Tribological behavior of plasma-enhanced CVD a-C:H films. Part I: effect of processing parameters

A systematic study was conducted on the effect of plasma-enhanced CVD processing parameters, namely bias voltage, pressure and CH₄/Ar flow ratio, on the characteristics and tribological response of amorphous hydrogenated carbon (a-C:H) films. Film hardness, intrinsic stress, structure, composition and tribological response were characterized. Variation of processing parameters was found to produce a-C:H films with a range of characteristics with the CH₄/Ar ratio exercising a dominant effect. A low ratio produced harder films with more sp³ bonding, low hydrogen content and low wear rate; whereas a high ratio produced softer films, with more sp² bonding, higher hydrogen content and low friction. Film characteristics were found to affect the wear mechanism with softer films showing a layer-by-layer removal and harder films involving formation of fine debris. These two diverse types of films offer the opportunity to synthesize multilayered films combining desirable properties from each component.     

Ultra-High Tribological Performance of Magnetron Sputtered a-C:H Films in Sand-Dust Environment

The hydrogenated amorphous carbon (a-C:H) films were prepared on AISI 440C steel substrates using a RF magnetron sputtering graphite target in the CH₄ and Ar mixture atmosphere. The friction and wear behavior of a-C:H films were comparatively investigated by pin-on-disc tester under dry sliding and simulated sand-dust wear conditions. In addition, the effects of applied load, amount of sand and sand particle sizes on the tribological performance of a-C:H films were systemically studied. Results show that a-C:H films exhibited ultra-high tribological performance with low friction coefficient and ultra-low wear rate under sand-dust environments. It is very interesting to observe that the friction coefficient of a-C:H film under sand-dust conditions was relatively lower when compared with dry sliding condition, and the wear rate under sand-dust conditions kept at the same order of magnitude (×10⁻¹⁹ m³/N m) with the increase of applied load and particle size as a comparison with the dry sliding condition. Based on the formation of “ridge” layer (composite transfer layer), a transfer layer-hardening composite model was established to explain the anti-wear mechanisms and friction-reducing capacity of a-C:H solid lubrication films under sand-dust conditions.     

Tribological Properties of Nanocomposite CrC<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>/a-C:H Thin Films

Recently we showed that coatings, prepared by unbalanced magnetron sputtering from a metallic Cr target in an Ar/CH₄ discharge are composed of nanocrystalline CrC _x embedded in an a-C:H matrix. This work investigates the structural correlation of such nanocomposite CrC _x /a-C:H coatings to their tribological properties. Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) were used to characterize the phase composition and the chemical bonding in the films deposited at different experimental conditions. The coating microstructure was investigated on selected samples by high-resolution transmission electron microscopy. For CrC _x -dominated coatings deposited at CH₄ partial to total pressure ratios (p_CH4/p_t) < 0.42, only minor changes regarding the friction coefficients and the abrasive wear rates were observed although microstructural changes towards a higher degree of crystallinity were proven by transmission electron microscopy and substantiated with XPS results. For a-C:H dominated coatings deposited at p_CH4/p_t > 0.42, the friction coefficients and abrasive wear rates were shown to decrease with increasing a-C:H phase content and its more sp²-like bonding configuration. It can be concluded that the microstructural changes in terms of CrC _x crystallite coarsening and bonding configuration of the a-C:H matrix phase are responsible for the observed changes of the friction coefficients and wear rates.     

Influence of CH4 Flow Rate on Microstructure and Properties of Ti-C:H Films Deposited by DC Reactive Magnetron Sputtering

