Chameleon Coatings: Adaptive Surfaces to Reduce Friction and Wear in Extreme Environments

Adaptive nanocomposite coating materials that automatically and reversibly adjust their surface composition and morphology via multiple mechanisms are a promising development for the reduction of friction and wear over broad ranges of ambient conditions encountered in aerospace applications, such as cycling of temperature and atmospheric composition. Materials selection for these composites is based on extensive study of interactions occurring between solid lubricants and their surroundings, especially with novel in situ surface characterization techniques used to identify adaptive behavior on size scales ranging from 10−10 to 10−4 m. Recent insights on operative solid-lubricant mechanisms and their dependency upon the ambient environment are reviewed as a basis for a discussion of the state of the art in solid-lubricant materials

Effect of Substrate Support on Dynamic Graphene/Metal Electrical Contacts.

Recent advances in graphene and other two-dimensional (2D) material synthesis and characterization have led to their use in emerging technologies, including flexible electronics. However, a major challenge is electrical contact stability, especially under mechanical straining or dynamic loading, which can be important for 2D material use in microelectromechanical systems. In this letter, we investigate the stability of dynamic electrical contacts at a graphene/metal interface using atomic force microscopy (AFM), under static conditions with variable normal loads and under sliding conditions with variable speeds. Our results demonstrate that contact resistance depends on the nature of the graphene support, specifically whether the graphene is free-standing or supported by a substrate, as well as on the contact load and sliding velocity. The results of the dynamic AFM experiments are corroborated by simulations, which show that the presence of a stiff substrate, increased load, and reduced sliding velocity lead to a more stable low-resistance contact

Nitrogen and Hydrogen Plasma Treatments of Multiwalled Carbon Nanotubes

This article investigates plasma treatment of vertically aligned multiwall carbon nanotube (CNT) films in different plasma environments for modification of surface chemistry and morphology. The surfaces of the CNTs were functionalized with a pulsed dc plasma treatment, where the power was directly applied to the nanotube film in low pressure argon/nitrogen and argon/hydrogen backgrounds. Optical emission spectroscopy was used to detect atomic and molecular excitations in the gas mixtures as well as in pure gases in the vicinity of the CNT films. In situx-ray photoelectron spectroscopy was performed on the treated samples to examine CNT surface chemistry after treatment. The analysis of CNT films after nitrogen and hydrogen treatment indicated formation of both C-N and C-H bonds, respectively. Correlations of the plasma characteristics to the surface chemistry and morphology of the CNT surfaces are discussed

Photo-Sensitivity of Large Area Physical Vapor Deposited Mono and Bilayer MoS2

We present photosensitivity in large area physical vapour deposited mono and bi-layer MoS2 films. Photo-voltaic effect was observed in single layer MoS2 without any apparent rectifying junctions, making device fabrication straightforward. For bi-layers, no such effect was present, suggesting strong size effect in light-matter interaction. The photo-voltaic effect was observed to highly direction dependent in the film plane, which suggests that the oblique deposition configuration plays a key role in developing the rectifying potential gradient. To the best of our knowledge, this is the first report of any large area and transfer free MoS2 photo device with performance comparable to their exfoliated counterparts

Electrostatic Quadrupole Plasma Mass Spectrometer Measurements during Thin Film Depositions using Simultaneous Matrix Assisted Pulsed Laser Evaporation and Magnetron Sputtering

A hybrid plasma deposition process, combining matrix assisted pulsed laser evaporation (MAPLE) of carbon nanopearls (CNPs) with magnetron sputtering of gold was investigated for growth of composite films, where 100 nm sized CNPs were encapsulated into a gold matrix. Composition and morphology of such composite films was characterized with x-ray photoelectron spectroscopy, scanning electron microscopy, and transmission electron microscopy (TEM) analysis. Carbondeposits on a gold magnetron sputter target and carbon impurities in the gold matrices of depositedfilms were observed while codepositing from gold and frozen toluene-CNP MAPLE targets in pure argon. Electrostatic quadrupole plasma analysis was used to determine that a likely mechanism for generation of carbon impurities was a reaction between toluene vapor generated from the MAPLE target and the argon plasma originating from the magnetron sputtering process. Carbon impurities of codeposited films were significantly reduced by introducing argon-oxygen mixtures into the deposition chamber; reactive oxygen species such as O and O+ effectively removed carbon contamination of gold matrix during the codeposition processes. Increasing the oxygen to argon ratio decreased the magnetron target sputter rate, and hence hybrid process optimization to prevent gold matrix contamination and maintain a high sputter yield is needed. High resolution TEM with energy dispersive spectrometry elemental mapping was used to study carbon distribution throughout the gold matrix as well as embedded CNP clusters. This research has demonstrated that a hybrid MAPLE and magnetron sputtering codeposition process is a viable means for synthesis of composite thin films from premanufactured nanoscale constituents, and that cross-process contaminations can be overcome with understanding of hybrid plasma process interaction mechanisms

Temporally and Spatially Resolved Plasma Spectroscopy in Pulsed Laser Deposition of Ultra-Thin Boron Nitride Films

