Tribological properties of MoS2 nano-balls as filler in polyoxymethylene-based composite layer of three-layer self-lubrication bearing materials

POM/MoS₂ nano-balls composite was prepared by adding MoS₂ nano-balls synthesized from Na₂MoO₄ and CH₃CSNH₂ into polyoxymethylene (POM). The comparative POM-based composite blended with micro-MoS₂ particles was also prepared. The obtained POM/MoS₂ composites were used as the polymerical layer in the three-layer self-lubrication materials. The results of tribological tests showed that the POM with MoS₂ nano-balls presented better tribological properties than that with micro-MoS₂. When the content of MoS₂ nano-balls was not more than 1.0 wt%, the POM/MoS₂ nano-balls samples presented lower friction coefficients and smaller wear volumes. However, higher contents of MoS₂ nano-balls than 1.0 wt% were very disadvantageous to the tribological performances. DSC results showed the excessive MoS₂ nano-balls affected the POM crystallinity, and accordingly, the self-lubricating capabilities of these samples were influenced as well. SEM micrographs for wear scars confirmed that the worn manner of the POM sample was changed when the content of MoS₂ nano-balls was increased. XPS analysis showed that MoS₂ nano-balls was transferred to the mated friction surface, on which Mo(IV) was oxidized into Mo(VI) via tribochemical reaction. TEM micrographs of worn debris proposed a wear manner concerning the exfoliation of nano-sheets from MoS₂ nano-balls. The reason for the stable self-lubrication properties of POM/MoS₂ nano-balls composite was ascribed to the forming-destroying of debris clusters in a long-time sliding process.     

Influence of solid lubricants and fibre reinforcement on wear behaviour of polyethersulphone

Polyethersulphone (PES), is an amorphous, brittle and high temperature engineering thermoplastic. Two composites of PES containing short glass fibres (GF) and solid lubricants viz. PTFE and MoS₂; and two composites containing short carbon fibre (CF) [30% and 40%] were selected for the present studies. Compositional analysis of selected materials was done with various techniques such as gravimetry, solvent extraction and thermal analysis viz. thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC). These materials were studied for adhesive and abrasive wear performance by sliding against a mild steel disc and silicon carbide abrasive paper respectively, under different loads. It was observed that GF reinforcement along with incorporation of solid lubricants (PTFE and MoS₂) enhanced the wear performance of PES by an order of two. In the case of solid lubricants, PTFE proved to be more beneficial than MoS₂. CF reinforcement, however, proved to be the most effective in enhancing wear performance of PES. PES reinforced with 40% CF exhibited a specific wear rate in the order of 10⁻¹⁶m³/Nm which is considered to be very good for the thermoplastic composite. In the case of abrasive wear behaviour, however, incorporation of fibres or solid lubricants deteriorated the performance of the neat matrix. SEM was employed to investigate the wear mechanisms.     

Comparative Study on the Tribological Performances of Barium Perrhenate,Molybdenum Disulfide,and Calcium Carbonate as Lubricant Additives in a Wide Temperature Range

Barium perrhenate [Ba(ReO₄)₂], a compound used as an oil additive, was synthesized via the aqua-solution method. Its tribological properties were examined using the four-ball test and ball-on-disc tribotester in a wide temperature range and compared with those of oil that contained the additive molybdenum disulfide (MoS₂) and calcium carbonate (CaCO₃) compound. X-ray diffraction analysis, scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and differential thermal analysis/thermogravimetry were performed to determine the possible mechanism of the antifriction behavior of the lubricants. Results of the four-ball test showed that all of the additives can improve the extreme pressure property of the base oil and decrease the wear scar diameters of low-carbon steel balls. The results of the ball-on-disc test suggested that the MoS₂ additive exhibited better lubrication property than the Ba(ReO₄)₂ and CaCO₃ additives at below 450°C. The CaCO₃ additive displayed moderate performance in friction reducing in the high-temperature period. The Ba(ReO₄)₂ additive exhibited preferable comprehensive antifriction performance in a wide temperature range because of its intrinsic shear-susceptible property and crystalline change with varied temperatures, which could form a protective layer with some native oxides of the disc sample and thus effectively prevented direct contact between rubbing parts. The detailed friction-reducing mechanism of the three additives is also discussed.     

