1,720,998 research outputs found
Nanofluid lubrication and high pressure Raman studies of oxygen functionalized graphene nanosheets
No full textUltralow friction coefficient in reduced graphene oxide (rGO) nanofluid was observed at high pressure lubrication conditions. High pressure Raman spectroscopic studies of graphene oxide (GO) and rGO in a hydrostatic pressure medium in diamond anvil cell (DAC) showed an increase in G-band linewidth in GO but this value was decreased in rGO at same pressure range due to the defect relaxation in sp(2) networks. Moreover, loss of recovery of G band linewidth in decompression cycles was clearly noticed in both the samples but it was significantly higher in GO due to the irreversible sp(3) into sp(2) planar phase. (C) 2017 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserve
Tribofilm formation in ultrananocrystalline diamond film
No full textFriction and wear properties of ultrananocrystalline diamond (UNCD) films are found to be superior, and therefore it could be useful for various applications. However, understanding of the tribological properties with respect to boundary phase composition in this material is not yet well understood. Here, the grain boundary phases such as graphite and amorphous carbon (a-C) of UNCD films were tailored during the chemical vapor deposition process by altering the Argon and Nitrogen gases in CH4 plasma medium. The significance of these grain boundary phases in UNCD film was discussed to explain the tribological properties. In run-in, friction coefficient was high in UNCDAr film deposited in CH4 (1%)/Ar plasma condition and it was decreased to lower value after longer sliding distance. However, ultrahigh wear resistance of this film was observed. Here, graphite and a-C phases were insignificant in the grain boundary region as evident from high resolution transmission electron microscope (HRTEM). Further, chemical bonding of these phases was quantitatively described by electron energy loss spectroscopy (EELS). In contrast, ultralow value of friction coefficient with significantly shorter run-in high friction regime was observed in UNCDN film deposited in CH4 (6%)/N2 plasma. Such a unique characteristic was described by the nanographite phase encasing the needle-like diamond grain of UNCDN film, forming core-shell granular structure. Atomic force microscopy (AFM) showed nucleation of two dimensional (2D) nanographite particles in the deformed wear track after run-in. This was possible due to the presence of core-shell granular structure in UNCDN film. Graphitic nature of the shell for needle-like diamond grains in the wear track was investigated by micro-Raman spectroscopy. Moreover, graphite and a-C tribofilm phase in the wear track was investigated by X-ray photoelectron spectroscopy (XPS) having spatial resolution micrometer scale. Needle-like diamond grains and graphite phase of tribofilm could be one of the primary reasons for the marked reduction in the friction coefficient. © 2017 Elsevier B.V3
Enhanced tribo-chemical properties of oxygen functionalized mechanically exfoliated hexagonal boron nitride nanolubricant additives
No full textTwo dimensional (2D) materials with layered lattice structure as nanofluid additives are useful to improve the tribological properties of metallic sliding interfaces. To enhance the tribological efficiency, the bulk crystalline hexagonal boron nitride (h-BN) powder sample was mechanically exfoliated by ball milling, and further processed through ultrasonication for de-aggregation. High resolution X-ray diffraction (HR XRD) and high resolution transmission electron microscopy (HR TEM) results clearly indicate the exfoliation of bulk h-BN into thinner two-dimensional (2D) crystalline sheets without creating noticeable structural defects. The topography of exfoliated nanosheets is well confirmed by atomic force microscopy (AFM). Oxygen functionalization into the h-BN nanosheets after the mechanical exfoliation was investigated by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). Oxygen functionalization of h-BN improved the cohesive compatibility between h-BN and commercial 10W30 lubricant oil for stable dispersion. Friction coefficient and wear of sliding metallic interfaces were reduced significantly in the presence of few layered exfoliated h-BN nanofluid as compared to neat lubricated oil. Micro- XPS and energy-disperse X-ray spectroscopy (EDX) analysis demonstrated the presence of adsorbed h-BN tribofilm in the metallic wear track. Thin 2D sheets of h-BN nanofluid was effective as an additive for low shear resistance under the tribo stressed condition which is the main reason for significant reduction in friction coefficient. Moreover, the enhanced wear resistance of exfoliated h-BN additives was explained by low shear resistance and high compressive/tensile strength of planer sheets which restricted the mechanical damage and protected the metallic interfaces against deformation and wear. © 2017 Elsevier B.
