Synergetic Lubricating Research Articles

Effect of WS2 addition on electrical sliding wear behaviors of Cu–graphite–WS2 composites

Four kinds of Cu-based composites with different mass ratios of graphite and WS2 as lubricants were fabricated by hot-pressing method. Electrical sliding wear behaviors of the composites were investigated using a block-on-ring tribometer rubbing against Cu–5%Ag alloy ring. The results demonstrated that 800 °C was the optimum sintering temperature for Cu–graphite–WS2 dual-lubricant composites to obtain the best comprehensive properties of mechanical strength and lubrication performance. Contact voltage drops of the Cu-based composites increased with increasing the mass ratio of WS2 to graphite. The Cu-based composite with 20% graphite and 10% WS2 showed the best wear resistance due to the excellent synergetic lubricating effect of graphite and WS2. The reasonable addition of WS2 into the Cu-graphite composite can remarkably improve the wear resistance without much rise of electrical energy loss which provides a novel principle of designing suitable sliding electrical contact materials for industrial applications.

Ionic liquids gels with in situ modified multiwall carbon nanotubes towards high-performance lubricants

Two ionic liquids (ILs) gels were prepared by grinding multiwall carbon nanotubes (MWCNTs) in two kinds of ILs, and their physical and tribological properties were investigated in detail. Results demonstrate that ILs gels possess high conductivity and excellent tribological performance which mainly depends on the synergy of ILs and MWCNTs with their respective outstanding characteristics. ILs modified MWCNTs through van der Waals and π–π stacting interactions significantly improve the dispersibility and compatibility with lubricants, which greatly enhances the conductivity and tribological properties of the lubricants. The friction mechanism for the ILs gels is attributed to the synergetic lubrication of ILs and MWCNTs.

“Bio-glues” to enhance slipperiness of mucins: improved lubricity and wear resistance of porcine gastric mucin (PGM) layers assisted by mucoadhesion with chitosan

A synergetic lubricating effect between porcine gastric mucin (PGM) and chitosan based on their mucoadhesive interaction is reported at a hydrophobic interface comprised of self-mated polydimethylsiloxane (PDMS) surfaces. In acidic solution (pH 3.2) and low concentrations (0.1 mg mL(-1)), the interaction of PGM with chitosan led to surface recharge and size shrinkage of their aggregates. This resulted in higher mass adsorption on the PDMS surface with an increasing weight ratio of [chitosan]/[PGM + chitosan] up to 0.50. While neither PGM nor chitosan exhibited slippery characteristics, the coefficient of friction being close to 1, their mixture improved considerably the lubricating efficiency (the coefficient of friction is 0.011 at an optimum mixing ratio) and wear resistance of the adsorbed layers. These findings are explained by the role of chitosan as a physical crosslinker within the adsorbed PGM layers, resulting in higher cohesion and lower interlayer chain interpenetration and bridging.

Wear and Friction of TiAl Matrix Self-Lubricating Composites against Si3N4 in Air at Room and Elevated Temperatures

More durable, low-friction bearing materials over a wide temperature range are needed for turbine components and other high-temperature bearing applications. The current study reported the tribological properties of TiAl matrix self-lubricating composites (TMC) containing MoS2 (a low-temperature lubricant, below 500°C), hBN (a medium-temperature lubricant, below 600°C), and Ti3SiC2 (a high-temperature lubricant, above 600°C) designated as MhT against an Si3N4 counterface at temperatures ranging from 25 to 800°C in air. The load was 10 N and the sliding speed was 0.2 m/s for all tests. Tribological studies indicated that TMC containing MhT showed a lower friction coefficient and wear rate in comparison to TiAl-based alloy at all test temperatures, which was attributed to the excellent synergetic lubricating effect of MoS2, hBN, and Ti3SiC2. TMC containing 5 wt% MhT exhibited the best tribological properties over a wide temperature range.

Tribological behaviors of NiAl based self-lubricating composites containing different solid lubricants at elevated temperatures

An investigation was conducted on the possible beneficial synergistic effects of multiple additives in self-lubricating composites for use in high temperature friction and wear applications. The sliding friction and wear rates of NiAl based self-lubricating composites with different solid lubricant additions (PbO, Ti3SiC2–MoS2, Ti3SiC2–WS2) were obtained from room temperature to 800°C with a pin-on-disc configuration. Dry sliding wear tests against Si3N4 balls (the pin specimens) were run using a constant load of 10N and a constant speed of 0.2m/s. The results showed that the composites with Ti3SiC2–MoS2 exhibited excellent self-lubricating and anti-wear properties over a wide temperature range. MoS2 lubricated better at low temperatures, while Ti3SiC2 lubricated better at high temperatures. The Ti3SiC2–MoS2 binary lubricant presented the best synergetic lubricating effect of the three candidate compositions.

