Carbon-based Tribofilms Research Articles

Influence of a carbon-based tribofilm induced by the friction temperature on the tribological properties of impregnated graphite sliding against a cemented carbide

Impregnated graphite has attracted considerable attention and has been widely used as an ideal friction material in many fields. However, the influence of the friction temperature on its tribological properties has not been clearly studied; furthermore, the evolution mechanism of transferred tribofilm is unknown. In this study, the tribological properties of impregnated graphite were investigated at different friction temperatures, and the evolution of the carbon-based tribofilm was also determined. The results revealed that the tribological properties significantly improved with an increase in friction temperature. The friction coefficient and wear depth of impregnated graphite reduced by 68% and 75%, respectively, at a high temperature of 160 °C compared with those of non-impregnated graphite. The significant properties of the impregnated graphite can be attributed to a transferred carbon-based tribofilm with an ordered structure induced by the friction temperature, which uniformly and stably adsorbs on friction interfaces. This study provides an important basis for designing graphite-based friction materials with improved properties suited for industrial applications.

Ultralow-Friction and Ultralow-Wear TiN-Ag Solid Solution Coating in Base Oil.

Lubrication plays a pivotal role in reducing energy consumption and machinery wear, profoundly impacting technological and economic development and the environment. A recent study (Erdemir, A., et al. Nature 2016, 536, 67) reported the effective extraction of carbon-based tribofilms from lubricating oil by catalytic activation of the coating material, opening new possibilities for innovative lubrication material research and development. Here, we showcase a solute-atom-strengthened and catalytically functionalized coating design and demonstrate its implementation in a TiN-Ag solid solution film that exhibits concurrent ultralow friction and ultralow wear. Indentation tests and Raman and X-ray photoelectron spectroscopy combined with quantum mechanical simulations uncover the rare superhard nature of the TiN-Ag film along with a solute-Ag-atom-induced self-oxidation mechanism for its outstanding catalytic capacity. These findings identify an outstanding type of mechanically strong and catalytically active coating material with simultaneous superior protective and lubricating functionality, holding great promise for applications ranging from microdevices to large-scale industrial equipment.

2D black phosphorus dotted with silver nanoparticles: An excellent lubricant additive for tribological applications

Black phosphorus (BP) is a new two-dimensional (2D) material with attractive nanoscale chemical and physical properties. Herein, black phosphorus dotted with silver nanoparticles (Ag/BP) was synthesized via a facile and in-suit chemical reduction approach without reductant. The tribological properties of as-prepared Ag/BP nanocomposite as lubricant additive in poly-alpha-olefin (PAO6) based oil were investigated by a ball-on-disc tribometer. The results demonstrate that the oil dispersed with trace amount of Ag/BP nanoadditives shows excellent lubricating performance for the steel/steel contact. The friction coefficient and wear rate of the oil dispersed with 0.075 wt% Ag/BP nanoadditives are reduced by 73.4% and 92.0% respectively, as compared to the based PAO6 oil. The possible friction-reducing and anti-wear mechanisms of Ag/BP nanoadditives in oil are illustrated by analyzing the wear scar surfaces. During scratching, the Ag/BP nanomaterials serves not only as a friction-reducing and anti-wear additive, but also as a catalyst to decompose the based PAO oil for forming a carbon-based tribofilm under ambient condition that further reduces the friction and wear. The findings here provide a novel insight into the design of lubricant additive incorporated in base oil and suggests a new potential application of black phosphorus in lubrication and tribology.

