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ABSTRACT: Reactively sputtered Mo2N/MoS2/Ag nanocomposite coatings were deposited from three individual Mo, MoS2, and Ag targets in a nitrogen environment onto Si (111), 440C grade stainless steel, and inconel 600 substrates. The power
to the Mo target was kept constant, while power to the MoS2 and Ag targets was varied to obtain different coating compositions. The coatings consisted of Mo2N, with silver and/or sulfur additions of up to approximately 24at%. Coating chemistry and crystal structure were evaluated
using X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD), which showed the presence of tetragonal Mo2N and cubic Ag phases. The MoS2 phase was detected from XPS analysis and was likely present as an amorphous inclusion based on the absence of characteristic
XRD peaks. The tribological properties of the coatings were investigated in dry sliding at room temperature against Si3N4, 440C stainless steel, and Al2O3. Tribological testing was also conducted at 350 and 600°C against Si3N4. The coatings and respective wear tracks were examined using scanning electron microscopy (SEM), optical microscopy, profilometry,
energy dispersive X-ray spectroscopy (EDX), and micro-Raman spectroscopy. During room temperature tests, the coefficients
of friction (CoF) were relatively high (0.5–1.0) for all coating compositions, and particularly high against Si3N4 counterfaces. During high-temperature tests, the CoF of single-phase Mo2N coatings remained high, but much lower CoFs were observed for composite coatings with both Ag and S additions. CoF values
were maintained as low as 0.1 over 10,000 cycles for samples with Ag content in excess of 16at% and with sulfur content in
the 5–14at% range. The chemistry and phase analysis of coating contact surfaces showed temperature-adaptive behavior with
the formation of metallic silver at 350°C and silver molybdate compounds at 600°C tests. These adaptive Mo2N/MoS2/Ag coatings exhibited wear rates that were two orders of magnitude lower compared to Mo2N and Mo2N/Ag coatings, hence providing a high potential for lubrication and wear prevention of high-temperature sliding contacts.
Tribology Letters 04/2012; 29(2):95-103. · 1.58 Impact Factor
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ABSTRACT: Monolithic single phase cubic (c) Ti1- x Al x N thin films are used in various industrial applications due to their high thermal stability, which beneficially effects lifetime and performance of cutting and milling tools, but also find increasing utilization in electronic and optical devices. The present study elucidates the temperature-driven evolution of heat conductivity, electrical resistivity and optical reflectance from room temperature up to 1400 °C and links them to structural and chemical changes in Ti1- x Al x N coatings. It is shown that various decomposition phenomena, involving recovery and spinodal decomposition (known to account for the age hardening phenomenon in c-Ti1- x Al x N), as well as the cubic to wurtzite phase transformation of spinodally formed AlN-enriched domains, effectively increase the thermal conductivity of the coatings from ∼3.8 W m-1 K-1 by a factor of three, while the electrical resistivity is reduced by one order of magnitude. A change in the coating color from metallic grey after deposition to reddish-golden after annealing to 1400 °C is related to the film structure and discussed in terms of film reflectivity.
Acta Materialia 03/2012; 60(5):2091-2096. · 3.76 Impact Factor
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ABSTRACT: The solid-liquid phase transition of silica encapsulated bismuth nanoparticles was studied by in situ transmission electron microscopy (TEM). The nanoparticles were prepared by a two-step chemical synthesis process involving thermal decomposition of organometallic precursors for nucleating bismuth and a sol-gel process for growing silica. The microstructural and chemical analyses of the nanoparticles were performed using high-resolution TEM, Z-contrast imaging, focused ion beam milling, and X-ray energy dispersive spectroscopy. Solid-liquid-solid phase transitions of the nanoparticles were directly recorded by electron diffractions and TEM images. The silica encapsulation of the nanoparticles prevented agglomeration and allowed particles to preserve their original volume upon melting, which is desirable for applications of phase change nanoparticles with consistently repeatable thermal properties.
Nanoscale 07/2011; 3(9):3700-4. · 5.91 Impact Factor
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MRS Proceedings. 12/2010; 1329.
