A comparison of oxidation and oxygen substitution in MoS2 solid film lubricants

Significant advancements in the production of low friction, long wear life, sputter-deposited MoS₂ lubricant coatings have been made in the last decade. The introduction of multi-layered coatings, the establishment of careful controls on doping during DC and magnetron sputter deposition, and the implementation of ion assisted deposition have resulted in lubricants with substantially longer wear lives (up to a factor of ten greater than in the early 1980s) and lower sliding friction coefficients. A major research effort, designed to improve the performance of solid lubricants, involved a number of laboratories during this time period, resulting in these major breakthroughs. However, even with this concentrated effort, the typical investigation involved making an educated guess, based on previous experience, of the deposition conditions, target compositions, or post treatments that might be expected to provide improved performance of resulting coatings. One notable discovery during this time period was that typical MoS₂ films contain large quantities (up to 20 atom %) of oxygen substituted for sulfur in individual crystal lattices. In this paper we will compare the effects of this oxygen substitution with the effects of oxidation which involves a change in the oxidation number of the central molybdenum atoms within the crystals. A discussion of the relationship(s) between chemistry and coating structure and tribological performance will be presented with emphasis on defect chemistry and multiple phase interactions. Speculations on the role of coating chemistry in determining coating performance in applications such as in ball bearings will be presented.     

In Situ Analysis of Third Body Contributions to Sliding Friction of a Pb–Mo–S Coating in Dry and Humid Air

Reciprocating sliding tests of ion-beam deposited (IBD) Pb–Mo–S coatings were performed with an in situ tribometer that allows real-time visualization and Raman analysis of the sliding contact through a transparent hemisphere. Experiments were performed in dry air, ambient air (∼50% RH) and mixtures of dry and humid air cycled between low and high humidity. Third bodies formed in the sliding contact were monitored through an optical microscope and analyzed by Raman Spectroscopy. Third body velocity accommodation modes were identified and correlated with friction behavior in dry and ambient air. The dominant velocity accommodation mode in both dry and humid air was interfacial sliding between the outer surface of the transfer film and the wear track; this interface, based on present and earlier studies, is crystalline MoS₂. Therefore, the friction coefficient was controlled by the interfacial shear strength of MoS₂ sliding against MoS₂. Humid air sliding was accompanied by a rise in the friction coefficient and a small but observable second velocity accommodation mode: shear/extrusion of the transfer film. It is concluded that the friction rise in humid air was due to an increase in the interfacial shear strength, and that the rise in friction caused the third body to deform rather than the deformation causing the friction to rise.     

单层MoS2和WS2的太赫兹近场显微成像研究

采用太赫兹散射式扫描近场光学显微镜(THz s-SNOM)研究了化学气相沉积法制备的单层MoS₂和WS₂晶粒的太赫兹近场响应。在没有可见光激发时,未探测到可分辨的太赫兹近场响应,说明晶粒具有较低的掺杂载流子浓度。有可见光激发时,由于光生载流子的太赫兹近场响应,能够测得与晶粒轮廓完全吻合的太赫兹近场显微图。在相同的光激发条件下,MoS₂的太赫兹近场响应强于WS₂,反映了两者之间载流子浓度或迁移率的差异。研究结果表明,THz s-SNOM兼具超高的空间分辨率和对光生载流子的灵敏探测能力,对二维半导体材料和器件光电特性的微观机理研究具有独特的优势。     

Friction and Wear Properties of Cyanex 302-Modified MoS2 Micro-Sized Spheres as Additive in Liquid Paraffin

The friction and wear properties of MoS ₂ micro-sized spheres (MS-MoS₂) modified by Cyanex 302 (bis(2,4,4-trimethylpentyl) monothiophosphinic acid) as additive in liquid paraffin (LP) were studied and compared with those of commercial colloidal MoS ₂ (CC-MoS ₂ ) on a four-ball tester and an Optimol SRV oscillating friction and wear tester in a ball-on-disc contact configuration. The worn surfaces were examined with SEM and XPS, respectively. The results showed that the Cyanex 302-modified MS-MoS ₂ was a better extreme-pressure and antiwear and friction-reducing additive in LP than CC-MoS ₂ . The boundary lubrication mechanism could be deduced as the effective chemical adsorption film formed by the alkyl and active elements (S, O, and P) in Cyanex 302 and tribochemical reaction and deposition film containing MoS ₂ , Fe ₂ O ₃ , and FePO ₄ . Moreover, the coexistence of sliding and rolling frictions in the lubricant of MS-MoS ₂ /LP also contributed to the enhanced tribological properties.     