Synergetic lubricating effect of MoS2 and Ti3SiC2 on tribological properties of NiAl matrix self-lubricating composites over a wide temperature range

NiAl matrix self-lubricating composites with MoS2 and Ti3SiC2 lubricants were prepared by spark plasma sintering. The tribological behaviors of the NiAl–Ti3SiC2–MoS2 composites against Si3N4 were investigated from room temperature to 800°C. The results showed that the composites exhibited excellent self-lubricating and anti-wear properties over a wide temperature range. At 400°C, the composites containing 5Ti3SiC2–5MoS2 (wt.%) had a very low friction coefficient of about 0.13 and a low wear rate of 4.5×10−5mm3N−1m−1. MoS2 played the main role of lubrication at low temperatures, while Ti3SiC2 was responsible for low friction at high temperatures. Ti3SiC2 and MoS2 lubricants in the NiAl–Ti3SiC2–MoS2 composites showed the excellent synergetic lubricating effect over a wide temperature range from room temperature to 800°C.

Effect of electrical current on the tribological behavior of the Cu-WS2-G composites in air and vacuum

As the traditional graphite-based composites cannot meet the requirement of rapid developing modern industry, novel sliding electrical contact materials with high self-lubricating performance in multiple environments are eagerly required. Herein a copper-based composite with WS2 and graphite as solid lubricant are fabricated by powder metallurgy hot-pressed method. The friction and wear behaviors of the composites with and without current are investigated under the condition with sliding velocity of 10 m/s and normal load of 2.5 N/cm2 in both air and vacuum. Morphologies of the worn surfaces are observed by optical microscope and compositions of the lubricating films are analyzed by XPS. Surface profile curves and roughness of the worn surfaces are obtained by 2205 surface profiler. The results of wear tests show that the friction coefficient and wear volume loss of the composites with current are greater than that without current in both air and vacuum due to the adverse effects of electrical current which damaged the lubricating film partially and roughed the worn surfaces. XPS results demonstrate that the lubricating film formed in air is composed of oxides of Cu, WS2, elemental S and graphite, while the lubricating film formed in vacuum is composed of Cu, WS2 and graphite. Because of the synergetic lubricating action of oxides of Cu, WS2 and graphite, the composites show low friction coefficient and wear volume loss in air condition. Owing to the fact that graphite loses its lubricity which makes WS2 become the only lubricant, severe adhesive and abrasive wear occur and result in a high value of wear rate in vacuum condition. The formation of the lubricating film on the contact interface between the brush and ring is one of the factors which can greatly affect the wear performance of the brushes. The low contact voltage drop of the composites in vacuum condition is attributed to the high content of Cu in the surface film. This study fabricated a kind of new sliding electrical contact self-lubricating composite with dual-lubricant which can work well in both air and vacuum environments and provides a comprehensive analysis on the lubrication mechanisms of the composite.

Effect of the Synergetic Action on Tribological Characteristics of Ni-Based Composites Containing Multiple-Lubricants

Ni-based self-lubricating composites with multiple-lubricants addition were prepared by a powder metallurgy technique, and the effect of multiple-lubricants on tribological properties was investigated from room temperature to 700 °C. The synergetic effects of graphite, MoS2, and metallic silver lubricants on the tribological characteristics of composites were analyzed. XRD analysis showed that new CrxSy and Mo2C phase were formed in the composites containing graphite, MoS2 and metallic Ag lubricants during the sintering process. The average friction coefficients (0.69–0.22) and wear rates (11.90–0.09 × 10−5 mm3 N−1 m−1) were obtained when rubbing against Inconel 718 alloy from room temperature to 700 °C due to synergetic lubricating action of multiple-lubricants. A smooth lubricating was gradually generated on the worn surface, and the improving of tribological properties was attributed to the formation of lubricious glaze film on the worn surface and their partially transferred to the counterface. The graphite played the main role of lubrication at room temperature, while molybdate phase and graphite were responsible for low friction coefficients and wear rates at mid/high temperatures. The synergetic lubricating effect of molybdate (produced in the rubbing process at high temperatures) iron oxide (transfer from disk material to the pin) and remaining graphite multiple-lubricants play an important lubricating role during friction tests at a wide temperature range.

Vacuum tribological performance of DLC-based solid–liquid lubricating coatings: Influence of sliding mating materials

The friction and wear behaviors of DLC-based solid–liquid lubricating coatings for three liquid lubricants sliding against different counterface materials were examined under high vacuum conditions. Seven kinds of balls with the diameter of 3mm were chosen as counterparts, which were GCr15, bronze, ZrO2, Al2O3, SiC, WC, and Si3N4. Under high vacuum condition, the friction coefficient (COF) and wear rate of carbon-based solid–liquid lubricating coatings sliding against different counterparts were diverse, due to different liquid lubrications and counterface materials. In analyzing the friction and wear mechanism, the contact radius and the contact pressure were introduced. The COF showed the inverse varied trend with Hertzian contact radius for the three liquid lubricants. COF decreased with the increase of contact radius, which was different from the dry sliding of DLC film. The possible reason was that the synergetic lubrication between the DLC films and the liquid lubricants formed a new solid–liquid synergistic system (DLC-oil-DLC), which improved the ability of plasticity-resistant deformation, reduced the real contact area and friction coefficient. The contact pressure was consistent with the wear rate varied trend for the three liquid lubricants, and this varied trend was in good agreement with each other for MACs lubricant.

