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.     

Tribochemistry of MoS3 Nanoparticle Coatings

The tribology of nanoparticles based on transition metal dichalcogenides has been studied extensively. However, evaluation of metal chalcogenides with other stoichiometries has been lacking. We have studied the friction, endurance, and tribochemistry of bonded molybdenum trisulfide (MoS₃) nanoparticle-based coatings for the first time. A facile aqueous chemistry method was used to fabricate the MoS₃ nanoparticles. Pin-on-disk tribometry of an MoS₃ coating using phenolic resin as the binder was conducted in a dry N₂ atmosphere (0.06 % RH, using normal loads of 5 N and 10 N). The results were compared with two types of commercial bonded coatings based on the solid lubricant molybdenum disulfide (MoS₂), as well as a bonded coating we formulated with commercial MoS₂ nanoparticles. Surprisingly, the MoS₃ coating showed similar lubricating ability to the MoS₂-based coatings, exhibiting average μ _k < 0.05 and endurance greater than a million cycles. To evaluate the tribochemistry occurring in the contact region, tribotesting of an MoS₃ coating was halted when steady-state low friction was achieved (i.e., prefailure). Raman spectroscopy, X-ray photoelectron spectroscopy, and X-ray diffraction on the surface of this wear track showed that the MoS₃ had undergone a tribochemical reaction to form the solid lubricant MoS₂, which explains the excellent lubricity of the coating. This result opens up the possibility of developing MoS₃ nanoparticle-based solid lubricant coatings and MoS₃ nanoparticle additives for oils and greases that are synthetically easier and lower cost than formulations based on MoS₂ nanoparticles.     

High Wear Resistance of Magnetron Sputtered Cr<Subscript>80</Subscript>Si<Subscript>20</Subscript>N Nanocomposite Coatings: Almost Independent of Hardness

Hardness has been popularly considered as an essential factor defining the wear resistance of hard coatings. Here, we report magnetron sputtered Cr₈₀Si₂₀N nanocomposite coatings, of widely varied packing densities, that exhibited identical specific wear rates, while the hardness changed over a wide range (from ~12 to ~36 GPa). All the Cr₈₀Si₂₀N coatings were free of extended and uninterrupted columnar boundaries, and retained low specific wear rates in the ball-on-plate sliding tests against Al₂O₃ counterpart with a normal load of 5 N (less than 3.0?×?10^?16 m³/N m under ambient condition and less than 2.0?×?10^?15 m³/N m under 3.5 wt% NaCl solution, respectively). Post examination reveals extensive interruption or termination of cracks in the wear tracks of the under-dense coatings, indicative of extrinsic toughening mechanisms by effective relief of local contact stress. Our results suggest that a critical role of toughening rather than hardening, played in enhancing the wear resistance of hard coatings, and thus would pave a way to develop highly wear-resistant coatings with a low hardness.     

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.     

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.     

Effects of ZnO and MoS<Subscript>2</Subscript> Solid Lubricants on Mechanical and Tribological Properties of M50-Steel-Based Composites at High Temperatures: Experimental and Simulation Study

Prospective beneficial effects of mixtures of temperature-adaptive solid lubricants (ZnO–MoS₂) on mechanical and tribological properties of M50 alloy steel were investigated at temperatures from 25 to 800 °C. ZnO and MoS₂ were mixed with M50 (designated as M) to create composites MZ (M50 steel plus ZnO), MM (M50 steel plus MoS₂), and MZM (M50 steel plus both additives). Sliding friction and wear experiments were performed at different temperatures using a pin-on-disk at a sliding speed of 0.2 m s^?1 and a load of 12 N. Silicon nitride and M50 steel were used as the pin materials. In order to understand the friction and wear behavior of composites, analyses of their surfaces were done using XRD, EPMA, FESEM, EDS line/mapping, and XPS tests. A dynamic simulation model based on the finite element method was built to simulate the different stresses on the contact pairs. Results elucidated that MZM attained the least friction (0.17), compared to M (0.40), MZ (0.26), or MM (0.29) at 800 °C. The increase in surface roughness of MZM due to sliding was reduced by 37.3% compared to that of MZ (11.9%) or MM (22.7%). The good lubricating behaviors were referred to the synergetic effects of ZnO, MoS₂, and formed lubricating components on worn surfaces.     