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ABSTRACT: This paper describes a new way to control temperatures of heterogeneous exothermic reactions such as heterogeneous catalytic reaction and polymerization by using encapsulated nanoparticles of phase change materials as thermally functional additives. Silica-encapsulated indium nanoparticles and silica encapsulated paraffin nanoparticles are used to absorb heat released in catalytic reaction and to mitigate gel effect of polymerization, respectively. The local hot spots that are induced by non-homogenous catalyst packing, reactant concentration fluctuation, and abrupt change of polymerization rate lead to solid to liquid phase change of nanoparticle cores so as to avoid thermal runaway by converting energies from exothermic reactions to latent heat of fusion. By quenching local hot spots at initial stage, reaction rates do not rise significantly because the thermal energy produced in reaction is isothermally removed. Nanoparticles of phase change materials will open a new dimension for thermal management of exothermic reactions to quench local hot spots, prevent thermal runaway of reaction, and change product distribution.
Nanoscale 10/2010; 2(12):2790-7. · 5.91 Impact Factor
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ABSTRACT: Field-induced phonon tunneling, a previously unknown mechanism of interfacial thermal transport, has been revealed by ultrahigh vacuum inelastic scanning tunneling microscopy (STM). Using thermally broadened Fermi-Dirac distribution in the STM tip as in situ atomic-scale thermometer we found that thermal vibrations of the last tip atom are effectively transmitted to sample surface despite few angstroms wide vacuum gap. We show that phonon tunneling is driven by interfacial electric field and thermally vibrating image charges, and its rate is enhanced by surface electron-phonon interaction.
Physical Review Letters 10/2010; 105(16):166101. · 7.37 Impact Factor
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ABSTRACT: Continuous titanium carbide (TiC) nanofibers that possess an intriguing nanoribbon morphology with a width and thickness of approximately 300 nm and approximately 40 nm, respectively, and containing TiC crystallites with sizes ranging from 5 nm to 30 nm were synthesized through electrospinning followed by carbothermal reduction.
Nanoscale 09/2010; 2(9):1670-3. · 5.91 Impact Factor
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ABSTRACT: This paper describes a new method to enhance the heat-transfer property of a single-phase liquid by adding encapsulated phase-change nanoparticles (nano-PCMs), which absorb thermal energy during solid-liquid phase changes. Silica-encapsulated indium nanoparticles and polymer-encapsulated paraffin (wax) nanoparticles have been made using colloid method, and suspended into poly-alpha-olefin (PAO) and water for potential high- and low-temperature applications, respectively. The shells prevent leakage and agglomeration of molten phase-change materials, and enhance the dielectric properties of indium nanoparticles. The heat-transfer coefficients of PAO containing indium nanoparticles (30% by mass) and water containing paraffin nanoparticles (10% by mass) are 1.6 and 1.75 times higher than those of corresponding single-phase fluids. The structural integrity of encapsulation allows repeated use of such nanoparticles for many cycles in high heat generating devices.
ACS Applied Materials & Interfaces 06/2010; 2(6):1685-91. · 4.53 Impact Factor
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Applied Organometallic Chemistry 03/2010; 24(8):590 - 599. · 2.06 Impact Factor
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ABSTRACT: Solid lubricants (SLs) characterized by low coefficients of friction (mu) and wear rates (w) drastically improve the life span of instruments that undergo extreme frictional wear. However, the performance of SLs such as sputtered or nanoparticulate molybdenum disulfide (MoS(2)), tungsten disulfide (WS(2)), or graphite deteriorates heavily under extreme operational conditions such as elevated temperatures and high humidity. Here, we present our preliminary results, which demonstrate that composites of carbon nanotubes (CNTs) and MoS(2) produced by electrodeposition of MoS(2) on vertically aligned CNT films have low mu ( approximately 0.03) and w (approximately 10(-13) mm(3)/N.mm) even at 300 degrees C, which are about 2 orders of magnitude better than those of nanoparticulate MoS(2)-based coatings. The high load-bearing capacity of CNTs provides a strong enduring support to MoS(2) nanoclusters and is responsible for their ultralow w. The incorporation of these composites in liquid lubricants reduces the friction coefficient of the liquid lubricants by approximately 15%. The technique described here to produce SL coatings with extremely appealing frictional properties will provide valuable solutions for a variety of tribological applications where the coatings encounter high temperature, reduced pressure, and/or low- and high-humidity conditions.
