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Modern automotive engines operate at higher power densities than ever before, driving a need for new lubricant additives capable of reducing friction and wear further than ever before while not poisoning the catalytic converter. Reported in this paper is a new class of molecular friction modifier (FM), represented by 1,4,7,10-tetradodecyl-1,4,7,10-tetraazacyclododecane (1a), designed to employ thermally stable, sulfur- and phosphorus-free alkyl-substituted nitrogen heterocycles with multiple nitrogen centers per molecule. The multiple nitrogen centers enable cooperative binding to a surface which provides strong surface adsorption and lubricant film durability in the boundary lubrication (BL) regime. A 1 wt % loading of the cyclen FM 1a in Group III base oil exhibits strong surface adsorption, leading to excellent reductions in friction (70%) and wear (95%) versus the pure Group III oil across a wide temperature range. The lubricant with the new FM additive also outperforms two commercially available noncyclic amine-based FMs and a fully formulated commercial 5W30 motor oil.

The development of oxide dispersion strengthened ferrous alloys has shown that microstructures designed for excellent irradiation resistance and thermal stability ideally contain stable nanoscale precipitates and dislocation sinks. Based upon this understanding, the microstructures of conventionally manufactured ferritic and ferritic-martensitic steels can be designed to include controlled volume fractions of fine, stable precipitates and dislocation sinks via specific alloying and processing paths. The concepts proposed here are categorized as advanced high-Cr ferritic-martensitic (AHCr-FM) and novel tailored precipitate ferritic (TPF) steels, which have the potential to improve the in-reactor performance of conventionally manufactured alloys. AHCr-FM steels have modified alloy content relative to current reactor materials (such as alloy NF616/P92) to maximize desirable precipitates and control phase stability. TPF steels are designed to incorporate nickel aluminides, in addition to microalloy carbides, in a ferritic matrix to produce fine precipitate arrays with good thermal stability. Both alloying concepts may also benefit from thermomechanical processing to establish dislocation sinks and modify phase transformation behaviors. Alloying and processing paths toward designed microstructures are discussed for both AHCr-FM and TPF material classes.

Most hard coatings are based on ceramic materials and are generally brittle. It is desirable to have coatings that are both hard and tough. Here, we review several strategies that can be employed to increase coating toughness while maintaining hardness. Various nanocomposite and multilayer coatings (Ti/TiB 2 , FeMn/TiB 2 , Fe/VC and W/VC) were synthesized to explore three such toughening strategies: coherency strain, transformation toughening, and nanograined metals. Practical methods used to measure coating toughness in this work were presented: scratch testing, nanoindentation, and modified Vickers. Results demonstrate that coating systems that exploit these strategies show significantly enhanced toughness compared with those that do not. In particular, the strategy of using nanolayers of a metal with high elastic modulus alternating with spacer layers much thinner than the metal appears to be the most effective. In principle, one can reach hardness values up to 10% of the elastic modulus, while attaining toughness comparable to most nanocrystalline metals. Given that most metals with high elastic moduli are refractory materials, such coatings may also be useful for high-temperature applications.

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.

High-strength low-carbon ferritic steels attaining a maximum yield strength of 1600 MPa by combined Cu and NiAl precipitation-strengthening were developed. The yield strength of the alloys increases monotonically with the total concentration of the principal alloying elements i.e. Mn, Cu, Ni and Al. At 12.40 at.%, a 1600 MPa yield strength is achieved after solution treatment at 950 °C followed by aging at 550 °C for 2 h. For all three alloys investigated, the hardness reached a maximum after 1–2 h aging at 500–550 °C. At peak hardness, the combined precipitation of the body-centered cubic (bcc) Cu-alloy and B2-ordered NiAl-type intermetallic precipitates is observed by atom probe tomography (APT). The morphology, composition and structure of the Cu-alloy and NiAl-type precipitates were characterized using APT and transmission electron microscopy, as a function of aging time at 550 °C. In peak hardness conditions, the equiaxed bcc Cu-alloyed precipitates contain substantial amounts of Fe and are enriched in Ni, Al and Mn. Ni and Mn segregate at the Cu-alloy precipitate/ferrite matrix interface. In addition to the segregation, B2 NiAl-type precipitates nucleate at the Cu-alloy precipitates. After aging for 2 h, most Cu-alloy precipitates have a NiAl-type precipitate attached to their side. On subsequent further aging, the Cu-alloyed precipitates enrich progressively with Cu and elongate, indicating a transformation to the 9R or face centered cubic structure. The Cu-alloyed precipitates coarsen slower than the NiAl-type precipitates due to interfacial energy differences between the two types of precipitates, slower diffusion kinetics of Cu through the NiAl precipitates, different matrix equilibrium solubility and solute transfer from Cu-alloyed precipitates to NiAl-type precipitates. The relatively slow growth and coarsening of Cu-alloyed precipitates are consistent with the observation of an only modest decrease of hardness with extended aging.