Testing Atmosphere Effect on Friction and Wear Behaviors of Duplex TiC/a-C(Al) Nanocomposite Carbon-Based Coating

Due to strongly tribological atmosphere sensitivity of carbon-based coatings, it is of extreme significance to investigate their friction and wear behaviors in different atmospheres. In this letter, duplex nc-TiC/a-C(Al) nanocomposite carbon-based coating coupled with high hardness, low internal stress and high adhesion strength was successfully fabricated using magnetron sputtering process. The friction and wear behaviors of as-fabricated coating were evaluated in dry N₂, humid N₂, air, dry O₂, and humid O₂ atmospheres, respectively. Results show that the as-fabricated coating possesses very high friction and wear due to the strong covalent bond interactions at the sliding interface caused by the free ??-bonds on the coating surface in dry N₂ atmosphere. Whereas the free ??-bonds can be efficiently terminated and passivated by water and/or oxygen molecules to weaken the strong covalent bond interactions to result in low friction and wear of coating in humid N₂, air, dry O₂, and humid O₂ atmospheres. The compact and homogeneous carbonaceous tribo-layer on the counterpart is mainly responsible for the lowest friction and wear of coating in humid N₂ atmosphere. Whereas the tribo-layer can be restrained to a certain extent by the tribo-chemical reaction, especially it results in a nearly negligible carbonaceous tribo-layer on the counterpart in dry O₂ atmosphere, which is mainly responsible for largely increased friction and wear of coating.     

Dry Sliding Friction and Wear Characteristics of Cu-Sn Alloy Containing Molybdenum Disulfide

The wear and sliding friction response of a hybrid copper metal matrix composite reinforced with 10 wt% of tin (Sn) and soft solid lubricant (1, 5, and 7 wt% of MoS₂) fabricated by a powder metallurgy route was investigated. The influence of the percentages of reinforcement, load, sliding speed, and sliding distance on both the wear and friction coefficient were studied. The wear test with an experimental plan of six loads (5–30 N) and five sliding speeds (0.5–2.5 m/s) was conducted on a pin-on-disc machine to record loss in mass due to wear for two total sliding distances of 1,000 and 2,000 m. The results showed that the specific wear rate of the composites increased at room temperature with sliding distance and decreased with load. The wear resistance of the hybrid composite containing 7 wt% MoS₂ was superior to that of the other composites. It was also observed that the specific wear rates of the composites decreased with the addition of MoS₂. The 7 wt% MoS₂ composites exhibited a very low coefficient of friction of 0.35. The hardness of the composite increased as the weight percentage of MoS₂ increased. The wear and friction coefficient were mainly influenced by both the percentage of reinforcement and the load applied. Wear morphology was also studied using scanning electron microscopy and energy-dispersive X-ray analysis.     

Stable and super‐low friction of amorphous carbon nitride coatings in nitrogen gas by using two‐step ball‐on‐disk friction test

Effect of running‐in process on friction behaviour of carbon nitride (CNx) coating in N₂ gas stream was investigated with a newly introduced two‐step ball‐on‐disk friction test, where the rubbed Si₃N₄ ball in the pre‐sliding (step 1) was replaced by a new CNx‐coated Si₃N₄ ball in the subsequent sliding stage under N₂ gas (step 2). The two‐step friction test is clarified to be a simple but effective technique for obtaining contact material combination of self‐mated CNx coatings and for achieving stable and low frictions of CNx coatings. Friction coefficients of CNx/CNx in N₂ gas stream decrease greatly from 0.07 without pre‐sliding to less than 0.025 in two‐step friction tests. The minimum friction coefficient of 0.004 was obtained by introducing 500 cycles of pre‐sliding in ambient air. These stable and low frictions are attributed to the generation of self‐mated CNx coatings and the formation of a lubricious layer on the disk surface. Copyright © 2014 John Wiley & Sons, Ltd.     

Tribological behavior of RH ceramics made from rice husk sliding against stainless steel,alumina, silicon carbide,and silicon nitride

The tribological behavior of rice husk (RH) ceramics, a hard, porous carbon material made from rice husk, sliding against stainless steel, alumina, silicon carbide, and silicon nitride (Si₃N₄) under dry conditions was investigated. High hardness of RH ceramics was obtained from the polymorphic crystallinity of silica. The friction coefficients for RH ceramics disks sliding against Si₃N₄ balls were extremely low (<0.1), irrespective of contact pressure or sliding velocity. Transfer films from RH ceramics formed on Si₃N₄ balls. Wear-mode maps indicated that the wear modes were powder formation under all tested conditions, resulting in low specific wear rates (<5×10⁻⁹ mm²/N).     

