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The thermal conductivity of a metallic test piece is one of the principal parameters that influence the temperature buildup at tribocontacts and this normally plays an important role in the unlubricated dry sliding wear of metallic materials. It is, however, not clear whether thermal conductivity is an equally important parameter in the case of wear of metals at cryogenic temperatures, in particular, at liquid nitrogen temperature ([LN.sub.2]) of -196[degrees]C. In order to assess the influence of such a physical property of selected nonferrous metals on their tribological behavior in the [LN.sub.2] environment, we have studied the friction and wear properties of high purity copper (Cu) and titanium (Ti) against the bearing grade steel. These two materials have been processed to produce samples of comparable hardness that have widely different thermal conductivities at room temperature and at test temperature. Wear tests were conducted at three different sliding speeds (0.89 m/s, 1.11 m/s, and 1.34 m/s) under 10 N load, and the friction and wear data were compared. Ti exhibited an order of magnitude higher wear rate (~[10.sup.-3] [mm.sup.3]/N m) as compared with Cu in identical test conditions. While evidences of abrasive wear and adhesive wear, without any oxidative wear, were found in worn Cu surfaces, worn Ti surfaces showed evidences of significant oxidative wear and mechanical damage of tribolayers. Higher wear rate in Ti appeared to be a result of oxidative wear of Ti, which seemed to be driven by the depletion of [LN.sub.2] blanket at the tribocontacts under the influence of high flash temperature (14-76 [degrees]C) as compared with the boiling temperature of [LN.sub.2] (-196[degrees]C). These results demonstrate that the materials with similar hardness subjected to identical [LN.sub.2] wear test conditions can have significantly different wear rates because of the difference in the flash temperatures, which depend on the thermal conductivity of the test pieces. [DOI: 10.1115/1.4002503] Keywords: cryogenic wear, titanium, copper, flash temperature

Copper-tungsten composite materials are developed for applications such as electrical contacts, resistance electrodes, and contact tips in welding guns as well as for components requiring higher wear resistance. In addition to the aspect of improved performance, it is scientifically interesting to assess the tribological properties, and therefore the objectives of the present work include, to determine the role of W additions in improving the fretting wear resistance of Cu for electrical applications, to determine the optimum concentration for W additions, and to identify the mechanisms responsible for fretting wear improvements. In addressing these issues, a planned set of fretting wear tests were conducted on powder metallurgically processed Cu-W composites (maximum W content of 20 wt %) against steel counterbody under varying load (up to 10 N) for 10,000 cycles. It has been observed that at lower loads of 2 N, the coefficient of friction (COF) recorded was ~0.9 for the Cu-20 wt % W/steel, whereas it was ~0.85 for a pure Cu/steel couple. Under similar operating conditions with the increase in load, the COF decreases to 0.5 at 10 N load, irrespective of the composition of the Cu-W composite. Furthermore, the incorporation of 5 wt % W has reduced the volumetric wear loss by 4-6 folds in comparison to unreinforced Cu. The addition of even higher percentage of W has led to increase its wear resistance by ~10 folds. Under the investigated conditions, the wear rate systematically decreases with the increase in load for all the tested Cu-W composites. Based on the topographical observation of worn surfaces, it is observed that wear mechanisms for the Cu and Cu-W composites are tribochemical wear, adhesive wear, and abrasive wear. The incorporation of harder W particles (5 wt % or more) help in abrading the steel ball and in forming a dense tribolayer of [Fe.sub.x][O.sub.y], which effectively reduces wear rate and hence, increases wear resistance of the Cu-W composite surface in reference to unreinforced Cu. [DOI: 10.1115/1.3204776] Keywords: Cu, W, repeated pressing and sintering, fretting, wear, tribolayers, powder metallurgy

The crosstalk effects in single- and double-walled carbon-nanotube (SWCNT and DWCNT) bundle-interconnect architectures are examined. The studies have shown that, compared with the SWCNT bundle, the DWCNT bundle interconnect has led to a reduction of crosstalk-induced time delay, while the peak voltage of the crosstalk-induced glitch in SWCNT and DWCNT bundle interconnects is in the same order as that of Cu wires.

