Tribofilm Formation Research Articles

Influence of Atmosphere on Carbonaceous Film Formation in Rubbing, Metallic Contacts

Many previous researchers have reported the formation of carbonaceous tribofilms from organic lubricants on rubbing metallic surfaces. This paper shows that a very important factor in the formation of such tribofilms is the presence or absence of molecular oxygen. When steel surfaces are rubbed in saturated hydrocarbon lubricants in the absence of oxygen, for example in nitrogen or hydrogen gas, carbonaceous films form very readily, resulting in low friction and wear. However, when a significant amount of oxygen is present, as is the case in air, carbonaceous tribofilms are not generally formed, so friction and wear are very high, with values comparable to those seen when no lubricant is present. In situ Raman analysis combined with gas-switching experiments show that the carbonaceous films formed during rubbing when no oxygen is present are rapidly removed during rubbing in air, while tests in which lubricant is removed during a test in N2 indicate that the films are quite weak. This suggests that these carbonaceous films are being continually removed and replenished during rubbing in oxygen-free conditions. It is proposed that these carbonaceous films are formed from hydrocarbyl free radicals that are generated mechanochemically from hydrocarbon molecules during rubbing. In the absence of oxygen, these free radicals then react together to form a carbonaceous film. However, when oxygen is present, the hydrocarbyl free radicals react extremely rapidly with oxygen molecules to produce hydroperoxyl free radicals and so are no longer available to generate a carbonaceous tribofilm.Graphical abstract

Recycling Waste Glyceroborate to Aqueous Lubricant for Tribological Applications.

The present study attempts to explore the direct recyclability of glyceroborate from medicine pharmaceutical production wastewater into an aqueous lubricant instead of conventional waste processing methods from the tribological view. In order to determine the tribological feasibility, the physicochemical properties of crude pharmaceutical wastewater are investigated and compared with those of pure glycerol to access their potential lubrication properties. The results demonstrated that the crude pharmaceutical wastewater has better friction-reducing and antiwear properties under the same working conditions. Besides outstanding lubricating properties, the friction-induced formation of borate tribo-film and intermediate FeOOH compound favors lowering of the shear stress between the rubbing surfaces. This finding better provides an alternative to transform glyceroborate from medicine pharmaceutical production wastewater after simple distillation processing to a potential aqueous lubricant.

Oil-soluble deep eutectic solvent functionalized graphene oxide towards synergistic lubrication in non-polar PAO 20 lubricant

Although deep eutectic solvents (DESs) have shown excellent surface modification and lubrication properties, there have been seldom reports concerning the effect of those characters on the tribological behavior of nanoparticles as non-polar oil additives. Here, we grafted tetrabutylammonium chloride and octanol-based oil-soluble DES onto the base plane or interlayer of GO through a long aliphatic chain, chloride ion and C-N covalent functionalization, forming functionalized GO (DES/GO). The DES/GO could well disperse in a nonpolar polyalphaolefin base oil (PAO 20) to construct DES/GO hybrid PAO 20. Moreover, the friction and wear of DES/GO hybrid PAO 20 oil decreased by 35% and 41% compared with those of pure PAO 20 oil. The splendid interfacial lubrication depends on the formation of uniform, dense, and strong graphene-like tribofilm to shield the tribo-interfaces against wear. And the grafted DES on GO further participates in the tribochemical reaction to form the FexOy, FeCl2 or FeCl3 and tetraalkylammonium salt tribochemical film. The self-lubricating performance of GO and tribochemical reaction of DES together achieve a synergistic lubrication effect.

Study of Adaptation Processes in Tribofilms during Friction of Antifriction Aluminum Alloys for Journal Bearings

This paper provides results on the tribological behavior of experimental Al–Sn–Pb–Si–Cu–Mg–Zn aluminum alloys and describes the adaptation phenomena that reduce wear intensity during friction with steel. The main focus is on tribofilm formation, which plays an important role in friction energy dissipation. The alloys were tested in a rig imitating a journal-bearing shaft couple, and the friction surfaces were studied by the scanning electron microscopy, energy-dispersive analysis and X-ray photoelectron spectroscopytechniques. Based on the analysis, a conclusion on processes and tribochemical reactions was made. Compared to the initial state, eight new compounds were found on the friction surface. In the most wear-resistant alloy, magnesium precipitated from a solid solution with the subsequent oxidation. The same process was detected for zinc in the least wear-resistant alloy due to its low magnesium content. Furthermore, CuSn3 and PbS compounds, which require >600 °C temperature to compose, were found in tribofilms, indicating that the rubbing body lost thermodynamic equilibrium during friction. The revealed processes are non-spontaneous and decrease the wear intensity of the alloys, as they are accompanied by negative entropy production and dissipation of friction energy. Stepwise depth XPS analysis also showed the functional levels of the tribofilms.