Tribochemistry of contact interfaces of nanocrystalline molybdenum carbide films
No full textTransition metal carbides (TMC) are known for their improved tribological properties and are sensitive to the tribo-atmospheric environment. Nanocrystalline molybdenum carbide (MoC) thin films were deposited by DC magnetron sputtering technique using reactive CH4 gas. The friction and wear resistance properties of MoC thin films were significantly improved in humid-atmospheric condition as compared to high-vacuum tribo-condition. A comprehensive chemical analysis of deformed contact interfaces was carried out by X-ray photoelectron spectroscopy (XPS), energy dispersive X-ray spectroscopy (EDX) and Raman spectroscopy. XPS and Raman spectroscopy showed the formation of stable molybdenum-oxide (MoO), molybdenum carbide (MoC) and amorphous carbon (a-C) tribo-phases. Moreover, during the sliding in humid-atmospheric condition, these phases were extensively deposited on the sliding steel ball counter body which significantly protected against undesirable friction and wear. © 2018 Elsevier B.
Tribochemistry of TaN, TiAlN and TaAlN coatings under ambient atmosphere and high-vacuum sliding conditions
Full text link© 2019 Elsevier B.V.Tribochemical analysis of monolithic TaN, TiAlN, and TaAlN coatings deposited by reactive magnetron sputtering onto 316LN stainless steel (SS) substrates are described. Tribology experiments were carried out in ambient atmospheric and high-vacuum sliding conditions to investigate the tribo-atmospheric dependent friction and wear characteristics of these coatings. The lower friction coefficient and improved wear-resistant properties were observed for TaN and TiAlN coatings in the humid atmosphere than in high-vacuum testing condition. Interestingly, lower friction and wear resistance properties of TaAlN coated SS are significantly enhanced in atmospheric as well as high-vacuum sliding conditions because of their highly dense and fine-grained microstructure with stable cubic B1 TaAlN phase. Energy dispersive X-ray spectroscopy elemental mapping and micro-focused X-ray photoelectron spectroscopy were carried out on the wear tracks to explore the comprehensive tribo-environment dependent tribochemistry. Formations of alumina (Al2O3) rich tribolayer reduced the friction and enhanced the wear resistance of TaAlN/SS sample tested in atmospheric condition; whereas this coating is highly stable in the high-vacuum condition with higher wear resistance11sciescopu
Nanoscale investigation of enhanced electron field emission for silver ion implanted/post-annealed ultrananocrystalline diamond films
Full text linkSilver (Ag) ions are implanted in ultrananocrystalline diamond (UNCD) films to enhance the electron field emission (EFE) properties, resulting in low turn-on field of 8.5 V/mu m with high EFE current density of 6.2 mA/cm(2) (at an applied field of 20.5 V/mu m). Detailed nanoscale investigation by atomic force microscopy based peak force-controlled tunneling atomic force microscopy (PF-TUNA) and ultra-high vacuum scanning tunneling microscopy (STM) based current imaging tunneling spectroscopy (CITS) reveal that the UNCD grain boundaries are the preferred electron emission sites. The two scanning probe microscopic results supplement each other well. However, the PF-TUNA measurement is found to be better for explaining the local electron emission behavior than the STM-based CITS technique. The formation of Ag nanoparticles induced abundant sp(2) nanographitic phases along the grain boundaries facilitate the easy transport of electrons and is believed to be a prime factor in enhancing the conductivity/EFE properties of UNCD films. The nanoscale understanding on the origin of electron emission sites in Ag-ion implanted/annealed UNCD films using the scanning probe microscopic techniques will certainly help in developing high-brightness electron sources for flat-panel displays applications© The Author(s) 201711sciescopu
Tribological Properties of Ultrananocrystalline Diamond Films in Inert and Reactive Tribo-Atmospheres: XPS Depth-Resolved Chemical Analysis
No full textTribological
properties of diamond films are sensitive to the chemically
reactive and inert tribo-atmospheric media, and therefore, it is difficult
to understand the underlying tribological mechanisms. In the present
work, tribological properties of surface-modified ultrananocrystalline
diamond (UNCD) thin films were investigated in four distinct tribo-environmental
conditions of ambient humid-atmosphere, nitrogen (N2),
argon (Ar), and methane (CH4) gases. The in situ depth-resolved
X-ray photoelectron spectroscopy (XPS) showed the desorption of oxygen
and oxy-functional additives and sputtering of weakly bonded amorphous
carbon species from the UNCD film surface after the Ar+-ion sputtering process. After desorption of these chemical entities,
friction and wear were decreased and run-in regime cycles became shorter
in UNCD films. Friction in the ambient humid-atmosphere was higher
compared to other tribo-environmental conditions, and it was explained
by the oxidation mechanism of the sliding interfaces and the formation
of the oxidized carbon transferfilm. However, low friction and wear
in the N2 atmosphere was associated with the adsorption
of N2 species, forming nitrogen-terminated carbon bonds
at the sliding interfaces. This was directly investigated by XPS and