As in other P-type ATPases, metal binding to transmembrane metal-binding sites (TM-MBS) in [Cu.sup.+]-ATPases is required for enzyme phosphorylation and subsequent transport. However, [Cu.sup.+] does not access [Cu.sup.+]-ATPases in a free (hydrated) form but is bound to a chaperone protein. [Cu.sup.+] transfer from [Cu.sup.+] chaperones to regulatory cytoplasmic metal-binding domains (MBDs) present in these ATPases has been described, but there is no evidence of a proposed subsequent [Cu.sup.+] movement from the MBDs to the TM-MBS. Alternatively, we postulate the parsimonious [Cu.sup.+] transfer by the chaperone directly to TM-MBS. Testing both models, the delivery of [Cu.sup.+] by Archaeoglobus fulgidus [Cu.sup.+] chaperone CopZ to the corresponding [Cu.sup.+]-ATPase, CopA, was studied. As expected, CopZ interacted with and delivered the metal to CopA MBDs. [Cu.sup.+]-loaded MBDs, acting as metal donors, were unable to activate CopA or a truncated CopA lacking MBDs. Conversely, [Cu.sup.+]-loaded CopZ activated the CopA ATPase and CopA constructs in which MBDs were rendered unable to bind [Cu.sup.+]. Furthermore, under nonturnover conditions, CopZ transferred [Cu.sup.+] to the TM-MBS of a CopA lacking MBDs. These data are consistent with a model where MBDs serve a regulatory function without participating in metal transport and the chaperone delivers [Cu.sup.+] directly to transmembrane transport sites of [Cu.sup.+]-ATPases. CopA | CopZ | Cu homeostasis | Cu-ATPase | metal binding

Experiments are conducted to explain the microstructure and mechanical property characterizations of the metal foils after the microscale laser dynamic forming (LDF). The analysis shows that the high strain rate microforming process exhibits improved formability and hence can be used as an important microforming technology for a large number of materials.

Various test patterns are used to analyze the stress migration (SM) behavior in narrow copper (Cu) damascene interconnects, connecting in narrow or wide lines. The results of the analysis shows that the SM phenomenon in such lines is highly affected by the placing of the vias at the edge of the M1 line, which is then used to explain the effect of the via arrangement close to the edge of the lower lines on the SM failure in the interconnects.

The instrumented nanoindentation test is used to investigate the mechanical properties and deformation behaviors of copper with different grain sizes. The results have shown that the critical shear stresses for the initiation of plastic deformation in the copper specimens with large grain sizes are close to the theoretical value and comparatively much lower for electroless copper films with an ultrafine grain size.

Quantum mechanical calculations are used to study the relative Cu(super 2+)/Cu(super +) reduction potentials of six type -1 copper sites. Studies reveal that in human ceruloplasmin, the reduction potential is increased due to the structural distortion of equatorial ligand orientation.

Digenite quantum dots are synthesized by using a single source precursor type process and the optical transitions and dynamic properties of the photoinduced charge carriers in these novel nanomaterials characterized by femtosecond time-resolved spectroscopy. Results reveal that the larger quantum dots have longer excited-state lifetimes, which implies a strong surface-induced effect on the relaxation dynamics.

Copper(I) and silver(I) Fischer carbenes are synthesized in the gas phase. Trapping of the Fischer carbene by various functional groups attached through the open coordination site of the metal is investigated.

The magnetic properties of heat-treated, copper (Cu)-containing high-strength low-alloy (HSLA-100) steel and the variations of the magnetic properties during tensile deformation were studied. For samples aged at temperatures below 775 K, precipitation of nanosized coherent Cu particles occurred, causing an increase in hardness but a decrease in coercivity. For higher aging temperatures, the Cu precipitates coarsened and then were able to pin domain walls. As a result, magnetic coercivity increased. During tensile deformation of selected heat treated samples, the interaction of dislocations with the coherent and incoherent Cu precipitates influenced the magnetic properties. Systematic variation of coercivity with plastic strain was found for brittle and ductile types of fracture of the materials. Index Terms--Fracture, high-strength low-alloy steel (HSLA), magnetic properties, nondestructive evaluation, plastic deformation.

A high-speed tensile facility (HSTF) is applied to interrupting the tests for pure copper specimen bars controlled locally at different levels of elongation. The results of scanning electron microscopical (SEM) investigation of the recovered pure copper specimens have shown that the void evolution near the surface of the minimum cross-section of the necking area is more severe than that at the middle of the necking area.

The micromechanisms related to ductile failure during dynamic loading of nanocrystalline Cu are examined in a series of large-scale molecular dynamics simulations. The studies have shown that higher impact velocities have resulted in higher strain rates and higher values of spall strengths for the metal as well as nucleation of larger number of voids in smaller times.

Molecular dynamics simulations are used for examining the mechanical deformation of bicrystalline metallic nanowires with different interfaces. The impact of interfacial microstructure on the mechanical behavior is elucidated by exploring the stress-strain property and the stress concentration along the tensile direction.

Several experimental studies are conducted to determine the resistivities and microstructures exhibited by the 50 nm wide Cu wire that is prepared by the high heating rate. The resistivity of the wires is shown to highly decrease with an increase in the annealing temperature.

Molecular dynamics simulations are used for examining the shock response of idealized hexagonal columnar nanocrystalline Cu, including plasticity, local shear, and spall damage during dynamic compression, release and tension. Shock loading is applied along three principal directions of the columnar Cu sample, which has displayed a strong anisotropy in the response to shock loading and release.

The stress evolution model (SEM) is used for calculating the void nucleation time in a large number of short interconnects. The assumption of a lognormal grain size distribution and a normal distribution of diffusion activation energies have shown the nucleation time distribution to be close to lognormal.