Effects of substrate roughness on the tribological properties of duplex plasma nitrided and MoS2 coated Ti6Al4V alloy

The poor surface shear resistance of Ti6Al4V alloy limits its applications involving moving components. To improve its tribological properties, a duplex treatment method involving plasma nitriding and MoS2 coating was proposed. The objective was to enhance load-bearing capacity, optimize surface roughness, and extend the lifetime of the MoS2 coating. Plasma nitriding significantly increased surface hardness by 2.3 times compared to untreated alloy. MoS2 coating on the hardened subsurface resulted in low friction coefficients of 0.05 at 25 % relative humidity. The duplex treated sample with a rough subsurface showed a lifetime of 11.90 h, six times better than the smooth subsurface sample. Higher subsurface roughness facilitated lubricant storage, release, and the formation of a continuous tribofilm, resulting in longer lifetime.

Tribological responses of steels to different surface films generated in oil lubricants

PurposeThe aim of this study is to experimentally analyze the friction and wear responses of different steels to different surface films generated in oil-lubricated tribological contacts.Design/methodology/approachTribological experiments were conducted using a 100Cr6 bearing ball sliding against a V155 carbon steel disk and 316L stainless steel disk, respectively. Lubricants with additives known to form zinc dialkyl-dithiophosphate (ZDDP) or Ca tribofilms were used.FindingsBoth of the ZDDP and Ca tribofilms helped stabilize the friction coefficient of the carbon steel and stainless steel. The ZDDP tribofilm could effectively protect the carbon steel from wear, in contrast to the stainless steel, whereas the wear of both carbon steel and stainless steel could be significantly reduced by the Ca tribofilm. In the case of neither ZDDP nor Ca tribofilms formation, the 100Cr6 ball was worn by the V155 disk and generated a special surface topography. A polishing wear mechanism was proposed to explain the wear of the 100Cr6 ball.Originality/valueThis study clearly shows the different friction and wear responses of steels to the different surface films and the response is dependent on the tested steel.

Tuning fluorine content of fluorinated graphene by an ionothermal synthesis method for achieving excellent tribological behaviors

Fluorinated graphene (FG) has been an excellent lubricant additive due to its wider layer spacing and weaker interlayer forces, and FG with different fluorine content can exhibit different tribological performance. Herein, an ionothermal synthesis method was provided to prepare FG nanosheets with different fluorine content. Under the dual function of the ionic liquids ([C16C1im][Br]) and the temperature, the [C16C1im][Br] can work well as both an intercalation agent and polar catalyst, thus achieving tuning fluorine content of FG. Surprisingly, due to the chemical stability of [C16C1im][Br], the [C16C1im][Br] used in the exfoliated process of FG can be recycled and reused at least 5 times and still remain favorable exfoliation effect. Moreover, it can be obtained that friction coefficient and wear track width are reduced by 53.1 % and 52.8 % through adding FG nanosheets into PAO4 oil, the function generated by breaking of C–F bonds can form an adsorbed film on the friction surface, thus offering a synergistic lubricant mechanism with formation of FG tribo-film. Therefore, the FG dispersed into PAO4 oil can greatly enhance the tribological behaviors during the friction process.

Microwave plasma-produced Al/Al2O3 microparticles as precursors for high-temperature high-strength composites

Microwave plasma-produced Al/Al2O3 microparticles as precursors for high-temperature high-strength composites

Influence of Artificially Altered Engine Oil on Tribofilm Formation and Wear Behaviour of Grey Cast Cylinder Liners

