Tribological behaviour of Mo-S-N solid lubricant coatings in vacuum, nitrogen gas and elevated temperatures

Molybdenum disulphide (MoS2) is well known for the exceptional tribological properties in inert and high vacuum environments. On the other hand, these properties rapidly degrade in humid and higher temperature (&gt;300 °C) environments which cause increase of the friction coefficient and substantial increase of the wear. Apart from this, MoS2 films also suffers of porous structure, very low hardness and low load-bearing capacity which limits its applications in terrestrial atmosphere and more demanding conditions. Mo-S-N coatings published in previous studies revealed greatly improved mechanical and tribological properties in humid environment – low coefficient of friction, very low wear rates and one order of magnitude higher hardness. However, to this date, knowledge about sliding response of these Mo-S-N coating is mainly limited to humid air. In this work, we tested Mo-S-N coatings with different nitrogen contents from 0 to 40 at.% of N in vacuum (10−2 Pa), nitrogen atmosphere and elevated temperatures. The coatings were deposited by High Target Utilisation Sputtering method. All tribological tests were performed using ball-on-disc testing rig. A maximum hardness of 8 GPa was measured for the 19 and 31 atomic % N-doped coatings. In all testing conditions, the pure MoS2 films had COFs in the range of 0.02–0.15 and wear rates of 1.2–22×10−6 mm3N−1 m−1. The COF and specific wear rates decreased with N additions. Mo-S-N films had COFs between superlubric 0.007 and 0.13 and wear rates of 0.08–3×10−6 mm3N−1 m−1. The best overall tribological performance was shown to be for the Mo-S-N films containing 31 at.% of N. This study concluded that Mo-S-N coatings performed better than MoS2 in all testing conditions.</p

A Facile Two-Step PVP-Assisted Deposition of Co-Activated Nanosized Nickel Hydroxide Directly on a Substrate for Large-Scale Production of Supercapacitor Electrodes

The self-decomposition reaction of the nickel ammonia complex was used for the nickel hydroxide formation on the nickel foam with further modification in several ways. The addition of polyvinyl pyrrolidone (PVP) and the electrochemical or chemical activation with cobalt hydroxide was used to modify the formation method. In all cases, structures with Ni(OH)2 nanoflakes were formed. It was found that the flower-like particles of Co(OH)2 were precipitated during chemical activation among the nanoflakes. It was shown that the presence of PVP during the nickel ammonia complex decomposition suppressed the highly branched particles. The absence of the highly branched particles increased the capacitive properties of the formed electrode at high current densities. The highest capacitance in 1408 F/g at 1 A/g was shown for the sample precipitated with the PVP presence and the further chemical activation by cobalt

Ti3+ doped anodic single-wall TiO2 nanotubes as highly efficient photocatalyst

In this work, a two-step treatment of TiO2 nanotube (TNT) layers towards enhanced photocatalytic performance is presented. TNT layers with a thickness of ~7 µm and an average inner diameter of ~190 nm were prepared via electrochemical anodization of Ti foil in a fluoride containing ethylene glycol-based electrolyte. To improve the photocatalytic activity of the produced TNT layers a two-step post-treatment was conducted. First, the inner shell of the native double-wall TNT layers was removed via a mild pre-annealing followed by a selective etching treatment of the inner shell in piranha solution yielding single-wall TNT layers. Second, reduction via annealing in H2/Ar atmosphere was performed. The resulting Ti3+ doped single-wall TNT layers possess 100% enhancement of photocatalytic activity compared to their non-treated counterparts

Structure, mechanical and tribological properties of MoSe₂ and Mo-Se-N solid lubricant coatings

Mo-Se-N films were prepared by pulsed direct current (DC) High Target Utilisation Sputtering (HiTUS) in reactive Ar + N2 atmosphere. Here, the effect of nitrogen doping was studied. MoSex film with Se/Mo atomic ratio ~ 2 exhibited polycrystalline structure and the lowest coefficients of friction (COFs) in humid air from 0.025 to 0.1 for loads in the range 2–45 N. Mo-Se-N coatings were deposited with N concentrations ranging from 1 to 40 at.%, whereas Se/Mo ratio varied from 0.6 to 2. Mo-Se-N coatings formed amorphous structures for the N contents above 7 at.% and increased hardness proportional with the N content up to 9 GPa. The addition of nitrogen also resulted in a general decrease in wear rate of two orders of magnitude when compared to pristine films while retaining a reasonably low coefficient of friction. Mo-Se-N films showed notable COF values in humid environment ranging from 0.22 to 0.015 when tested using loads from 2 to 45 N. The excellent friction properties of Mo-Se-N films were associated to the crystallisation of a MoSe2 tribofilm in the wear scar. Moreover, we showed that modern pulsed DC HiTUS technology represents a suitable way of producing thin films with a variety of elemental compositions and desired mechanical and tribological properties, even from sensitive, semi-conducting and extremely low thermally conductive MoSe2 targets.</p

Point contact spectroscopy of superconductors via nanometer scale point contacts formed by resistive switching

Point contact spectroscopy is commonly used to investigate electronic properties of superconductors. Here we show that nanometer scale point contacts, which enable to study the superconductor properties locally, can be created by means of the resistive switching phenomenon. Our experiments were performed on sandwiched MgB2/Al/TiO2/Pt structures, where multiple bipolar resistive switching cycles were conducted. The differential conductance as a function of voltage was measured at temperatures below the critical temperature of the MgB2 superconductor. In the low-resistance state the MgB2 and Pt electrodes are connected by an ultrathin metallic filament which creates at the MgB2 electrode the Sharvin point contact with diameter below 10 nm. In this case the differential conductance data demonstrate the Andreev reflections due to the carrier transport between the superconducting MgB2 electrode and filament. From these data the two-gap superconductivity of MgB2 is clearly visible which also confirms the fit by the Blonder-Tinkham-Klapwijk model. If the bottom electrode is made of a superconductor with known gap, our approach allows us to estimate from the Andreev reflection spectrum the resistance of both the filament and point contact. We can then determine from the Sharvin formula the cross-section size of the point contact and thus also the filament cross-section size. In the high resistance state when the filament is ruptured, the differential conductance data demonstrate the spectrum typical for tunneling between two normal metals, with a zero-bias anomaly due to the Altshuler-Aronov effect. This suggests that the filament is not ruptured at the superconducting MgB2 electrode but elsewhere

Fluoride-free synthesis of anodic TiO2 nanotube layers: a promising environmentally friendly method for efficient photocatalysts

TiO2 nanotube (TNT) layers are generally prepared in fluoride-based electrolytes via electrochemical anodization that relies on the field-assisted dissolution of Ti metal forming nanoporous/nanotubular structures. However, the usage of fluoride ions is considered hazardous to the environment. Therefore, we present an environmentally friendly synthesis and application of TNT layers prepared in fluoride-free nitrate-based electrolytes. A well-defined nanotubular structure with thickness up to 1.5 mu m and an inner tube diameter of similar to 55 nm was obtained within 5 min using aqueous X(NO3)(Y) electrolytes (X = Na+, K+, Sr2+, Ag+). For the first time, we show the photocatalytic performance (using a model organic pollutant), HO radical production, and thorough characterization of TNT layers prepared in such electrolytes. The highest degradation efficiency (k = 0.0113 min(-1)) and HO radical production rate were obtained using TNT layers prepared in AgNO3 (Ag-NT). The intrinsic properties of Ag-NT such as the valence band maximum of similar to 2.9 eV, surface roughness of similar to 6 nm, and suitable morphological features and crystal structure were obtained. These results have the potential to pave the way for a more environmentally friendly synthesis of anodic TNT layers in the future using the next generation of fluoride-free nitrate-based electrolytes.Web of Science1432117091170

Large-Area MoS2 Films Grown on Sapphire and GaN Substrates by Pulsed Laser Deposition

In this paper, we present the preparation of few-layer MoS2 films on single-crystal sapphire, as well as on heteroepitaxial GaN templates on sapphire substrates, using the pulsed laser deposition (PLD) technique. Detailed structural and chemical characterization of the films were performed using Raman spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction measurements, and high-resolution transmission electron microscopy. According to X-ray diffraction studies, the films exhibit epitaxial growth, indicating a good in-plane alignment. Furthermore, the films demonstrate uniform thickness on large areas, as confirmed by Raman spectroscopy. The lateral electrical current transport of the MoS2 grown on sapphire was investigated by temperature (T)-dependent sheet resistance and Hall effect measurements, showing a high n-type doping of the semiconducting films (ns from ~1 × 1013 to ~3.4 × 1013 cm−2 from T = 300 K to 500 K), with a donor ionization energy of Ei = 93 ± 8 meV and a mobility decreasing with T. Finally, the vertical current injection across the MoS2/GaN heterojunction was investigated by means of conductive atomic force microscopy, showing the rectifying behavior of the I-V characteristics with a Schottky barrier height of ϕB ≈ 0.36 eV. The obtained results pave the way for the scalable application of PLD-grown MoS2 on GaN in electronics/optoelectronics