Tribological Properties of MoS2 with Different Morphologies in High-Density Polyethylene

The tribological properties of high-density polyethylene (HDPE) modified by MoS₂ with different morphologies (nano-spheres, nano-platelets, and micro-platelets) were investigated using an end-face tribometer under dry friction and rapeseed oil lubrication. Under dry friction, MoS₂ nano-platelets and nano-spheres exhibited their best properties at 1.0 and 1.5?% (wt%) MoS₂ content, respectively. Under oil lubrication, the nano-spheres were better additives in HDPE than the other two. The melting of HDPE was the main wear mechanism under dry friction, whereas abrasive is the main wear mechanism under oil lubrication. The changing wear mechanisms led to anti-wear variations in HDPEs with increasing MoS₂ contents. The tribological properties were closely related to the crystallinity and thermo-mechanical properties of MoS₂/HDPE. The samples with lower damping factors and better crystallinity showed better tribological properties. The excellent anti-wear properties of nano-spheres can be attributed to the deformation and exfoliation of nano-spheres in the friction process. Nano-platelets and nano-spheres in HDPE are advantageous under dry friction and oil lubrication, respectively. This study better elucidated the relationship between the property and morphology of MoS₂ in a polymer.     

A study of the role of solid lubricant and fibrous reinforcement in modifying the wear performance of polyethersulphone

High‐temperature polymers are generally preferred for those tribology applications where cost is secondary and performance is the primary consideration. Since frictional heat dissipation limits the usefulness of polymers because of their poor thermal conductivity, high‐temperature polymers are preferred in applications which have harsh operating conditions. In this paper, a high‐temperature polymer, polyethersulphone (PES), was selected for some adhesive wear studies, along with two PES composites containing 18% glass‐fibre (GF) reinforcement and two solid lubricants, i.e., PTFE and MoS₂ (2% each). Adhesive wear studies of these materials on two pin‐on‐disc machines indicated that neat PES was not a good tribo‐material. However, incorporation of GF and solid lubricants enhanced the wear performance by an order of two. PTFE was found to behave better than MoS₂. However, after long sliding duration both the lubricants performed almost equally well. The topography of the surface of the pins and the disc was studied using SEM to investigate the wear mechanisms.     

The Effect of Morphology on the Tribological Properties of MoS<Subscript>2</Subscript> in Liquid Paraffin

The tribological properties of liquid paraffin (LP) containing molybdenum disulfide (MoS₂) additives, including nano-balls, nano-slices, and bulk 2H-MoS₂, are evaluated using a four-ball tribometer. Results show that all MoS₂ additives used can improve the tribological properties of LP, and that nanosized MoS₂ particles function as lubrication additives in LP better than micro-MoS₂ particles do. The LP with nano-balls presents the best antifriction and antiwear properties at the MoS₂ content of 1.5 wt%. This is ascribed to the chemical stability of the layer-closed spherical structure of nano-balls. The Stribeck curves confirm that the rotation speed of 1,450 rpm used is located at the mixed lubrication region under 300 N. MoS₂ nano-slices have small sizes and easily enter into the interface of the friction pair with a roughness of 0.032 μm, functioning as a lubricant in LP better than nano-balls do at the MoS₂ content of 1.0 wt%. The Stribeck curves also show that the differences between the two nano samples were magnified at high rotation speeds in hydrodynamic lubrication region. The application of nano-slices in high sliding speeds will be more advantageous. This work furthers the understanding of the relationship between the tribological properties and morphology of MoS₂.     

Multi-environmental lubrication performance and lubrication mechanism of MoS2/Sb2O3/C composite films

