Protective Tribofilm Research Articles

Novel phenylboronic acid derivatives containing phosphorous and nitrogen as lubricant additives: Synthesis, characterization, and tribological behaviors

Three phenylboronic acid derivatives containing phosphorous and nitrogen with chlorine (BNP1), methyl (BNP2), and nitro (BNP3) were successfully prepared and applied to evaluate the tribological behaviors as additives in liquid paraffin. Three derivatives exhibited high thermal stability. The tribological tests indicated that three derivatives lead to a substantial improvement in anti-friction and anti-wear properties of liquid paraffin. In comparison to BNP1 and BNP3, BNP2 performs best on friction-reducing and wear resistance performances. This is caused by the strong electron-donating ability of the methyl group in BNP2. The analysis of worn surfaces demonstrated that three derivatives could form a protective tribofilm containing boron, nitrogen, and phosphorus elements on the rubbing surfaces. Thus, three phenylboronic acid derivatives could be potential lubricant additives in industrial applications.

Understanding the role of surface textures in improving the performance of boundary additives, part I: Experimental

Tribological performance of systems in boundary lubrication is dependent on contact and lubricant conditions such as temperature, lubricant additive concentration, surface characteristics, contact pressure and additive-additive interactions. Although past parametric studies have investigated these various factors, there are limited studies on the effect of surface topography on the performance of boundary lubrication additives. This is pertinent because of the increased use of surface texturing in lubricated contacts. This study has employed laser surface texturing to achieve surfaces with different topographies. The influence of the different surface topographies on the tribological performance of a friction modifier additive, molybdenum dialkyldithiocarbamate (MoDTC), was investigated. Results show that laser-textured surfaces in some instances can offer improved friction performance compared to non-textured (polished) surfaces when working positively with a friction modifier additive. Anisotropic surfaces resulted in improved friction. The mechanism for the improved performance of the laser-textured surfaces is believed to be through the augmented local contact pressures at the asperities that accelerate additive decomposition to form protective low-friction tribofilms. This study demonstrates the feasibility of surface texturing in improving the performance of boundary additives.

How to get noWear? – A new take on the design of in-situ formed high performing low-friction tribofilms

Despite growing environmental concern, combustion engines are projected to stay the main technology of transportation for many years, so will unpopular S- and P- containing additives in lubricating oils. Therefore, it is essential to maximize efficiency and minimize the harmful impacts of these additives in the meantime. Coatings based on Mo and W hold great promise as they can chemically interact with S-bearing additives and form low friction tribofilms like MoS2 and in particular WS2. Here we highlight the reaction pathways involving first the formation of oxide intermediates, followed by the generation of a highly protective and low-shear transition metal disulfide tribofilm on a tailored WN-based coating, affording coefficients of friction as low as 0.05 combined with an extraordinary wear protection (i.e., no measurable wear on sliding surfaces even after 100 h of testing). High resolution TEM and Raman spectroscopy confirm the formation of WS2 sheets and hence the huge potential of these coatings for lubricated surfaces.

Ultrastable Lubricating Properties of Robust Self-Repairing Tribofilms Enabled by in Situ-Assembled Polydopamine Nanoparticles.

Aqueous lubrication in nature is attracting increasing attention in the tribological fields for reducing friction energy consumption and improving anti-wear durability. Generally, adding nanolubricant additives is one of the most important strategies to effectively enhance the interface performance under boundary lubrication via the formation of a protective tribofilm on rubbing surfaces. However, the adsorbed tribofilms are unstable and are prone to failure during friction, and the interaction mechanism between the tribofilms and frictional interfaces is partly disclosed. In this study, inspired by mussels, an in situ-assembled polydopamine (PDA) tribofilm is achieved with PDA nanoparticles as aqueous lubricant additives, which shows excellent lubrication properties. The coefficient of friction is interface-independent and is reduced by as much as 83%. The results show that the PDA tribofilm can not only form chemical bonding with metal interfaces but also present a synergistic lubrication effect with the upper ceramic surface. Especially, a self-repairing effect of the PAD tribofilm is observed, by which the ultrastable lubricating properties can be achieved during friction, and thus, the friction and wear can be effectively controlled. This work provides an effective method for improving the interface stability of friction pairs under aqueous lubrication and also shows great meaning for industrial applications.

Stability, