Moisture-resistant MoS2-based composite lubricant films

The moisture resistance of sputter-coated composite films of MoS₂ can be markedly increased when the MoS₂ is sputter deposited with water-repellent additives. To increase the adhesion of such coatings and to prevent corrosive attack of the substrate, generally steel, a thin corrosion-resistant sulphide-forming intermediate layer is applied previously on the functional surface and serves as an interlayer.With a sputtering process coatings of these lubricants, which are durable in the Earth's atmosphere and adhere well to their support, are obtained. Friction and wear results for such composite lubricant films on different interlayers and various substrates, which were obtained in dry and humid air from pin and disc experiments and from functional bearing elements of precision engineering systems, confirm the improvements.     

Tribological characteristics of sputtered thin films in rolling hertzian contacts

The results of tribological studies carried out on hardened steel test bodies coated with solid lubricants are reported. The tests were carried out to assess the suitability of solid lubricant films in rolling hertzian contacts. A specially constructed concentrated contact simulator was used to determine the traction (frictional) characteristics of sputtered MoS₂ and WSe₂-In-Ga films. Test results obtained show that sputtered solid lubricant films are satisfactory for heavily loaded rolling contact applications as long as the slide-roll ratio is not very much larger than 0.02. Comparative tests show that burnished films yield lower traction coefficients than sputtered films indicating that there is room to improve sputtering practice further in order to enhance the tribological characteristics of sputtered solid lubricant films.     

Effects of deposition parameters on hardness and lubrication properties of thin MoS2 films

The purpose of the present study is the development of a sputtered MoS₂‐coating suitable for the use in humid atmospheres. Influences of the process parameters argon pressure, temperature, distance between substrate and target and bias voltage on tribological properties and hardness are presented. By means of these parameter studies, different microstructures with widely varying wear behavior were achieved. Life cycle tests show that exclusively basal oriented coatings with dense, non columnar structures show the highest durability and a hardness of about 400 HV 0.01. However, dendritic needle like structures show lower endurance and lower hardness (<100 HV 0.01). Films with a comparatively high hardness (up to 800 HV 0.01) exhibit lowest sliding distances until failure due to an appearance of spalling in the ball‐on‐disc tests. Those coatings also show a dense but amorphous structure and sulfur deficiencies. Consolidation during sputtering, i. e. film hardness and porosity, can be controlled by certain process parameters and is probably affected by varying amounts of internal stress.     

Nanostructural, electrical, and tribological properties of composite Au-MoS2 coatings

Considerable research has been done on the tribological properties of cosputtered metal/MoS₂ solid lubricant films with low metal content (< 20 at.%) because of their usefulness in applications at high Hertzian contact stress (around 1 GPa). However, cosputtered Au-MoS₂ coatings with a much higher range of metal contents up to (95 at.%) have shown surprisingly good performance at low contact stresses (as low as 0.1 MPa). In the present study, transmission electron microscopy, X-ray diffraction and electrical resistance measurements of cosputtered Au-MoS₂ coatings reveal them to be composites of nanocrystalline Au particles within an amorphous MoS₂ matrix. Electrical conductivity images of the coatings displayed metallic (Au) and semi-conducting (MoS₂) domains of nanometer dimensions. Auger Nanoprobe analyses confirmed that sliding on the coatings causes the formation of a pure MoS₂ layer about a nanometer thick on top of the bulk of the coatings. Lattice resolution atomic force microscopy revealed that this nanometer-thick MoS₂ layer is crystalline, and oriented with the basal plane (0001) parallel to the coating surface. Electrical resistance obtained during sliding and pull-off force measurements was consistent with the structure of the coatings. Sliding friction data on the coatings support previous results showing that performance at different Hertzian contact stresses correlated strongly with Au content.     

Transfer of molybdenum sulphide coating material onto corundum balls in fretting wear tests

Transfer films on corundum balls from sulfur deficient molybdenum disulfide (MoS_x) coatings with different crystallographic orientations were investigated after fretting tests performed in ambient air of different humidity levels. The morphology of wear tracks on MoS_x coatings and of transfer films on corundum balls were investigated by light optical microscopy with Normarski contrast. The thickness of transfer films was measured by scanning white light and optical phase-shifting interferometry, and their composition was analyzed by X-ray photoelectron spectroscopy. The effect of relative humidity in fretting tests on the composition of the transfer films as well as the effect of the transfer film on the tribological performance of MoS_x coatings in fretting wear tests is discussed.     

Synergistic effects of metals co-sputtered with MoS2

Sputtered MoS₂ films providing satisfactory lubricating properties can be achieved by proper control of processing parameters. The sputtering process is ideally suited to applications which require very thin films of MoS₂ as it yields coatings with excellent substrate adhesion and which have endurances per unit thickness in excess of those of conventional dry film coatings. Sputtered interfacial coatings of other materials can be deposited onto certain substrates to enhance lubricating qualities and adherence.In the work presented here, the evaluation method was a sliding friction test run at a normal load of 63.4 kgf and a speed representing 800 load applications per minute. With this set of test parameters sputtered MoS₂ films produced under widely reported process parameters manifest irregular coefficients of friction and variability of endurance. The films also exhibit time-related changes in their coefficients of friction which stabilize from 6 to 12 h after sputtering. The observed irregularities are not peculiar to any particular method of sputtering and are most probably caused by residual stresses or inhomogeneous crystal growth as the film increases in thickness.The observed frictional instabilities and irregularities can be eliminated or greatly reduced by the codeposition of certain metals with MoS₂. These metal additives appear to increase the size of the acicular crystallites by approximately 35%, and in appropriate concentrations they provide synergistic effects such as a lowering of the coefficient of friction and an increase in the endurance per unit thickness.The ability to vary the ratio of metals to MoS₂ enables films to be produced which are controllably harder than those obtained with MoS₂ alone. Up to about 10% the metal additives do not adversely affect the lubricating qualities. The codeposited films have stable coefficients of friction with increased endurance per unit thickness and are clearly superior to standard sputtered MoS₂ films.     

Solid lubricant behavior of MoS2 and WSe2-based nanocomposite coatings

Abstract

Tribological coatings made of MoS₂ and WSe₂ phases and their corresponding combinations with tungsten carbide (WC) were prepared by non-reactive magnetron sputtering of individual targets of similar composition. A comparative tribological analysis of these multiphase coatings was done in both ambient air (30–40% relative humidity, RH) and dry nitrogen (RH<7%) environments using the same tribometer and testing conditions. A nanostructural study using advanced transmission electron microscopy of the initial coatings and examination of the counterfaces after the friction test using different analytical tools helped to elucidate what governs the tribological behavior for each type of environment. This allowed conclusions to be made about the influence of the coating microstructure and composition on the tribological response. The best performance obtained with a WSe_x film (specific wear rate of 2 × 10^?8 mm³ N^–1m^–1 and a friction coefficient of 0.03–0.05) was compared with that of the well-established MoS₂ lubricant material.     

The Structure and Properties of Ti–B–N, Ti–Si–B–N, Ti–Si–C–N, and Ti–Al–C–N Coatings Deposited by Magnetron Sputtering Using Composite Targets Produced by Self-Propagating High-Temperature Synthesis (SHS)

The microstructures and compositions of multicomponent Ti–B–N, Ti–Si–B–N, Ti–Si–C–N, and Ti–Al–C–N films deposited by reactive magnetron sputtering using composite targets and produced by self-propagating high-temperature synthesis (SHS) have been investigated by means of transmission electron microscopy. Auger spectroscopy, and X-ray diffraction. Depending on the chemical composition of the film deposited, different single-phase crystalline films were observed. The sputtering process included sputter cleaning prior to the DC magnetron sputter deposition of Ti and TiN interlayers prior to DC magnetron sputter deposition of the multicomponent films from multicomponent targets. The films produced were characterized in terms of their microhardness, wear resistance, high-temperature oxidation conducted in air. and corrosion resistance in a solution of 5NH₂SO₄ at room temperature.     