DLC-based solid–liquid synergetic lubricating coatings for improving tribological behavior of boundary lubricated surfaces under high vacuum condition

► Five DLC-based solid–liquid lubricating dual-layer coatings were successfully fabricated. ► Solid–liquid lubricating coatings of MACs, IL, Zdol exhibited excellent tribological properties. ► Solid–liquid lubricating coatings of PAO and silicon oil showed the poor tribological behaviors. ► Such difference depends on nature of liquid lubricant, and synergy lubricating mechanism with DLC. In the present study, five kinds of liquid lubricants, including MACs, IL, Zdol, PAO and silicon oil, were homogeneously spun on DLC coatings and consequently the DLC-based solid–liquid lubricating dual-layer coatings were successfully fabricated. The tribological behaviors of the DLC/steel combinations and steel/steel combinations lubricated with above five liquid lubricants were comparatively investigated using a high-vacuum tribometer simulated for space environments. The SEM, 3D surface profiler, Raman spectrum and EDS were employed to analyze the worn surfaces of the friction pairs. The analysis results showed that three kinds of solid–liquid lubricating coatings including MACs, IL, Zdol exhibited excellent tribological properties, in which DLC/steel combinations lubricated with these three liquid lubricants exhibited smoother running-in and much lower steady-state friction than steel/steel combinations, and the wear rates of DLC/steel combinations were in the range of 1–3 orders of magnitude lower than that of steel/steel combinations. But for PAO and silicon oil, they demonstrated the poorest tribological behaviors with high friction coefficients and wear rates. Such significant improvement in tribological performance of DLC-based solid–liquid lubricating dual-layer coatings under high vacuum can be attributed to the synergy lubrication mechanism by the combination of the solid lubrication effect of DLC film and the boundary lubrication of the liquid lubricants. Moreover, the differences among different liquid lubricants in tribological properties of DLC-based solid–liquid lubricating dual-layer coatings under high vacuum were possible due to the differences in viscosity and components of the five kinds of liquid lubricants.

Preparation and characterization of reactively sintered Ni 3Al–hBN–Ag composite coating on Ni-based superalloy

Ni 3Al–hBN–Ag intermetallic matrix composite coating was prepared on Ni-based superalloy by reactive sintering. The crystalline phase and microstructure of the coating were examined by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). The results showed that the hBN particles pretreated by electroless nickel plating and hot-dip aluminizing and Ag particles with a size of several micrometers were homogeneously dispersed in the Ni 3Al matrix, and the composite coating had strong interfacial bonding with a fine and dense microstructure. During the reactive sintering, an exothermic reaction between Ni and Al associated with a transient liquid phase occurred, leading to in situ synthesis of the densified Ni 3Al matrix in the coating with a high tensile strength of more than 70 MPa. The high-temperature tribological properties evaluated on a ball-on-disc test rig showed that the coating possessed self-lubricating properties from room temperature to 800 °C due to a synergetic lubricating action of Ag and hBN.

Tribological properties of nickel-based self-lubricating composite at elevated temperature and counterface material selection

Solid lubricating materials are necessary for development of new generation gas turbine engines. Nickel-based self-lubricating composites with graphite and molybdenum disulfide as lubricant were prepared by powder metallurgy (P/M) method. Their tribological properties were tested by a MG-2000 high-temperature tribometer from room temperature to 600 °C. The structure of the composite was analyzed by XRD and worn surface morphologies were observed by optical microscope. The effects of counterface materials on tribological behavior of composites were investigated. It was found that chromium sulfide and tungsten carbide were formed in the composite by adding molybdenum disulfide and graphite, which were responsible for low-friction and high wear-resistance at elevated temperatures, respectively. The average friction coefficients (0.14–0.27) and wear rates (1.0–3.5 × 10 −6 mm 3/(N m)) were obtained for Ni–Cr–W–Fe–C–MoS 2 composite when rubbed against silicon nitride from room temperature to 600 °C due to a synergetic lubricating action of graphite and molybdenum disulfide. The optimum combination of Ni–Cr–W–Fe–C–MoS 2/Ni–Cr–W–Al–Ti–C showed lower friction than other counter pairs. The graphite played the main role of lubrication at room temperature, while sulfides were responsible for low friction at high temperature.