ACS Applied Materials & Interfaces 03/2009; 1(3):735-9. · 4.53 Impact Factor
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ABSTRACT: A model has been developed to study the stress distribution in Ti1 − xCx multilayered functionally gradient (FG) coatings, with a top coating of diamond-like carbon (DLC), on 440C stainless steel substrates. Using the finite element method, these gradient coatings were assumed as a series of perfectly bonded layers with unique material properties and layer thickness. In addition, a matrix of nanoindentation experiments were performed to measure material properties of each Ti1 − xCx layer on separate coating blocks. The yield strength of the coating materials was then determined by coupling the finite element analysis model in connection with the nanoindentation technique. Once developed, this model was used to examine the threshold of plasticity and identify the plastic deformation zone inside the multilayered coatings and substrate. This work shows how the multilayered FG Ti/TiC/DLC coating system improves the coating integrity under heavy loading conditions.
Tribology Transactions. 11/2008; 51(6):817-828.
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ABSTRACT: Reliability continues to be a critical issue in microelectromechanical systems (MEMS) switches. Failure mechanisms include high contact resistance (R), high adhesion, melting/shorting, and contact erosion. Little previous work has addressed the lubrication of MEMS switches. In this study, bimetallic nanoparticles (NPs) are synthesized using a biotemplated approach and deposited on Au MEMS switch contacts as a nanoparticle-based lubricant. Bimetallic nanoparticles are comprised of a metallic core (∼10 nm diameter gold nanoparticle) with smaller metallic nanoparticles (∼2-3 nm diameter Pd nanoparticles) populating the core surface. Adhesion and resistance (R) were measured during hot switching experiments at low (10 µA) and high (1 mA) current. The Au/Pd NP coated contacts led to reduced adhesion as compared to pure Au contacts with a compromise of slightly higher R. For switches held in the closed position at low current, R gradually decreased over tens of seconds due to increased van der Waals force and growth of the real area of contact with temporal effects being dominant over load effects. Contact behavior transitioned from 'Pd-like' to 'Au-like' during low current cycling experiments. Melting at high current resulted in rapid formation of large real contact area, low and stable R, and minimal effect of load on R. Durability at high current was excellent with no failure through 10(6) hot switching cycles. Improvement at high current is due to controlled nanoscale surface roughness that spreads current through multiple nanocontacts, which restricts the size of melting regions and causes termination of nanowire growth (prevents shorting) during contact opening. Based on these results, bimetallic NPs show excellent potential as surface modifiers/lubricants for MEMS switch contacts.
Nanotechnology 10/2008; 19(40):405705. · 3.98 Impact Factor
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ABSTRACT: Encapsulated nanoparticles were produced by the pulsed laser ablation of MoS2−Te composite targets in a high vacuum chamber. Transmission electron microscope and X-ray energy dispersive spectrometer measurements showed that core-like Mo-rich nanoparticles were encapsulated with shell-like Te-rich materials. Layer structures of the hexagonal MoTe2 phase were formed on the nanoparticle surface that was linked to the diffusion-controlled migration and crystallization. The mechanism of nanoparticle syntheses was discussed in relationship with the pulsed laser ablation of MoS2−Te target materials, the formation of Mo-rich cores, and the growth of layer-structured MoTe2 shells.
07/2008;
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Andrey A Voevodin,
Richard A Vaia,
Steven T Patton,
Steven Diamanti,
Mark Pender,
Mitra Yoonessi,
Jennifer Brubaker,
Jian-Jun Hu,
Jeffrey H Sanders,
Benjamin S Phillips,
Robert I MacCuspie
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ABSTRACT: Submonolayer coatings of noble-metal nanoparticle liquids (NPLs) are shown to provide replenishable surfaces with robust asperities and metallic conductivity that extends the durability of electrical relays by 10 to 100 times (depending on the current driven through the contact) as compared to alternative approaches. NPLs are single-component materials consisting of a metal nanoparticle core (5-20 nm Au or Pt nanoparticles) surrounded by a covalently tethered ionic-liquid corona of 1.5 to 2 nm. Common relay failure modes, such as stiction, surface distortion, and contact shorting, are suppressed with the addition of a submonolayer of NPLs to the contact surfaces. This distribution of NPLs results in a force profile for a contact-retraction cycle that is distinct from bare Au contacts and thicker, multilayer coatings of NPLs. Postmortem examination reveals a substantial decrease in topological change of the electrode surface relative to bare contacts, as well as an indication of lateral migration of the nanoparticles from the periphery towards the contact. A general extension of this concept to dynamic physical interfaces experiencing impact, sliding, or rolling affords alternatives to increase reliability and reduced losses for transmittance of electrical and mechanical energy.
