Macroscopic Work Research Articles

Nano Forming of Glasses by Indentation

This paper reports ultra fine plastic forming (nano forming) of hard brittle materials by indentation of diamond tools. Soda glass, firelite glass and quartz glass were examined by a nano indentation tester specially designed for this study. Two types of diamond tools were used for the experiment : one was a knife edge type tool and the other was a stamp type tool which was machined by a focused ion beam equipment. Indentation tests by the knife edge tool showed that glasses are available for ductile plastic deformation where no cracks are generated when indentation depth is smaller than their critical depth. Deformation characteristics of nano forming were found to be almost similar to macroscopic plastic work even on a hard brittle material. Indentation test of the stamp type tool succeeded in fabricating dot array structures on the glasses. Deformation of work material was however not uniform but depth of dots and density of cracks were distributed in a imprinted structure. Such distributions were explained by stress variation under the tool in imprinting.

The distinction between heat and work: an approach based on a classicalmechanical model

The distinction between work and heat is obvious in most typical situations, butbecomes difficult in certain critical cases. The subject is discussed in texts onthermodynamics and has long given rise to debate. This paper presentsan approach based on a mesoscopic analysis, using a simple mechanicalmodel, in which bodies are made up of particles (representing atoms and/ormolecules) treated as material points interacting with forces that obeyNewton’s laws. The sum of the work done by these microscopic forces is splitinto two terms representing the macroscopic quantities work and heat.

Particle size effect in metallic materials: a study by the theory of mechanism-based strain gradient plasticity

It is a significant fact that the size of second-phase particles has an important effect on the macroscopic plastic work hardening behavior of metals and their alloys or metal–matrix composites. The classical plasticity theories cannot explain this size effect since their constitutive laws possess no internal material lengths. We use the theory of mechanism-based strain gradient (MSG) plasticity to investigate the particle size effect and find good agreements with the experiments of aluminum matrix reinforced by silicon carbide particles as well as with prior numerical studies by other strain gradient plasticity theories. It is shown that, at a fixed particle volume fraction, smaller particles give larger plastic work hardening of the composite than large particles do.

Modeling of large plastic deformation in crystalline polymers

When a crystalline polymer grain undergoes plastic deformation, four independent slip systems may operate with the chain direction inextensible. The inextensibility leads to texture evolution for a polycrystalline polymer under large plastic deformation. This paper develops a micromechanics model for the three-dimensional large plastic deformation of crystalline polymers. A continuous orientation distribution function is introduced to characterize the distribution of the chain axes. Restrictions on the orientation distribution imposed by material symmetry are considered. A micro–macro interaction law for deformation accounting for molecular chain inextensibility is proposed by employing tensor function representation theorems. By the equivalence between the microscopic and macroscopic work rate, an expression for the macroscopic stress is obtained. The evolution for the orientation distribution is quantified. The proposed model enables us to simulate not only the rotating process of chain axes toward the direction of the maximum stretch but also the texturing hardening under axi-symmetric straining and simple shear.

Crack velocity dependent toughness in rate dependent materials

Mode I, quasi-static, steady-state crack growth is analyzed for rate dependent materials under plane strain conditions in small scale yielding. The solid is characterized by an elastic–viscoplastic constitutive law and the plane ahead of the crack tip is embedded with a rate dependent fracture process zone. The macroscopic work of fracture of the material is computed as a function of the crack velocity and the parameters characterizing the fracture process zone and the solid. With increasing crack velocity a competition exists between the strain rate hardening of the solid, which causes elevated tractions ahead of the crack tip that tend to drive crack propagation, and the rate strengthening of the fracture process zone which tends to resist fracture. Results for material parameters characteristic of polymers show that the toughness of the material can either increase or decrease with increasing crack velocity. To motivate the model, the cohesive zone parameters are discussed in terms of failure mechanisms such as crazing and void growth ahead of the crack tip. The toughness of rubber modified epoxies is explained by employing the fracture model along with micromechanical void cell calculations.

A strain gradient-strengthening law for particle reinforced metal matrix composites

The influence on macroscopic work hardening of small, spherical, elastic particles dispersed within a matrix is studied using an isotropic strain gradient plasticity framework. An analytical solution for strain hardening, i.e. the flow stress as a function of plastic strain, based on a recently developed model for initial yield strength is proposed. The model accounts for random variations in particle size and elastic properties, and is numerically validated against FE solutions in 2D/3D unit cell models. Excellent agreement is found as long as the typical particle radius is much smaller than the material length scale, given that the particle volume fraction is not too large (<10%) and that the particle/matrix elastic mismatch is within a realistic range. Finally, the model is augmented to account for strengthening contribution from shearable particles using classic line tension models and successfully calibrated against experimental tensile data on an Al−2.8wt%Mg−0.16wt%Sc alloy.

