V. Perfilyev

Controlled Doping of MS2 (M=W, Mo) Nanotubes and Fullerene‐like Nanoparticles

Doping of semiconductor nanocrystals and nanowires with minute amounts of foreign atoms plays a major role in controlling their electrical, optical, and magnetic properties. In the case of carbon nanotubes, subsequent doping with oxygen and potassium leads to a p-type and n-type behavior, respectively. In another work, VOx nanotubes were transformed from spin-frustrated semiconductors to ferromagnets by doping with either electrons or holes. Calculations indicated that nand p-type doping of multiwall MoS2 nanotubes (INT) could be accomplished by substituting minute amounts of the Mo lattice atoms with either Nb (p-type) and Re (n-type), respectively. Substituting (< 0.1 at%) molybdenum by rhenium atoms and sulfur by halogen atoms was shown to produce n-type conductivity in MoS2 crystals. To synthesize rhenium-doped nanoparticles (NP) and nanotubes both in situ and subsequent doping methods were used. Figure 1a shows the quartz reactor used for in situ synthesis of rhenium doped MoS2 NP with fullerene-like structure (Re:IF-MoS2). The formal Re concentration was varied from 0.02 to 0.7 at%. The precursor RexMo1 xO3 (x< 0.01) powder was prepared in a specially designed auxiliary reactor (see Supporting Information). Evaporation of this powder takes place in area 1 at 770 8C (Figure 1 a). The oxide vapor reacts with hydrogen gas in area 2 (Figure 1a) at 800 8C which leads to a partial reduction of the vapor and its condensation into Re-doped MoO3 y nanoparticles. The resulting NP react with H2/H2S gas in area 3 at 810–820 8C to produce reduced oxide nanoparticles engulfed with a few closed layers of Re:MoS2, which protect it against ripening into bulk 2H-MoS2. [7] To complete this oxide to sulfide conversion a long (25–35 h) annealing process at 870 8C in the presence of H2S and forming gas (H2 10 wt %; N2) was performed. At the end of this diffusion-controlled process a powder of Re-doped MoS2 NP with a fullerene-like (IF) structure (Re:IF-MoS2) was obtained. In addition, doping of IF-WS2 NP and INT-WS2 was subsequently carried out by heating the pre-prepared IF/INT in an evacuated quartz ampoule also containing ReO3, or ReCl3 and iodine. In the case of ReCl3, both the rhenium and the chlorine atoms (substitution to sulfur atoms) served as ntype dopants. Typical high-resolution scanning electron microscopy (HRSEM) and transmission electron microscopy (HRTEM) micrographs of the Re-doped fullerene-like NP are shown in Figure 1b. The Re:IF-MoS2 consists of about 30 closed (concentric) MoS2 layers. No impurity, such as oxides, or platelets (2 H) of MoS2 could be found in the product powder. The line profile and the Fourier analyses (FFT) (inset of Figure 1b) show an interlayer spacing of 0.627 nm (doped). Furthermore, the layers seem to be evenly folded and closed with very few defects and cusps, demonstrating the Re-doped NP to be quite perfectly crystalline. HRTEM did not reveal any structural changes even for the samples with high Re concentration (0.71 at%). However, owing to its quasispherical shape and size, this analysis cannot completely rule-out the presence of a small amount of the ReS2 phase in the IF NP. Figure 1c shows a typical TEM image of Re(Cl) (post synthesis) doped multiwall WS2 nanotube. There is no [*] L. Yadgarov, Dr. R. Rosentsveig, Dr. A. Albu-Yaron, Prof. R. Tenne Department of Materials and Interfaces, Weizmann Institute Rehovot 76100 (Israel) E-mail: reshef.tenne@weizmann.ac.il

Dislocation Structure and Stick–Slip Phenomenon

Abstract Friction is a complex process involving multi-scale asperity contact and large plastic deformation associated with the development of a dislocation structure. Friction is closely associated with the stick–slip phenomenon. In spite of the large number of papers, dedicated to stick–slip effects, little effort has been directed toward elucidating the development of the dislocation structure during stick–slip phenomena. Here, we report some new systematic investigations into the dislocation nature of stick–slips during low-velocity friction of a lithium fluoride single crystal rubbed against a spherical diamond indenter. It is shown that the average velocity of the indenter in the stick phase is about 300 times lower than the maximal velocity in the slip. This difference in velocities leads to entirely different dislocation behavior and damage development in the stick and slips phases. The stick phase is mainly determined by time-dependent strain (creep) wherein, as in metals and alloys, three stages of creep are observed. Based on the analysis of the dislocation structure, a model of the dislocation distribution in the regions of stick (creep) and slip is proposed.

Tribological and Adhesion Properties of CVD Diamond Films Grown on Steel with a Cr‐N Interlayer

In the present work we investigate friction and adhesion properties of microcrystalline diamond films grown on steel substrates with a Cr‐N interlayer prepared at 500° C and 800° C. Scratch tests were performed at indentation loads of 1–4 N and sliding velocity of 0.08 mm/s. The friction coefficient and wear loss were assessed. The diamond film structure and composition are analyzed using different microscopic and spectroscopic techniques. The effect of the diamond structure on adhesion of the deposited diamond films to the substrates, as well as the friction coefficient and wear were evaluated. Continuous, good crystalline quality diamond films with a similar phase content and surface morphology were obtained on both, high and low temperature nitridated substrates, provide similar tribological properties of films. Thus, the temperature of interlayer preparation does not affect the friction coefficient and the size of wear profiles. Substantially different results were obtained when evaluating the adhesio...