Tribological properties of composite multilayer coating

Abstract

The use of surface coatings is emerging as one of the most important approaches in reducing friction and wear in various tribological applications. Even though single layer coatings have a wide range of applications, the performance of the single layer alone may not always be adequate to meet the desired tribological property requirements. Hence, coatings consisting of multilayers to meet different property requirements in demanding applications are required. In this study, the tribological properties of a graded composite multilayer coating, with a specific layer sequence of MoS₂/Ti–MoS₂/TiBN–TiBN–TiB₂–Ti deposited on tool steel substrate, have been investigated at temperatures of 40 and 400°C respectively. The experimental results from the tests at 40°C have shown that the friction coefficient value ranges between 0·02 and 0·034. It was found that the deposition parameters influenced the friction and durability of the coatings. Higher substrate bias was found to result in higher friction, and the coating deposited at high substrate bias and low N₂ flow showed the lowest durability. The friction coefficient and durability of the coatings were found to be highly dependent on temperature. At high temperature, the friction coefficient increases almost threefold, and the durability decreases significantly.     

Adaptive Mo2N/MoS2/Ag Tribological Nanocomposite Coatings for Aerospace Applications

Reactively sputtered Mo₂N/MoS₂/Ag nanocomposite coatings were deposited from three individual Mo, MoS₂, and Ag targets in a nitrogen environment onto Si (111), 440C grade stainless steel, and inconel 600 substrates. The power to the Mo target was kept constant, while power to the MoS₂ and Ag targets was varied to obtain different coating compositions. The coatings consisted of Mo₂N, with silver and/or sulfur additions of up to approximately 24 at%. Coating chemistry and crystal structure were evaluated using X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD), which showed the presence of tetragonal Mo₂N and cubic Ag phases. The MoS₂ phase was detected from XPS analysis and was likely present as an amorphous inclusion based on the absence of characteristic XRD peaks. The tribological properties of the coatings were investigated in dry sliding at room temperature against Si₃N₄, 440C stainless steel, and Al₂O₃. Tribological testing was also conducted at 350 and 600 °C against Si₃N₄. The coatings and respective wear tracks were examined using scanning electron microscopy (SEM), optical microscopy, profilometry, energy dispersive X-ray spectroscopy (EDX), and micro-Raman spectroscopy. During room temperature tests, the coefficients of friction (CoF) were relatively high (0.5–1.0) for all coating compositions, and particularly high against Si₃N₄ counterfaces. During high-temperature tests, the CoF of single-phase Mo₂N coatings remained high, but much lower CoFs were observed for composite coatings with both Ag and S additions. CoF values were maintained as low as 0.1 over 10,000 cycles for samples with Ag content in excess of 16 at% and with sulfur content in the 5–14 at% range. The chemistry and phase analysis of coating contact surfaces showed temperature-adaptive behavior with the formation of metallic silver at 350 °C and silver molybdate compounds at 600 °C tests. These adaptive Mo₂N/MoS₂/Ag coatings exhibited wear rates that were two orders of magnitude lower compared to Mo₂N and Mo₂N/Ag coatings, hence providing a high potential for lubrication and wear prevention of high-temperature sliding contacts.     

Synergistic Effect of Nano-MoS<Subscript>2</Subscript> and Anatase Nano-TiO<Subscript>2</Subscript> on the Lubrication Properties of MoS<Subscript>2</Subscript>/TiO<Subscript>2</Subscript> Nano-Clusters

A MoS₃ precursor deposited on anatase nano-TiO₂ is heated at 450 °C in an H₂ atmosphere to synthesize MoS₂/TiO₂ nano-clusters. The nano-clusters are then characterized, and their tribological properties are evaluated. MoS₂ is found to be composed of layered structures with 1–10 nm thicknesses, 10–30 nm lengths, and 0.63–0.66 nm layer distances. The MoS₂ sizes in the MoS₂/TiO₂ nano-clusters are smaller and their layer distances are larger than those of pure nano-MoS₂. The MoS₂/TiO₂ nano-clusters also present a lower average friction coefficient than pure nano-MoS₂, but the anti-wear properties of both the nano-clusters and pure nano-MoS₂ are similar. X-ray photoelectron spectroscopy indicates that nano-TiO₂ and the element Mo are transferred to the friction surface from the MoS₂/TiO₂ nano-clusters through a tribochemical reaction. This produces a lubrication film containing TiO₂, MoO₃, and other chemicals. The nano-MoS₂ changes in size and layer distance when combined with nano-TiO₂, producing a synergistic effect. This may further be explained using a micro-cooperation model between MoS₂ nano-platelets and TiO₂ solid nanoparticles.     

