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Deterministic contact modeling based on half-space theories has satisfied a wide range of applications. However, the half-space theories themselves do not involve shape effects of roughness on Green's functions/influence coefficients; in deterministic rough-surface contact analyses, the roughness is considered in gap function. This approach can be called the “roughness simplification”. One needs to answer two questions about the validity of the roughness simplification: How appropriate is the roughness simplification in modeling rough-surface contacts? How accurate can the commonly-included contact-plasticity behavior be captured under the roughness simplification? This work utilized a double-scale representation of an asperity--a microscopic deformable asperity stacked on a deformable half-space, to obtain their combined contact responses in both elastic and plastic regimes. The deformation and contact behaviors of asperities thus configured were obtained with finite element method (FEA) and rough-surface half-space contact solvers. Three stages of asperity contact were discovered: the Hertzian stage, the single-region elastoplastic stage, and the two-region elastoplastic stage where the surrounding base material also takes part in the contact. The comparisons of contact deformation and pressure results from both the FEA and half-space contact solvers support the validity of the half-space theories with the roughness simplification for various ellipsoid-shape asperities with circular-bases in both elastic and elastoplastic rough-surface asperity modeling. The research also reveals that when significant plastic deformations occur, asperities with different aspect ratios can bear different maximum elastoplastic contact pressures.

We report here the impact of different alloying elements in steels on friction and wear behavior by performing ball-on-flat lubricated reciprocating tribotesting experiments on 52100 ball on steel flats with different compositions (52100, 1045, A2, D2, M2, and a specialty Cu-alloyed steel) heat-treated to give similar hardness and microstructure, with polyalphaolefin (PAO-4) as the lubricant. There are small variations of coefficient of friction among these alloys. The major observation is that steels containing high concentrations (≥ 10 wt.%) of Cr, Mo, and V gave rise to markedly reduced wear compared with 52100 or plain carbon steels. D2 steel, which contains 11.5 wt.% Cr as the major alloying element was the most wear-resistant. The wear resistance is strongly correlated with the efficiency of formation of carbon-containing oligomeric films at specimen surfaces as determined by Raman spectroscopy. This correlation holds for steels heat-treated to have higher hardness and with n-dodecane, a much less viscous lubricant compared with PAO-4. Given the strong affinity of chromium to oxygen, chromium should exist as Cr2O3 at the steel surfaces during testing. We have performed molecular dynamics simulation on Cr2O3 and demonstrated its ability to catalyze the formation of carbon-containing oligomeric films from hydrocarbon molecules, consistent with its known catalytic activity in other hydrocarbon reactions. We believe that chromium-containing alloys, such as D2, and coatings, such as CrN, derive their wear resistance in part from the efficient in-situ formation of wear-protective carbon tribofilms at contacting asperities.

Under severe conditions, solid contacts take place even when parts are lubricated. Precise mathematical conditions are needed to describe the interior interface between fluid lubrication and solid-contact zones. In order to distinguish the conditions for this interface from conventional lubrication boundary conditions, they are named lubrication–contact interface conditions (LCICs). In this work, mathematical LCICs are derived with local flow continuity from the continuum mechanics point of view and pressure inequality across the interface. Numerical implementations are developed and tested with problems having simple geometries and configurations, and they are integrated into a new mixed/boundary elastohydrodynamic lubrication solver that uses a new method to determine solid-contact pressures. This solver is capable of capturing film thickness and pressure behaviors involving solid contacts.

The long-term performance of current heat-resistant ferritic steels used in steam generators is primarily limited by microstructure degradation as a result of coarsening or transformation of precipitates. To improve the microstructural stability, a heat-resistant ferritic steel was designed via computational thermodynamics to exclusively contain metal carbonitride (MX) precipitates as the primary means for strengthening at elevated temperatures. The volume fraction of precipitates is 0.35 vol pct, about one-seventh of P91. These MX precipitates are either V-rich or Nb-rich with plate-like or spheroidal morphology, respectively. The precipitate size remains almost constant at 973 K for up to 3000 hours aging. Electron diffraction analysis revealed a Baker-Nutting orientation relationship between the precipitate and the matrix. Consistent with the thermodynamics-based design, M23C6-, Laves-, and Z phase were not detected. The creep threshold stress, derived from high-temperature compressive creep tests, are evaluated to be 63 ± 1 and 43 ± 2 MPa at 923 K and 973 K, respectively, on par with or slightly better than P91. This study reveals that MX precipitates in ferritic steels coarsen slowly at temperatures up to 973 K and that a relatively small volume fraction of MX precipitates can provide effective long-term creep performance at elevated temperatures.