MoS2/ta‐C‐Kombinationsschichten hergestellt durch Laser‐Arc‐Technologie

MoS₂/ta‐C coatings produced by laserarc‐technology A series of MoS₂ and combined MoS₂/ta‐C coatings were prepared by lasercontrolled arc evaporation (Laser‐Arc) in order to study the tribological coating behaviour under vacuum and atmospheric conditions. Very low friction coefficients down to 0.005 were measured under high vacuum. By using a ta‐C underlayer beneath the MoS₂ a increased lifetime up to 5×10⁵ load cycles could be obtained. Also under atmospheric conditions the underlayer had a beneficial effect on coating performance.     

Preparation of multilayered nanocrystalline thin films with composition-modulated interfaces

The properties of multilayer thin film structures depend on the morphology and structure of interfaces. A broad interface, in which the composition is varying, can enhance, e.g., the hardness of multilayer thin films. In the present experiments multilayers of TiAlN and CrN as well as TiAlN, CrN and MoS₂ were studied by using unbalanced magnetron sputter sources. The sputter sources were arranged side by side on an arc. This arrangement permits development of a transition zone between the layers, where the composition changes continuously. The multilayer system was deposited by one-fold oscillating movement of substrates in front of sputter sources. Thicknesses of layers could be changed both by oscillation frequency and by the power applied to sputter sources. Ti/Al: 50/50 at%, pure chromium and MoS₂ targets were used in the sputter sources. The depositions were performed in an Ar–N₂ mixture at 0.22 Pa working pressure. The sputtering power of the TiAl source was feed-back adjusted in fuzzy-logic mode in order to avoid fluctuation of the TiAl target sputter rate due to poisoning of the target surface. Structure characterization of films deposited on 1 0 0 Si wafers covered by thermally grown SiO₂ was performed by cross-sectional transmission electron microscopy. At first a 100 nm thick Cr base layer was deposited on the substrate to improve adhesion, which was followed by a CrN transition layer. The CrN transition layer was followed by a 100 nm thick TiAlN/CrN multilayer system. The TiAlN/CrN/MoS₂ multilayer system was deposited on the surface of this underlayer system. The underlayer systems Cr, CrN and TiAlN/CrN were crystalline with columnar structure according to the morphology of zone T of the structure zone models. The column boundaries contained segregated phases showing up in the under-focused TEM images. The surface of the underlayer system was wavy due to dome-shaped columns. The nanometer-scaled TiAlN/CrN/MoS₂ multilayer system followed this waviness. Crystallinity of the TiAlN and CrN layers in the multilayer system decreases with increasing thickness of the MoS₂ layer.     

MoSx-Ta composite coatings on steel by d.c magnetron sputtering

Sputter-deposited MoS₂ films have been often used as dry lubricant in various industrial fields, such as space application and much attention has been paid to reduction of friction coefficient and improvement of mechanical properties in recent decades. One way to achieve this is to deposit a MoS₂ film doped with another metal. The MoS_x-metal films were found to be denser, more adhesive and more oxidation-resistant than pure MoS₂. In this study, MoS_x-Ta composite films were synthesized by Electron Cyclotron Resonance microwave source enhanced DC sputtering with different target powers. The effects of doping Ta on mechanical properties of MoS_x-Ta films were investigated. The morphology and structure of films were investigated using a scanning electron microscope (SEM), X-ray diffraction (XRD) and atomic force microscopy (AFM). The microhardness was evaluated using microhardness test instrument, and the adhesion strengths were obtained using a scratch tester. The results showed that the S/Mo ratio was influenced by the dc sputtering target power. Typical MoS₂ (100) (103) (002) orientations were present in pure MoS_x films, but disappeared with the increase in doped Ta, with the S/Mo content ratios decreasing from 1.52 to 0.84, and the hardness increasing from 3.55 to 15.23 GPa. The roughness and surface topography, friction coefficient and adhesion were significantly affected by the Ta, Mo and S content. The content of doped Ta plays a dominant role on the change in the Mo/S ratio, thereby influencing the mechanical and tribological properties of the MoS_x-Ta composite films.     