Tribological properties of spark-plasma-sintered ZrO2(Y2O3)-CaF2-Ag composites at elevated temperatures

Spark-plasma sintering is employed to synthesize self-lubricating ZrO₂(Y₂O₃) matrix composites with different additives of CaF₂ and Ag as solid lubricants by tailoring the composition and by adjusting the sintering temperature. The friction and wear behavior of ZrO₂(Y₂O₃) matrix composites have been investigated in dry sliding against an alumina ball from room temperature to 800 °C. The effective self-lubrication at different temperatures depends mainly on the content of various solid lubricants in the composites. The addition of 35 wt.% Ag and 30 wt.% CaF₂ in the ZrO₂(Y₂O₃) matrix can promote the formation of a well-covered lubricating film, and effectively reduce the friction and wear over the entire temperature range studied. The friction coefficients at low temperatures were at a minimum value for the composite containing 35 wt.% of silver. At this silver concentration, low and intermediate temperature lubricating properties are greatly improved without affecting high-temperature lubrication by the calcium fluoride in ZrO₂(Y₂O₃) matrix composites. The worn surfaces and transfer films formed during wear process have been characterized to identify the synergistic lubrication behavior of CaF₂ and Ag lubricants at different temperatures.     

Tribological Behavior of Novel Ti3SiC2 (Natural Nanolaminates)-Reinforced Epoxy Composites during Dry Sliding

In this article, we report for the first time the synthesis and characterization of Ti₃SiC₂–epoxy (MAXPOL) composites. Three novel composites were designed by adding 20.7, 30.6, and 71.6 vol% Ti₃SiC₂ particulates to an epoxy matrix. The microstructure evaluation by scanning electron microscopy (SEM) showed that the Ti₃SiC₂ particles are well dispersed in the epoxy matrix. The addition of Ti₃SiC₂ enhanced the ultimate yield strength (UYS) and hardness of all of the composites compared to epoxy. Tribological studies were performed by a tab-on-disc method against Inconel 718 and alumina substrates. In both cases, the mean friction coefficient (µ_mean) decreased as the concentration of Ti₃SiC₂ in the epoxy matrix was increased. The concomitant wear rates (WRs) also decreased steadily and then increased slightly after reaching a concentration of ~32.6 vol% Ti₃SiC₂. The tribological studies proved conclusively that the addition of Ti₃SiC₂ in the epoxy matrix imparts self-lubricity to the composites. The tribofilms formed on different tribosurfaces were also characterized by detailed SEM investigations.     

Tribological Performance of Diamond and Diamondlike Carbon Films at Elevated Temperatures

In this study, the authors investigated the tribological performance of diamond and diamondlike carbon (DLC) films as a function of temperature. Both films were deposited on silicon carbide (SiC) by microwave plasma chemical vapor deposition and ion-beam deposition processes. Tribological tests were performed on a reciprocating wear machine in open air (20 to 30% relative humidity) and under a 10 N load using SiC pins. For the test conditions explored, the steady-state friction coefficients of test pairs without a diamond or DLC film were 0.7 to 0.9 and the average wear rates of pins were 10^?5 to 10^?7 mm³/N·m, depending on ambient temperature. DLC films reduced the steady-slate friction coefficients of the test pairs by factors of three to five and the wear rates of pins by two to three orders of magnitude. Low friction coefficients were also obtained with the diamond films, but wear rates of the counterface pins were high due to the very abrasive nature of these films. The wear of SiC disks coated with either diamond or DLC films was virtually unmeasurable while the wear of uncoated disks was substantial. Test results showed that the DLC films could afford low friction up to about 300° C. At higher temperatures, the DLC films graphitized and were removed from the surface. The diamond films could withstand much higher tempera-lures, but their tribological behavior degraded. Raman spectroscopy and scanning electron microscopy were used to elucidate the friction and wear mechanisms of both films at high temperatures.     