Microscopical analysis of structure and work function of Ba-covered Si(111)-(7×7) surface

Microscopic `work function' is evaluated by local tunneling barrier height (LBH) measurements of clean and Ba-deposited Si(111) surfaces in order to clarify the mechanism of work function reduction by Ba deposition. Atomically resolved LBH images are obtained corresponding to scanning tunneling microscopy (STM) images. The difference in the LBH measurement for clean Si(111)-(7×7) and Ba deposited Si(111)-(3×1) surfaces agrees with macroscopic work function reduction due to Ba adsorbate. The result suggests that the work function is strongly related to the atomic arrangement of the Si(111) surface induced by Ba deposition.

Measurement of the local work function of dispenser cathodes using an SPM technique

An STM technique has been developed which makes possible direct quantitative measurements of the magnitude and distribution of the electronic property of surfaces known as the local work function (barrier height), φ. The technique was used, first, to measure the barrier height of several well defined surfaces for which values of the macroscopic work function, Φ, are available in the literature, viz. graphite, platinum and gold. The variation of measured barrier height (≡mean local work function φ ̄ of the tip and the sample) with tip-sample distance s was found to have a similar shape for each of these surfaces. Characteristic features of these φ ̄ vs. s dependencies were found to correlate well with the respective mean Φ values, Φ ̄ , derived from the published Φ values for the tip and the surface in question. The technique was used then to investigate an unprocessed CD-type and a processed B-type dispenser cathode. The results discriminated strongly between the two cathodes. In particular, the barrier height image of the B-type cathode was found to be rich in detail compared with that from the CD-type, evidence that in the former there are several different surface states discernible on the basis of their different electron emissive properties. The results demonstrate that the highly spatially resolving capabilities of SPM techniques are commensurate with length scales associated with surface variations of the local work function of cathode materials. The technique is a basis for investigations of cathode science and technology.

Steady-state crack growth and work of fracture for solids characterized by strain gradient plasticity

Mode I steady-state crack growth is analysed under plane strain conditions in small scale yielding. The elastic-plastic solid is characterized by a generalization of J 2 flow theory which accounts for the influence of the gradients of plastic strains on hardening. The constitutive model involves one new parameter, a material length l, specifying the scale of nonuniform deformation at which hardening elevation owing to strain gradients becomes important. Gradients of plastic strain at a sharp crack tip result in a substantial increase in tractions ahead of the tip. This has important consequences for crack growth in materials that fail by decohesion or cleavage at the atomic scale. The new constitutive law is used in conjunction with a model which represents the fracture process by an embedded traction-separation relation applied on the plane ahead of the crack tip. The ratio of the macroscopic work of fracture to the work of the fracture process is calculated as a function of the parameters characterizing the fracture process and the solid, with particular emphasis on the role of l.

Group work of distributed microactuators

SUMMARYThis paper proposes and demonstrates a method toobtain macroscopic work out of distributed microactuatorsfabricated by IC-compatible micromachiningprocesses. We have coordinated the simple and smallmotion of microactuators in order to perform a task. Theconcept and a control scheme are discussed first. In orderto show the feasibility, the fabrication and operation ofarrayed microactuators for conveyors are described. One uses thermally driven cantilevers and the other uses controlled air flow from micronozzles to carry flat objects.

Mode II fracture toughness of a brittle adhesive layer

This paper concerns the analytical estimation of the macroscopic mode II fracture toughness of a brittle adhesive layer sandwiched between and bonding together stiff substrates. The process of failure involves the propagation and coalescence of microscopic tensile cracks ahead of the macroscropic mode II crack tip. The basic problem at the heart of the analysis is the plane strain problem for a layer subject to shear and containing a periodic array of micro-cracks which grow and coalesce under the condition that their tips advance under pure mode I conditions. A numerical solution to this basic problem is obtained and is then used to make detailed predictions for conditions for tunneling of the micro-cracks and for the evolution of their shape and spacing. These predictions are used in turn to develop the shearing traction-displacement relation for the brittle adhesive layer. The work per unit length of layer needed to drive the microcracks to coalescence can be identified with the macroscopic work of fracture in mode II, ΓIIc, as is discussed via a cohesive zone model. The macroscopic model II toughness is predicted to be between three and four times the mode I fracture toughness, ΓIC, depending on constraints provided by substrates and very slightly on Poisson's ratio, ν. The theoretical predictions are compared with experimental data reported in the literature. Also discussed are the consequences of the assumption underlying the analysis that there exists an ample population of initial flaws whose largest dimension is roughly comparable to the thickness of the layer.

