Influence of the Chemical Composition and Local Atomic Packing of Nanostructured MoSx and MoSex Solid Lubricant Coatings on Their Tribological Properties under Complicated Conditions

Technical Physics Letters - We present the results of a comparative investigation of the friction and wear characteristics of MoSx and MoSex coatings in an oxidizing medium (argon–air...     

Influence of hydrogen on the thermoelectric voltage signal in a Pt/WO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>/6<Emphasis Type="Italic">H</Emphasis>-SiC/Ni/Pt layered structure

The possibility of detecting H₂ by registering the thermal electromotive force signal, which arises between the surfaces of 6H-SiC plates with a thickness of 400 μm, is established. The working surface of the plates is modified by deposition of a WO _x film and catalytic Pt. An ohmic contact (Ni/Pt) is created on the rear surface of the plate, and this surface is maintained at a stabilized temperature of 350°C. The temperature gradient through the plate thickness arises due to the cooling of the working surface with the air medium. The delivery of H₂ into this medium up to a concentration of 2% gives rise to a 15-fold increase in the electric signal, which considerably exceeds the Pt/WO _x /SiC/Ni/Pt system’s response registered in the usual way by measuring the current–voltage dependence. In this case, an additional power source for the registration of the thermal electromotive force is not required.     

Modification of the chemical composition,morphology, and antireflection properties of WSe x films formed by pulsed laser deposition

We have studied the possibility of controlling important structural characteristics of WSe _x films, which belong to the class of layered materials and have good prospects for application in modern nano- and optoelectronic devices. It is established that, by using thermal treatment and ion irradiation during pulsed laser deposition (PLD) in the shadow of an antidroplet shield, it is possible to vary the Se/W atomic ratio from 5 to 1.5, change the character of atomic packing, and obtain films with either smooth or rough surfaces. An increase in the height of parabolic protrusions on the surface up to 200–500 nm leads to a decrease in the optical reflection coefficient in a broad wavelength range from 30% (typical of smooth films) to 6%, which can favor a significant increase in the efficiency of solar cells based on semiconductor films of this type.     

On the features of hydrogen detection by a semiconductor structure grown on a 6H-SiC substrate by the combined method of platinum ion implantation and deposition

The results of a comparative study of the electrical properties of gas-sensitive semiconductor structures grown by the pulsed laser deposition of platinum, platinum ion implantation, and a combined method of platinum implantation and deposition onto an n-6H-SiC substrate are presented. Double-layer structures show a stronger response to hydrogen gas with a more pronounced diode behavior of the currentvoltage characteristics at high temperatures of ～500°C than single-layer ion-implanted structures. Furthermore, double-layer structures exhibit higher reproducibility of the current-voltage characteristic parameters during thermal cycling in a hydrogen-containing medium than ordinary thin-film structures on SiC substrates. The chemical state of ion-implanted platinum and the structure of thin-film layers after long-term testing are studied under harsh conditions. Possible mechanisms of the effect of platinum on the current flow in the ion-implanted layer and its dependence on the composition of the surrounding gaseous medium are considered.     

Structure and mechanical properties of W-Se-C/diamond-like carbon and W-Se/diamond-like carbon bi-layer coatings prepared by pulsed laser deposition

Bi-layer W-Se-C/diamond-like carbon (DLC) and WSe_x/DLC coatings were obtained by standard and shadow-masked pulsed laser co-deposition from WSe₂ and graphite targets. W-Se-C coatings appeared as nanocomposites containing quasi-amorphous WSe₂, WC, spherical β-W nanocrystalline particles encapsulated in WSe₂ amorphous shell, and amorphous carbon phases. In WSe_x/DLC coatings, the formation of chemical bonds between W and C atoms was noticed at the interface. An increase of the C concentration over 40 at.% increases hardness and elasticity (up to 2 times at ~ 60 at.%C), and the Se/W ratio was always close to 1.4. The use of shadow-masked configuration avoids the deposition of micro- and nanoparticles. However, this method leads to a substantial increase of the Se content (Se/W ≥ 4), and the coatings became softer.     

On the mechanism of encapsulated particle formation during pulsed laser deposition of WSe x thin-film coatings

We have studied factors influencing the formation of particles with the structure of a spherical metal W core inside a WSe₂ shell during pulsed laser deposition (PLD) of thin films of tungsten diselenide under variable conditions (buffer gas (Ar) pressure, substrate temperature). It is established that the metal core is formed at the stage of laser ablation of a synthesized WSe₂ target, while the shell grows as a result of condensation, migration, and redistribution of atoms during deposition of a laser-initiated atomic flow on the surface of a growing film. Retardation of the atomic flow by a buffer gas at pressures within 2–10 Pa does not ensure activation of the shell condensation process on the metal core in the gas phase. Increasing the substrate temperature from room temperature up to 250°C leads to transformation of the shell structure from amorphous into laminar.     