While positive correlation between fracture toughness and H/E and H3/E2 has been reported for some ceramic coatings, we outline an argument that such correlation cannot be true in general, especially for nanolayer and nanocomposite coatings that present multiple interfaces to impede crack growth. Literature data are presented to support this argument.

The objective of this research is to characterize the elastic modulus, hardness, fracture toughness, and structure of multilayer coatings consisting of alternating nanolayers of Fe 1 − x Mn x and TiB 2 coatings ( x = 0, 0.18, and 0.35). These coatings were synthesized by dc magnetron sputtering. X-ray diffraction showed that Fe 0.82 Mn 0.18 contains 79% bcc and 21% fcc phases, while Fe 0.65 Mn 0.35 contains close to 100% fcc phase. The hardness of these multilayer coatings was found to exhibit a small enhancement (~ 2 GPa) over the rule-of-mixture values. The most striking finding is that the fracture toughness of Fe 0.82 Mn 0.18 /TiB 2 is about twice of that for Fe 0.65 Mn 0.35 /TiB 2 and Fe/TiB 2 with comparable hardness. In addition, Fe 0.82 Mn 0.18 /TiB 2 exhibits a quasi-elastic response in nanoindentation experiments. Given that Mn addition to Fe is known to result in the formation of metastable fcc phases at room temperature (with the degree of metastability controlled by the Mn content) and that the fcc phase may transform to the bcc phase under stress, such a transformation is likely to play a role in the increased toughness and quasi-elastic nanoindentation response observed in these coatings and may provide a strategy in the synthesis of hard coatings with improved toughness.

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.

We have synthesized Ti–TiB 2 nanocomposite coatings by dual-cathode magnetron sputtering in argon onto polished titanium and silicon substrates with the TiB 2 volume fraction varying between 6 and 45%. Over this volume fraction range, TiB 2 is present as nanoparticles with diameter ~ 3 to 5 nm. When deposited on titanium substrates, the coatings are relatively stress-free. Through most of the composition range, the coating hardness is enhanced over the rule-of-mixture value by 5 to 10 GPa. We used two methods to determine coating fracture toughness: nanoindentation and nanoscratch. Results from these two methods are consistent and demonstrate that Ti–TiB 2 nanocomposite coatings exhibit a “flatter” hardness–toughness relationship compared with metal carbide/DLC and TiN/ a –Si 3 N 4 nanocomposite coatings, i.e. , the hardness decreases slower with increasing toughness. We speculate that the improved toughness of Ti–TiB 2 nanocomposite coatings is due to the formation of coherent Ti–TiB 2 interfaces, which in turn produce stress fields activating the motion of nearby dislocations within the Ti matrix and hence improved fracture toughness.

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.

One of the pressing needs for the UV-Vis is a design to allow even larger mirrors than the JWST primary at an affordable cost. We report here the results of a NASA Innovative Advanced Concepts phase 1 study. Our project is called A Precise Extremely large Reflective Telescope Using Reconfigurable Elements (APERTURE). The idea is to deploy a continuous membrane-like mirror. The mirror figure will be corrected after deployment to bring it into better or equal lambda/20 deviations from the prescribed mirror shape. The basic concept is not new. What is new is to use a different approach from the classical piezoelectric-patch technology. Instead, our concept is based on a contiguous coating of a so called magnetic smart material (MSM). After deployment a magnetic write head will move on the non-reflecting side of the mirror and will generate a magnetic field that will produce a stress in the MSM that will correct the mirror deviations from the prescribed shape.