MoS₂–Sb₂O₃–C composite films exhibit adaptive behavior, where surface chemistry changes with environment to maintain the good friction and wear characteristics. In previous work on nanocomposite coatings grown by PVD, this type of material was called a “chameleon” coating. Coatings used in this report were applied by burnishing mixed powders of MoS₂, Sb₂O₃ and graphite. The solid lubricant MoS₂ and graphite were selected to lubricate over a wide and complementary range including vacuum, dry air and humid air. Sb₂O₃ was used as a dopant because it acts synergistically with MoS₂, improving friction and wear properties. The MoS₂–Sb₂O₃–C composite films showed lower friction and longer wear life than either single component MoS₂ or C film in humid air. Very or even super low friction and long wear-life were observed in dry nitrogen and vacuum. The excellent tribological performance was verified and repeated in cycles between humid air and dry nitrogen. The formation of tribo-films at rubbing contacts was studied to identify the lubricating chemistry and microstructure, which varied with environmental conditions. Micro-Raman spectroscopy and Auger electron spectroscopy (AES) were used to determine surface chemistry, while scanning electron microscopy and transmission electron microscopy were used for microstructural analysis. The tribological improvement and lubrication mechanism of MoS₂–Sb₂O₃–C composite films were caused by enrichment of the active lubricant at the contact surface, alignment of the crystal orientation of the lubricant grains, and enrichment of the non lubricant materials below the surface. Sb₂O₃, which is not lubricious, was covered by the active lubricants (MoS₂ – dry, C – humid air). Clearly, the dynamics of friction during environmental cycling cleaned some Sb₂O₃ particles of one lubricant and coated it with the active lubricant for the specific environment. Mechanisms of lubrication and the role of the different materials will be discussed.     

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.     

MoS2‐based alloys and nanocomposites for solid lubrication

The development of MoS₂ coatings has involved the modification of substrate surfaces, the addition of metals or compounds to the MoS₂, and variation in the deposition process parameters affecting the properties of deposited films. More recently, multilayer and periodic nanolayer coating structures have also been investigated. At present, work is concentrated on alloys of MoS₂, mainly with various metals, and targeted at terrestrial (ambient air) applications. The addition of metals or compounds to physical‐vapour‐deposited MoS₂ has led to improvements in coating performance, for example, greater stability of friction coefficient, greater film endurance, and increased temperature/oxidation resistance. The metal or compound can be either in the form of nanoscale multilayers or mixed with the MoS₂, sometimes leading to nanoclusters within a MoS₂ matrix. Microstructural analysis seems to show that the primary function of these additives is to suppress the formation of low‐density, columnar structures. At certain concentrations an added metal can also enhance the formation of the tribologically favourable (002) orientation of the MoS₂ crystallites. Other changes in the properties of MoS₂—metal composites may be due to their oxidation resistance, as indicated by the stability of these films against storage in air and their increased endurance when in sliding contacts at elevated temperatures.     

Erratum to: Study the Sensitivity of Solid Lubricating Additives to Attapulgite Clay Base Grease

The focus of this study was the development of a new lubricating grease, using surface-modified attapulgite clay as thickener and synthetic oil (PAO 40) as the base oil. The tribological sensitivity of the new grease was investigated by studying the effect of adding three solid additives [KB₃O₅, MoS₂, graphite and a graphite/MoS₂ mixture (mass ratio 3:2)]. Its tribological behavior was compared with that of traditional bentone grease by adding MoS₂. The dropping point and the cone penetration of the new grease were also investigated and analyzed. The wear scar diameter of the base grease was measured on an MRS-1 J (G) four-ball tester, and the tribological sensitivity of solid lubricating additives to attapulgite clay base grease was evaluated using an Optimol SRV reciprocating friction and wear tester. The addition of MoS₂ and the graphite/MoS₂ mixture to the new lubricating grease improved its friction-reducing ability, while the addition of KB₃O₅ improved its antiwear ability. The additives MoS₂ and the graphite/MoS₂ mixture also increased the load-carrying capacity of the base grease. The attapulgite clay grease containing MoS₂ had a better friction-reducing ability than the traditional bentone grease containing MoS_2.     

Study the Sensitivity of Solid Lubricating Additives to Attapulgite Clay Base Grease

The focus of this study was the development of a new lubricating grease, using surface-modified attapulgite clay as thickener and synthetic oil (PAO 40) as the base oil. The tribological sensitivity of the new grease was investigated by studying the effect of adding three solid additives [KB₃O₅, MoS₂, graphite and a graphite/MoS₂ mixture (mass ratio 3:2)]. Its tribological behavior was compared with that of traditional bentone grease by adding MoS₂. The dropping point and the cone penetration of the new grease were also investigated and analyzed. The wear scar diameter of the base grease was measured on an MRS-1 J (G) four-ball tester, and the tribological sensitivity of solid lubricating additives to attapulgite clay base grease was evaluated using an Optimol SRV reciprocating friction and wear tester. The addition of MoS₂ and the graphite/MoS₂ mixture to the new lubricating grease improved its friction-reducing ability, while the addition of KB₃O₅ improved its antiwear ability. The additives MoS₂ and the graphite/MoS₂ mixture also increased the load-carrying capacity of the base grease. The attapulgite clay grease containing MoS₂ had a better friction-reducing ability than the traditional bentone grease containing MoS_2.     