Deformation and Fracture of Copper and Silicon During Indentation Acoustic Emission Measurements

The acoustic emission (AE) signals during different phases in indentation (loading, dwell, and unloading times) of ductile Cu and brittle Si were studied. The effect of load and time of loading on the AE parameters were studied. The waveforms and dominant frequency ranges were analyzed. In order to clarify the effect of friction during indentation on the AE signals, the indentation was performed through lubricant film. It was shown that the main AE signals were observed just in the loading phase in indentation of Cu. The low-amplitude wide band noise-like high-frequency-range AE signals, observed during indentation of Cu, can be associated with breakaway and jump in the motion of dislocations. Much larger duration of the AE signals in indentation of Cu indicates a substantially larger dislocation glide in annealed Cu in comparison with limited deformation in Si. The waveform during the indentation of brittle Si presents strong burst-type AE signals. Indentation of Si is accompanied by high-energy and low frequency AE signals characterizing development of the dominant damage and limiting level of plastic deformation. Indentation through the lubricant film leads to the formation of flaky cleavage and swelling around the impression.

The Failure and Damage Mechanisms Under Friction of Copper in the EHL and Mixed EHL Regions

Recently, friction and wear behaviour of Copper (Cu)–steel pairs rubbed under different lubrication conditions were studied. The Stribeck curve was used to identify the different regimes of friction of copper with different virgin grain sizes: the elasto-hydrodynamic lubrication (EHL), mixed lubrication and boundary lubrication (BL) regimes. The aim of this work is the detailed analysis of the damage evolution under friction in the EHL and mixed EHL regions. The effects of load on the friction and the wear and damage mechanisms have been studied. The surfaces of Cu samples before and after friction have been analysed using SEM and AFM techniques as well as roughness and hardness measurements. It was shown that the mechanisms of damage and failure of Cu samples under friction in the EHL region are similar to the damage and failure mechanisms in Very High Cycle Fatigue. Friction in the EHL region is accompanied by initiation and coalescence of pores and microcracks. The effects of the coalescence of pores and microcracks observed in the EHL region are enlarged with a subsequent loading in the mixed EHL region. The effect of the loading rate on the transition from the EHL to BL region has been studied. It was shown that decreasing the loading rate increases significantly the load of the transition from the EHL to BL region. The pore and microcrack coalescence remain as the dominant damage mechanism under friction in the mixed EHL region with the low loading rate, while a lot of ploughing tracks and large delaminated regions appeared on the surface of Cu sample after friction with the higher loading rate.

Friction and Wear of MoS2 Films on Etched Steel Surfaces

Surface texturing is one of modern technologies using for improvement friction and wear properties of rubbed surfaces. The simple and effective methods of surface texturing are mechanical or chemical treatments of friction surfaces. In this work we deal with chemical treatment of steel surfaces with following filling the profile by solid lubricant particles as MoS2 . The steel surfaces were etched with phosphoric acid. The effect of time and temperature of etching on the surface roughness parameters was studied. Some standard roughness parameters of etched surface were measured. It was found that the parameters of roughness as Rz and Sm characterizing the depth of valleys and their size can be used for description of etched surfaces. Optimal parameters, Rz and Sm, providing the best tribological properties were determined: Rz = 5 ∼ 7 m and Sm = 50–70 m. The thickness of solid lubricant film was about 1 m. Finally, the tribological properties of MoS2 films on the steel surfaces textured by chemical treatment were performed. The friction coefficient and wear life of solid lubricant films was assessed. Friction and wear properties of MoS2 films on etched surfaces were compared with similar films on virgin steel surface. The wear life of MoS2 films on etched surfaces was 5–7 times larger than for solid lubricant film on mechanically treated steel surface.Copyright

Friction and Wear of MoS2 Films on Laser Textured Steel Surfaces

Incorporation of solid lubricant into micro-reservoirs produced by Laser Surface Texturing (LST) and its effect on the tribological properties of surfaces under dry friction is studied. The density of the dimple reservoirs and the height of the bulges around them are investigated in terms of the longevity of solid lubricant films burnished on LST steel surfaces. Friction tests were performed using a ball-on-flat device. Optimum density (40–50%) of the dimples is revealed. It is shown that the adhesion of solid lubricant in the space between the dimples is provided by mechanical engagement of particles in the rough surface and by smearing the solid lubricant around the dimples. Best results are obtained with the surfaces that were lapped to half of the height of bulges. Long wear life of burnished film on LST steel surfaces is apparently provided by preservation of thin MoS2 film around the bulges and by supply of solid lubricant from the dimples to the surface. The effect of repeated burnishing on wear life of solid lubricant films was studied. Repeating burnishing leading to increasing the density of solid lubricant films increases the wear life.Copyright