Effect of engine oil additives reduction on the tribofilm structure of a cylinder liner model surface

PurposeThe purpose of this paper is to provide a general picture for describing the formed tribofilm, including chemical and physical aspects in the micro-scale and the nano-scale. In a previous study, the durability of zinc dialkyl dithiophosphate (ZDDP) tribofilms on cylinder liner samples has been investigated in a tribometer model system by using fresh and aged fully formulated oils and replacing them with PAO8 without additives. Analyses of the derived tribofilms by means of X-ray photoelectron spectroscopy and scanning electron microscopy could give some hints about the underlying mechanisms of the tribofilm build-up and wear performance, but a final model has not been achieved.Design/methodology/approachThus, characterisation of these tribofilms by means of focused ion beam-transmission electron microscopy (FIB-TEM) and energy dispersive X-ray spectroscopy is presented and a concluding model of the underlying mechanisms of tribofilm build-up is discussed in this paper.FindingsFor tribotests running first with fresh fully formulated engine oil, a rather homogeneous ZDDP-like tribofilm is found underneath a carbon rich tribofilm after changing to non-additivated PAO8. However, when the tests run first with aged fully formulated engine oil, no ZDDP-like tribofilm has been found after changing to non-additivated PAO8, but a wear protective carbon rich tribofilm.Originality/valueThe obtained results provide insights into the structure and durability of tribofilms. Carbon-based tribofilms are built up on the basis of non-additivated PAO8 because of the previously present ZDDP tribofilms, which suggests an alternative way to reducing the consumption of antiwear additives.

Elastic Lubricious Effect of Solidlike Boundary Films in Oil-Starvation Lubrication

Herein, a boundary lubrication (BL) mechanism based on the carbon-based tribofilms formed via Ni(Cu)-catalytic dehydrogenation of hydrocarbon oils was queried by performing standard ball-on-disk tr...

Formation of Carbon-Based Tribofilm Under Palm Methyl Ester

The popularity of biodiesel is continuously increasing nowadays because of fossil fuel’s shortage and environment concerns. Biodiesel is renewable, carbon neutral, and, most importantly, usable in diesel engines without modification. Particularly, the biodiesel from palm oil, palm methyl ester (PME), has become the main source of the world’s biodiesel due to its availability. One of the main concerns in biodiesel fuel usage is its ability to lubricate the mechanical components of the fuel system such as injection pump and nozzle sprayer since those parts are mainly lubricated by the fuel. In this research, an investigation has been conducted to analyze the characteristic of a tribofilm (boundary film) produced by friction of self-mated stainless steel at boundary lubrication condition using palm methyl ester as the lubricant. The friction tests were conducted using a ball on disk tribometer. The results show that excessive wear of the material was prevented; thanks to the tribofilm formed on the contact interface. The analysis shows that the tribofilm has a unique characteristic indicated by the presence of D-band’s peak but the absence of G-band’s peak in its Raman spectra. It seems that the carbon-based tribofilm was formed on the solid surface as a result of carbon decomposition from the palm methyl ester assisted by the tribo-chemical reaction. Although its real structure is still not clear, the tribofilm has an excellent ability to protect the material from severe friction and wear.

Friction and wear behaviour of silicon carbide/graphene composites under isooctane lubrication

The tribological performance of silicon carbide (SiC)/graphene nanoplatelets (GNPs) composites is analysed under oscillating sliding tests lubricated with isooctane, looking to explore their potential as components for gasoline direct injection (GDI) engines. High graphene filler contents (20 vol.% of GNPs) are required to substantially reduce the friction coefficient of SiC ceramics, attaining decreases on friction up to 30% independently of the applied load. For all materials and testing conditions a mild wear regime is evidenced. SiC/20 vol.% GNPs composite also enhances the wear resistance up to 35% at low load, but the addition of GNPs produces a deleterious effect as the load augments. The tribological behaviour depends on the formation and destabilization of a solid lubricant carbon-based tribofilm and strongly correlates with the mechanical properties of the tested materials.