Environmental Effects on the Tribology and Microstructure of MoS<Subscript>2</Subscript>–Sb<Subscript>2</Subscript>O<Subscript>3</Subscript>–C Films

The tribology of molybdenum disulfide (MoS₂)–Sb₂O₃–C films was tested under a variety of environmental conditions (ambient 50% RH, 10⁻⁷ Torr vacuum, 150 Torr oxygen, and 8 Torr water) and correlated with the composition of the surface composition expressed while sliding. High friction and low friction modes of behavior were detected. The lowest coefficient of friction, 0.06, was achieved under vacuum, while sliding in 8 Torr water and ambient conditions both yielded the highest value of 0.15. Water vapor was determined to be the environmental species responsible for high friction performance. XPS evaluations revealed a preferential expression of MoS₂ at the surface of wear tracks produced under vacuum and an increase in Sb₂O₃ concentration in wear tracks produced in ambient air (50% RH). In addition, wear tracks produced by sliding in vacuum exhibited the lowest surface roughness as compared to those produced in other environments, consistent with the picture of low friction originating from well-ordered MoS₂ layers produced through sliding in vacuum.     

On the possibility of improving antifriction properties of MoS<Subscript>2</Subscript> coatings by alloying

The paper presents an explanation of the improved antifriction properties of MoS₂ in vacuum compared to their properties in air. It is shown that the effect of superlow friction upon intensive irradiation results from the formation of a “two-dimensional gas” consisting of sulfur atoms knocked out of their positions. The possibility of the alloying of MoS₂ by elements which do not react with sulfur is analyzed. The alloying of MoS₂ coatings by an excess number of sulfur atoms to realize the effect of superlow friction in vacuum and air is substantiated.     

The Tribological Mechanism of MoS<Subscript>2</Subscript> Film under Different Humidity

Molybdenum disulfide (MoS₂) has been widely used in vacuum environment as an excellent solid lubricant. However, the application of MoS₂ is greatly limited in terrestrial atmosphere due to the sensitivity to humidity. Although the sensitivity of MoS₂ to water vapor has been widely recognized, the mechanism is not clear. To explore the tribological mechanism of MoS₂ in the presence of water vapor, a series of experiments were performed to investigate the effect of N₂ (inert gas), O₂ (active gas), air (a combination of both) and cyclic humidity change in air on the frictional response of MoS₂ to humidity. According to the results, a model that described water adsorption enhanced by active sites in MoS₂ and formed oxides, and an adsorption action change in water molecules with humidity was proposed. The model was applied to explain the recovery and instantaneous response of friction coefficient to humidity change.     

Dry and oil-lubricated sliding wear of Si<Subscript>3</Subscript>N<Subscript>4</Subscript> and Si<Subscript>3</Subscript>N<Subscript>4</Subscript>/BN fibrous monoliths

A high-temperature ball-on-flat tribometer was used to investigate dry and oil-lubricated friction and wear of sintered Si₃N₄ and Si₃N₄/hexagonal boron nitride (H-BN) fibrous monoliths. The friction coefficients of base Si₃N₄ flats sliding against Si₃N₄ balls were in the range of 0.6–0.8 for dry and 0.03–0.15 for lubricated sliding, and the average wear rates of Si₃N₄ were 10^–5 mm³ N^–1 m^–1 for dry sliding and 10^–10–10^–8 mm³ N ^–1m^–1 for lubricated sliding. The friction coefficients of Si₃N₄ balls against composite fibrous monoliths were 0.7 for dry sliding and 0.01–0.08 for lubricated sliding. The average specific wear rates of the pairs were of the same order as those measured for the conventional Si₃N₄ pairs. However, the fibrous monoliths, in combination with sprayed dry boron nitride, resulted in reduction in the lubricated friction coefficients of the test pairs and significant reduction in their wear rates. The most striking result of this study was that the coefficients of friction of the Si₃N₄/H-BN fibrous monolith test pair were 70–80 lower than those of either roughened or polished Si₃N₄ when tests were performed under oil-lubricated sliding conditions over long distances (up to 5000 m). The results indicated that Si₃N₄/H-BN fibrous monoliths have good wear resistance and can be used to reduce friction under lubricated sliding conditions.     