Elevated temperature fracture of RS/PM alloy 8009: part i. fracture mechanics behavior

Increasing temperature and decreasing loading rate degrade the plane strain initiation (KICifrom theJ integral) and growth (tearing modulus,TR) fracture toughnesses of RS/PM 8009 (Al-8.5Fe-1.3V-1.7Si, wt pct).KICidecreases with increasing temperature from 25 ‡C to 175 ‡C (33 to 15 MPa√m for an extrusion and 28 to 11 MPa√m for hot cross-rolled plate) and further declines to 10 MPaVm at 316 ‡C without a minimum.TRis greater than zero at all temperatures and is minimized at 200 ‡C. A four order-of-magnitude decrease in loading rate, at 175 ‡C, results in a 2.5-fold decrease inKICiand a 5-fold reduction inTR.KICiandTRare anisotropic for extruded 8009 but are isotropic for cross-rolled plate. Cross rolling does not improve the magnitude or adverse temperature dependence of toughness. Delamination occurs along oxide-decorated particle boundaries for extruded but not cross-rolled 8009. Delamination toughening plays no role in the temperature dependence ofKICi, however,TRis increased by this mechanism. Macroscopic work softening and flow localization do not occur for notch-root deformation; such uniaxial tensile phenomena may not be directly relevant to crack-tip fracture. Micromechanical modeling, employing temperature-dependent flow strength, modulus, and constrained fracture strain, reasonably predicts the temperature dependencies ofKICiandTRfor 8009. While E and σys decrease with increasing temperature for all aluminum alloys, the strain to nucleate crack-tip damage dominates the fracture toughness of 8009 and decreases with increasing temperature for a range of constraint. Damage mechanisms for this novel behavior are evaluated in Part II.

Photochemistry of adsorbed molecules. XII. Photoinduced ion–molecule reactions at a metal surface for CH3X/RCl/Ag(111) (X=Br, I)

A photoinduced ion–molecule reaction is reported between superimposed molecular layers of alkyl halides on a metal substrate CH3X/RCl/Ag(111) (where X=Br or I and R=CCl3, CHCl2, or CH2Cl) to form CH3Cl(ad) (wavelengths 193, 248, and 350 nm). The reaction is mediated by charge-transfer (CT) photodissociation, in which photoelectrons from the metal surface transfer to the lower layer of adsorbate RCl to form RCl−. These negative ions then react with the upper layer CH3X in an ion–molecule reaction to form CH3Cl+X−. The yield of product CH3Cl is found to be enhanced at ∼1 ML of adsorbed CH3X (upper layer) due to a decrease in the local potential in the region of the adsorbate–adsorbate interface that enhances the probability of CT to the lower layer. In addition to lowering the local potential at the interface, the adsorbed CH3X also lowers the surface work function; as a result changes in the microscopic local potential correlate (via the CT reaction rate) with changes in the observed macroscopic work function. The yield of CH3Cl decreases at still higher CH3X coverage in the upper layer as the work function increases. The ion–molecule reaction gives evidence of being a concerted process in which the Cl− reacts as it separates from RCl− rather than following separation. The reagent RCl−, as in the surface reaction discussed in the previous paper, is formed by CT from ‘‘hot’’ electrons rather than free photoelectrons.

Fabrication and operation of polyimide bimorph actuators for a ciliary motion system

In order to extract macroscopic mechanical work out of microelectromechanical systems, we have proposed the concept of distributed micromotion systems (DMMS). The key idea of DMMS is to coordinate simple motions of many microactuators in order to perform a task. Design, fabrication, and operation of a type of DMMS, called a ciliary motion system, are presented. A bimorph thermal actuator using two types of polyimides with different thermal expansion coefficients and a metallic microheater in between them was fabricated. The cantilever-shaped actuator curled up from the substrate owing to the residual stress in polyimides which built up during the cooling process after they were cured at 350 degrees C. It flattened and moved downward by flowing current in the heater. The dimensions of the cantilever were 500 mu m in length, 100 mu m in width, and 6 mu m in thickness. The tip of the cantilever moved 150 mu m in the direction vertical to the substrate and 80 mu m in the horizontal direction; these were the maximum displacements obtained with 33 mW dissipated in the heater. The cut-off frequency was 10 Hz. On a 1-cm-square substrate, 512 cantilevers were fabricated to form an array. Two sets of cantilevers were placed opposing to each other. We operated them in coordination to mimic the motion and function of cilia and carried a small piece of a silicon wafer (2.4 mg) at 27-500 mu m/s with 4-mW input power to each actuator. >

