the connecting cell walls between the surface pores deform globally, resembling a beam or a plate, and a decreasing trend in MRR is observed with increasing particle size and concentration. The predicted MRR trends are compared to existing experimental observations.

Surface evolution during the chemical mechanical planarization of copper

Stressed surfaces are configurationally unstable under chemical etching wherein they may evolve to reduce their total energy. This paper investigates how such an effect may influence the planarization rate in a Chemical Mechanical Planarization (CMP) process. Nano-wear experiments on electro-plated copper surfaces have been conducted with systematic exposures to chemically active slurry. The nano-wear experiments have been first performed to generate local variation of the residual stress levels, followed by chemical etching to investigate the variation of the wear depth and the evolution of surface topography. It is found that the residual stress caused by the mechanical wear enhances the chemical etching rate.

Simulation of chemical mechanical planarization of copper with molecular dynamics

With an aim to understanding the fundamental mechanisms underlying chemical mechanical planarization (CMP) of copper, we simulate the nanoscale polishing of a copper surface with molecular dynamics utilizing the embedded atom method. Mechanical abrasion produces rough planarized surfaces with a large chip in front of the abrasive particle, and dislocations in the bulk of the crystal. The addition of chemical dissolution leads to very smooth planarized copper surfaces and considerably smaller frictional forces that prevent the formation of bulk dislocations. This is a first step towards understanding the interplay between mechanistic material abrasion and chemical dissolution in chemical mechanical planarization of copper interconnects.

Modeling Wear Process of Electroplated CBN Grinding Wheel

The wear of Cubic Boron Nitride (CBN) grinding wheel directly affects the workpiece surface integrity and tolerances. This paper summarizes a combined experimental-modeling framework for CBN grinding wheel life expectancy utilized in both cylindrical and surface grinding. The presented fatigue type model is based on grit pullout mechanism and the associated state of damage percolation. The unique grit-workpiece interaction process leads to a non-uniform spatial distribution of the grit wear. The life expectancy model can be described as a function of the process parameters, grinding wheel geometry and topology, workpiece material properties, etc. The developed modeling framework will greatly enhance the understanding of electroplated CBN grinding wheel wear mechanism.Copyright

Experimental Study on Electric-Current Induced Damage Evolution at the Crack Tip in Thin Film Conductors

The time dependent temperature distribution induced by electric current heating in a double edge cracked, unpassivated thin aluminum or gold film interconnect lines is monitored using a high resolution infrared imaging system. A pure aluminum or gold film, with a thickness of 0.2 μm, is deposited by high vacuum evaporation coating and patterned into test structures of varying widths. The operative mechanisms of mass transport are assessed in view of the monitored temperature profile. The pre-cracked aluminum film shows fine crack growth towards the positive electrode, which originates from the initial crack tips. The crack-tip temperature is close to melting, during propagation. After the initial crack propagation, a hot spot is formed between the two elongated cracks, and leads to failure. The crack growth generates a backward mass flow towards the negative electrode. The gold film shows a different pattern, in which the original cracks propagate towards each other with a slight tilt towards the negative electrode. The tip temperature is lower than the melting temperature. These time dependent failure mechanisms are rationalize using a proposed critical current intensity factor and a normalized current intensity rate, similar to the fracture toughness KIC for brittle fracture.

Experimental Characterization of Electroplated CBN Grinding Wheel Wear: Topology Evolution and Interfacial Toughness

The wear rate of a grinding wheel directly affects the workpiece surface integrity and tolerances. This paper summarizes a combined experimental-modeling framework for life cycle prediction of an electroplated Cubic Boron Nitride (CBN) grinding wheel, typically utilized in nickel-based superalloy grinding. The paper presents an experimental framework to facilitate the formulation of a micro-mechanics based modeling framework. The presented work investigates the topological evolution of the grinding wheel surface and mechanisms of grit failure via depth profiling, digital microscopy and scanning electron microscopy. The results are used to elucidate the statistical evolution of the grinding wheel surface. Different modes of grit failure, including grit attritious wear, fracture and pull out haven been identified. The analysis of the surface topological features indicates a unique grit activation process, leading to a non-uniform spatial distribution of the grit wear. Additionally, single grit pull out experiment has been conducted to assess the residual strength of the grit-wheel interface and the associated state of damage percolation. The experimental results can be utilized in developing a life expectancy model for the CBN grinding wheel to assess the grit mean time to failure as well as grit surface topological evolution as a function of the process parameters.Copyright