Small 12/2007; 3(11):1957-63. · 8.35 Impact Factor
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ABSTRACT: Contact failures in microelectromechanical systems (MEMS) switches prevent widespread use of MEMS technology for current handling in miniature devices. A self-assembled monolayer (SAM) lubricant was applied to MEMS switch surfaces in this paper as a possible approach for preventing contact failure. Chemical and physical processes on SAM lubricated contact surfaces were investigated at low (10 μA) and high (1 mA) current using a micro/nanoadhesion apparatus as a switch simulator with in situ monitoring of contact resistance and adhesion force. This was coupled with ex situ analytical analyses of the contacts using x-ray photoelectron spectroscopy (XPS) and micro-Raman techniques. Diphenyl disulfide was chosen as a lubricant due to its thermal stability, enhanced conductivity, and its ability to form a 3.4 Å thick SAM on the gold electrode surface. Hot switching experiments were conducted in humid air (45% RH) and dry nitrogen using a MEMS-scale contact force of 200 μN and 5 Hz frequency. At low current, lubricated contacts failed by growth in both adhesion and contact resistance (R) at about 105 cycles. A multi-step degradation mechanism was suggested which includes (1) SAM debonding under electron flow with formation of charged molecular species and dipole molecular structures, (2) migration and trapping of charged molecular species and/or molecular dipoles in the contact zone, (3) decomposition of molecular structures under Joule heating and repeated mechanical impact, and (4) increased R due to carbonaceous film formation that further accelerates thermal decomposition of the SAM. At high current, switch contacts failed immediately due to SAM thermal decomposition. Failure mechanisms and durability were similar in either air or dry nitrogen, indicating a minimum influence of the environment chemistry on the contact processes. This study establishes degradation mechanisms of SAM based lubricants in MEMS electrical contacts and results can be used in designing contact switch lubrication materials.
Journal of Applied Physics 07/2007; 102(2):024903-024903-5. · 2.17 Impact Factor
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ABSTRACT: High inorganic volume fraction, solventless nanoparticle liquids have many potential applications, including reconfigurable electronic materials. Materials such as conductive lubricants could find applications in MEMS devices to increase relay switch performance as one example. The ability of the conductive nanoparticles to reconfigure themselves and fill voids in damaged areas can increase the lifespan of devices where local defects can cause failure states. Solventless solid nanoparticles with liquid-like properties are an area of recent research interest. For example, Giannelis and colleagues have reported metal oxide and metal nanoparticle liquids which contain no free solvent but still can flow in a liquid-like fashion. These materials contain large organic ligands bound to the surface of the nanoparticle through a combination of covalent and electrostatic bonds. By optimizing the attractive and repulsive forces between the nanoparticles through the surface chemistry of these organic ligands, the properties of the resulting liquids can be tailored.
03/2007;
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ABSTRACT: Lubrication of Micro Electro Mechanical Systems (MEMS) became very important as the devices became complex and more and more parts had interacting areas. In previous reports, self-assembled monolayers and long chains of flurocarbons were used to lubricate MEMS components with significant success. In this report, a method based on atomic force microscopy is described that measures and compares ionic liquid lubricity. Effect of ring structure is studied in the case of substituted pyridinium and imidazolium rings as cations in ethyl methyl pyridinium and ethyl methyl imidazolium ethyl sulfate. Effect of alkyl chain length on friction was studied for butyl methyl pyrrolidimium and hexyl methyl pyrrolidinium bis(trifluro methyl sulfonyl) imide. Some of the ionic liquids that exhibited promising results from AFM study are tested on MEMS test devices. The friction and wear data obtained for these liquids applied on hydrogenated silicon showed ample correlation to the failure life span of hydrogenated MEMS test devices. This shows that AFM-liquid cell based tests of ionic liquid lubricity is a good characterization technique for screening lubricants for MEMS devices.