Low-energy electron diffraction observations of the thermal evolution of 0.5–3.0 ML Pt/Ni(111)

The near-surface geometric structure of the system 0.5–3.0 ML Pt/Ni(111) has been monitored with low-energy electron diffraction (LEED) as a function of annealing treatment. In this coverage range a variety of structures are documented. Evidence is presented for commensurate submonolayer Pt (despite the large misfit) and incommensurate (but aligned) second and third layer Pt, at low temperatures. The second and third layers of Pt yield (111)-type LEED patterns but with spacing at an intermediate value between Ni(111) and Pt(111). In a very narrow range of coverage and annealing treatment (1.5–2 ML PT and 673–773 K) a hexagonal satellite appears around each primary beam. These satellites are interpreted as evidence for partially rotated incommensurate second layer Pt domains. Annealing at 873 K or above of even the thickest deposits (3 ML) produces diffraction spots indistinguishable from Ni(111). Structural evidence is correlated with the thermal evolution of the surface electronic structure in the same system (using photoemission for local and macroscopic work function measurements). The photoemission results complement the LEED information and together they provide evidence for complex diffusional rearrangements even at low temperatures in this system.

Thermal evolution of very thin platinum films deposited on Ni(111): A PAX/work function study

The thermal evolution of very thin Pt films deposited on Ni(111) has been studied by photoemission of adsorbed xenon and measurements of the macroscopic work function. The observed surface electronic structure has been correlated with structural and compositional changes occurring during low temperature annealing. Evidence for a number of distinct surface domains has been found.

Plastic deformation and the flow stress of aluminium-lithium alloys

Activation analysis has been used to investigate plastic deformation in Al-Li alloys. The flow stress can be resolved into a component produced by work hardening and a friction stress produced by solid-solution or precipitation hardening. Macroscopic work softening is observed at larger strains in compression tests on precipitation hardened specimens and the results of the analysis indicate that softening is caused by a strain-induced reduction in the friction stress. It is believed that work softening results from the shear of ordered precipitates but an argument is presented to show that macroscopic work softening can only occur when Stage III work hardening is established. At higher temperatures, alloys exhibit the Portevin-Le Chatelier (P-LC) effect in both the solution treated and precipitation hardened condition, but, on the basis of the experimental evidence, it is believed that the detailed mechanism responsible is different in precipitation hardened specimens to that which occurs in the classical P-LC effect, and is associated with the shear of ordered precipitates.

Photoemission of adsorbed xenon on Pt xNi 1− x(111) single crystal alloy surfaces

Photoemission of adsorbed xenon and work function measurements have been performed on three fcc, (111) oriented, PtNi single crystal alloys showing a strong platinum surface segregation. For Pt-rich alloys, the first layer has been found a quasi-homogeneous layer but different from a pure Pt(111) surface. No platinum islands have been observed. The local and the macroscopic work function are nearly the same and do not change linearly with the alloy composition but remain nearly constant and close to 5.4 eV, a value significantly lower than that on a pure Pt(111) face (5.95 eV).

Auger electron spectroscopy, X-ray photoelectron spectroscopy, work function measurements and photoemission of adsorbed xenon on thin films of Pt-Re(111) alloys

Auger electron spectroscopy (AES), low energy electron diffraction (LEED) and measurements of the macroscopic work function and the photoemission of adsorbed xenon (PAX) were used to investigate the deposition of 0–15 monolayers of platinum onto an Re(0001) single crystal and subsequent annealing above 1000 K. During deposition, the macroscopic work function reached the bulk value after deposition of 1 monolayer. The PAX and LEED studies indicated that Pt(111) grows epitaxially on Re(0001), which is consistent with the small difference in lattice parameters. Annealing led to the formation of a homogeneous alloy whose work function varied linearly with the platinum content.

Electrostatic interactions and their role in coadsorption phenomena

A theoretical framework for comparing binding energies and activation energies for adsorption on surfaces is presented. It is applied to microscopic description of the mechanisms underlying the promotion of the molecular adsorption process on metal surfaces by coadsorbed species. Estimates of the range of this effect, its magnitude and the important factors which differentiate promotional efficiency within a particular chemical group are discussed. It is shown that while correlating surface activity with the macroscopic work function may give useful trends, it is in fact the local derivative of the electrostatic potential with respect to the surface normal which governs the free energy difference when a molecule is adsorbed in the presence or absence of a promoter.