Tribological properties of molybdenum disulfide nanosheets by monolayer restacking process as additive in liquid paraffin

Molybdenum disulfide nanosheets were prepared by monolayer restacking process. Results of transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and scanning electron microscopy (SEM) showed that the obtained MoS₂ nanosheets had a thickness about 30-70 nm. The tribological properties of the so-prepared MoS₂ nanosheets were investigated on a MQ-800 four-ball tribometer. The results showed the base oil with MoS₂ nanosheets had better friction reduction, wear resistance and extreme pressure than those with commercial micro-MoS₂. The good tribological properties of MoS₂ nanosheets were mainly ascribed to the surface effect and the dimension effect of nanoparticles. Moreover, the formation of MoO₃ and FeSO₄ complex film on the rubbed surface also played an important role in friction reduction and wear resistance.     

Torsional Fretting Wear Behavior of Bonded MoS2 Solid Lubricant Coatings

The effects of applying a bonded MoS₂ solid lubricant to a 1050 steel substrate were investigated using a torsional fretting wear apparatus. Tests were conducted under a normal load of 50 N with angular displacement amplitudes ranging from 0.1 to 5°. Wear scars were examined using scanning electron microscopy, energy-dispersive X-ray spectrometry, optical microscopy, and surface profilometry. The MoS₂ coating exhibited different torsional fretting regimes than those of the substrate. Fretting regimes of the coating were primarily in the partial slip regime (PSR) and the slip regime (SR) with no mixed fretting regime. The width of the PSR narrowed. Due to the lubricating effects of the coating, the friction torque was consistently lower than that of the substrate. The damage to the coating in the PSR was very slight, and its granular structure remained even after 1,000 cycles. The damage mechanism to the SR coating was a combination of abrasive wear, oxidative wear, and delamination. The MoS₂ coating had potential to alleviate torsional fretting wear.     

Tribological Behaviour of Nanostructured Al2O3-3 wt% TiO2 Coating Against Steel in Dry Sliding

Nanostructured and conventional Al₂O₃-3 wt% TiO₂ coatings were deposited by atmospheric plasma spraying. The wear and friction properties of both coatings against a steel ball under dry friction conditions were examined. It was found that the wear resistance of the nanostructured Al₂O₃-3 wt% TiO₂ coating was superior to that of the corresponding conventional counterpart. The improvement in wear resistance of the nanostructured coating was attributed to its higher toughness and cohesion strength between splats. As for the nanostructured coating, the wear mechanism was mainly adhesion with micro-abrasion at low loads (20 N). At high loads (80 N), the wear of the nanostructured coating was controlled by plastic deformation and associated delamination along the splat boundaries, which was similar to that of the conventional coating at low loads. However, the failure of the conventional coating was predominantly brittle fracture within the splats and delamination between splats at high loads.     

Wear and Friction of Filled Polytetrafluoroethylene Compositions in Liquid Nitrogen

Wear and friction behavior of slider materials at cryogenic temperature is important to the development of seals and bearings for missile powerplants. Data were obtained in liquid nitrogen (?320°F) with a series of molded and extruded polytetrafluoroethylene (PTFE) compositions containing various filler materials. A 3/16-in. radius rider specimen (PTFE materials) was caused to slide in a circumferential path on the flat surface of a rotating 2½-in. diameter disk specimen (usually type 304 stainless steel). The sliding velocity was usually 2300 ft per min and the load was 1000 grams.

As compared with reference steels and carbons used in conventional seals and bearings, the filled PTFE compositions gave low wear and friction (friction coefficients from 0.06 to 0.13) in liquid nitrogen. Several extruded compositions have particular promise for seal and bearing materials. An extruded glass-filled material gave wear and friction that was essentially unaffected by sliding velocities to 6000 ft per min.     