The influence of the local atomic packing of thin MoS x films on their electrocatalytic properties in hydrogen reduction

Technical Physics Letters - We have studied the characteristics of the structure, composition, and electrocatalytic properties of thin MoS x films obtained by pulsed laser deposition using an Mo...     

Pulsed laser deposition of composite Mo–Se–Ni–C coatings using standard and shadow mask configuration

Thin film Mo–Se–Ni–C coatings for tribologycal applications were prepared by pulsed laser co-deposition from two targets — MoSe₂(Ni) and graphite. Two methods of deposition from the MoSe₂(Ni) target were used: deposition with unhindered expansion of the laser plume (standard PLD) and deposition of a plume scattered in collisions with Ar gas (pressure 2 Pa) in the shadow of a mask (shadow-masked PLD — SMPLD). Doping with carbon by PLD was used in all cases, and carbon content in the composite Mo–Se–Ni–C coatings was varied in the range 35–85.5 at.%. Pure MoSe_x(Ni) coatings were also prepared by PLD and SMPLD. Rutherford backscattering spectroscopy of helium ions, scanning electron and atomic force microscopy, electron probe microanalysis, transmission electron microscopy and micro-diffraction, micro-Raman spectroscopy, X-ray photoelectron spectroscopy and hardness evaluation were all used for comparative studies of the coatings obtained by PLD and SMPLD/PLD. In the PLD coatings, micron-sized particles were found, consisting of pure Ni or MoSe₂, as well as nanometre-sized particles of monocrystalline Mo. The nanoparticles were distributed on the surface and in an amorphous matrix in all PLD coatings. Within the amorphous matrix of the Mo–Se–Ni–C coatings, local ordering of atoms was detected, causing the formation of a mixture of amorphous carbon, Mo–C, and Mo–Se phases. Increasing the carbon content caused an increase in the content of sp³ bonds in the carbon phase, and an increase in the hardness of the coatings. SMPLD/PLD coatings had no micro- and nanoparticles, but these coatings were characterized by high selenium content and reduced density. Doping with carbon in the SMPLD/PLD configuration caused the formation of composite coatings containing Mo–Se and amorphous carbon phases as in the PLD coatings, but the hardness of the composite SMPLD/PLD coatings was significantly lower than even the hardness of the pure PLD MoSe_x(Ni) coatings.     

The Effect of Boron on the Structure and Conductivity of Thin Films Obtained by Laser Ablation of Diamond with Deposition at 700°C

Structural features of CB_x films obtained by pulsed laser ablation of targets made of pressed diamond powder with boron-powder additions at B/C atomic ratio of x = 0.33 have been studied. The films were deposited on heated substrates, so that diffusion processes involving C and B atoms on the surface and in the volume of films were possible. Selected conditions of film deposition ensured their effective doping with boron (0.4 ≤ x ≤ 0.6). The incorporation of B atoms was accompanied by the formation of B–C chemical bonds, whereas the formation of sp² graphite bonds and their ordering in clusters with laminar packing was suppressed. The films possessed very low resistivity (~1.4 mΩ cm) at room temperature and exhibited metallic type of conductance on decreasing the temperature to 77 K.     

Pulsed laser deposition of antifriction thin-film MoSex coatings at the different vacuum conditions

MoSe_x coatings were obtained by pulsed laser deposition in vacuum at the pressure of background Ar gas up to 10 Pa. The deposition temperature was 200 °С. The films were studied by means of X-ray diffraction, scanning and transmission electron spectroscopy, X-ray photoelectron spectroscopy, and Rutherford backscattering spectroscopy of helium ions. The tribological properties of thin-film coatings were investigated by pin-on-disk testing in air with 50% relative humidity. In addition, wear tracks were studied by micro-Raman spectroscopy. Chemical composition, structure, and tribological properties of the coatings were found to be sensitive to the presence of the inert gas. Thus, increasing the gas pressure from 10^− 4 to 10 Pa changes the chemical composition, so that the ratio of the atomic concentrations of Se and Mo (x = Se/Mo) increases from 1.5 to 2.4 in the principal deposition zone. The changing of the structure concerns the accumulation of distortions in the lattice of MoSe_x nano-crystals as increasing the distance between the basal planes and intensive formation of nano-sized inclusions of the amorphous phase and Mo nano-crystals in the volume of the coatings. At the optimal gas pressure (∼ 2 Pa), the composition of the coating was close to the stoichiometric one, and the layer adjacent to the substrate consisted of MoSe_x nano-crystals with the basal planes parallel to the substrate surface or oriented at small angles to the surface. The thickness of the oriented layer in such coatings was greater than the thickness of the similar layer in the coatings deposited in vacuum (10^− 4 Pa). The tribological properties of MoSe_x coatings deposited on substrates of stainless steel type 95 × 18 (18 at.% Cr) depend on the gas pressure. The friction coefficient in air decreases from 0.08 for deposition at the background pressure of 10^− 4 Pa to 0.04 for deposition at the optimal pressure. This change in the deposition conditions has only a marginal effect on the coating durability. Means to increase the durability are also considered.