We recently reported the use of cyclopropanecarboxylic acid (CPCa) as a model additive that can readily react under the combined effect of flash heating and stress in steel tribocontacts to form tribopolymers, along with marked improvement in tribological performance. In this paper, we present results of how chemical structural modification of CPCa may impact on the formation of tribopolymers and hence friction and wear properties, both by experiments and molecular dynamics simulation. Four lubricant additives, viz., CPCa, cyclobutanecarboxylic acid (CBCa), cyclopropane-1,1-dicarboxylic acid (CPDCa), and cyclobutane-1,1-dicarboxylic acid (CBDCa) consisting of a metastable ring structure and one or two carboxyl groups dissolved in an ester base oil were studied. Friction and wear rate using these additives rank in the order of CPDCa

To improve safety in case of building fires, stricter building codes have been proposed requiring structural steels to maintain two-thirds of their room-temperature yield strength after exposure to 873 K (600 °C) for longer than 20 minutes. To address this need, we have designed lightly alloyed structural steels, employing computational thermodynamics in combination with fundamental principles of precipitation strengthening and its temperature dependence, precipitate stability, characterization by optical microscopy and atom probe tomography (APT), and mechanical testing at room and elevated temperatures. The design process resulted in low-carbon ferritic steels with small alloying additions of V, Nb, and Mo that maintain over 80 pct of room-temperature yield strength in compression, and nearly 70 pct in tension, after 2 hours of exposure at 873 K (600 °C). APT demonstrates the formation of nanoscale MX and M2X (where M = V + Nb + Mo and X = C + N) precipitates after exposure to 873 K (600 °C). The favorable high-temperature mechanical properties are discussed with a model of precipitation strengthening by detachment-stress-mediated dislocation pinning at nanoscale semi-coherent MX precipitates.

In this work, we explored macroscale liquid superlubricity using glycerol as the base lubricant to form aqueous solutions with different water/glycerol weight ratios. We find that glycerol aqueous solutions with water/glycerol weight ratio of 0.2 achieves superlubricity (coefficient of friction

Mechanochemical reactions induced by external stress provide a unique approach for in situ synthesis of carbon tribofilms that can improve friction and wear performance. In this work, we studied how tribofilm formation and tribological performance might be related to the adsorption strength of three additives in polyalphaolefin (PAO4) as base oil, viz., cyclopropanecarboxylic acid (CPCa), cyclopropanemethanol (CPMA), and 1-cyclopropylethanol (CPEA) as characterized by two different surface-active groups –COOH and –OH. Tribo-testing results reveal that addition of 2.5 wt% CPCa to PAO4 gave the lowest friction coefficient and wear volume. FTIR and Raman analysis demonstrate substantial tribofilm formation only in the case when CPCa was used as the oil additive, not CPMA or CPEA, in spite of the fact that all three additives contain the same metastable cyclopropane ring. Thermogravimetric analysis and molecular dynamics simulations indicate the stronger adsorption of CPCa on the iron oxide surface compared with CPMA and CPEA. Weak adsorption of the latter molecules results in their desorption from the surface due to flash heating during tribotesting before they have the chance to participate in mechanochemical reactions required for tribofilm formation. The stronger binding of CPCa to the steel surface is a necessary condition for this type of surface mechanochemistry and appears critical to the efficient formation of carbon-containing tribofilms under our tribo-testing conditions.

Diamond-like carbon (DLC) is a class of amorphous carbon materials used as wear-resistant coatings in tribo-components. The chemical inertness of DLC surfaces, while important in many applications, makes them incompatible with additives in commercial lubricants. Specifically, DLC surfaces do not permit strong adsorption of friction modifier molecules. This study aims at improving the adsorption of these friction modifier molecules on DLC surfaces through an acid treatment. Water contact angle measurements show that such a treatment results in improved hydrophilicity of the DLC. XPS analysis demonstrates 50% increase in the uptake of ArmeenT, an organic friction modifier, on DLC after the acid treatment. This increased ArmeenT adsorption is accompanied by marked decrease in friction in micro-scale friction experiments.

The demand for increased energy efficiency continuously drives the development of new lubricants. Here we report the design and synthesis of hexahydrotriazine, triazine, and cyclen derivatives as friction modifiers (FMs) for enhanced fuel economy. This series of sulfur- and phosphorus-free oil-soluble heterocyclic ring-based molecules exhibits differing thermal and chemical stability depending on the degree of aromatization and number of linking spacers within the central heterocyclic ring. Thermally stable triazine and cyclen FMs significantly increase friction performance in the boundary lubrication regime. Cyclens in particular reduce friction by up to 70% over a wide temperature range. Detailed experimental investigations of the newly synthesized FMs at elevated temperatures demonstrate their favorable tribological performance under four operating conditions: variable-temperature sliding, linear speed ramping, reciprocating sliding, and rolling–sliding contact. These latest experimental findings suggest the potential of the application of “designer” heterocyclic FMs for reducing frictional loss in motor vehicles.

