•Two coatings (CrN/GLC and CrN) were evaluated in different lubrication conditions.•The potential of reducing friction and wear was addressed by GLC surface.•Use of GLC surface improved the operating reliability significantly.•The compatibility of GLC surface with the oil additives was issued.Modern automotive systems strictly demands increased mechanical and thermal loads, a longer service life and the weight reduction. Low friction hard coatings are extensively used in power train and engine applications by wear inhibition and friction reduction. The present study evaluates friction and wear responses of CrN and CrN/GLC coatings in different lubricated conditions. Results showed that, the presence of GLC surface not only lowered friction significantly by 67% under oil-starved or even dry conditions, but also obtained a pronounced decrease in wear by 70% for the opposite surface in the oil-lubricated environment, as compared to CrN coating. GLC application thus allowed the engine components to operate more reliably and durably when improper conditions happened. A tribochemical layer with the responsive character accounted for superior performance of GLC/CrN coating, which derived from the interactions of the additive in oil with the rubbing surfaces. The information on the correlation of performance with physics and chemistry of the rubbing interface was provided.

•Integration of traditional ceramic coatings and hard low friction surface is proposed.•The load-carrying capacity of engineered coating materials is evaluated by the scuffing tests.•Amorphous carbon improves reliability and robustness of chromium-based surface during rubbing.Continuous optimization of advanced coatings is required to achieve technology advances and strict emission standards in automotive systems. Integration of conventional ceramic coatings and hard amorphous graphite-like carbon (GLC) with low friction is an economically feasible way of achieving superior efficiency of oil and durability as well as scuffing resistance. This work evaluates the scuffing resistant capacity and durability of engineered coating materials. The presence of GLC not only combats the scuffing damage and running instability effectively for conventional chromium-based coatings, and also improves the reliability and robustness of the piston rings. The scuffing mechanism of the engineered rings with and without GLC surface will be discussed by the observation of the damaged characteristics and the chemistry of the rubbing parts. This will potentially benefit to optimize the coating material in the piston assembly of engine.

•Ultrathin period thickness results in superior mechanical and tribological properties.•Columnar structure originates from the surface irregularities of intermediate layer.•The more heterointerfaces, the lower internal stress.•Interfacial sliding between WO3 rich tribofilm and C rich surface generates low friction.Nanomultilayered WC/a-C coatings with modulation period raning from 1.3 to 11.5 nm were successfully fabricated using unbalanced magnetron sputtering process and the evolution of their microstructure, mechanical and tribological properties with the modulation period were systemically investigated. It has been demonstrated that the columnar structure in WC/a-C nanomultilayers originates from the surface irregularities of intermediate Cr/WC/C layer and the column diameter is correlated with the initial Cr layer thickness in this intermediate stage. Hardening and toughening through the nanomultilayer structure design have been achieved and particularly pronounced when the modulation period ranges from 5.8 to 10 nm. The enhanced hardness and fracture toughness are mainly attributed to dislocation/crack pinning effect from the thin individual layer and the heterointerfaces between WC and a-C layers. It is also found that the more heterointerfaces, the lower internal stress. What's more, the WC/a-C nanomultilayers with small modulation period (≤5.8 nm) exhibit low COF about 0.05 and good wear resistance due to the interfacial sliding between a WO3-rich tribofilm and a carbon-rich wear surface; once the modulation period above 5.8 nm, it fails to build up such a low friction interface between the tribopairs, which combining with the high intrinsic friction caused by its own structure characteristics result in high COF about 0.18–0.2 and poor wear resistance.Download high-res image (281KB)Download full-size image

•B4C/a-C coatings with various boron carbide contents were synthesized.•Both the carbon matrix and the introduced boron carbide were amorphous.•Improved mechanical and tribological properties were simultaneously achieved.•The relevant low friction mechanism was deeply investigated.Boron carbide doped DLC coating can combine the property superiority of these two materials to create high-performance carbon-based coating, while the final property of the composite coating is strongly dependent on the proportion of coating components. The aim of this study was to investigate the effect of boron carbide content on the mechanical and tribological properties of the magnetron sputtered B4C/a-C coatings, as well as the relevant low friction mechanism. It was found that both the carbon matrix and introduced boron carbide were amorphous and the composite coating exhibited typical columnar structure. Appropriate doping of boron carbide significantly enhanced coating hardness, toughness as well as the adhesion strength and carrying ability, and all of them reached the optimal value when the B concentration was 2.92 at.%. It was also found that the B4C/a-C coating with B 2.92 at.% exhibited the lowest steady friction coefficient about 0.05 and highest wear resistance at 20% RH. None of boric acid was formed on the sliding interface under the tested humidity range. Thus, the improved tribological properties were mainly attributed to the enhanced mechanical properties and friction-induced formation of an intact graphitized carbon transfer layer on the counterpart under specific humidity condition.

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.

The MoS2 coatings were prepared by unbalanced magnetron sputtering system. Ti was used as the dopant to improve its mechanical properties and oxidation resistance. The microstructure of the coatings changed from coarse columnar platelet structure at low Ti content (0–1.8 at.%) to a denser columnar microstructure at increased Ti content. The hardness and elastic modulus of the MoS2/Ti composite coatings gradually increase as the Ti content increases. The tribological properties of the coatings were evaluated against a GCr15 ball under different relative humidity atmosphere to verify the oxidation resistance. The results show that doping of Ti can significantly improve the oxidation resistance of the coatings. Particularly, the friction coefficient for the composite coating with 10.8 at.% Ti exhibited almost the same friction coefficient under three different humidity. However, excessive Ti induced high brittleness and the coatings flaking easily from the substrate; therefore, the coatings show poor wear resistances under both dry and humidity atmospheres.