Nanocomposite Ti-containing hydrogenated carbon films (Ti-C:H) were prepared using a DC reactive magnetron sputtering system. The relationship between CH₄ flow rate and the film characterization and tribological behaviors in both ambient air and deionized water conditions were investigated. Results showed that the Ti content in the as-deposited Ti-C:H films decreased and the sp³ content increased with an increase in CH₄ flow rate. TiC nanocrystallites can be formed at a relatively low CH₄ flow rate, whereas there was almost no formation of TiC in the amorphous carbon matrix at the highest CH₄ flow rate. The hardness, elastic modulus, and internal stress of the films were decreased firstly and then increased as the CH₄ flow rate increased, whereas their adhesion presented an inversely changing trend. The friction coefficients and wear rates of Ti-C:H films in both ambient air and deionized water conditions decreased with increasing CH₄ flow rate from 8 to 12 sccm and then increased as the CH₄ flow rate continually increased. In particular, the nanocomposite Ti-C:H film deposited with a CH₄ flow rate of 12 sccm could achieve superior combining mechanical properties and low friction and high antiwear behaviors in both ambient air and deionized water conditions, indicating potential applications as a protective and lubricating film for mechanical components.     

Effect of CH4/H2 plasma ratio on ultra-low friction of nano-crystalline diamond coating deposited by MPECVD technique

Nano-crystalline diamond coatings were deposited on the silicon substrate using Microwave Plasma Enhanced Chemical Vapor Deposition (MPECVD). Experiments were performed by varying the H₂ content in CH₄/H₂ plasma during synthesis. Raman spectral analysis revealed that with decrease in hydrogen content in the CH₄ plasma, the I_D/I_G ratio decreases with the formation of smaller crystallites. Such a film possesses a large grain boundary fraction containing hydrogenated amorphous carbon (a-C:H). During tribological test, sufficient amount of hydrogen present in the grain boundary passivates the dangling σ-bond causing ultra-low friction and extremely low wear evident by improvement in microstructure.     

Lubrication at 750°C in a vacuum by a transfer film from self‐lubricating composites for roll/slide contact of Si3N4

Roll/slide friction tests were carried out at a temperature of 750°C in a vacuum. Disc specimens were made of Si₃N₄ with or without a sputtered MoS₂ film. A pin specimen was rubbed against one disc to supply a lubricating transfer film. With a pin made of an MoS₂‐based composite, the friction coefficient was around 0.3 and almost no wear of the discs was observed after 24 h of operation at a load of 50 N, a rotating speed of 0.5 m/s, and a slip ratio of 10%. Transferred patchy MoS₂ films were observed on the friction track. With a pin made of Ni‐based composite containing BN and graphite, the friction coefficient increased from 0.2 to 0.7 over a test time of about 8 h and severe disc wear was found. In an additional test using Si₃N₄ discs with a sputtered MoS₂ film without a pin, the friction coefficient was about 0.3, and no wear of the discs was found after 24 h of operation. The appearance of the friction track was similar to that in the test using the MoS₂‐based composite pin. It seems that the sputtered MoS₂ film wore, but wear particles reattached on the friction path to develop an effective lubricating film. These results demonstrate the effectiveness of transfer film lubrication for long‐term operation in a high‐temperature vacuum, and the superior ability of MoS₂ to develop an effective transfer film.     

Tribochemical effects on the friction and wear behaviors of a-C:H and a-C films in different environment

Friction and wear behaviors of hydrogenated amorphous carbon (a-C:H) and hydrogen-free amorphous carbon (a-C) films sliding against Si₃N₄ balls were investigated in different testing environments. The result showed that two films with extreme chemical disparity (one hydrogenated, and the other hydrogen free) showed distinct different friction and wear behaviors, and the friction and wear behaviors of the both films were strongly dependent on the environment. For a-C:H films, much low friction coefficient and wear rate were obtain in dry N₂. In the water and/or oxygen containing environments, the friction coefficient and wear rate of a-C:H films were obviously increased. On the contrary, a-C films only provided low friction coefficient and wear rate in the presence of water and/or oxygen in the test chamber. In dry N₂, the highest friction coefficient and wear rate were observed for a-C films. By investigating the worn surfaces of the films using XPS, it was proposed that the environment dependence of the friction and wear behaviors of the films was closely related with the friction-induced chemical interactions between the films and water and/or oxygen molecules. The specific roles of hydrogen, water and oxygen molecules and their tribochemical effects on the friction and wear mechanism of the films are discussed.     

