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It is currently a challenge for space tribology to develop a long lifetime and high bearing capacity lubricant meeting the requirements of space applications. Herein, we dispersed graphene into ionic liquid, prepared novel composite coatings of diamond-like carbon (DLC)/ionic liquid (IL)/graphene with different graphene concentrations, and investigated its space performance under high vacuum and space radiation conditions. IL/graphene nanofluids with different concentrations were examined by Fourier transform infrared spectroscopy (FTIR). Furthermore, IL/graphene nanofluids after friction tests were investigated by X-ray photoelectron spectroscopy (XPS), and high-resolution transmission electron microscopy (HRTEM). The results showed that the graphene concentration would obviously affect the spatial tribology performance of the composite coatings. Because the excess graphene in the IL would tend to form irreversible agglomerates, leading to reduction of the effective graphene dose, an optimum graphene concentration (0.075 mg ml À1 ) in IL for the composite coatings was required to exhibit the lowest friction coefficient, the highest bearing capacity and the strongest anti-irradiation in a simulated space environment. In addition, XPS spectra further confirmed that the formation of a fluorinated oilcontaining carbon-rich tribofilm between the friction pairs further ensured the good antifriction and wear resistance performance of DLC/IL/graphene.

From daily intuitions to sophisticated atomic-scale experiments, friction is usually found to increase with normal load. Using first-principle calculations, here we show that the sliding friction of a graphene/graphene system can decrease with increasing normal load and collapse to nearly zero at a critical point. The unusual collapse of friction is attributed to an abnormal transition of the sliding potential energy surface from corrugated, to substantially flattened, and eventually to counter-corrugated states. The energy dissipation during the mutual sliding is thus suppressed sufficiently under the critical pressure. The friction collapse behavior is reproducible for other sliding systems, such as Xe/Cu, Pd/graphite, and MoS/MoS, suggesting its universality. The proposed mechanism for diminishing energy corrugation under critical normal load, added to the traditional structural lubricity, enriches our fundamental understanding about superlubricity and isostructural phase transitions and offers a novel means of achieving nearly frictionless sliding interfaces.

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A remarkable synergetic effect between the graphene oxide (GO) layers and multiwalled carbon nanotubes (MWCNTs) in improving friction and wear on sliding diamond-like carbon (DLC) surfaces under high vacuum condition (10(-5) Pa) and low or high applied load is demonstrated. In tests with sliding DLC surfaces, ionic liquid solution that contains small amounts of GO and MWCNTs exhibited the lowest specific friction coefficient and wear rate under all of the sliding conditions. Optical microscope images of the wear scar of a steel ball showed that GO/MWCNT composites exhibited higher antiwear capability than individual MWCNTs and GO did. Transmission electron microscopy images of nanoadditives after friction testing showed that MWCNTs support the GO layers like pillars and prevent assembly between the GO layers. Their synergistic effect considerably enhances IL-GO/MWCNT composites.

Superthick diamond-like carbon (DLC) films [(Six-DLC/Siy-DLC)n/DLC] were deposited on 304 stainless steel substrates by using a plane hollow cathode plasma-enhanced chemical vapor deposition method. The structure was investigated by scanning electron microscopy and transmission electron microscopy. Chemical bonding was examined by Raman, Auger electron, and X-ray photoelectron spectroscopy techniques. Mechanical and tribological properties were evaluated using nanoindentation, scratch, interferometry, and reciprocating-sliding friction testing. The results showed that implantation of a silicon ion into the substrate and the architecture of the tensile stress/compressive stress structure decreased the residual stress to almost 0, resulting in deposition of (Six-DLC/Siy-DLC)n/DLC films with a thickness of more than 50 μm. The hardness of the film ranged from 9 to 23 GPa, and the adhesion strength ranged from 4.6 to 57 N depending on the thickness of the film. Friction coefficients were determined in three tested environments, namely, air, water, and oil. Friction coefficients were typically below 0.24 and as low as 0.02 in a water environment. The as-prepared superthick films also showed an ultrahigh load-bearing capacity, and no failure was detected in the reciprocating wear test with contact pressure higher than 3.2 GPa. Reasons for the ultrahigh load-bearing capacity are proposed in combination with the finite-element method.

Based on the microbumps of graphene nanosheets and the nanostructure of carbon nanotubes (CNTs), novel graphene/CNTs composite films with hierarchical micro-and nanoscale surface roughness were successfully fabricated by simply spraying the mixed acetone dispersion of graphene nanosheets and CNTs onto stainless steel substrates. The as-prepared composite films exhibited controlled surface hydrophobic, adhesive and electrowetting properties via altering the film surface structure and surface energy. Among them the composite film with a 1 : 5 mass ratio of graphene to CNTs showed high hydrophobicity and conductivity, low water adhesion and contact angle sensitivity to the external electric field, which would help to resolve the surface electrostatic problems and unstable hydrophobicity under applied potential that exist in many conventional insulating hydrophobic materials, and could be useful in some application fields.