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.

A major challenge in lubrication technology is to enhance lubricant performance at extreme temperatures that exceed conventional engine oil thermal degradation limits. Soft noble metals such as silver have low reactivity and shear strength, which make them ideal solid lubricants for wear protection and friction reduction between contacting surfaces at high temperatures. However, achieving adequate dispersion in engine lubricants and metallic silver deposition over predetermined temperatures ranges presents a significant chemical challenge. Here we report the synthesis, characterization, and tribological implementation of the trimeric silver pyrazolate complex, [Ag(3,5-dimethyl-4-n-hexyl-pyrazolate)]3 (1). This complex is oil-soluble and undergoes clean thermolysis at ∼310 °C to deposit lubricious, protective metallic silver particles on metal/metal oxide surfaces. Temperature-controlled tribometer tests show that greater than 1 wt % loading of 1 reduces wear by 60% in PAO4, a poly-α-olefin lubricant base fluid, and by 70% in a commercial fully formulated 15W40 motor oil (FF oil). This silver-organic complex also imparts sufficient friction reduction so that the tribological transition from oil as the primary lubricant through its thermal degradation, to 1 as the primary lubricant, is experimentally undetectable.

Ultrafine particles are often used as lubricant additives because they are capable of entering tribological contacts to reduce friction and protect surfaces from wear. They tend to be more stable than molecular additives under high thermal and mechanical stresses during rubbing. It is highly desirable for these particles to remain well dispersed in oil without relying on molecular ligands. Borrowing from the analogy that pieces of paper that are crumpled do not readily stick to each other (unlike flat sheets), we expect that ultrafine particles resembling miniaturized crumpled paper balls should self-disperse in oil and could act like nanoscale ball bearings to reduce friction and wear. Here we report the use of crumpled graphene balls as a high-performance additive that can significantly improve the lubrication properties of polyalphaolefin base oil. The tribological performance of crumpled graphene balls is only weakly dependent on their concentration in oil and readily exceeds that of other carbon additives such as graphite, reduced graphene oxide, and carbon black. Notably, polyalphaolefin base oil with only 0.01-0.1 wt % of crumpled graphene balls outperforms a fully formulated commercial lubricant in terms of friction and wear reduction.