Low-friction carbon-based tribofilm from poly-alpha-olefin oil on thermally oxidized Ti6Al4V

Titanium alloy shows a poor tribological performance even under lubrication. In this paper, we report the formation of carbon-based tribofilms from poly-alpha-olefin (PAO) base oil molecules on Ti6Al4V (Ti64) samples after thermal oxidization treatment, where PAO enabling base oils to provide not only the fluid but also the solid tribofilm. Ball-on-disk tests at contact pressures of 2.0 GPa reveal that these carbon-based tribofilms on thermally oxidized Ti64 samples decrease the friction coefficient by about 3 times and wear rate by up to 2 orders magnitude as comparing to untreated one.

Toward low friction in high vacuum by designing textured a-C/IL duplex lubricating film

The advanced space technologies urge us to develop a superior lubricant that owns low friction coefficient and high anti-wear abilities in high vacuum environment. Herein, we designed a novel textured a-C/ionic liquid (IL) film and investigated its vacuum tribological performance. The worn surfaces of as-deposited film and corresponding counterpart ball were investigated in detail. The results showed that the combined effect of textured grooves and IL obviously improved tribological performance of a-C film in vacuum environment. The grooves acted as IL reservoirs that promoted retention of the lubricating film and prevented direct contact between the solid surfaces. In addition, these textured grooves also acted as buffers that trapped and collected wear debris, thereby preventing the break down of the tribological system. The characterization results confirmed the formation of IL containing carbon-based tribofilm between the rubbed surfaces under joint effect of textured patterns and IL lubrication.

Comparative study on tribological mechanisms of polyimide composites when sliding against medium carbon steel and NiCrBSi

Tribological behaviors of various polyimide (PI) composites when rubbing with medium carbon steel (MCS35) and NiCrBSi, were comprehensively investigated. When the conventional PI composite filled with carbon fibers and graphite was concerned, the carbon-based tribofilm formed on NiCrBSi surface resulted in obvious friction- and wear-reduction. However, no lubricating tribofilm was formed on MCS35 surface. Chelation of polymeric molecular radicals with the metallic counterparts was identified on the worn surfaces. Theoretical calculations corroborated that the Ni-based metal–organic compound showed a higher stability than the Fe-based one. With respect to the sliding of the hybrid nanocomposites containing silica or h-BN nanoparticles, the nanoparticles released onto the interface significantly mitigated tribo-oxidation of metallic counterparts, and were finally tribo-sintered into a compact layer after being mixed with remnant polymer and tribo-oxidation products. The effect of counterpart material was less pronounced for the tribological mechanisms of the nanocomposites than for the conventional composite.

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 sp2-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 sp3-sp2 phase transformation.

Carbon-based tribofilms from lubricating oils.

Moving mechanical interfaces are commonly lubricated and separated by a combination of fluid films and solid 'tribofilms', which together ensure easy slippage and long wear life. The efficacy of the fluid film is governed by the viscosity of the base oil in the lubricant; the efficacy of the solid tribofilm, which is produced as a result of sliding contact between moving parts, relies upon the effectiveness of the lubricant's anti-wear additive (typically zinc dialkyldithiophosphate). Minimizing friction and wear continues to be a challenge, and recent efforts have focused on enhancing the anti-friction and anti-wear properties of lubricants by incorporating inorganic nanoparticles and ionic liquids. Here, we describe the in operando formation of carbon-based tribofilms via dissociative extraction from base-oil molecules on catalytically active, sliding nanometre-scale crystalline surfaces, enabling base oils to provide not only the fluid but also the solid tribofilm. We study nanocrystalline catalytic coatings composed of nitrides of either molybdenum or vanadium, containing either copper or nickel catalysts, respectively. Structurally, the resulting tribofilms are similar to diamond-like carbon. Ball-on-disk tests at contact pressures of 1.3 gigapascals reveal that these tribofilms nearly eliminate wear, and provide lower friction than tribofilms formed with zinc dialkyldithiophosphate. Reactive and ab initio molecular-dynamics simulations show that the catalytic action of the coatings facilitates dehydrogenation of linear olefins in the lubricating oil and random scission of their carbon-carbon backbones; the products recombine to nucleate and grow a compact, amorphous lubricating tribofilm.