Extreme pressure (EP) lubricant additives form protective tribofilms at the site of contact using the heat and pressure of contact and relative motion. Common EP additives contain undesirable elements such as phosphorus and sulfur. A novel EP lubricant additive, which contains no phosphorus and sulfur, is presented for generating lubricious carbon films. The additive consists of a surface-active molecule with a metastable cycloalkane ring, which dissociates readily during tribological contact to form lubricious carbon films. Friction and wear performance of PAO4 with this additive under a range of loads and speeds were shown to be superior to that without the additive. Optical and scanning electron microscopy and Raman spectroscopy were used to analyze the tribofilms formed on post-test contact surfaces, providing direct evidence for the formation of carbon films. Quantitative kinetics for the carbon tribofilm formation was analyzed as a function of temperature and stress, from which the activation energy for carbon tribofilm formation was obtained.

Electrophoretic deposition (EPD) is used to assemble metal-organic framework (MOF) materials in nano- and micro-particulate, thin-film form. The flexibility of the method is demonstrated by the successful deposition of 4 types of MOFs: NU-1000, UiO-66, HKUST-1, and Al-MIL-53. Additionally, EPD is used to pattern the growth of NU-1000 thin films that exhibit full electrochemical activity.

Modern lubricants face the task of providing lubrication over a wide range of temperatures, and extreme engine temperatures can exceed the thermal degradation limits of many engine oils. Soft metal additives can extend the life of engine oils at very high temperatures by providing solid lubrication to contacting surfaces. We report a new silver–organic complex which contains a high metal content and minimal supporting organic ligands. This silver pyrazole–pyridine complex is evaluated as a friction-reducing and anti-wear additive in engine oil at testing temperatures which thermally degrade the base oil. Two sets of ball-on-disk tests are performed: the first at a constant temperature of 200 °C and the second while increasing the chamber temperature from 180 to 330 °C. At 200 °C, the wear is considerably reduced compared with the base oil when the silver-organic additive is present at 2.5–5.0 wt%. Furthermore, the silver-based additive at 20 wt% in oil induces a remarkable friction reduction during the temperature ramp test, so much, so that the tribological transition from the oil as the primary lubricant to its degradation, and to the silver additive as the primary lubricant, is imperceptible.

Several new siloxane lubricants were synthesized with linear and ring-shaped branch structures of various lengths and branch contents, aiming at a search for better molecular design for lower boundary friction and more effective surface protection against wear of materials. Their molecular structure and mass were measured by means of nuclear magnetic resonance and gel permeation chromatography, respectively. The new lubricants were compared with commercially available polysiloxanes, poly-α-olefins, and perfluoropolyether in lubricating a steel ball-on-steel disk interface using a tribotester at a load of 1.76 GPa and an entrainment speed of 0.025 m/s. The results are used to explain the effects of alkyl branch length L, pendant type J, percent of branch functional monomers Q, and degree of polymerization DP on siloxane design for the most effective boundary lubrication.

This study investigates the rheologic properties, elastohydrodynamic film, and friction coefficients of several siloxane-based lubricants to assess their shear stability and their potential for energy efficient lubrication. Several siloxane-based polymers with alkyl, aryl, and alkyl-aryl branches were synthesized in order to examine the relationship between their molecular structures and tribological performance. Nuclear magnetic resonance spectroscopy and gel permeation chromatography were used to characterize the molecular structures and masses, respectively. Density, viscosity, elastohydrodynamic film thickness, and friction measurements were measured from 303 to 398 K. Film thickness and friction measurements were made at loads and speeds that cover the boundary, mixed, and full film lubrication regimes. These results illustrate that the shear characteristics of siloxane lubricants vary significantly with polymer length as well as branch structure and content. The findings provide quantitative insight into the features of siloxane molecular structure conducive to optimum film formation with minimum wear and elastohydrodynamic friction to enhance energy efficiency.

This study investigates the rheological properties, elastohydrodynamic (EHD) film-forming capability, and friction coefficients of low molecular mass poly-α-olefin (PAO) base stocks with varying contents of high molecular mass olefin copolymers (OCPs) to assess their shear stability and their potential for energy-efficient lubrication. Several PAO–OCP mixtures were blended in order to examine the relationship between their additive content and tribological performance. Gel permeation chromatography (GPC) and nuclear magnetic resonance (NMR) spectroscopy were used to characterize the molecular masses and structures, respectively. Density, viscosity, EHD film thickness, and friction were measured at 303 K, 348 K, and 398 K. Film thickness and friction were studied at entrainment speeds relevant to the boundary, mixed, and full-film lubrication regimes. The PAO–OCP mixtures underwent temporary shear-thinning resulting in decreases in film thickness and hydrodynamic friction. These results demonstrate that the shear characteristics of PAO–OCP mixtures can be tuned with the OCP content and provide insight into the effects of additives on EHD characteristics.

