Microencapsulated paraffin as a tribological additive for advanced polymeric coatings

Numerous tribological applications,wherein the use of liquid lubricants is infeasible,require adequate dry lubrication.Despite the use of polymers as an effective solution for dry sliding tribological applications,their poor wear resistance prevents the utilization in harsh industrial environment.Different methods are typically implemented to tackle the poor wear performance of polymers,however sacrificing some of their mechanical/tribological properties.Herein,we discussed the introduction of a novel additive,namely microencapsulated phase change material(MPCM)into an advanced polymeric coating.Specifically,paraffin was encapsulated into melamine-based resin,and the capsules were dispersed in an aromatic thermosetting co-polyester(ATSP)coating.We found that the MPCM-filled composite exhibited a unique tribological behavior,manifested as“zero wear”,and a super-low coefficient of friction(COF)of 0.05.The developed composite outperformed the state-of-the-art polytetrafluoroethylene(PTFE)-filled coatings,under the experimental conditions examined herein.

Achieving superlubricity in DLC films by controlling bulk, surface, and tribochemistry

Superlubricity refers to a sliding regime in which contacting surfaces move over one another without generating much adhesion or friction[1].From a practical application point of view,this will be the most ideal tribological situation for many moving mechanical systems mainly because friction consumes large amounts of energy and causes greenhouse gas emissions[2].Superlubric sliding can also improve performance and durability of these systems.In this paper,we attempt to provide an overview of how controlled or targeted bulk,surface,or tribochemistry can lead to superlubricity in diamond-like carbon(DLC)films.Specifically,we show that how providing hydrogen into bulk and near surface regions as well as to sliding contact interfaces of DLC films can lead to super-low friction and wear.Incorporation of hydrogen into bulk DLC or near surface regions can be done during deposition or through hydrogen plasma treatment after the deposition.Hydrogen can also be fed into the sliding contact interfaces of DLCs during tribological testing to reduce friction.Due to favorable tribochemical interactions,these interfaces become very rich in hydrogen and thus provide super-low friction after a brief run-in period.Regardless of the method used,when sliding surfaces of DLC films are enriched in hydrogen,they then provide some of the lowest friction coefficients(i.e.,down to 0.001).Time-of-flight secondary ion mass spectrometer(TOF-SIMS)is used to gather evidence on the extent and nature of tribochemical interactions with hydrogen.Based on the tribological and surface analytical findings,we provide a mechanistic model for the critical role of hydrogen on superlubricity of DLC films.

Nano-Structured Carbide-Derived Carbon Films and Their Tribology

Carbide-derived carbon （CDC） is a form of carbon produced by reacting metal carbides, such as SiC or TiC, with halogens at temperatures high enough to produce fast kinetics, but too low to permit the rearrangement of the carbon atoms into an equilibrium graphitic structure. The structure of CDC is derivative of the original carbide structure and contains nanoscale porosity and both sp2 and sp3 bonded carbon in a variety of nanoscale structures. CDC can be produced as a thin film on hard carbides to improve their tribological performance. CDC coatings are distinguished by their low friction coefficients and high wear resistance in many important industrial environments and by their resistance to spallation and delamination. The tribology of CDC coatings on SiC surfaces is described in detail.

Influence of tribofilm on superlubricity of highly-hydrogenated amorphous carbon films in inert gaseous environments

In this study,we mainly focus on the structural morphology and inter-atomic bonding state of tribofilms resulting from a highly-hydrogenated amorphous carbon(a-C:H) film in order to ascertain the underlying mechanisms for its superlubric behavior(i.e.,less than 0.01 friction coefficient).Specifically,we achieved superlubricity(i.e.,friction coefficients of down to 0.003) with this film in dry nitrogen and argon atmospheres especially when the tribo-pair is made of an a-C:H coated Si disk sliding against an a-C:H coated steel ball,while the a-C:H coated disk against uncoated ball does not provide superlubricity.We also found that the state of superlubricity is more stable in argon than in nitrogen and the formation of a smooth and uniformly-thick carbonaceous tribofilm appears to be one of the key factors for the realization of such superlubricity.Besides,the interfacial morphology of sliding test pairs and the atomic-scale bond structure of the carbon-based tribofilms also play an important role in the observed superlubric behavior of a-C:H films.Using Raman spectroscopy and high resolution transmission electron microscopy,we have compared the structural differences of the tribofilms produced on bare and a-C:H coated steel balls.For the a-C:H coated ball as mating material which provided superlow friction in argon,structural morphology of the tribofilm was similar or comparable to that of the original a-C:H coating;while for the bare steel ball,the sp^2-bonded C fraction in the tribofilm increased and a fingerprint-like nanocrystalline structure was detected by high resolution transmission electron microscopy(HRTEM).We also calculated the shear stresses for different tribofilms,and established a relationship between the magnitude of the shear stresses and the extent of sp^3-sp^2 phase transformation.

Guest editorial: Special issue on superlubricity

Energy and material losses due to friction and wear in mechanical systems account for huge economic and environmental burdens.Approximately one-third of the world’s primary energy consumption is attributed to friction;in addition,about 80%of the equipment failure is caused by wear in friction processes.Even relatively small improvements in the tribology of mechanical systems would reap enormous societal benefits.Superlubricity is a state in which two contacting surfaces exhibit almost no resistance to sliding,and the friction force between the two sliding surfaces nearly vanishes.Improvement of superlubricity technology and our understanding of its mechanism play an important role for saving energy in industry as well as our daily life.Consequently,superlubricity has attracted a large amount of attention from researchers in many fields,which is leading to a revolution in engineering technology.