Different reinforcement strategies of hybrid surface composite AA6061/(B4C+MoS2) produced by friction stir processing

Aluminum surface composites have gained huge importance in material processing due to their noble tribological characteristics. The reinforcement of solid lubricant particles with hard ceramics further enriches the tribological characteristics of surface composites. In the current study, friction stir processing was chosen to synthesize hybrid surface composites of aluminum containing B₄C and MoS₂ particles with anticipated improved tribological behavior. B₄C and MoS₂ powder particles in 87.5: 12.5 ratio were reinforced into the AA6061 by hole and groove method. Microstructural observations indicated that reinforcement particles are well distributed in the matrix. The hardness and wear resistance of hybrid surface composites improved as compared to the base material, due to well distributed abrasive B₄C and solid lubricant MoS₂ particles in AA6061. The hybrid surface composites achieved ∼32 % increased average hardness as compared to the base material. Hole method revealed ∼13 % better wear resistance compared to the groove method for friction stir processed hybrid surface composite, attributing to an improved homogeneity of particle distribution shown by zigzag hole pattern. Moreover, friction stir processed AA6061 without reinforcement particles exhibited reduced hardness and wear resistance due to loss of strengthening precipitates during multi-pass friction stir processing.     

Lubrication mechanisms of C‐MoS2‐Fe2O3 (Fe3O4) nano‐composite lubricants at the rubbing interfaces of non‐copper coated solid wires against the contact tube

Graphite‐MoS₂‐Fe₂O₃ (Fe₃O₄) nano‐composite lubricating coatings were prepared on the surfaces of non‐copper coated solid wires by a mechanical coating technique. The tribological behaviours of graphite‐MoS₂‐Fe₂O₃ (Fe₃O₄) coatings at the rubbing interfaces of welding wires against the contact tube were investigated. The results demonstrate that the lubricating properties of graphite‐Fe₃O₄ coatings outperform the lubricating properties of graphite‐Fe₂O₃ coatings. The anti‐wear performance of the contact tube is strengthened with increasing nano‐MoS₂ contents. Layers of protective tribofilms are formed at the rubbing interfaces of welding wires against a contact tube by tribochemical reaction among lubricants. The tribofilms are composed of FeO, MoO₃ and FeMoO₄ with excellent lubricating properties. They can avoid direct contact of welding wires against the contact tube, thus decreasing contact tube wear. With the transition of the contact tube wear from mild to severe, the dominant wear mechanisms of contact tube change from fatigue peeling and oxidative wear to abrasive wear and arc ablation.     

Frictional and morphological properties of AuMoS2 films sputtered from a compact target

AuMoS₂ films 0.02–1.2 μm thick were sputtered from a target compacted from 5 wt.% Au plus 95 wt.% MoS₂ to investigate the frictional and morphological film growth characteristics. The gold dispersion effects in MoS₂ films are of interest to increase the densification and strengthening of the film structure. Three microstructural growth stages were identified on the nano-micro-macrostructural level. During sliding both sputtered AuMoS₂ and sputtered MoS₂ films have a tendency to break within the columnar region. The remaining or effective film, about 0.2 μm thick, performs the lubrication. The AuMoS₂ films displayed a lower friction coefficient with a high degree of frictional stability and less wear debris generation compared with pure MoS₂ films. The more favorable frictional characteristics of the AuMoS₂ films are attributed to the effective film thickness and the high density packed columnar zone which has a reduced effect on the fragmentation of the tapered crystallites during fracture.     

Structure and tribological property of MoSx-CrTiAlN film by unbalanced magnetron sputtering

MoS_x-CrTiAlN film was deposited on Mg alloy substrates using unbalanced magnetron sputtering. First of all, the CrTiAlN layer was synthesized in a gas mixture of Ar + N₂, and then the MoS_x layer on CrTiAlN were deposited by a single MoS_x target. The composition, structure and tribological property of MoS_x-CrTiAlN film were characterized by X-ray photoelectron spectrometry, X-ray diffraction, transmission electron microscopy and ball-on-disc tester. The experimental results show that crystallography structure of CrTiAlN layer is FCC whilst the MoS_x layer has a mixed microstructure with hexagonal and amorphous state. The coefficient of friction of MoS_x-CrTiAlN film is a function of load and shows a steady decreasing with the increasing in applied load.     