Advanced ultra-high strength steels are highly desirable for a wide range of engineering applications. Nanoscale co-precipitation strengthening in steels has received increasing attention in recent years and has become a new cornerstone for the development of advanced steels with superior combination of mechanical, welding, and irradiation properties for a new era. In this review, we highlight recent advances in computation-aided alloy design, nanostructural characterization, and unique properties of newly developed nanoscale co-precipitation-strengthened steels. In particular, our emphasis is on elucidating alloy design strategies, the co-precipitation mechanism, and cooperative evolution of multiple types of nanoparticles, and the correlation between nanostructures and bulk steel properties. Finally, future research areas for this class of nanostructured steels are critically discussed.

The molecular structures, rheological properties, and friction coefficients of several new siloxane-based polymers were studied to explore their traction characteristics. The molecular structures including branch content were established by nuclear magnetic resonance spectroscopy, while the molecular mass distributions were determined by gel permeation chromatography. Density, viscosity, elastohydrodynamic film formation, and friction were investigated over a temperature range of 303–398 K. Film thickness and friction measurements were studied under the conditions that are representative of boundary, mixed, and full-film lubrication regimes, aiming at maximizing traction performance and temperature stability by simultaneous optimization of the size and content of ring-shaped branch structures. This study provides quantitative insight into the effect of siloxane molecular structure on the tribological performance for traction drive applications such as continuously variable transmissions.

The mechanical properties of Cu-rich nano-cluster-strengthened ferritic steels with and without boron doping were investigated. Tensile tests at room temperature in air showed that the B-doped ferritic steel has similar yield strength but a larger elongation than that without boron doping after extended aging at 500 degrees C. There are three mechanisms affecting the ductility and fracture of these steels: brittle cleavage fracture, week grain boundaries, and moisture-induced hydrogen embrittlement. Our study reveals that boron strengthens the grain boundary and suppresses the intergranular fracture. Furthermore, the moisture-induced embrittlement can be alleviated by surface coating with vacuum oil. (C) 2010 Elsevier B.V. All rights reserved.

The steels used for structural and other applications ideally should have both high strength and high toughness. Most high-strength steels contain substantial carbon content that gives poor weldability and toughness. A theoretical study is presented that was inspired by the early work of Weertman on the effect that single or clusters of solute atoms with slightly different atom sizes have on dislocation configurations in metals. This is of particular interest for metals with high Peierls stress. Misfit centers that are coherent and coplanar in body-centered cubic (bcc) metals can provide sufficient twisting of nearby screw dislocations to reduce the Peierls stress locally and to give improved dislocation mobility and hence better toughness at low temperatures. Therefore, the theory predicts that such nanoscale misfit centers in low-carbon steels can give both precipitation hardening and improved ductility and fracture toughness. To explore the validity of this theory, we measured the Charpy impact fracture energy as a function of temperature for a series of low-carbon Cu-precipitation-strengthened steels. Results show that an addition of 0.94 to 1.49 wt pct Cu and other accompanying elements results in steels with high Charpy impact energies down to cryogenic temperatures (198 K [–75 °C]) with no distinct ductile-to-brittle transition. The addition of 0.1 wt pct Ti results in an additional increase in impact toughness, with Charpy impact fracture energies ranging from 358 J (machine limit) at 248 K (–25 °C) to almost 200 J at 198 K (–75 °C). Extending this concept of using coherent and coplanar misfit centers to decrease the Peierls stress locally to other than bcc iron-based systems suggests an intriguing possibility of developing ductile hexagonal close-packed alloys and intermetallics.

This article reports an investigation of the effect of melting and microstructure on the microscale friction of several silver–bismuth alloys using a high-temperature nanoindentation-tribotesting system. These studies showed that friction increases with temperature before melting. We modeled these results as due to the softening of the alloys with increasing temperature, which appears to adequately explain the experimental trend. The friction behavior upon melting depends on the alloy composition. For some alloy composition, friction was observed to exhibit a sharp decrease upon melting, while for another alloy composition, friction was observed to keep increasing with temperature. This unusual behavior can be explained by the difference in microstructure and phase composition as a function of temperature among different Ag–Bi alloys.