This work investigates the influence of altered engine oil on the tribological performance, focusing in particular on wear and interconnected tribofilm formation. For this purpose, Zinc dialkyldithiophosphate (ZDDP) additivated engine oils of different degradation levels, produced in an artificial oil alteration process, were used in tribometer tests with a nitride steel piston ring against a grey cast iron cylinder liner model contact. Parameters were chosen to simulate the boundary and mixed lubrication regime typical for the top dead centre conditions of an internal combustion engine of a passenger car. Wear of the cylinder liner specimens was continuously monitored during the tribometer tests by the radio-isotope concentration (RIC) method, and tribofilms were posteriorly investigated by X-ray photoelectron spectroscopy (XPS). The results clearly show that the steady-state wear rates for experiments with altered lubricants were significantly lower than for the experiments with fresh lubricants. XPS analysis on the formed tribofilms revealed a decrease in sulphide and an increase in sulphate states for altered oils evaluated at 120 °C oil temperature, correlating with a decrease in steady-state wear rate. This finding emphasizes the role of sulphate species in the tribofilm formation process and its anti-wear capabilities, in contrast to the sulphide species and the (poly-)phosphate species, as outlined in most of the ZDDP literature. Moreover, the RIC signal that represents the amount of wear in the engine oil showed a decrease over time for specific altered lubricants and test conditions. These “negative” trends in the wear signal are remarkable and have been identified as an incorporation of wear particles from the lubricant into the tribofilm. This finding is supported by XPS results that detected an iron-oxide layer with a remarkably similar quantity within the tribofilm on the surface. Based on these findings, an assessment of the minimum film formation rate and particle incorporation rate was achieved, which is an important basis for adequate tribofilm formation and wear models.

Self-assembled hybrid phosphate nanoflowers as oil-based lubricant additive: Interfacial adsorption and lubrication mechanism

In this work, a self-assembled hybrid phosphate nanoflower (HPN) containing Ba3(PO4)2 and NaBaPO4 was synthesized via BaCO3 primary crystals inducement. A ball-on-disc reciprocating tribometer was used to explore the tribological behavior of HPN as an additive for polyalphaolefin 8 (PAO8) on titanium alloy. Compared with the neat PAO8, after being lubricated with PAO8 containing HPN, the friction coefficient and wear rate decreased by 74.98% and 99.89%, respectively. The tribofilm at the friction interface was characterized and conformed by SEM, EDS, XPS, and cross-sectional TEM. Asan additive toPAO8, HPN can participate in the formation of tribofilm and exhibit superior friction-reducing and anti-wear properties for titanium alloy. It was demonstrated that due to the P-O-Ti bonds, HPN can easily adsorb and deposit at the friction interface to form a tribofilm against wear. Besides, the simulation experiments showedthat the repulsive force at the solid-liquid interface between HPN and oil molecules is the key to friction reduction and lubrication, and the comparative tribological experiments of different types of base oils were performed to verify the results of molecular dynamics.

Insight into tribocorrosion resistance and tribofilm formation on titanium boride coatings in a phosphate buffer saline solution

TiB2+TiB coatings were formed on Ti–6Al–4V through a molten-salt diffusional process at temperatures of 850 °C and 900 °C for 16 h. The thickness of the TiB layer increased by about two times at the higher process temperature, while the thickness of the TiB2 layer remained constant. Tribocorrosion rate of Ti–6Al–4V and the coated samples was evaluated in a reciprocating sliding under a normal load of 15 N in a phosphate buffer saline (PBS) solution for 3600–36000 cycles against an alumina ball. The results showed that the tribocorrosion rate of the borided samples decreased up to 56 times compared with Ti–6Al–4V after 36000 cycles. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies showed the presence of oxide-rich patches on the wear surface of Ti–6Al–4V with a thickness of up to 1 μm. TEM and atomic force microscopy (AFM) images also revealed the presence of a tribofilm with a thickness of up to 200 nm on the surface of the boride coating. In addition, X-ray photoelectron spectroscopy (XPS) analysis confirmed the tribofilm formed on the wear surface of coated samples contained B2O3/H3BO3 compound, resulting in a more stable fluctuation in coefficient of friction and open circuit potential during sliding. The SEM images of the alumina wear surface showed cracks and titanium transfer layers in contact with the borided layers and Ti–6Al–4V, respectively.

Tribological behavior of Al matrix composites reinforced by hybrid Cu and SiC under SN500 + G lubricant

Al matrix composites reinforced with hybrid SiC and Cu reinforcement were prepared by powder metallurgy and sintering. The presence of Cu in the composite microstructure has caused the formation of a three-dimensional network of metallic reinforcing phase. Wear tests were performed (50 and 100 N, 1 mm/s, 1000 m) as dry wear and lubricated wear under SN500 base oil containing graphite particles. The friction results showed that the COF under lubricant compared to the dry wear test was decreased by ∼80 %. With increasing SiC/Cu content the wear rate was decreased up to 30 %. The SEM observation of the lubricated worn surfaces showed that adhesive wear was the dominant wear mechanism, which significantly decreased with the tribofilm formation.