Effect of current density,MoS2 content and bath agitation on tribological properties of electrodeposited nanostructured Ni-MoS2 composite coatings

ABSTRACT

Nanostructured nickel coatings with molybdenum disulphide particles were electrodeposited to form composite coatings. Three different current densities, i.e. 3, 5 and 7?A/dm² were investigated initially. The best results were obtained with 5 A/dm² for codeposition of nanostructured Ni-MoS₂ composite coatings. With the addition of 1–4?g/L molybdenum disulphide to the bath, the weight percentages of MoS₂ particles in the coatings were 23–38%. This increase of MoS₂ content was accompanied with decrease in friction coefficient of the coatings from 0.35 to 0.08. Wear resistance of the coatings was increased with increasing MoS₂ content and the weight loss was decreased from 1.4 to 0.7?mg. Hardness was decreased from 585 to 400 VHN with increasing the MoS₂ content. By increasing bath agitation speed up to 150?rpm, more MoS₂ particles were embedded in the matrix and the coatings showed better wear resistance. However, increase of agitation speed from 150 to 200?rpm caused a decrease of MoS₂ particles in the nickel matrix due to the turbulent motion of particles in the bath. Overall, it was shown that the lubricating effect of MoS₂ in the coating was more influential than the nanocrystallinity of the nickel matrix in improving tribological properties of these composite coatings.     

Antiwear Properties of Bonded MoS2 Solid Lubricant Coating under Dual-Rotary Fretting Conditions

Bonded MoS₂ solid lubricant coatings are widely used in tribology for their friction-reducing and antiwear properties. However, such coatings have been rarely investigated in complex fretting conditions, such as dual-rotary fretting (DRF). DRF is a complex fretting wear mode that combines torsional fretting with rotational fretting. In this work, the antiwear properties of bonded MoS₂ solid lubricant coating under dual-rotary fretting conditions were studied. Results indicated that the MoS₂ coating had better friction-reducing and antiwear properties than the substrate for alleviating DRF wear. The coating can greatly influence the fretting regimes and reduce the coefficient of friction. Furthermore, the service life of the coating was strongly dependent on the competition of the two fretting components and was reduced as the rotational fretting component increased.     

Friction and Wear Properties of IF–MoS2 as Additive in Paraffin Oil

Inorganic fullerene-like (IF) MoS₂ nanoparticles with diameters ranging from 70 to 120 nm were synthesized by desulphurizing the MoS₃ precursor and characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Tribological properties of the IF–MoS₂, as lubricating oil additive, were evaluated using a MMW-1 four-ball tribotester. The wear scar was examined with an optical microscope and scanning electron microscopy (SEM). The wear resistance of the paraffin oil was improved and the friction coefficient of the oil was decreased by addition of the IF–MoS₂ nanoparticles. The mechanism of friction and wear of the IF–MoS₂ nanoparticles was discussed.     

Residual stresses in TiN, DLC and MoS2 coated surfaces with regard to their tribological fracture behaviour

Thin hard coatings on components and tools are used increasingly due to the rapid development in deposition techniques, tribological performance and application skills. The residual stresses in a coated surface are crucial for its tribological performance. Compressive residual stresses in PVD deposited TiN and DLC coatings were measured to be in the range of 0.03-4 GPa on steel substrate and 0.1-1.3 GPa on silicon. MoS₂ coatings had tensional stresses in the range of 0.8-1.3 on steel and 0.16 GPa compressive stresses on silicon. The fracture pattern of coatings deposited on steel substrate were analysed both in bend testing and scratch testing. A micro-scale finite element method (FEM) modelling and stress simulation of a 2 μm TiN-coated steel surface was carried out and showed a reduction of the generated tensile buckling stresses in front of the sliding tip when compressive residual stresses of 1 GPa were included in the model. However, this reduction is not similarly observed in the scratch groove behind the tip, possibly due to sliding contact-induced stress relaxation. Scratch and bending tests allowed calculation of the fracture toughness of the three coated surfaces, based on both empirical crack pattern observations and FEM stress calculation, which resulted in highest values for TiN coating followed by MoS₂ and DLC coatings, being K_C = 4-11, about 2, and 1-2 MPa m^1/2, respectively. Higher compressive residual stresses in the coating and higher elastic modulus of the coating correlated to increased fracture toughness of the coated surface.     