Physical vapor deposition (PVD) has recently been investigated as a viable, alternative growth technique for two-dimensional materials with multiple benefits over other vapor deposition synthesis methods. The high kinetic energies and chemical reactivities of the condensing species formed from PVD processes can facilitate growth over large areas and at reduced substrate temperatures. In this study, chemistry, kinetic energies, time of flight data, and spatial distributions within a PVD plasma plume ablated from aboron nitride (BN) target by a KrF laser at different pressures of nitrogen gas were investigated. Time resolved spectroscopy and wavelength specific imaging were used to identify and track atomic neutral and ionized species including B+, B*, N+, N*, and molecular species including N2*, N2 +, and BN. Formation and decay of these species formed both from ablation of the target and from interactions with the background gas were investigated and provided insights into fundamental growth mechanisms of continuous, amorphous boron nitride thin films. The correlation of the plasma diagnostic results with film chemical composition and thickness uniformity studies helped to identify that a predominant mechanism for BN film formation is condensation surface recombination of boron ions and neutral atomic nitrogen species. These species arrive nearly simultaneously to the substrate location, and BN formation occurs microseconds before arrival of majority of N+ ions generated by plume collisions with background molecular nitrogen. The energetic nature and extended dwelling time of incident N+ ions at the substrate location was found to negatively impact resulting BN film stoichiometry and thickness. Growth of stoichiometric films was optimized at enriched concentrations of ionized boron and neutral atomic nitrogen in plasma near the condensation surface, providing few nanometer thick films with 1:1 BN stoichiometry and good thicknesses uniformity over macroscopic areas

Организация проведения аварийно-спасательных и других не отложенных работ при наводнении на территории г. Юрги

Целью работы является оценка и прогнозирования паводковой обстановки на примере города Юрги, изучены и проанализированы организация аварийно-спасательных работ при наводнении, проведен расчет сил и средств мероприятий ликвидации ЧС, проанализирован характер наводнения, спрогнозирован наиболее неблагоприятный участок затопления для человека, выбран вид сооружения, который максимально предотвратит затопления участка в городе Юрга.The aim of the work is to estimate and forecast the flood situation on the example of the city of Yurga, the organization of emergency rescue operations in the flood was studied and analyzed, the forces and means of emergency response activities were calculated, the nature of the flood was analyzed, the most unfavorable site of flooding for a person was predicted, Maximally prevent flooding of the site in the city of Yurga

Limited Thermal Conductance of Metal-Carbon Interfaces

The thermal conductance for a series of metal-graphite interfaces has been experimentally measured with time-domain thermoreflectance (TDTR). For metals with Debye temperatures up to ∼400 K, a linear relationship exists with the thermal conductance values. For metals with Debye temperatures in excess of ∼400 K, the measured metal-graphite thermal conductance values remain constant near 60 MW m−2 K−1. Titanium showed slightly higher conductance than aluminum, despite the closeness of atomic mass and Debye temperature for the two metals. Surface analysis was used to identify the presence of titaniumcarbide at the interface in contrast to the aluminum and gold-carbon interfaces (with no detectablecarbide phases). It was also observed that air-cleaved graphite surfaces in contact with metals yielded slightly higher thermal conductance than graphite surfaces cleaved in vacuo. Examination of samples with scanning electron microscopy revealed that the lack of absorbed molecules on the graphite surfaceresulted in differences in transducer film morphology, thereby altering the interface conductance.Classical molecular dynamic simulations of metal-carbon nanotube thermal conductance values were calculated and compared to the TDTR results. The upper limit of metal-graphite thermal conductance is attributed to the decreased coupling at higher frequencies of the lighter metals studied, and to the decreased heat capacity for higher vibrational frequency modes

Oxygen Plasma Treatment and Deposition of CNx on a Fluorinated Polymer Matrix Composite for Improved Erosion Resistance

The use of polymer matrix composites in aerospace propulsion applications is currently limited by insufficient resistance to erosion by abrasive media. Erosion resistant coatings may provide necessary protection; however, adhesion to many high temperature polymer matrix composite (PMC) materials is poor. A low pressure oxygen plasma treatment process was developed to improve adhesion of CNx coatings to a carbon reinforced, fluorinated polymer matrix composite. Fullerene-like CNx was selected as an erosion resistant coating for its high hardness-to-elastic modulus ratio and elastic resilience which were expected to reduce erosion from media incident at different angles (normal or glancing) relative to the surface. In situ x-ray photoelectron spectroscopy was used to evaluate the effect of the plasma treatment on surface chemistry, and electron microscopy was used to identify changes in the surface morphology of the PMC substrate after plasma exposure. The fluorine concentration at the surface was significantly reduced and the carbon fibers were exposed after plasma treatment. CNx coatings were then deposited on oxygen treated PMC substrates. Qualitative tests demonstrated that plasma treatment improved coating adhesion resulting in an erosion resistance improvement of a factor of 2 compared to untreated coated composite substrates. The combination of PMC pretreatment and coating with CNx reduced the erosion rate by an order of magnitude for normally incident particles

Domain Engineering of Physical Vapor Deposited Two-Dimensional Materials

Physical vapor deposited two-dimensional (2D) materials span larger areas compared to exfoliated flakes, but suffer from very small grain or domain sizes. In this letter, we fabricate freestanding molybdenum disulfide (MoS2) and amorphous boron nitride (BN) specimens to expose both surfaces. We performed in situ heating in a transmission electron microscope to observe the domain restructuring in real time. The freestanding MoS2 specimens showed up to 100× increase in domain size, while the amorphous BN transformed in to polycrystalline hexagonal BN (h-BN) at temperatures around 600 °C much lower than the 850–1000 °C range cited in the literature