Structure and tribological property of MoSx-CrTiAlN film by unbalanced magnetron sputtering

MoS_x-CrTiAlN film was deposited on Mg alloy substrates using unbalanced magnetron sputtering. First of all, the CrTiAlN layer was synthesized in a gas mixture of Ar + N₂, and then the MoS_x layer on CrTiAlN were deposited by a single MoS_x target. The composition, structure and tribological property of MoS_x-CrTiAlN film were characterized by X-ray photoelectron spectrometry, X-ray diffraction, transmission electron microscopy and ball-on-disc tester. The experimental results show that crystallography structure of CrTiAlN layer is FCC whilst the MoS_x layer has a mixed microstructure with hexagonal and amorphous state. The coefficient of friction of MoS_x-CrTiAlN film is a function of load and shows a steady decreasing with the increasing in applied load.     

Heterogeneous growth in transition metal-rare earth films during bias sputter deposition

Unusual multiphase microstructures formed in thick Ni-La and Ni-Y films that were produced by high rate sputter deposition, when the substrate was biased at several hundred volts during deposition. Films biased in this range were bombarded with an intense flux of Kr⁺ ions (approximately 5 × 10¹⁶ cm^-2 s^-1) because of the dense thermionically supported plasma used here. In these films large oriented precipitates formed near certain defects or irregularities in the substrate or film surface. The matrix in which these growths formed was amorphous or microcrystalline. No heterogeneities were observed in films deposited with a negative substrate bias lower than about 50 V. Those films were amorphous and were homogeneous in composition. Examples of the heterogeneous microstructures are presented here, and reasons for their formation are discussed.     

Microstructure and properties of nc-TiC/a-C∶H films deposited by radio frequency reactive sputtering

Abstract

Amorphous carbon films containing titanium carbide (nc-TiC/a-C∶H) were deposited onto n-type silicon (100) by radio frequency reactive sputtering titanium target in an Ar–CH₄ mixed atmosphere. The composition and microstructure of the films were characterised by means of X-ray photoelectron spectroscopy, field emitted SEM, XRD and Raman spectra. The mechanical and tribological properties of the films were measured by a nanoindentation tester and a ball-on-disc UMT–2MT tribometer. By adjusting the CH₄ flowrate, Ti content in the films could be controlled, and a transition in structures of the films from loose polymer-like to glassy and dense nanostructure was observed. The density of coatings was improved by the introduction of TiC nanocrystalline particles. The mechanical and lubricious properties were different accordingly.     

Morphological and frictional behavior of sputtered MoS2 films

From the texture and growth patterns of sputtered MoS₂ films deposited onto substrates, three regions can be distinguised: (1) a ridge formation region, (2) an equiaxed transition zone and (3) a columnar-fiber-like structure. The lubricating properties of sputtered MoS₂ films can be visually identified with respect to optical changes before and after rubbing. The orientation of the surface microcrystallites is identified, and the change in optical properties is explained. In sliding contact the sputtered film tends to break up at the base of the columnar region. Effective lubrication occurs with the film remaining on the substrate. This film is 0.18–0.22 microm thick.     

Microstructure and tribological behavior of WS2-Ag composite films deposited by RF magnetron sputtering

WS₂-Ag composite films were deposited on medium carbon steel substrate by RF magnetron sputtering method. The morphology and microstructure of the composite films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The tribological behavior was investigated using a ball-on-disk tribometer in vacuum and in humid air. In the range of Ag content of the film from 4.2 at.% to 40.4 at.%, Ag phase dispersed in amorphous WS₂ matrix, and it changed from amorphous to crystalline structure with the increase of Ag content. The friction coefficients of composite films in humid air were lower and more stable than those of pure WS₂ film, and the environmental sensitivity of tribological behavior decreased obviously with the addition of Ag in the films. At the content of 16.2 at.% Ag, the composite film was dense and adherent, and exhibited excellent tribological performance both in vacuum and in humid air.