Comparative study on tribological performance and mechanism of eco-friendly solvent-free covalent MXene nanofluids in glycerin and polyethylene glycol

Solvent-free covalent MXene nanofluids (MXene NFs) were prepared as eco-friendly additives to improve the dispersibility of MXene and the tribological performance of lubricants. Here, the tribological properties and tribo-mechanisms of MXene NFs additives in glycerin and polyethylene glycol (PEG 400) were evaluated, and pronounced differences were observed. In glycerin, the excellent anti-friction (28.9%) and anti-wear (98.0%) properties were realized after 0.5 wt% MXene NFs addition, for MXene NFs preferentially adsorbing onto the surface of friction pair and participating in the formation of tribofilm. However, in PEG 400, the poor friction-reducing and negative anti-wear performance were related to PEG 400 exhibiting high affinity towards steel substrate. This work provides a new approach for fabricating eco-friendly additives with excellent dispersion and lubricity.

Deep eutectic solvent intercalation graphene oxide with strong interfacial adsorption capacity towards efficient lubrication

The high-performance lubrication effect of graphene materials is mainly due to the formation of friction film. However, the poor compatibility and interfacial adsorption of graphene oxide restrict the density of the friction film, thus losing the expected lubrication performance. Herein, the novel oil-based lubricant additives were configured by deep eutectic solvents (made of choline chloride and ethylene glycol) intercalation graphene oxide (DES-GOs). The strong hydrogen bonding between deep eutectic solvents (DESs) and graphene oxide nanosheets was detected. Precipitate experiment and UV–vis absorption spectra demonstrated high dispersion stability of DES-GOs1:3 in PEG 200 (no precipitate after 120 days). Significantly, DES-GOs1:3 with excellent electrical conductivity possessed the outstanding lubrication performance, whose average friction coefficient and wear volume decrease by 38.3% and 91.2%, compared with PEG 200, respectively. The proposed reasons to this enhanced performance were the synergistic effect of high dispersion stability, formation of the dense tribo-films and low inter-layers shear effect. Considering the diversity of DESs components and two-dimensional materials, the novel DESs functionalized two-dimensional materials possess great potential in industrial application as lubricant additives for different working conditions and base oils.

Tribological Performance of Steel/W-DLC and W-DLC/W-DLC in a Solid–Liquid Lubrication System Additivated with Ultrathin MoS2 Nanosheets

In this paper, MoS2 nanosheets with an ultrathin structure were fabricated using a solvothermal method and further added into PAO oil, which was further combined with W-DLC coating to constitute a solid–liquid lubricating state. The influences of MoS2 concentration, applied load and counter surfaces on the lubricating of the solid–liquid hybrid lubricating system were explored through a ball-on-disk tribometer. The friction results indicated that the steel/W-DLC and W-DLC/W-DLC tribopairs lubricated with ultrathin MoS2 possessed better friction reduction and wear resistance behaviors in comparison to pure PAO oil. However, compared to the steel/steel couple case, the prepared MoS2 nanosheets exhibited a more efficient lubricating effect for the W-DLC/W-DLC couple. The beneficial boundary lubricating impact of MoS2 nanosheets on self-mated W-DLC coated rubbing surfaces could be attributed to the tribochemical reaction between MoS2 and doping W element in DLC, resulting in a formation of a thin tribofilm at both counterparts. Meanwhile, the extent of graphitization of W-DLC film induced by friction was alleviated because of the lubrication and protection from the formation of MoS2-based tribofilm at both counterparts.

Tribological performance of polymeric friction modifiers under sliding rolling contact condition

AbstractThe tribological performance of two polymeric friction modifiers, one based on an ester‐based compound and another based on an ethoxylated fatty ester and an organic friction modifier, oleamide, was studied at 50, 90 and 140°C using a Mini Traction Machine equipped with optical interferometry and electrical contact resistance. The ability to form surface film is found to vary among the friction modifiers and with temperature and rubbing duration. Despite a thinner film being formed, polymeric friction modifier (PFMs) exhibited lower friction and wear than oleamide at all the studied temperatures. Further, the PFMs reduced boundary friction more effectively at higher temperature. In accordance with lower boundary friction, a smoother surface topography characterized by low wear was exhibited by PFM lubricated surfaces at higher temperatures. Scanning electron microscopy‐energy dispersive x‐ray analysis and time‐of‐flight secondary ion mass spectrometry provided insights on the tribofilm formation. The improvement in the tribological performance of PFMs is attributed to temperature‐induced conformation transition of adsorbed polymer chains on the surface. The results are corroborated by data obtained from dynamic light scattering and gel permeation chromatography.