The quest for improved engine performance and reduced emissions drives the design of increasingly sophisticated lubrication technologies. Lubricating oils and greases are engineered to function over a broad range of temperatures and loading conditions. Modern engines operate at higher temperatures, speeds and pressures than previous engines, and therefore require lubricants capable of handling harsher conditions. Reliable performance in extreme conditions is also necessary in emergency and combat situations. Thus, a major challenge for next-generation lubrication technology is to improve performance at extreme temperatures exceeding the thermal degradation limits of conventional engine oils.In automotive engines, the surface temperature of critical tribological components can easily reach 200°C, while asperity contacts can generate ‘flash temperatures’ up to 1000°C. These extreme pressures and temperatures in the contact zones can lead to plastic deformation, wear away mating surfaces, and catalyze chemical reactions which damage the surfaces and lubricant. Tests carried out on PAO4 and 15W40 motor oils show that they decompose at 275°C, irreversibly losing viscosity and generating oil-insoluble acids and salts that corrode surfaces and form sludges.Surface coatings, such as diamond-like carbon, and texturing can be used to reduce friction at temperatures which lead to motor oil thermal degradation. However, such treatments are costly for large components, and these coatings cannot be replenished without dismantling the treated machinery. Soft metal ductility can also be utilized in lubrication. The low shear-strengths of metallic films can form smooth “glaze layers” on tribosurfaces which lubricate sliding contact. Noble metals have oxidative stability, enabling lubricious performance at extreme temperatures. Silver-coated contact surfaces exhibit reduced friction and wear from 25–750°C. However, a method is required to dissolve metallic silver precursors in base oil for deposition at high temperatures.Silver nanoparticles are known to increase surface fatigue life, decrease friction, and wear, and work synergistically with other lubricant additives. However, silver nanoparticles are expensive, difficult to suspend in nonpolar media, and typically require a surfactant to prevent agglomeration. An alternative, described here, is to use a silver-containing molecular precursor. Organic ligands impart solubility to silver atoms and control the organosilver complex decomposition temperature to deposit silver only when and where it is needed. Controlled silver deposition is arguably more economical than full protective coatings. Also, a lubricant additive can be replenished during oil changes to provide more lubricious silver to high asperity engine contact regions. We report here the synthesis, characterization, and tribological implementation of a silver-pyrazole complex, silver 3,5-dimethyl-4-n-hexyl-pyrazolate (HPzAg)3. This complex is oil-soluble and undergoes clean thermolysis at ∼310°C to deposit lubricious, protective metallic silver on mechanical surfaces. Temperature controlled tribometer tests show that an optimized 2.5 wt% (HPzAg)3 loading reduces wear by 60% in PAO4 (poly-α-olefin lubricant) and 70% in a commercial fully-formulated motor oil (military grade 15W40). This organosilver complex also imparts sufficient friction reduction that the tribological transition from oil as the primary lubricant through its thermal degradation, to (HPzAg)3 as the primary lubricant, is experimentally undetectable.

In order to advance significantly scientific objectives, future x-ray astronomy missions will likely call for x-ray telescopes with large aperture areas (≍ 3 m2) and fine angular resolution (≍ 12). Achieving such performance is programmatically and technologically challenging due to the mass and envelope constraints of space-borne telescopes and to the need for densely nested grazing-incidence optics. Such an x-ray telescope will require precision fabrication, alignment, mounting, and assembly of large areas (≍ 600 m2) of lightweight (≍ 2 kg/m2 areal density) high-quality mirrors, at an acceptable cost (≍ 1 M$/m2 of mirror surface area). This paper reviews relevant programmatic and technological issues, as well as possible approaches for addressing these issues-including direct fabrication of monocrystalline silicon mirrors, active (in-space adjustable) figure correction of replicated mirrors, static post-fabrication correction using ion implantation, differential erosion or deposition, and coating-stress manipulation of thin substrates....

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.

This work examines the thermal behavior and failure mechanisms of TiN/CNx multilayer coatings deposited by DC magnetron sputtering. The samples were examined by transmission electron microscopy before thermal analysis. During thermal analysis, the samples were heated up to 1070 K at a constant rate of 10 K min−1 in a N2 atmosphere and their thermal stability was evaluated by thermo gravimetric (TGA) and differential thermal analyses (DTA). These analyses indicate that the multilayer coating is thermally stable up to 950 K, followed by stress relaxation when the temperature exceeds 950 K. After this thermal treatment, coating surfaces were observed by scanning electron microscopy. Buckling and fractured surface were seen for multilayers deposited with and without substrate bias voltage applied during growth process, although samples deposited with substrate bias are more resistant to crack formation and propagation.

Many machines operate in harsh environments where elevated temperatures require careful consideration of the lubricant for optimal performance. Lubricant additives can be designed to improve properties of base oil at specific temperature ranges. In the present work, two [tris(phosphino)borate]AgL (L ¼PEt3; NHC) complexes are synthesized and added to engine oil at various concentrations. The complexes thermolyze between 200 and 3008C, yielding metallic Ag. A mixture of engine oil and the silver-based nanoparticles provides fully flooded lubrication for pin-on-disk friction tests. A thermo-elastohydrodynamic model for point contact is utilized to predict the pin loads at which flash temperatures between 200 and 3008C occur, thus inducing thermal decomposition of the complexes. Results of the friction tests and wear measurements indicate a significant reduction in wear at 0.5‐1.0% Ag complex weight concentrations and little change in friction. The improved wear performance is attributed to the thermolysis and deposition of the silver-based complexes in the wear scar, as confirmed by energy-dispersive X-ray analysis.