Fullerene-like MoS<Subscript>2</Subscript> Nanoparticles and Their Tribological Behavior

Using a new quartz-made reactor, large amounts of fullerene-like (IF) MoS₂ nanoparticles were synthesized by reacting MoO₃ vapor with H₂S in a reducing atmosphere. The nanoparticles were found to be of high crystalline order; with an average size of 70 nm and consist of more than 30 closed shells. Extensive tribological testing of the nanoparticles in two types of synthetic oils- poly-alpha olefins (PAO)- was carried out and compared to that of bulk (2H platelets) MoS₂ and IF-WS₂. These tests indicated that under high pressure and relatively low humidity, the IF-MoS₂ exhibited a friction coefficient as low as 0.03 and the smallest wear rate of the measured systems. However, its performance was found to be lower in comparison to IF-WS₂ after 2500 cycles, due probably to its inferior chemical stability. This study indicates that the tribological performance of the IF nanoparticles depends strongly on their crystalline order and size.     

Nanotribology and lubrication mechanisms of inorganic fullerene-like MoS<Subscript>2</Subscript> nanoparticles investigated using lateral force microscopy (LFM)

Inorganic fullerene-like (IF) MoS₂ nanoparticles were produced by arc discharge in water, and their tribological properties were investigated using a lateral force microscope in dry nitrogen and humid air. Two types of tips – Si and Si₃N₄ tips were used in this work. The sharp Si tip produced a much higher contact stress than the blunt Si₃N₄ tip. The measurement of lateral forces using a Si₃N₄ tip resulted in almost no wear, while the measurement made using a Si tip resulted in MoS₂ transfer due to the high contact stress. For comparison, measurements were also made on MoS₂ films grown by pulsed laser deposition (PLD). The experimental results demonstrated that IF-MoS₂ nanoparticles had significantly lower friction than the MoS₂ films prepared by PLD. Variation of the test environment from dry to wet did not affect the tribological performance of the IF material as much as it did PLD films due to the chemical inert structure of the IF-MoS₂ nanoparticles. The multi-wall-encapsulated structure of inorganic fullerenes has a nearly isotropic geometry. They can supply a slippery surface in all orientations, though only the basal planes of 2H–MoS₂ crystals are optimum for lubrication. Therefore, the inorganic fullerenes do not have to be oriented by rubbing as does most layer-structured solid lubricants. However, the lack of reactive edge planes impedes bonding of the lubricant to the surface. The lubrication mechanisms of IF-MoS₂ nanoparticles are discussed in detail.     

Comparative study of the tribological behavior of self-lubricating W–S–C and Mo–Se–C sputtered coatings

Transition metal dichalcogenides (TMD) have been one of the best alternatives as low friction coatings for tribological applications, particularly in dry and vacuum environments. However, besides their deficient behavior in humid containing atmospheres, their extensive application has also been restricted due to their low load-bearing capacity. In order to overcome these problems, recently the alloying with C has been tried with the expectation of simultaneously improving the coatings hardness and reaching sliding contacting phases more convenient for achieving low friction in humid environments.The practical application of this concept was extensively studied with the W–S–C system, with the C addition being achieved either by reactive or co-sputtering processes. The best tribological results were obtained by co-sputtering from a C target embedded with an increasing number of WS₂ pellets. Excellent results were reached from the more than one order of magnitude increase in the coatings hardness up to friction coefficients which are close to those of the references of self-lubricating coatings: TMD for dry or vacuum atmospheres or C-based coatings for terrestrial sliding conditions.Following the good results achieved with W–S–C system, other TMDs systems have been envisaged to be studied. The main focus was placed on the Mo–Se–C system.In this paper, the general comparison between W–S–C and Mo–Se–C coatings is presented. The main effort is pointed on the tribological behavior of both systems when tested by pin-on-disk against steel counterpart balls under different testing conditions: applied normal loads, temperatures and relative humidity of the atmospheres. Both coatings were deposited by co-sputtering from a C target with a varying number of TMD pellets which could lead to C contents in the films in the range from 30  up to 70 at.%. A Ti interlayer was interposed between the films and the substrates for improving the adhesion.Typically, W–S–C films are harder than Mo–Se–C films. From the tribological point of view, W–S–C films are more thermally stable than Mo–Se–C films although the friction coefficients of these last ones are lower when tested in humid containing atmospheres.