Surface tension, wettability and tribological properties of a low viscosity oil using CaCO3 and CeF3 nanoparticles as additives

The rise of the electric vehicles as a more sustainable transport alternative makes it necessary to study new transmission fluids that adapt to their needs and improve their efficiency. The thermophysical properties (contact angle, surface tension and rheology) and tribological properties (friction and wear) of potential PAO8 transmission nanofluids using CaCO3 and CeF3 nanoparticles as additives are evaluated at mass concentrations of 0.05, 0.10, 0.15 and 0.20 wt%. Low contact angle and surface tension values are obtained, indicating good wettability. When the nanoparticles are used as PAO8 additives, small reductions in surface tension are achieved, but no differences in contact angle are observed. All nanolubricants and base oil exhibit Newtonian behavior. Concerning the tribological behavior, reductions in the friction coefficient are achieved for both types of nanoparticles, with the largest reductions comparing to PAO8 base oil being 13 % and 10 % for nanolubricants containing 0.05 wt% CaCO3 and 0.10 wt% CeF3 nanoparticles (NPs), respectively. In terms of antiwear performance, maximum reductions are obtained for the CaCO3 nanolubricant of 28 % (0.15 wt%), 41 % (0.10 wt%) and 59 % (0.15 wt%) and for the CeF3 nanolubricant of 19 % (0.20 wt%), 53 % (0.10 wt%) and 58 % (0.20 wt%) for the parameters of diameter, depth, and area of the worn track, respectively. Through Raman microscopy, the tribological mechanisms of tribofilm formation, repairing, and rolling can be proposed. A discontinuous tribofilm formed by the CaCO3 and CeF3 nanoparticles reduces the contact area between the two surfaces protecting them.

Pd nanoparticles lubricant additive catalyze the construction of carbon-based tribofilm to reduce graphitization-induced wear of DLC films under boundary lubrication

To explore approaches of reducing graphitization-induced wear on the friction surface of DLC films under boundary lubrication to increase their service life, Pd nanoparticles (PdNPs) prepared by chemical reduction method were used as lubricant additive of DLC films and GCr15 ball under boundary lubrication. Tribotests, reactive force field molecular dynamics (ReaxFF MD) and surface characterization methods, such as SEM, XPS, Raman and FIB-TEM, were used to study the effects of PdNPs as lubricant additive on the graphitization-induced wear of DLC films and the effect mechanisms. The results show that, PdNPs lubricant additive can reduce the friction coefficient (COF) of the tribosystems, and more importantly, they can obviously increase the lifespan of DLC films. Characterization results indicate that PdNPs can promote the tribochemical reaction on the friction surface and catalyze the formation of carbon-based tribofilm on the friction surface, which significantly reduces the graphitization-induced wear of DLC films. ReaxFF MD studies show that PdNPs effectively catalyze the massive dehydrogenation and chain-breaking of base oil hydrocarbon molecules, providing the basis for the formation of carbon-based tribofilm. These results indicate that PdNPs are promising as effective additive to prolong the service life of DLC films under boundary lubrication.

Improving tribological properties of water/glycerol lubricating fluid by the synergy of nanoparticles and protic ionic liquid

Improvement in wettability and lubricity of water/glycerol base fluid by the synergy of nanoparticles (NPs) and protic ionic liquid (PIL) was experimentally investigated. The hybrid additives were made by combining graphene nanoplatelets or silicon oxide nanoparticles with tert-octylamine oleate PIL. A lubricating sample preparation procedure included ultrasonication, centrifugation and magnetic stirring. Kinematic viscosity, pH, and wettability were evaluated. The tribo-tests were performed on a ball-on-plate reciprocating tribometer. It was observed that NPs alone could not improve the lubricity of the base fluid. However, the lubricity was substantially enhanced by using the PIL additive. Furthermore, the combination of NPs and PIL provided an even better tribological response. The hybrid additives improved wettability and facilitated composite tribo-film formation.

In situ tribological regulation behavior induced by MSH@LaF3 in phosphate/MoS2 bonded solid lubricating coatings

Composite powders consisted of tubular MSH and LaF3 NPs were incorporated to improve the tribological performance of phosphate/MoS2 composite coatings. It suggested that tribological performance of composite coatings can be greatly promoted by MSH@LaF3, and at the same time, the enhancement effect can be further facilitated by high-temperature treatment on MSH@LaF3. The concrete reason was attributed to that LaF3 stimulated dehydration of structural water and stripping of silicon-oxygen tetrahedra sheet from MSH matrix, through which MSH@LaF3 can easily undergo tribo-chemical reactions with the worn surface and initiated in-situ formation of cermet tribo-film. What’s more, MSH@LaF3 that transferred into the tribo-film also induced the formation of MoS2 lubrication film on the top worn surface.