Nanocomposite WC/a-C coatings were successfully fabricated using a magnetron sputtering process, and post-deposition annealing was conducted in vacuum for 1 h at the annealing temperatures ranging from 100 to 500 °C. The changes in coating structure, internal stress, hardness, toughness, friction coefficient and wear have been investigated to assess the effects of annealing on microstructure, mechanical and tribological properties of the WC/a-C coatings. The results show that the nanocrystalline WC1−x partially decays to metastable W2C when annealing at 300–500 °C and no graphitization of amorphous carbon matrix starts up to 500 °C. This structural change results in a slightly increased hardness and an improved toughness as well as a gradually decreased internal stress. In addition, the time for the annealed coatings to achieve a low steady friction coefficient decreases with the increase of annealing temperature. An optimized tribological property with low friction coefficient of about 0.06 and enhanced wear resistance of the WC/a-C coating is obtained by annealing at 400 °C. Friction reduction and wear resistance caused by annealing can be attributed to the friction-induced WO3-rich tribofilm which slides against a thin carbon-rich layer on the coating surface resulting in a low friction, and the partition effect of the stationary WO3-rich tribofilm combining with the improved mechanical properties generates a high wear resistance.

•Low F content in a-C:H:F films reduces the friction coefficient.•High F content in a-C:H:F films increases the friction coefficient.•A composite-like tribo-layer on Al surface changes with the F content.•Adhesion, shear and abrasion competitively control the friction.Fluorinated amorphous carbon (a-C:H:F) films synthesized from C2H2 and CF4 were examined by Raman spectra and X-ray photoelectron spectroscopy (XPS). Their tribological properties were tested against aluminum balls in dry sliding. The a-C:H:F films with low F content showed lower friction coefficient than hydrogenated amorphous carbon (a-C:H) film, whereas high F content in films resulted in a significant friction increase. Contact surfaces were analyzed in detail to elucidate the possible sliding mechanism. Results indicated that the friction behavior was closely related to the nature of a composite-like tribo-layer consisting of Al compounds and carbon components formed on Al ball, relying on the tribochemical processes of contact interface. The accumulated F atoms on tribo-layer reduced the adhesion across sliding interface because of strong repulsion between F atoms, and thereby lowered the friction of a-C:H:F films. However, with increasing F contents in films, the enhanced tribochemical reaction between Al and F caused crack, delamination and fragmentation of the tribo-layer, and then a marked abrasive process at the sliding interface. Consequently, the increased shearing and abrasive actions strongly opposed the contribution of the reduced adhesion to friction, and result in a rather high friction of highly fluorinated a-C:H:F films.

The anticorrosion and tribocorrosion properties of a multilayer diamond-like carbon (DLC) film were systematically investigated in NaCl solution. Electrochemical measurements suggest that the corrosion performance of the multilayer DLC film is superior to those of the substrate and single layer DLC film in NaCl solution, which is attributed to the successively multilayered structure with a well-bonded interface and the formation of Si oxides. An extremely high Warburg impedance value, higher than 107 Ω cm2, of the multilayer DLC film has been observed. Tribocorrosion tests show that the multilayer DLC film presents lower wear rate in NaCl solution, with the substrate and single layer DLC film as comparisons. We demonstrate that the multilayer DLC film is an excellent protective material for improving both corrosion and wear performance of the substrate.

•Combining LST technology with DLC coatings achieved a low friction and wear.•An appropriate number of dimples trapped wear debris and maintained coating hardness.•Dimples and dimples-induced DLC graphitization generated the friction-reducing.•Entrapment of wear debris and low DLC graphitization improved the wear-resistance.Titanium alloys are characterized by poor tribological performance, and their conventional use has been restricted to non-tribological applications. Surface films and surface texturing are effective methods to improve the tribological properties of sliding surfaces. In this study, the patterns of micro-dimples with different densities and diamond-like carbon (DLC) films were fabricated on the surface of titanium alloy by laser surface texturing and magnetron sputtering, respectively. The effects of dimple densities and DLC phase transformation on the tribological behavior of the titanium alloy under dry friction and liquid lubrication conditions were investigated. The results showed that DLC film with appropriate dimple area density (44%) are effective in enhancing reducing-friction property of titanium alloy substrate because of the entrapment of wear particles in the dimples and dimple-induced graphitization during sliding motions, while the 24% textured specimen exhibited outstanding wear resistance.

•The structure, mechanical and tribological properties of the CrSiN films were systematically investigated.•The wear mechanisms in air and water were systematically investigated.•Found an alternative approach to overcome the problem of water lubrication.The CrSiN films with different Si contents were deposited on stainless steel and silicon substrate (100) by reactive magnetron sputtering. The microstructures, mechanical properties and tribological properties were investigated as a function of Si/(Cr+Si) ratio. It was found that the hardness reached its maximum value of approximately 22 GPa. The wear mechanism of CrSiN films were investigated as the Si/(Cr+Si) ratio changed in the air and water environment. The tribochemical reaction was found to occur in the wear contact area, and this caused CrSiN films to form the amorphous oxide layer which was the reason for the low fiction coefficient and low wear rate.