Because of the high residual compressive stress normally accompanying the growth of diamond-like carbon (DLC) coatings and the large mismatch in the thermal expansion coefficient between DLC and steel, it is difficult to grow DLC coatings much thicker than 0.25 μm on steels. This paper describes our attempt to overcome this thickness limitation by a sequence of carbonitriding, carburizing and equilibration pre-treatments of the steel surface, followed by DLC coating deposition, all conducted within the same deposition system without breaking vacuum. These pre-treatments resulted in a surface with a graded composition and hardness profile. Such a graded interface is expected to reduce the interfacial energy, decrease thermal mismatch between the coating and the substrate, and thus improve coating adhesion. X-ray diffraction revealed the formation of various hard carbide and nitride phases. Raman spectroscopy showed that the modified steel surface just before DLC deposition exhibits local carbon bonding characteristics similar to DLC. Pulsed dc plasma-enhanced chemical vapor deposition was used to deposit one-micron thick DLC on these steel surfaces. The coating hardness was ~ 18–19 GPa. Its adhesion on the steel substrate was measured by scratch testing and was found to be comparable to thick, adherent DLC coatings deposited by other methods.

This article summarizes the discussions and deliberations on transformative research issues and opportunities in energy efficiency identified by a panel of experts assembled for the Civil, Mechanical, and Manufacturing Innovation Division of the US National Science Foundation. The discussions were confined to two areas – reducing energy consumption in buildings and improving energy efficiency in transportation. While these represent only a very small segment of important areas in energy efficiency, the panel considered them to be the most promising in terms of return on investment in research efforts. In the area of reducing energy consumption in buildings, high-priority research topics include information technology infrastructure for fundamental data gathering, processing and management, whole system and process integration for design and operation of smart buildings, and high-performance building components and sub-systems. In the area of energy efficiency in transportation, high-priority research topics include development of high-temperature high-performance ferrous alloys, systems design of protective coatings, fundamental understanding of surface texturing effects on friction and wear, and development of oxidatively stable bio-based lubricants. The energy challenge is serious. We need sustained investment in renewable energy, energy efficiency, and talent development in these new technologies for the future of our civilization.

This article presents a summary of research issues and opportunities in alternative energy source research identified by panels of experts assembled by the Engineering Directorate of the US National Science Foundation. The objective was to identify transformative research issues and opportunities to make alternative energy sources viable. The article presents motivations for energy research, grand challenges, and specific challenges in the research areas covered. The grand challenges identified for the United States include supplying 30% of US electricity from photovoltaics by 2030, supplying 25% of US electricity from wind by 2025, displacing 30% of US hydrocarbon use by 2030 with bio-based products, and providing a practical 250–300 W h/kg energy storage system by 2025. Similar challenges could be outlined along the same lines for the remainder of the world. Examples of specific areas of research focus identified as promising include high performance p-type transparent conductors, multijunction thin-film photovoltaic devices, defects in chalcogenide semiconductors, experimental study and numerical modeling of the fluid mechanics of airflow as applied to wind turbines, improved materials for wind turbines, methods for creating high energy density transportable biological feedstocks, biorefinery processes yielding infrastructure-compatible biofuels and biochemicals directly, and improved electrodes and electrolytes for Li ion batteries. Arguments for each of these as research priorities are given.

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.

Titanium dioxide (TiO 2 ) films with varying concentrations of gold particles were synthesized using pulsed DC magnetron sputtering, with the intent to develop infrared reflecting films for use on cars and planes to reduce solar heat load. Under our deposition conditions, the films are smooth (RMS roughness on the order of 1.0–2.0 nm) and consist of rutile TiO 2 with embedded gold. The average gold particle diameter on the sample surface was found to change from 60 to 200 nm as the volume fraction of gold in the films increased from 1.9 to 4.3% (3.5 to 7.9 mol% Au). The maximum reflectance of these films in the infrared region (800–2500 nm) is > 50%, compared with 30% for pure TiO 2 . The Maxwell–Garnett equation does not model the reflectance data very well, due to the relatively large gold particle size. Instead, by assuming that the contribution of gold particles to the reflectance response is proportional to their projected areal fraction in an effective medium approximation, we were able to fit the observed reflectance data quite well.