The Solubility of Helium in Various Lubricating Oils at High Temperatures and Pressures

The helium solubility characteristics of six low-viscosity lubricating oils have been investigated; namely, diester, aryl phosphate ester, dimethyl silicone, methyl phenyl silicone, paraffin base oil, and aromatic base oil. Solubility measurements were made over a 1000 psi pressure range and a 350 F temperature range. Results show that the solubility of helium in all fluids studied increases linearly not only with pressure but also with temperature. All fluids except the silicones exhibited heavy foaming tendencies upon decompression.     

Experiments with Hydrodynamic Journal Bearings of Various Materials and Designs in Sodium at Temperatures to 800 F

Experiments were conducted with 1.5 inch diameter hydrodynamic journal bearings in liquid sodium at 500 and 800 F, speeds to 12,000 rpm and unit loads to 31 psi. The stability characteristics of five different geometries and the wear and seizure properties of several material combinations were investigated. Tilting pad bearings were most stable. Combinations of a cobalt alloy with nickel alloys or with a titanium carbide cermet showed the best wear and seizure properties.     

Study on the tribological properties of surfactant-modified MoS2 micrometer spheres as an additive in liquid paraffin

Tribological properties of MoS₂ micrometer spheres modified by self-prepared surfactant as an additive in liquid paraffin (LP) are studied and compared with those of the commercial colloidal MoS₂ on a four-ball tester and an Optimol SRV oscillating friction and wear tester. The worn surfaces are examined with SEM and XPS, respectively. Results show that MoS₂ micrometer sphere is a much better extreme-pressure additive and anti-wear and friction-reducing additive in LP than the commercial colloidal MoS₂. The boundary lubrication mechanism can be deduced as an effective chemical adsorption protective film formed by the long chain alkyl and active elements (S and N) in the prepared surfactant and tribochemical reaction film composed of the tribochemical reaction products of the additive. Moreover, sliding and rolling frictions exist simultaneously in the MoS₂ micrometer spheres /LP lubricating system, which also do more contributions to the good tribological properties.     

Tribological behaviour and wear mechanism of MoS2–Cr coatings sliding against various counterbody

MoS₂–Cr coatings with different Cr contents have been deposited on high speed steel substrates by closed field unbalanced magnetron (CFUBM) sputtering. The tribological properties of the coatings have been tested against different counterbodies under dry conditions using an oscillating friction and wear tester. The coating microstructures, mechanical properties and wear resistance vary according to the Cr metal-content. MoS₂ tribological properties are improved with a Cr metal dopant in the MoS₂ matrix. The optimum Cr content varies with different counterbodies. Showing especially good tribological properties were MoS₂–Cr8% coating sliding against either AISI 1045 steel or AA 6061 aluminum alloy, and MoS₂–Cr5% coating sliding against bronze. Enhanced tribological behavior included low wear depth on coating, low wear width on counterbody, low friction coefficients and long durability.     

Synergistic effect on frictional characteristics under rolling-sliding conditions due to a combination of molybdenum dialkyldithiocarbamate and zinc dialkyldithiophosphate

The frictional characteristics of molybdenum dialkyldithiocarbamate (MoDTC) and/or zinc dialkyldithiophosphate (ZnDTP) in a paraffinic mineral oil were studied using a two-roller machine, in which the synergistic effect for reducing friction was found for the oil containing ZnDTP together with MoDTC. Variation in chemical composition of the surface film over time indicated preferential formation of the products through decomposition of ZnDTP. The reduction in friction was mainly due to MoS₂ derived from MoDTC, while ZnDTP had a role in enhancing the wear resistance and promoting the formation of MoS₂. More severe conditions such as an increase in sliding speed tended to more significantly decrease friction.