Effect of deposition pressure on microstructure and tribological behavior of MoS<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>/MoS<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>-Mo nanoscale multi-layer films

MoS _x /MoS _x -Mo multi-layer films consisted of several bilayers and a surface layer on steel substrate were deposited by d.c. magnetron sputtering at different deposition pressures. Each bilayer contained a MoS _x layer with 80 nm in thickness and a MoS _x -Mo composite layer with 20 nm in thickness. With the increase of deposition pressure, the perpendicular orientation of the basal plane prevailed while the parallel orientation decreased. The tribological properties of the multi-layer films were investigated by using a ball-on-disk tribometer both in vacuum and in humid air. The multi-layer film deposited at 0.24 Pa had a compact, consistent layered structure with high intensity of (002) plane and low S content compared to the others deposited at 0.32 and 0.40 Pa, and showed the lowest friction coefficient and wear rate in humid air.     

Structure characterization of pulsed laser deposited MoS x –WSe y composite films of tribological interests

Pulsed laser deposition (PLD) was employed to grow MoS _x –WSe _y composite films, where x = 1.18, y = 0.78. Scanning electron micrographs show that the films have a dense granular morphology. Crystallization, d-spacing and hexagonal sheet curvature within the film were studied with X-ray diffraction, electron diffraction and transmission electron microscopy. A predominant hexagonal MoS _x phase was formed but contained W and Se, which were most likely present as substituents for Mo and S. There was no evidence for two separate crystalline phases. MoS _x –WSe _y composite films exhibited a larger expansion along the c-axis (d-spacing between basal planes) than PLD MoS₂ and WSe₂ films grown by laser ablation of pure targets. The lattice spacing along the a-axis was expanded in comparison to the MoS₂ film, and compressed in comparison to the WSe₂ film. X-ray photoelectron spectroscopy showed a significant sulfur deficiency, and verified both of S and Se bonding in the film. High-resolution electron microscope images exhibited significant curvatures of the (002) basal planes in the films. The bending behavior of basal planes was explained by S vacancies and Se substitution on the atomic site of S layers. The tribological properties of the composite films were measured in dry and wet conditions using a ball-on-disc tribometer. The reduced friction was correlated with the increased crystallinity and increased separation of basal planes in the composite films.     

The Structural and Tribological Properties of MoS2-Ti Composite Solid Lubricants

MoS ₂ -Ti composite solid lubricant films were deposited on an AISI D2 tool steel and silicon wafer by CFUBMS (closed field unbalanced magnetron sputtering). The deposition process was performed for nine different test conditions at various levels of target current, working pressure, and substrate voltage using the Taguchi L ₉ (3 ⁴ ) experimental method. It was observed that the chemical composition of MoS ₂ -Ti composite films was significantly affected by sputtering parameters. It was determined that the microstructure of composite films is neither crystalline nor amorphous; in other words, it is quasi-amorphous, and (002) and (100) planes characteristic of MoS ₂ occurred. The friction coefficients of the films were determined over 1800 s and at a loading of 10 N by means of a pin-on-disk tribotester. The changes in friction coefficient were related to structural changes based on Ti addition and the different levels of deposition parameters.     

Effects of Deposition Conditions and Counter Bodies on the Tribological Properties of Pulsed DC Magnetron Sputtered TiN–MoS<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> Composite Coating

In the current study, TiN–MoS _x composite coatings were deposited by co-sputtering of MoS₂ and Ti targets under a mixture of Ar and N₂ gas environment using pulsed DC closed-field unbalanced magnetron sputtering. The tribological response of TiN–MoS _x composite coatings was studied against two different counter bodies: cemented carbide (WC–6% Co) ball and pin made of aluminium alloy (AlSiMg). First, the effect of substrate bias was studied on tribological properties using cemented carbide ball. Lowest coefficient of friction in the range of 0.03–0.04 was obtained for the specimen deposited at a substrate bias of −60 V. Wear coefficient was also found to be minimum for the same specimen. Coatings were further deposited at an optimum bias of −60 V in order to vary MoS _x content of TiN–MoS _x composite coating. Effect of variation of chemical composition of the coating was then studied on tribological performance of the coating against aluminium alloy counterface. Excellent anti-sticking property of MoS _x was found to have enabled the TiN–MoS _x composite coating to achieve considerably low coefficient of friction against aluminium alloy. It was shown that with optimum MoS _x content of TiN–MoS _x composite coating, it was possible to attain as low coefficient of friction as 0.09 against aluminium alloy even under normal atmospheric condition.     

