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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.

Early transition metal catalysts produce high-density and linear low-density polyethylenes with spectacular efficiency. Nevertheless, these catalysts are ineffective in producing low-density polyethylene homopolymers with large –(CH2)xCH3 branch densities (x ≥ 5) or low-molecular-mass (Mn

A simple and robust method for fabrication of superamphiphobic surfaces by template-free chemical deposition of Cu micro-cone arrays, followed by air plasma activation and room-temperature treatment in tridecafluoro-1,1,2,2-tetrahydrooctyl trichlorosilane (FTS) is reported in this paper. After only treatment with 15 min of FTS, a water contact angle of 166.1° and an oil (diiodomethane, CH2I2) contact angle of 160.5° are reached on the Cu surfaces. The highest oil contact angle of 168.1° is achieved after 1 h treatment with FTS. Such surfaces also display enhanced corrosion resistance due to the combination of the FTS layer and air pockets which prevent ion diffusion to the internal copper.

It has been hypothesized that introduction of proper solute atoms, which preferentially segregates to grain boundaries, can stabilize the nanoscale grain size at elevated temperatures. To explore this possibility, we synthesized pure Cu and Cu-3 at. % W nanocrystalline films using magnetron sputtering. Scanning electron microscopy and x-ray diffraction showed that while grains in pure Cu coarsen rapidly when annealed at 673 K, Cu-3% W maintains a stable microstructure when annealed at the same temperature, achieving grain size of 37 nm after 120 min annealing. Nanoindentation measurements gave results consistent with the grain size evolution: upon annealing at 673 K, the hardness of the pure Cu film decreases to 40% of its initial value after 20 min, while the Cu-3% W film retains more than 90% of its initial hardness even after 120 min. Atom probe analysis shows W segregation in the Cu-3% W film. Results of this study indicate that solute segregation can be used as a method to fabricate nanocrystalline materials that are stable against grain coarsening 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.

Minimizing friction and wear at a rubbing interface continues to be a challenge and has resulted in the recent surge toward the use of coatings such as diamond-like carbon (DLC) on machine components. The problem with the coating approach is the limitation of coating wear life. Here, we report a lubrication approach in which lubricious, wear-protective carbon-containing tribofilms can be self-generated and replenishable, without any surface pretreatment. Such carbon-containing films were formed under modest sliding conditions in a lubricant consisting of cyclopropanecarboxylic acid as an additive dissolved in polyalphaolefin base oil. These tribofilms show the same Raman D and G signatures that have been interpreted to be due to the presence of graphite- or DLC films. Our experimental measurements and reactive molecular dynamics simulations demonstrate that these tribofilms are in fact high-molecular weight hydrocarbons acting as a solid lubricant.

Electricity in many countries such as the US and China is produced by burning fossil fuels in steam-turbine-driven power plants. The efficiency of these power plants can be improved by increasing the operating temperature of the steam generator. In this work, we adopted a combined surface science and computational thermodynamics approach to the design of high-temperature, corrosion-resistant steels for this application. The result is a low-carbon ferritic steel with nanosized transition metal monocarbide precipitates that are thermally stable, as verified by atom probe tomography. High-temperature Vickers hardness measurements demonstrated that these steels maintain their strength for extended periods at 700 °C. We hypothesize that the improved strength of these steels is derived from the semi-coherent interfaces of these thermally stable, nanosized precipitates exerting drag forces on impinging dislocations, thus maintaining strength at elevated temperatures.

Boron carbide thin films were deposited on (100) silicon substrates at ambient temperature via. pulse dc magnetron sputtering. Various frequency and duty cycles were applied to the hot-pressed B 4 C target in order to understand their influence on the structure and mechanical properties of the B 4 C films. X-ray Energy dispersive spectrum, Raman spectroscopy and Transmission electronic microscopy were used to characterize the composition and microstructure of the films. Nanoindenter was employed to measure the hardness and modulus. The film toughness was evaluated by a microindentation method. The results show that both pulse frequency and duty cycle significantly affect the B/C atomic ratio and then hardness and modulus in the boron carbide films. However, the amorphous structure of the films was maintained when the frequency and duty cycle changed. The maximum hardness of 29 GPa and modulus of 247 GPa combined with relative high toughness (3.3 MPa m 1/2 ) were achieved under 50 kHz frequency and 30% duty cycle. In addition, there was no evidence to prove that the graphite phase existed in the B 4 C films although exceeded C concentration was detected.

Fe/VC multilayer coatings with a fixed bilayer period (Λ = 8 nm) and variable Fe fraction (ΛFe/Λ ranging from 0.6 to 0.9) were deposited on Si substrates via dc magnetron sputtering. X-Ray diffraction and transmission electron microscopy showed the nanoscale layer structure of these coatings. High-resolution transmission electron microscopy imaging revealed the formation of coherent interfaces between Fe and VC (100). Throughout the entire range of Fe fractions investigated in this study, the hardness was enhanced over the rule-of-mixture trend line. Even at Fe fraction of 0.9, the hardness value was 16.3 GPa, enhanced by about 80% over the rule-of-mixture value of 9 GPa. While all Fe/VC coatings investigated in this work have statistically the same hardness, Fe/VC coating with Fe fraction of 90% has almost twice the fracture toughness of the other Fe/VC coatings. We attribute this enhanced toughness to the favorable generation of dislocations in the Fe layers and their activation by coherency strain emanating from Fe/VC interfaces. This study also demonstrates that H/E and H3/E2 are not good proxies for the toughness of these coatings.