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 explores the subtle effects of surfaces and interfaces on the mechanical properties of bulk metallic alloys using three examples: environmental effects on fatigue life, hydrogen embrittlement effects on the ductility of intermetallics, and the role of coherent precipitates in the toughness of steels. It is demonstrated that the marked degradation of the fatigue life of metals is due to the strong chemisorption of adsorbates on exposed slip steps that are formed during fatigue deformation. These adsorbates reduce the reversibility of slip, thus accelerating fatigue damage in a chemically active gas environment. For certain intermetallic alloys such as Ni(3)Al and Ni(3)Fe, the ductility depends on the ambient gas composition and the atomic ordering in these alloys, both of which govern the complex surface chemical reactions taking place in the vicinity of crack tips. Finally, it is shown that local stresses at a coherent precipitate-matrix interface can activate dislocation motion at low temperatures, thus improving the fracture toughness of bulk alloys such as steels at cryogenic temperatures. These examples illustrate the complex interplay between surface chemistry and mechanics, often yielding unexpected results.

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.

Multilayer oxide coatings consisting of amorphous Al 2 O 3 and crystalline TiO 2 nanolayers have been deposited using reactive pulsed d.c. magnetron sputtering at different partial pressures of oxygen. Hardness enhancement has been observed in oxide multilayer coatings with amorphous Al 2 O 3 as the majority component. These coatings had greater hardness-to-modulus ratios and showed greater resistance to wear over monolithic Al 2 O 3 and TiO 2 majority phase multilayers. Multilayer films retain their high hardness up to ~ 800 °C in air; some hardness enhancement in the Al 2 O 3 majority phase multilayer coating remains even after 1 h of air annealing at 1000 °C. The hardness decrease at elevated temperatures is due to the roughening of interfaces between nanolayers, which can be attributed to the annealing-driven change of crystallographic texture of TiO 2 layers.

Microsystems have become an integral part of our lives and can be found in homeland security, medical science, aerospace applications and beyond. Many critical microsystem applications are in harsh environments, in which long-term reliability needs to be guaranteed and repair is not feasible. For example, gyroscope microsystems on satellites need to function for over 20 years under severe radiation, thermal cycling, and shock loading. Hence a predictive-science-based, verified and validated computational models and algorithms to predict the performance and materials integrity of microsystems in these situations is needed. Confidence in these predictions is improved by quantifying uncertainties and approximation errors. With no full system testing and limited sub-system testings, petascale computing is certainly necessary to span both time and space scales and to reduce the uncertainty in the prediction of long-term reliability. This paper presents the necessary steps to develop predictive-science-based multiscale modeling and simulation system. The development of this system will be focused on the prediction of the long-term performance of a gyroscope microsystem. The environmental effects to be considered include radiation, thermo-mechanical cycling and shock. Since there will be many material performance issues, attention is restricted to creep resulting from thermal aging and radiation-enhanced mass diffusion, material instability due to radiation and thermo-mechanical cycling and damage and fracture due to shock. To meet these challenges, we aim to develop an integrated multiscale software analysis system that spans the length scales from the atomistic scale to the scale of the device. The proposed software system will include molecular mechanics, phase field evolution, micromechanics and continuum mechanics software, and the state-of-the-art model identification strategies where atomistic properties are calibrated by quantum calculations. We aim to predict the long-term (in excess of 20 years) integrity of the resonator, electrode base, multilayer metallic bonding pads, and vacuum seals in a prescribed mission. Although multiscale simulations are efficient in the sense that they focus the most computationally intensive models and methods on only the portions of the space–time domain needed, the execution of the multiscale simulations associated with evaluating materials and device integrity for aerospace microsystems will require the application of petascale computing. A component-based software strategy will be used in the development of our massively parallel multiscale simulation system. This approach will allow us to take full advantage of existing single scale modeling components. An extensive, pervasive thrust in the software system development is verification, validation, and uncertainty quantification (UQ). Each component and the integrated software system need to be carefully verified. An UQ methodology that determines the quality of predictive information available from experimental measurements and packages the information in a form suitable for UQ at various scales needs to be developed. Experiments to validate the model at the nanoscale, microscale, and macroscale are proposed. The development of a petascale predictive-science-based multiscale modeling and simulation system will advance the field of predictive multiscale science so that it can be used to reliably analyze problems of unprecedented complexity, where limited testing resources can be adequately replaced by petascale computational power, advanced verification, validation, and UQ methodologies.