Recent experimental studies showed that the ductility of NiTi is not affected by moisture, while addition of iron beyond 9 a/o in NiTi leads to moisture-induced embrittlement. To explore the nature of this embrittlement, we studied the chemical interaction between water vapor and (Ni,Fe)Ti(110) surfaces with 5 a/o and 10 a/o Fe. Temperature-programmed desorption and X-ray photoelectron spectroscopy show that decomposition of water to produce atomic hydrogen occurs on both surfaces. Activation energy for surface diffusion was calculated by density functional theory, showing that addition of Fe decreases H surface diffusivity, in agreement with experiment. Together with the observation that addition of 9 a/o Fe increases the strength of NiTi, this indicates that moisture-induced embrittlement in higher strength NiTi alloys is not due to faster H surface diffusion, but lower critical hydrogen concentration required for embrittlement.

The attempt of a recent paper to relate grease degradation with entropy generation is analysed. The analysis shows the deficiency of such an approach.

Siloxane-based polymers (polysiloxanes) are susceptible to temporary shear-thinning that manifests as a reduction of elastohydrodynamic film thickness with increasing entrainment speed or effective shear rate. The departure from Newtonian film thickness can be predicted with the power-law exponent ns, an indicator of the severity of shear-thinning in a polymeric fluid that is influenced by the macromolecular structure. In this paper, a combination of extant rheological and tribological models is applied to determine the power-law exponent of several polysiloxanes using film thickness measurements. Film thickness data at several temperatures and slide-to-roll ratios are used to validate the methodology for several siloxane-based polymers with alkyl and aryl branches.

We have synthesized the amorphous silicon/carbon (Si/C) multilayer thin films on Cu substrate and investigated their applications as the anode for high rate rechargeable Li-ion batteries. X-ray diffraction and Raman spectrum revealed that the films were amorphous. The electrochemical evaluation was performed with constant current charge/discharge test under various loading currents in PC. The results showed a high first initial capacity of 3107 mAh g−1 and a high first coulombic efficiency of 83% at 15C rate (60 A/g). In the following cycles, a capacity of 2000 mAh g−1 at 30C (120 A/g) and 1500 mAh g−1 at 60C (240 A/g) were attained with a retention of 99%. The test predicted that the Si/C multilayer thin films were a promising anode material for lithium ion batteries.

Boron carbide (B4C) is well known for its high hardness and favorable wear resistant properties. In dry sliding wear contact, it polishes its mating surface and provides fatigue resistance to coated parts. Employing such run-in coatings demands a thorough understanding of the parameters which directly influence the changes that occur in the coating abrasiveness during the polishing process. In this study, the effects of the overall coating thickness, overall coating roughness, substrate roughness and substrate roughness orientation are examined in connection with abrasiveness. The coating thickness only influences the initial abrasiveness, whereas the coating roughness drastically affects the rate at which the abrasiveness decreases. Finally, no significant changes are observed in the abrasiveness due to substrate roughness or substrate roughness orientation effects. This work provides further insight into the design of a finite-life run-in coating.

The recrystallization and associated nanoindentation behavior of a Zr–10Al–5Ti–17.9Cu–14.6Ni (at.%) bulk amorphous alloy was investigated using in situ transmission electron microscopy and high-temperature nanoindentation. The onset of recrystallization occurs around 710 K, with the formation of Zr 2 Cu, Al 3 Zr 4 , and Ni 10 Zr 7 . Nanoindentation hardness and elastic modulus were obtained as a function of temperature from 298 to 723 K. The hardness was measured to be ∼7.5 GPa and found unchanged at temperatures below 673 K, but softened rapidly and decreased to 5 GPa at 723 K. These results can be correlated very well with the onset of recrystallization of this alloy at ∼700 K. The present study demonstrates that transmission electron microscopy, hot-stage nanoindentation, and X-ray diffraction are supplementary techniques for the study of glass relaxation and crystallization of metallic glasses.

This paper reports the effects of thermal softening on asperity contacts. We modified the thermomechanical asperity model previously developed by Liu and Wang based on discrete convolution, fast Fourier transform and the conjugate gradient method and applied it to the current study. The yield strength was assumed to be a function of local asperity temperature. Asperity contact pressure and contact area with and without such temperature effects were analyzed and compared.

Iron addition beyond 9 a/o in NiTi leads to embrittlement by water or hydrogen. To explore the nature of this embrittlement, we studied the chemical interaction between water vapor and (Ni,Fe)Ti(110) surfaces with 5 a/o and 10 a/o Fe. Temperature-programmed desorption shows that decomposition of water to produce atomic hydrogen occurs on both surfaces. X-ray photoelectron spectroscopy further demonstrates that the decomposition occurs around 190 K in both cases. A statistical model was developed to explore the effect of dopant atoms on hydrogen diffusion, assuming that diffusion depends on local composition. Dopant atoms such as boron that bind strongly to hydrogen slow down diffusion and therefore should suppress the environmental effect, consistent with atomistic simulations. Fe binds to hydrogen less strongly and therefore should result in faster diffusion. However, the present model shows that this increase in diffusivity is minimal. Therefore, this result indicates that nonchemical effects must be at work in moisture-induced embrittlement of (Ni,Fe)Ti alloys when the Fe concentration exceeds 9 a/o.