Tribology of Co-sputtered Nanocomposite Au/MoS2 Solid Lubricant Films over a Wide Contact Stress Range

Co-sputtered nanocomposite metal/MoS₂ solid lubricant films are traditionally used in high contact stress applications (typically around 1 GPa) because they are hard and conform to the Hertzian contact model, i.e., the coefficient of friction () decreases with increasing contact stress. We are investigating whether appropriate modifications can be made in these films that could make them also work in low contact stress applications, especially sliding electrical contacts (e.g., slip rings) that would benefit from the higher conductivity and environmental robustness of these films. To this end, and also more generally to increase our understanding of how film composition affects performance, we studied the friction and endurance of co-sputtered Au/MoS₂ films in sliding contact in N₂ gas at two vastly different contact stresses, 730 and 0.1 MPa. Seven different film compositions were studied, with Au contents in the range 42-100 at.%, as well as pure MoS₂. The results showed that co-sputtered Au/MoS₂ films outperformed both pure sputtered MoS₂ films and pure sputtered Au films. Optimum films at high contact stress (i.e., those that exhibited the lowest and highest endurance) were films with lower Au contents (i.e., 42 and 59 at.% Au). In contrast, at low contact stress, films with moderately high Au contents (i.e., 75 and 89 at.% Au) performed the best. For films that did not fail by the end of the 2000 m test, Auger Nanoprobe analysis revealed that lubrication was provided by a thin film (1 nm thick) of relatively pure MoS₂, regardless of the contact stress. Based on these results, we hypothesize that at high contact stresses, the low Au content provides optimum amounts of MoS₂ in the contact region, while at low contact stresses, the higher Au contents limit the amount/size of MoS₂ particles that are transferred to the opposing surface, providing a thinner, more uniform transfer film. The results indicate that with appropriate optimization of the metal:MoS₂ ratio, co-sputtered nanocomposite metal/MoS₂ films can be applied to a much wider range of contact stresses than previously studied.     

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.     

Evaluation of Ion-Sputtered Molybdenum Disulfide Bearings for Spacecraft Gimbals

High-density, sputtered molybdenum disulfide films (MoS₂) were investigated as lubricants for the next generation of spacecraft gimbal bearings where low torque signatures and long life are required. Low friction in a vacuum environment, virtually no out-gassing, insensitivity to low temperature, and radiation resistance of these lubricant films are valued in such applications. One hundred and twenty five thousand hours of accumulated bearing lest time were obtained on 24 pairs of flight-quality bearings ion-sputtered with three types of advanced MoS₂ films. Life tests were conducted in a vacuum over a simulated duty cycle for a space pay bad gimbal. Optimum retainer and ball material composition were investigated. Comparisons were made with test bearings lubricated with liquid space lubricants.

Self-lubricating PTFE retainers were required for long life, i.e., > 40 million gimbal cycles. Bearings with polyimide retainers, silicon nitride ceramic balls, or steel balls sputtered with MoS₂ film suffered early torque failure, irrespective of the type of race-sputtered MoS₂ film. Failure generally resulted from excess film or retainer debris deposited in the ball track which tended to jam the bearing. Both grease lubricated and the better MoS₂ film lubricated bearings produced long lives, although the torque with liquid lubricants was lower and less irregular.     

Microstructure, chemical and tribological investigations of Mo x W1−x S y co-sputtered composite films

Mo _x W_1−x S _y composite films were co-sputtered by the combination of MoS₂ and WS₂ targets, which were shown to have much superior tribological performance with lower and more stable friction coefficient, longer durability and higher bearing resistance than pure MoS₂ films in room temperature air with a relative humidity of 45–50%. Especially for the Mo_0.6W_0.4S_1.6 (40 at.% WS₂) composite film, an increase in durability of more than a one order of magnitude was reached. X-ray diffraction (XRD), scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy were used to investigate the relationship between the microstructure and the tribological performance of the films. The composite films are shown to have a densified structure and accordingly improved oxidation resistance and lubrication properties. Moreover, the composite films have a lattice expansion in the c direction, along with a reduced the friction within the films.     