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.

Using pulsed dc magnetron sputtering, we synthesized hydrogenated amorphous carbon films with and without sulfur doping. These films were smooth and amorphous, with low compressive stress (

Water adsorption on hydrogenated carbon film surfaces can drastically affect friction behaviour. In this work, we investigate water adsorption and desorption properties of sulfur-doped hydrogenated films, which we have observed to retain ultra-low friction properties of hydrogenated carbon films in humid air. Water adsorption studies using a quartz crystal microbalance show that there is an almost threefold reduction in equilibrium water coverage at room temperature on hydrogenated carbon films doped with 5 at.% sulfur. Thermal desorption studies indicate that sulfur doping weakens the binding of water molecules on hydrogenated carbon film surfaces.

Ultrathin CN x overcoats were grown using pulsed dc magnetron sputtering. Substrates were mounted on a holder that allowed 45° tilt angle and rotation. Effects of process parameters on film growth were reviewed. AFM scans over large sampling areas show that thin CN x films obtained at -100 V substrate bias with 45° substrate tilt and 20-25 rpm rotation have r.m.s. roughness about 0.2-0.3 nm when sampled over 20 × 20 μm 2 areas, increasing to ∼0.45 nm when sampled over ∼0.05 × 3 cm 2 using X-ray reflectivity measurements. These 1-2 nm thick ultrasmooth coatings reduced corrosion damage compared with coatings of the same thickness grown without substrate tilt and rotation. This improved performance is likely a result of more efficient and uniform momentum transfer parallel to the surface during deposition in this configuration. In addition, detailed X-ray reflectivity measurements showed that the mass density of these CN x films is ∼2.0 g/cm 3 , independent of film thickness from ∼1 to 10 nm, consistent with ion beam analysis.

Boron carbide (B4C) and boron carbon nitride (BxCyNz) thin films were synthesized by pulsed DC magnetron sputtering. Effects of target power, target pulse frequency, substrate bias and pulse frequency on surface roughness were studied by AFM over sampling areas of 20 μm × 20 μm. For B4C, the combination of target power of 75 W/50 kHz and substrate bias of −100 V/2 kHz results in the smallest surface roughness. Compressive stress in these films is around 2.5 GPa. These B4C films have a hardness of 30 ± 5 GPa and reduced modulus of 250 ± 50 GPa. Nitrogen incorporation into B4C films, which gives BxCyNz thin films, has a beneficial effect. When deposited under similar conditions with substrate bias pulsed at −100 V / 20 kHz, the root-mean-square surface roughness decreases to 0.19 nm, compared with 0.28 nm for B4C. The hardness of BxCyNz is 20 ± 3 GPa, and reduced modulus is 210 ± 30 GPa. Auger electron spectroscopy was used to characterize the film composition. High-resolution cross-sectioned TEM images and diffraction show that both films are amorphous. Corrosion studies indicate that BxCyNz is a better protective coating for hard disk substrates than B4C and CNx films. This is attributed mainly to the smoother morphology of BxCyNz films.