A statistical model was developed to explain effects of alloying elements on moisture-induced embrittlement in intermetallics. This model shows that if alloying elements have strong binding to atomic hydrogen, they may suppress moisture-induced embrittlement by slowing hydrogen diffusion even at low alloying element concentrations. The concentration needed to effectively slow down the diffusion process decreases with temperature. Grain boundary segregation is not a necessary condition to suppress embrittlement as long as the alloying element binds to hydrogen sufficiently strongly and its concentration is above a certain level. In the other extreme, when the binding of hydrogen to the surface is so weak that hydrogen desorption occurs simultaneously as surface diffusion, the model demonstrates that above a certain desorption rate, there may be not enough hydrogen diffusing to the crack tip to cause embrittlement.

Theoretical studies of β-C3N4 and other crystalline carbon nitride structures indicate that several crystalline phases are metastable, with high bulk modulus values. There are many reports of successful synthesis of some crystalline carbon nitride phases. In spite of questionable data analysis in a significant number of such reports, there is sufficient evidence in the literature to indicate that some nanocrystalline carbon nitride phases have been synthesised. Usually, such crystalline phases are embedded in an amorphous carbon nitride matrix. Amorphous carbon nitride films can be synthesised readily using many different techniques and are dominated by sp2 carbon bonding, along with strong evidence of carbon-nitrogen bonding. Above a certain deposition temperature, fullerene-like structures can be formed with high hardness and excellent elastic recovery in nanoindentation testing. These thin films display excellent wear performance compared with carbon and hydrogenated carbon films under a wide ...

The interaction between water vapor (D2O) and boron-modified Ni3(Al,Ti)(110) was investigated using temperature-programmed desorption and X-ray photoelectron spectroscopy. Boron dosing was performed using a solid-state boron ion source. Results showed that boron reacts with water to form hydroxyls at temperatures as low as 130 K. D2 desorption occurs at ∼950 K from boron-modified Ni3(Al,Ti)(110), indicating strong B–D bonding. Surface diffusion coefficients of atomic D, obtained from dissociation of water, on clean and boron-modified Ni3(Al,Ti)(110) surfaces were measured at 270 K with electron-stimulated desorption. The surface diffusion coefficient of atomic D on 0.05 monolayer boron-modified surface is about 10 times smaller than that on the boron-free surface. This slower diffusion of atomic hydrogen may explain why boron improves the ductility of polycrystalline Ni3Al in moist environments.

Possible reasons for the high ductility of Ni3Fe and its insensitivity toward the testing environment have been investigated. Thermal desorption experiments have shown that water dissociates on clean Ni3Fe surfaces to produce atomic hydrogen. Ductility measurements of cast and cold-rolled polycrystalline Ni3Fe demonstrated that the reduced ductility was obtained only when the testing was performed with oxygen carefully removed. X-ray photoemission studies indicate that oxygen interacts with water to form hydroxyls, thereby suppressing the production of atomic hydrogen. Hydrogen desorption from the Ni3Fe surface requires a lower activation energy, resulting in a smaller surface hydrogen concentration at a given temperature. Hence it is possible that there is insufficient hydrogen to cause the nucleation and growth of brittle cracks for severe embrittlement.

Highly pure, ultralong, and uniform-sized semiconductor nanowires in bulk quantity were synthesized by thermal evaporation or laser ablation of semiconductor powders mixed with oxides. Transmission electron microscopy study shows that decomposition of semiconductor suboxides and defect structures play important roles in enhancing the formation and growth of high-quality nanowires. A new growth mechanism is proposed on the basis of microstructure and different morphologies of the nanowires observed.

Single- and dual-cathode DC magnetron sputtering was used to produce TiB 2 coatings and CN x /ZrN multilayers, respectively, with hardness exceeding 40 GPa. The composition, structure, topography, and mechanical properties were determined by various techniques, including Auger electron spectroscopy, X-ray diffraction, high-resolution electron microscopy, atomic force microscopy, and nanoindentation. An optimum combination of the sputtering pressure and substrate bias results in the production of ultrasmooth TiB 2 coatings with hardness up to 50 GPa and excellent wetting properties. The rationale for studying the CN x /ZrN system is that ZrN(111) provides excellent lattice match to the hypothetical β-C 3 N 4 (0001) face (β-C 3 N 4 was predicted to have mechanical properties comparable to diamond). Using a dual-cathode sputtering system, we produced crystalline multilayers of CN x /ZrN with bilayer thickness of 1-2 nm. Using various combinations of nitrogen partial pressure, target powers, and substrate bias, we found that the hardness of these coatings correlates very strongly with the occurrence of (111) texture of ZrN, consistent with the lattice-match strategy. Even with a ZrN volume fraction of 70 percent, such multilayer coatings have been synthesized with hardness in the 50 GPa regime.