energy dispersive X-ray spectroscopy techniques. Furthermore, low
friction in the Ar atmosphere was explained by the physical adsorption
of Ar gaseous species, which tend to avoid the covalent carbon bond
formation across the sliding interfaces. Moreover, ultralow friction
in the CH4 atmosphere was governed by the passivation of
dangling carbon bonds by dissociative CH4 complexes, which
creates hydrogen-terminated repulsive sliding interfaces. More importantly,
a shorter run-in regime with low friction and wear in Ar+-ion-sputtered UNCD films were explained by desorption of the oxygen
and oxy-functional groups, which are inherently present in the UNCD
films
Tribological Properties of Ultrananocrystalline Diamond Films: Mechanochemical Transformation of Sliding Interfaces
Full text linkAbstractImproving the tribological properties of materials in ambient and high vacuum tribo-conditions is useful for inter-atmospheric applications. Highly-hydrogenated and less-hydrogenated ultrananocrystalline diamond (UNCD) films with distinct microstructural characteristics were deposited on Ti–6Al–4 V alloy, by optimizing the plasma conditions in the chemical vapor deposition. Both the UNCD films showed less friction coefficient in ambient atmospheric tribo-contact conditions due to the passivation. This provides chemical stability to UNCD films under the tribo-mechanical stressed conditions which limits the transferlayer formation and conversion of UNCD phase into graphitization/amorphization. However, in the high vacuum tribo-conditions, highly-hydrogenated UNCD films showed low friction value which gradually increased to the higher magnitude at longer sliding cycles. The low friction coefficient was indicative of passivation provided by the hydrogen network intrinsically present in the UNCD films. It gradually desorbs and the dangling bonds are progressively activated in the contact regime, leading to a gradual increase in the friction value. In contrast, less-hydrogenated UNCD films do not exhibit low friction regime in high vacuum conditions due to the lack of internal passivation. In this case, the conversion of UNCD to amorphized carbon structure in the wear tracks and amorphous carbon (a-C) tribofilm formation on ball scars were observed.</jats:p
Nitrogen ion implanted ultrananocrystalline diamond films: A better electrostatic charge storage medium
No full textAtomic force microscopy (AFM) is used to induce electrostatically charged areas on nitrogen-ion-implanted ultrananocrystalline diamond (UNCD) films by applying a bias voltage to the AFM tip during the tapping mode scan. For local and intentional electrostatic charging, both positive and negative charges can be stored on pristine as well as nitrogen-ion-implanted UNCD films, as detected by Kelvin probe force microscopy. Interestingly, the charge storage capacity of pristine UNCD increases with the N-ion implantation dose. The potential amplitude and spatial distribution of the trapped charges can be controlled by the bias and load applied to the tip. The enhanced charge storage properties with doping are attributed to the formation of impurities, various defects, and the induction of an enhanced graphitic sp2 phase because of the high N-ion implantation dose, where all act as trapping centres for extra charge. The formation of various defect states and conducting sp2 nanographitic phases facilitate enhanced charge storage in N-ion-implanted UNCD films. This study underscores the potential importance of doping-dependent charge storage and the role of defects and nanographitic phases as a strategy for enhancing the electrostatic charge storage capability of diamond-based charge storage devices. (c) 2018 Elsevier Lt11sciescopu
Triboenvironment Dependent Chemical Modification of Sliding Interfaces in Ultrananocrystalline Diamond Nanowall Film: Correlation with Friction and Wear
No full textTribological properties of ultrananocrystalline diamond nanowall (UNCD NW) films were investigated quantitatively in three different and controlled triboenvironmental conditions, proposing the passivation and graphitization mechanisms. However, these mechanisms are rather complicated and possibly can be understood in well-controlled tribological conditions. It was shown that the friction and wear of these films were high in high-vacuum and room temperature (HV-RT) tribo conditions where the passivation of carbon dangling bonds were restricted and frictional shear-induced transformation of sp3 carbon into amorphous carbon (a-C) and tetrahedral amorphous carbon (t-aC) were noticed. However, the friction coefficients were reduced to the ultralow value in ambient atmospheric and room temperature (AA-RT) tribo conditions. Here, both passivation of dangling bonds through atmospheric water vapor and graphitization of the contact interfaces were energetically favorable mechanisms. Furthermore, the conversion of diamond sp3 into hydrogenated-graphitized phase was the dominating mechanism for the observed superlow friction coefficient and ultrahigh wear resistance of films in high-vacuum and high temperature (HV-HT) tribo conditions. These mechanisms were comprehensively investigated by micro-Raman and X-ray photoelectron spectroscopy analyses of the sliding interfaces. © 2017 American Chemical Society11sciescopu
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