A Raman Spectroscopic Study of MoS<Subscript>2</Subscript> and MoO<Subscript>3</Subscript>: Applications to Tribological Systems

Molybdenum disulfide (MoS₂) and molybdenum trioxide are investigated using Raman spectroscopy with emphasis on the application to tribological systems. The Raman vibrational modes were investigated for excitation wavelengths at 632.8 and 488 nm using both micro-crystalline MoS₂ powder and natural MoS₂ crystals. Differences are noted in the Raman spectra for these two different wavelengths, which are attributed to resonance effects due to overlap of the 632.8 nm source with electronic absorption bands. In addition, significant laser intensity effects are found that result in laser-induced transformation of MoS₂ to MoO₃. Finally, the transformation to molybdenum trioxide is explored as a function of temperature and atmosphere, revealing an apparent transformation at 375 K in the presence of oxygen. Overall, Raman spectroscopy is an useful tool for tribological study of MoS₂ coatings, including the role of molybdenum trioxide transformations, although careful attention must be given to the laser excitation parameters (both wavelength and intensity) when interpreting Raman spectra.     

Synthesis of nanometric molybdenum disulphide particles and evaluation of friction and wear properties

Nanometric molybdenum disulphide particles of about 30 nm diameter have been prepared by the hydrodesulphurisation of molybdenum trisulphide obtained from acidifying a mixed solution of Na₂MoO₄ and Na₂S at ambient temperature using a quick homogeneous precipitation method (QHPM). Using X‐ray diffraction and transmission electron microscopy (TEM), the size and crystallisation of the nano‐MoS₂ obtained by hydrodesulphurisation at different temperatures have been investigated. In addition, the tribological performance of nano‐MoS₂ has been investigated by means of a block‐on‐ring tribometer, X‐ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). The results show that base oils with nanometric MoS₂ as an additive provide higher wear resistance than other oils containing commercially available common MoS₂. However, the friction reduction is not obviously improved. SEM characterisation reveals a worn, smooth surface when using the nano‐MoS₂ additive, and XPS analysis indicates a greater amount of molybdenum oxide and iron sulphide in the antiwear thin film formed on the rubbed surface.     

Microtribological Performance of Au–MoS2 and Ti–MoS2 Coatings with Varying Contact Pressure

Solid lubricant coatings with co-sputtered metal and MoS₂ have shown favorable macrotribological properties at a wide range of contact stresses and humidity levels. These materials are also candidates for use in microcontacts and micro-electromechanical systems (MEMS), but their performance at this scale is poorly understood. For this study, microtribological properties of Au–MoS₂ and Ti–MoS₂ coatings, with varying metal additives of less than 15 at%, were examined using a nanoindentation instrument. Titanium and gold were chosen for this study as metal additives due to their different influence on the mechanical properties of the coating. The hardness and reduced modulus of the coatings increased with the addition of metal, when compared to pure MoS₂. Reciprocating microscratch tests were performed with two spherical diamond tips (50 and 10 μm radii) in dry air. A range of normal loads were used between 0.2 and 5.0 mN. Friction and wear measurements were analyzed with respect to the variation in the contact pressure and compared to literature studies performed at the macroscale. Correlations were found between the coating mechanical properties, tip-coating adhesion, interfacial shear strength, and the formation of transfer films and tribofilms.     

Chemical Reactivity Factor for Metallic Sulfide Lubricants

Wear data for molybdenum lubricated with various metallic sulfides have been analyzed. It is found that wear life in the Falex Lubricant Tester depends greatly upon chemical reactions, which apparently form MoS₂ in situ. With the possible exception of WS₂, none of the metal sulfides except MoS₂ are lubricants per se. Water solubility as an index of reactivity shows that compounds exhibiting pronounced water solubility function as more effective lubricants for molybdenum than less soluble compounds. There is also some evidence that wear life of alloy pins in the Falex wear test correlate with performance characteristics of ball bearings in high-speed bearing tests for several different alloy retainer materials.     