11/2006;
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ABSTRACT: We used water as an environmental friendly medium for the synthesis of hexagonal WS2 nanoparticles by the pulsed laser method. The materials collected on substrates were oriented with the 2H-WS2 basal planes parallel to the surface. The use of water, UV lasers, and large WS2 targets prevented the nanoparticles from restructuring into inorganic fullerenes, which were observed in research using hydrocarbon solvents, longer wavelength lasers, and dispersed powder targets. Fairly good dispersion of nanoparticles suggests that large surface areas are available for chemical reactivity.
The Journal of Physical Chemistry B 06/2006; 110(18):8914-6. · 3.70 Impact Factor
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ABSTRACT: An investigation was conducted to examine the friction and wear behavior of an amorphous diamondlike carbon (a-DLC or a-C) functionally gradient nanocomposite coating in sliding contact with a 6-mm-diameter silicon nitride ball and a 6-mm-diameter 440C stainless steel ball in ultrahigh vacuum and in humid air. The nanocomposite coatings, consisting of an A-DLC top layer and a gradient Ti-Ti(x),C(y)-DLC underlayer, were produced by the hybrid technique of magnetron sputtering and pulsed-laser deposition on 440C stainless steel substrates. The resultant coatings were characterized by Raman spectroscopy, scanning electron microscopy, energy-dispersive x-ray spectroscopy, and surface profilometry. All sliding friction experiments were conducted with a load of 0.98 N (100 g), average Hertzian contact pressures of 0.6 GPa with the 440C stainless steel balls and 0.8 GPa with the silicon nitride balls, and 120 revolutions per minute. The sliding velocity ranged from 31 to 107 mm/s because of the range of wear track radii involved in the experiments. The experiment was conducted at room temperature in two environments: ultrahigh vacuum (vacuum pressure, 7 x 10(exp 7) to 2 x 10(exp -6) Pa) and humid air (relative humidity, about 50%).A marked difference in friction and wear resulted from the environmental conditions and the combination of materials. The humid air caused mild wear with burnishing in the a-DLC top layer; the ultrahigh vacuum caused relatively severr wear with brittle fracture in both the A-DLC top layer and the Ti-Ti(x)C(y)-DLC underlayer. The humid air environment provided a low coefficient of friction, a low wear rate for the A-DLC top layer, and a low wear rate forthe 440C stainless steel ball (counterpart material). In ultrahigh vacuum the materials pair of a-DLC coated substrate and 440C stainless steel ball was superior in wear resistance to the pair of a-DLC and silicon nitride ball, although both pairs had high coefficients of friction. The wear rate was low in both environments for both the a-DLC coated substrate and the 440C stainless steel ball. Changes in the bonding state and structure of the A-DLC layer were observed during the sliding friction process. The amorphous, disordered nondiamond form of carbon was produced during sliding contact.
04/1998;
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ABSTRACT: Carbon nanopearls films were deposited onto silicon substrates using MAPLE. A 248 nm KrF excimer laser was directed onto a target consisting of ~ 150 nm-sized carbon nanopearls dispersed in a solvent solution and frozen in liquid nitrogen. The morphology of deposited carbon nanopearl films found to be influenced by matrix solvent, laser energy, repetition rate, background pressure, and substrate temperature. At ambient laboratory temperatures, the morphology of deposited films was characterized by highly concentrated areas of carbon nanopearls in the shape of hollow rings, caused by impingement of liquid droplets of the solvent/nanopearl suspension on the substrate surface followed by evaporation of the solvent. As the substrate temperature was increased, the size of the liquid droplets reaching the substrate surface decreased; however, the amount of material deposited via evaporation also decreased. The optimal deposition conditions were suggested and used for a hybrid process where laser ablation from frozen dispersion solution targets was combined with sputtering from gold targets. A nanocomposite coating consisting of carbon nanopearls encapsulated in a gold matrix was synthesized using MAPLE and magnetron sputtering simultaneously. This process makes it possible to synthesize nanocomposite films using a nanostructured dispersion solution.
Surface and Coatings Technology.