Electromigration results are described for a ducal damascene structure with copper metallization and a low-k dielectric material and it is shown that the threshold failure time has scaled differently with current density from the median time to failure. The observed current density dependency, along with scanning electron microscopy cross sections of stressed samples and Monte Carlo simulations of failure distributions, has shown that both void nucleation and void growth is considered for accurate modeling of the electromigration lifetime.

The resonant-ultrasound spectroscopy is used for observing very large and rapid evolution of elastic constants in deposited copper and silver films. Diffusion of atoms along the grain boundary region is one of the reasons for the evolution and it is shown that the activation energy is much smaller than that for grain-boundary diffusion in bulk materials.

First-principles study is used for analyzing the copper adhesion on Ta(110) and Ru(0001) surface with the density functional theory. The results have shown that Cu adsorbed on Ta(110) surface is desorbed and agglomerated on the top of the second Cu atom layer.

The presence of a boundary-localized mode with a phase velocity slightly below the Rayleigh wave velocity on the copper film observed near a linear boundary between copper and silica films on a silicon substrate is reported. The existence of the localized mode is attributed to the nonuniformity of the copper film thickness near its edge which results from polishing during fabrication.

The shock Hugoniot of single copper is described for stresses below 66 GPa and symmetric impact experiments are used for measuring the Hugoniots of three different crystal orientations. The studies have shown that at the lowest stresses, the sound speed in Cu is extracted from the photonic doppler velocimetry (PDV) data.

The spall damage of single crystal Cu under supported and decaying shock wave loading is examined by using molecular dynamics simulations. Multiple spall events are shown to be independent of each other at the early stage of spallation and the method has worked better for large target-to-flyer plate thickness ratio R (Taylor waves) than for small R (square waves).

Several deformability measurements are conducted to explain the impact of the superimposed ultrasonic vibration on the plastic deformation of the pure polycrystalline Cu. An increase in the ultrasound (US) power is shown to highly increase the residual softening effect of the system.

The significantly improved permeability and resonance frequency characteristics of metamaterial constructed by a Cu-wire coil wound on a ferrite core and controlled through varying the number of coil turns, N are reported. The special permeability spectrum shape and the dependence of permeability parameters on N for the metamaterial are demonstrated via use of an equivalent lumped parameter model.

The study results of a systematic atomic-scale computational analysis of strain relaxation mechanisms and the related defect dynamics in nanometer-scale thin or ultrathin Cu films that are subjected to a broad range of biaxial tensile strains are reported. The defect nucleation mechanisms and the high-strain response of the thin films are found to be significantly different from those observed in -oriented Cu thin films.

The first dynamic study of damage mechanisms in nanosized on-chip Cu interconnects caused by stress-induced voiding in advanced integrated circuits is described. The void growth rate has depended on the Cu thin film material and its microstructure and the Cu microstructure in the surroundings of the formed void has showed that Cu grains are (111) oriented to the wafer surface.

Creep tests are conducted on a nanocrystalline Cu at room temperature and the results at very low strain rates are consistent with Coble creep. The stress-strain rate behavior of the nanocrystalline Cu has shown that as the strain rate decreases, the deformation mechanism has changed from predominantly dislocation activity to diffusion Coble creep, as observed by the strain rate sensitivity on stress and activation volume.

The drift/diffusion model describing the effect of the transport and accumulation of copper ions on the time of failure of dielectrics is used for including metallic barriers and the effect of an elastic drift term. The change in the value of the field acceleration parameter is linked to many factors including the transport of oxidizing species through the barrier responsible for copper ion formation and the injection and motion of electrons and holes, which are changed in the presence of a barrier between the metal and dielectric.

Submicrosecond time scale underwater electrical explosions of planar Cu and Al wire arrays are studied by using hydrodynamic simulations. The studies have shown that electrical explosion of the Al wire array has allowed almost double the energy to be transferred to the water flow due to efficient combustion of the Al wires.

Severe plastic deformation processing of metals has created bulk nanostructured materials with a hierarchy of internal interfaces. Nonequilibrium grain boundaries, nonequilibrium triple junctions and microvoids/microcracks have made the percolating network of ultrafast diffusion channels, which has represented a newly recognized feature of severely deformed materials.

Silylated mesoporous silica (MPS) films capped with a Cu overlayer fracture near the Cu/MPS interface at a distance has correlated with the Cu penetration depth into MPS. Silylation has lowered the tensile stress and the fracture toughness of MPS films, but the extent of decreases in these properties has decreased the overall driving force for cracking.

Various approaches are used for measuring the strength of metals at ultrahigh strain stress, where the issues including heating due to plastic work and the effect of the superimposed hydrostatic stress on the strength are discussed. The effect of hydrostatic pressure in ramp loading experiments is similar for all three materials, but the effect is much greater in pressure-shear experiments for aluminum than the other materials.