TiB 2 /TiC multilayer coatings were synthesized in a dual-cathode unbalanced magnetron sputter–deposition system with substrate rotation. Our previous research has demonstrated that all coatings on M2 steel are polycrystalline with TiB 2 (001) preferred orientation and have a layer structure. Compressive stress in these coatings is less than 2 GPa, with hardness up to 60 GPa, which corresponds to 100% hardness enhancement over the rule-of-mixture value. This paper concentrates on the tribological and dry machining evaluation of these coatings. Coatings were deposited on Si(001), polished M2 steel, and C3 WC cutting tool inserts. From dry block-on-ring tribotesting, the 3:0.5 multilayer (i.e., the layer thickness is 3.0 nm for TiB 2 and 0.5 nm for TiC) coating provides four times improvement in wear resistance over the uncoated M2 steel substrate. In addition, dry machining was performed using AISI 1018 steel and 319 aluminum alloy cylindrical workpieces. Monolithic TiB 2 and 3:1 multilayer coatings (i.e., the layer thickness is 3.0 nm for TiB 2 and 1.0 nm for TiC) have the best performance: the flank wear was reduced by about a factor of ten compared with the uncoated tool after a cutting distance of 600 m. Results from cutting force measurements demonstrate that as far as dry machining is concerned, there is no direct correlation among cutting forces, room-temperature hardness, and flank wear. In dry machining of aluminum, the carbide tool deposited with the 3:1 multilayer, unlike other tools, has negligible buildup on the rake face. This results in better and more consistent surface finish of the final workpiece and less likelihood for premature tool breakage.

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.

Hydrogenated carbon (CH x ) films were grown by reactive magnetron sputtering of graphite in an argon–hydrogen plasma. Pulsed d.c. bias was applied to both the carbon target and the substrate to maintain a stable process and reasonable deposition rate. The resulting films were smooth and stress-free. The influence of hydrogen concentration and relative humidity on friction properties of the films was investigated. At 5% relative humidity, the lowest friction coefficient of 0.01 was obtained at a sputter-gas composition containing 25% hydrogen. Excessive incorporation of hydrogen produces softening and degrades its friction performance at high contact stresses. Molecular dynamics simulation studies showed the reduction of friction coefficient with surface hydrogenation. These studies indicate that pulsed d.c. magnetron sputtering can produce hydrogenated carbon films with friction properties similar to those prepared by chemical vapor deposition methods.

TiB2/TiC multilayer coatings were synthesized in a dual-cathode, non-reactive, unbalanced magnetron sputtering system with stationary and rotating substrates. All coatings are polycrystalline with TiB2(001) preferred orientation and have a multilayer structure. Coatings synthesized with stationary substrates have high compressive stress (4–7 GPa), and their hardness is slightly enhanced (∼25%) over the rule-of-mixture value. Coatings grown with substrate rotation have much lower compressive stress ( 60 GPa). Substrate rotation appears to improve mobility of surface species and allows more time for surface diffusion, thus resulting in denser films with lower void concentration. This may explain the substantial difference in hardness enhancement between these two cases. After being annealed in an inert environment (vacuum or argon) at 1273 K for 1 h, these multilayer coatings retain their layer structure. Moreover, most coatings have improved crystallinity and increased room-temperature hardness after such annealing.

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.

We conducted an international round-robin experiment to determine the consistency of nanoindentation hardness measurements of thin films among six different laboratories, using three different samples. These samples were chosen to present a challenge of indenting at small loads (μN range). They were: 250-nm-thick TiN x , 700-nm-thick TiC, and 500-nm-thick TiB 2 /TiC multilayer coatings (each layer being 3-nm thick), prepared at Northwestern University using magnetron sputtering on silicon (0 0 1) substrates. Each research team was free to use whatever nanoindentor and analysis methods at its disposal. This round-robin experiment demonstrates that for the hardness range of interest (15–35 GPa) and using well-documented procedures and analysis methods, the reported results from all laboratories are essentially the same, allowing for a statistical spread of approximately ±14%. For consistent hardness measurements, four precautions must be observed: (i) proper tip-area function calibration, (ii) using sharp indenters, (iii) performing nanoindentation measurements with minimal thermal drift and with drift correction, (iv) using smooth samples, and (v) measuring the full hardness–maximum penetration curve.