Titanium diboride has many interesting physical and chemical properties that make it attractive as a tribological coating material. We focus our study on the relationship between hardness, crystal structure and processing parameters for TiB2 thin films produced by conventional and ionized magnetron sputtering. When synthesized by conventional magnetron sputtering, TiB2 films with the highest degree of crystallinity have the highest hardness and are obtainable at an optimum combination of argon pressure and substrate bias. The films also show strong (0001) texture. Such high degree of crystallinity can be obtained without substrate bias by ionized magnetron sputtering, in which enhanced ionization of the plasma is obtained by inductively coupling RF power in the region between the magnetron target and the substrate.

Two common tribo-failure mechanisms are discussed in this paper.The first mechanism deals with the role of asperity flashtemperature in scuffing (severe adhesive wear) failure. Thesecond mechanism is concerned with the generation andaccumulation of wear particles in the contact zone in initiatingtribo-failure. These failure mechanisms can be suppressed bycontrolling the surface geometry, incorporating the properlubricant package and surface modification.

Siloxane-based polymers (polysiloxanes) exhibit a range of volume, viscosity, and pressure-viscosity behaviors that are strongly influenced by the macromolecular structure. In this report, a combination of extant rheological models is applied to develop a molecular-rheological modeling formalism that predicts polysiloxane rheological properties, such as specific volume, which means density, viscosity, and pressure-viscosity index variations with macromolecular structure, pressure, and temperature. Polysiloxane molecular features are described in terms of alkyl branch length L, pendant type J, density of branch functional monomers Q, and degree of polymerization DP. Both new and published data are used for model parameter determination and validation. Several siloxane-based polymers with alkyl, aryl, alkyl-aryl, cycloalkyl, and halogenated branches were synthesized to examine the modeled relationship between their molecular structures and rheological behaviors.

The interaction of water vapor and oxygen with polycrystalline Ni3Fe has been studied by Auger electron spectroscopy, X-ray photoelectron spectroscopy (XPS) and thermal desorption. Water adsorbs molecularly on the surface at or below 150 K. Between 150 and 200 K, some adsorbed water molecules desorb in vacuum, while others dissociate into hydroxyls, atomic oxygen, and hydrogen. XPS studies suggest iron to be the active species in controlling the dissociation of water on polycrystalline Ni3Fe surface. Water dissociation into atomic hydrogen is greatly suppressed by the presence of small concentrations of oxygen in water vapor.

Lubricant effects in the friction transition from boundary to microelastohydrodynamic lubrication were investigated by using a ballon-flat tribotester at sliding speeds from 0.02 to 0.88 mm/sec. Three lubricants—cyclophosphazine (X-IP), poly-alpha-olefin (PAO) and Z-DOL—were used, in this investigation. When X-IP was used at room temperature, a drop in friction coefficient from 0.22 to 0.12 at sliding speeds ≥0.10 mm/sec. (an unusually low speed) was observed, accompanied by a rise in the contact electrical resistance across the ball-fiat interface. The friction drop did not occur at temperatures ≥100°C. The friction transition was achieved at lower speeds when sliding perpendicular to the surface roughness texture. No transition occurred when PAO and Z-DOL were substituted as the lubricant. The latter him lubricants were working in the boundary lubrication regime as indicated by the contact resistance measurement. Presented at the 51st Annual Meeting in Cincinnati, Ohio May 19–23, 1996

Effects of the thermally grown wustite on the fatigue crack initiation and growth in HSLA steel are evaluated as a function of oxide thickness, strain amplitude, and gaseous environment in the push-pull plastic strain control mode, with special attention being given to the early stage of microcrack initiation. Specimens with a wustite surface layer thermally grown to 0.2 and 0.6 μm thicknesses show predominantly intergranular cracking at plastic strain amplitudes of 5×10−4 and 1×10−3 both in air and in ultrahigh vacuum (UHV), in contrast to the as-polished specimens where slip band cracking is the favoured mode. The cracking mode in the oxide layer is discussed in terms of the strain amplitude and the dislocation behaviour near the oxide/metal interface. The features of microcrack initiation in the oxide layer is not affected by the gaseous environment. Once, however, the surface oxide fractures, the rate of crack growth through the base metal is greatly reduced in UHV.