Effect of MoS2-based composite coatings on tribological behavior and efficiency of gear

MoS₂-based Ti composite coatings were deposited on the SCM420 alloy and gears using an RF magnetron sputtering (RFMS) system. While MoS₂ coating had been coated on the silicon substrate. The coatings structures were compared to each other to find the effect of Ti. The composite coatings have been tested in a ball-on-disk tribometer to investigate tribological behavior at various conditions. The scratch test was conducted to characterize adhesion force between composite coatings and substrates. The structure of the coatings has been extensively studied by a variety of techniques, including optical microscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM), etc. The composite coatings were also applied to the gears of a reduction gearbox. The efficiency of uncoated and MoS₂-based Ti coated gear was measured and compared at various input rotating speed under absorption oil film condition. It was found that the efficiency of gear had significantly improved after MoS₂-based Ti composite coatings deposition.     

Microstructural and wear properties of sputtered carbides and silicides

Sputtered Cr₃C₂, Cr₃Si₂ and MoSi₂ wear-resistant films (0.05–3.5 μm thick) were deposited on metal and glass surfaces. Electron transmission, electron diffraction and scanning electron microscopy were used to determine the microstructural appearance. Strong adherence was obtained with these sputtered films. Internal stresses and defect crystallographic growth structures of various configurations within the film have progressively more undesirable effects for film thicknesses greater than 1.5 μm. Sliding contact and rolling element bearing tests were performed with these sputtered films. Bearings sputtered with a duplex coating (a 0.1 μm thick undercoating of Cr₃Si₂ and subsequently a 0.6 μm coating of MoS₂) produced marked improvement (more than 10.5 × 10⁷ cycles) over straight MoS₂ films.     

Wear Resistance of Fine-Grained High Nitrogen Austenitic Stainless Steel Coated with Amorphous Carbon Films: The Soft X-ray Spectroscopy Approach

The wear resistance dependence on grain size of a high-nitrogen-alloyed austenitic stainless steels (HN) is investigated and compared to measurements for the same samples coated with amorphous carbon (a-C:H) and nitrogenated amorphous carbon (a-C:H(N)) films, deposited by means of plasma-enhanced chemical vapour deposition. A synergic effect between the grain refining and the film in the case of nitride amorphous carbon overcoats is observed in terms of increased low-friction performance duration. The temperature dependence of the wear resistance of a micro crystalline HN stainless steel coated with carbon films is also investigated. An overall decrease of the films' wear resistance is found with increasing temperature. Furthermore, a higher wear resistance is found in the a-C:H coated steel with respect to the a-C:H:N material. High-lateral-resolution photoemission microscopy reveals that inhomogeneities within the film after wear testing are correlated to an increase of the number of N-sp² C-bonded sites. The study of energy-distribution curves and high-lateral-resolution images on the nitrogenated samples shows that a modification of the surface chemistry occurs by mechanical action; in particular this implies that existing N-sp² C sites are beginning to cluster as temperature increases.     

The Lubricating Performance of an “in Situ” Process MoS2 Film in Air and in Liquids

The performance of a synthetic MoS₂ film, produced by electrodeposition of molybdic oxide followed by a temperature-pressure H₂S conversion to a molybdenum sulfide compound, is examined under extreme pressure conditions immersed in various fluids. Friction wear and EP characteristics, measured on various test machines, are compared to those of the fluids alone and also to conventional bonded films.

The fluids examined include: mineral oil, jet fuel, hydraulic fluid, silicone fluid.

The dry films include: burnished MoS₂ powder, MIL-L-8937 resin bonded film, MIL-L-8129 silicate bonded film and the synthetic “in situ” MoS₂ film.

The performance of the synthetic MoS₂ film on titanium and stainless steel is also examined.     

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.