Two experimental bcc -Cu- and B2–NiAl-precipitation-strengthened ferritic steels with 6.3 at. % and 12.4 at. % Cu + Mn + Ni + Al, 950 MPa and 1600 MPa yield strength respectively, were studied. Atom probe tomography showed that the volume fraction and number density of NiAl-type precipitates in the heavier alloyed steel (designated as CF-9 ) is ∼60–70 times greater than those in the lighter alloyed steel (designated as CF -2). This is attributed to the smaller lattice misfit between these NiAl-type precipitates and the ferritic matrix in CF- 9 due to more incorporation of Mn atoms on the Al sub-lattice in the B2 NiAl unit cell. Loading rate sensitivity of hardness was measured for CF -2, CF -9 and SAE -1090, which does not have bcc -Cu precipitates. Results show that even though CF -2 and CF -9 have double and triple the strength of SAE -1090 respectively, their hardness shows weaker dependence on loading rate. This is attributed to the presence of bcc -Cu precipitates in CF -2 and CF -9 providing athermal activation of nearby screw dislocation motion. Auger electron spectroscopy studies of CF -9 samples reveal Cu segregation on grain boundaries. The observed Cu segregation is believed to be partly responsible for the lower elongation-to-failure of CF -9 compared with CF -2.

It is desirable to increase the hardness of protective coatings for reduced abrasive wear and to increase the toughness for improved wear performance due to fatigue and formation of flaws or cracks. Unfortunately, there is an inverse relationship between hardness and toughness: the harder the coating, the lower the toughness. Using W/VC multilayer coatings, we demonstrated that it is possible to reverse this hardness–toughness trend. These coatings were synthesized by DC magnetron sputtering with bilayer periods of about 10 nm and different thickness fractions of W. They are crystalline, with hardness and toughness exceeding that of pure VC (25 GPa and 1.0 MPa-m 1/2 ). In particular, the W/VC multilayer coating with thickness fraction of 90% W achieves hardness of 28.5 GPa, comparable to many ceramics-based hard coatings, and toughness of 7.5 MPa-m 1/2 , similar to many nanocrystalline metals. This investigation shows that one can synthesize coatings as hard as ceramics and as tough as metals. These results also indicate that H / E and H 3 / E 2 ( H = hardness and E = elastic modulus) are not good proxies for coating toughness.

Larger mirrors are needed to satisfy the requirements of the next generation of UV–Vis space telescopes. Our study attempts to meet this requirement by demonstrating a technology that would deploy a large, continuous, high figure accuracy membrane mirror. The figure of the membrane mirror is corrected after deployment using a contiguous coating of a magnetic smart material (MSM) and a magnetic field. The MSM is a magnetostrictive material that is operable by magnetic write head(s), locally imposed on the nonreflective side of the membrane mirror. We report preparation, figure accuracy, stress analysis, and stability of the MSM coated CP1 polyimide substrate membrane mirror. The figure accuracy and magnetostrictive performance of the MSM coated membrane mirror are measured; furthermore, stability of the CP1 membrane for 48 h is observed and the results are found to be promising. In addition to membrane coating and the experimental procedure, the results of the surface profiling experiments are introduced and discussed.

Reported in this paper is a novel and facile room-temperature surface-hardening technique, capable of providing five-fold increase in the hardness of an aluminum alloy, by utilizing a cavitating waterjet laden with hard nanoparticles. Microstructural and composition analyses reveal several mechanisms responsible for surface hardening: strain hardening, grain refinement, and dispersion strengthening. The hardened alloy surface also exhibits about 50% reduction in friction in a series of microscale friction measurements. Without the need to treat the alloy at elevated temperatures, this technique obviates such problems as additional energy usage, part distortion, microstructural and composition changes, and thermal shock-induced cracking. Equally important, this new method could be further tailored to impart metals, polymers, composites, etc. with different surface functional properties.

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.

Larger mirrors are needed to satisfy the requirements of the next generation of UV-Vis space telescopes. Our NASA-NIAC funded project, titled A Precise Extremely large Reflective Telescope Using Reconfigurable Elements (APERTURE), attempts to meet this requirement. The aim of the project is to demonstrate technology that would deploy a large, continuous, high figure accuracy membrane mirror. The figure of the membrane mirror is corrected after deployment using a contiguous coating of a Magnetic Smart Material (MSM) and a magnetic field. The MSM is a magnetostrictive material which is driven by magnetic write head(s) (MWH), locally imposed on the non-reflective side of the membrane mirror. In this proceeding we report the figure accuracy of the MSM coated membrane mirror under various conditions using a Shack-Hartmann surface profiler. The figure accuracy and magnetostrictive performance of the membrane mirror is found to be significantly dependent on ambient temperature fluctuations, the tension load on the membrane, time, magnetic writing head orientation and magnetic field strength. The results and reproducibility of the surface profiling experiments under various conditions are introduced and discussed.

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.

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.