Nanoindentation hardness and compressive stress in amorphous carbon nitride thin films prepared by unbalanced magnetron sputter-deposition were studied. The coating hardness and compressive stress were found to be strongly dependent on processing parameters such as substrate bias and nitrogen partial pressure. Under optimized deposition conditions, carbon nitride thin films with nanoindentation hardness about 25 GPa have been coated onto Si wafers and M2 steels. A strong correlation between coating hardness and compressive stress in the coating was observed.

Dry sliding experiments of 3004 aluminum pins against H13 steel disks were performed at a load of 98 mN, sliding speed of 1 m/s and fixed sliding distance of 600 meters under vacuum and air with different relative humidities. Friction, microscopy and energy-dispersive X-ray studies showed that friction and wear increase with relative humidity and that material transfer occurs from the pin to the disk and vice-versa. The increase of friction and wear with relative humidity is believed to be due to the formation of oxide particles acting as hard third bodies ploughing into both contacting surfaces. Since oxidation is suppressed in vacuum, the relatively low wear in vacuum (compared with regular air ambients) is explained by the absence of such hard oxide particles. Presented as a Society of Tribologists and Lubrication Engineers paper at the STLE/ASME Tribology Conference in New Orleans, Louisiana, October 24–27, 1993

The interaction of water vapor with single crystal Ni 3 (Al, Ti) has been studied with Auger electron spectroscopy, photoemission spectroscopy and thermal desorption. Water adsorbs molecularly onto the (111) and (100) surfaces at 140 K. When warmed up to room temperature, water remains as an intact molecule on (111), but dissociates to surface hydroxyl and atomic oxygen on (100). The latter process is accompanied by the evolution of hydrogen and oxidation of Al. It appears that the dissociation of water is structure-sensitive and that Al is the active species in controlling the dissociation of water on Ni 3 Al surfaces.

This study investigates the rheological properties, elastohydrodynamic film thickness, and friction coefficients of several commercially available polyalphaolefin (PAO) and polydimethylsiloxane (PDMS)-based lubricants to assess relationships between molecular structure and lubricant performance. Molecular structures and masses were determined by nuclear magnetic resonance spectroscopy and gel permeation chromatography, respectively. Density and viscosity are measured from 303 to 398 K, while elastohydrodynamic lubricant film thickness and friction measurements were made at temperatures, loads, and speeds that are representative of boundary, mixed, and full-film lubrication regimes. The results show that PDMS-based lubricants are thermally and oxidatively more stable than PAOs, while the viscosity of PDMS-based lubricants is generally less temperature sensitive than PAOs, except for highly branched polysiloxanes. In particular, this study provides quantitative insight into the use of PDMS-based lubricants to obtain low friction through the entire lubrication regime (boundary to full film) by optimal tuning of the molecular mass and chain branching.

Low cycle fatigue behaviors in Nb-bearing high strength, low alloy (HSLA) steel have been studied at plastic strain amplitudes Δϵ p /2 ranging from 1 × 10 −5 to 1 × 10 −2 in air and in ultrahigh vacuum (UHV). Microcracks are initiated along slip lines both in air and in UHV, and grain boundaries act as effective barriers to the propagation of microcracks. Coffin-Manson relations are reasonably satisfied for the microcrack initiation life N i and the fatigue life N f both in air and in UHV. The accumulated total cyclic plastic strain energy W i required for fatigue crack initiation and the accumulated total cyclic plastic strain energy W f required for fatigue failure are expressed mathematically in the form of K ( Δϵ p /2) N . The computed values of W i and W f are in good agreement with the values determined experimentally. The vacuum environment enhances N i , N f , W i and W f . The environmental effect becomes greater as the plastic strain amplitude decreases.

Carbon nitride (CNX) thin films were prepared by DC magnetron sputtering of a graphite target in a nitrogen ambient onto Si(100) and M2 steel substrates held at ambient temperatures. All CNx coatings grown to a thickness of 1.5 μm are adherent and smooth. Nanoindentation studies showed that amorphous CNx thin films have hardness and effective Young's modulus comparable to those of amorphous hard carbon. In dry sliding against 52100 steel, CNx-coated M2 substrates give initial friction coefficients ˜0.1, rising to around 0.5 in the steady state. The wear life of coated M2 steels varies inversely with normal load, rising to about two orders of magnitude higher than that of uncoated M2 at a normal load of 1.3N and a sliding speed of 0.12 m/s. This improvement is attributed to the high yield strength of carbon nitride coatings. Presented as a Society of Tribologists and Lubrication Engineers paper at the STLE/ASME Tribology Conference in New Orleans, Louisiana, October 24–27, 1993