Surface and Subsurface Contributions of Oxidation and Moisture to Room Temperature Friction of Molybdenum Disulfide

Molybdenum disulfide (MoS₂), a lamellar solid lubricant, is used extensively in space applications due to its exceptional performance in vacuum and inert environments. The friction and wear of MoS₂, however, increase in the presence of atmospheric contaminants, such as water. Despite numerous studies of the moisture-sensitive friction response of MoS₂ over the decades, important fundamental questions remain unanswered. Two leading hypotheses suggest that water affects friction by causing the MoS₂ to oxidize or by physically bonding to edge sites, and thereby disrupting easy lamellar shear. This paper presents a parametric study to (1) isolate the effects of water and oxygen on ambient MoS₂ friction, (2) identify the effect of water and oxygen on MoS₂ oxidation, and (3) distinguish between the effects of water diffusion and surface oxidation on the frictional response of MoS₂ coatings. The experimental findings were used to develop a qualitative model for the effects of environment on MoS₂ friction; the model is used to explain transients, hysteretic effects, oxidation effects, and effects of physically bound water.     

Discussions on “Some Basic Concepts Regarding the Separation of Oily Water Mixtures”

Examination of adsorptive properties of graphite and MoS₂ revealed that their surfaces are composed of two distinct sites, i.e., basal plane and edge sites, having different affinities for polar compounds and paraffmic hydrocarbons.

It was therefore of interest to investigate the relationship between the proportions of the individual surface sites and the lubricating action of these solids.

This was done by preparing special graphite and MoS₂ powders having a high proportion of basal plane surface and comparing their lubricating properties to those of the powders having a relatively high ratio of edge to basal plane surface area.

It was found that the proportion of basal plane surface is an important factor in the lubricating performance of both graphite and MoS₂ and that the powders having predominantly basal plane surface have significantly better antiwear properties than the powders having a high ratio of polar to basal plane surface. The basal plane surface in MoS₂ plays an exceptionally important role in lubrication, and its contamination with strongly adsorbed paraffins reduces substantially the lubricating action of the powder.     

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.     

The Importance of Variable Speeds under Extreme Pressure Loading in Molybdenum Disulfide Greases Using Four-Ball Wear Tests

There is recent concern regarding grease behavior in extreme pressure applications. The research described here is aimed at providing good friction and wear performance while optimizing rotational speeds under extreme loading conditions. A design of experiment (DOE) was used to analyze molybdenum disulfide (MoS₂) greases and their importance in reducing wear under extreme loading and various speeds conditions (schedule 1 and schedule 2 speeds). The lamellar structure of MoS₂ provides very good weld protection by forming a layer that can be easily sheared under the applications of extreme pressures. An extreme load of 785 N was used in conjunction with different schedules of various rotational speeds to examine lithium-based grease with and without MoS₂ for an equal number of revolutions. A four-ball wear tester was utilized to run a large number of experiments randomly selected by the DOE software. The grease was heated to 75°C and the wear scar diameters were collected at the end of each test.

The results indicated that wear was largely dependent on the speed condition under extreme pressure loading, and thus a lower MoS₂ concentration is needed to improve the wear resistance of lithium-based greases. The response surface diagram showed that the developed molybdenum disulfide greases exhibited both extreme pressure as well as good wear properties under various rotational speeds when compared to steady-state speed. It is believed that MoS₂ greases under schedule 1 speeds perform better and provide an antiwear film that can resist extreme pressure loadings.     

The effect of space irradiation on the lubricating performance of perfluoropolyether greases in simulated space environment

In this paper, 2 kinds of commercial perfluoropolyether (PFPE) greases were coated on the polyimide (PI) blocks, which were placed within simulated space environment including atomic oxygen (AO), proton (Pr), ultraviolet (UV), and electron (El) irradiations, and then the tribological performance has been investigated with a ball‐on‐disc tribometer. Results indicated that the MoS₂‐grease showed better lubrication performance than the PTFE‐grease. The changes in infrared spectroscopy induced by Pr and El irradiations were more obvious than that by AO and UV irradiations. Results of energy dispersive X‐ray spectroscopy indicated that Pr and El irradiations caused carbonation of greases, and AO and UV irradiations induced oxidation of greases. Referred to the tribological properties of PI coated with PFPE oil, PI coated with PFPE greases showed minor changes in friction coefficient and wear rate, and the MoS₂ additives could significantly improve the lubrication properties of PFPE greases in simulated space environment.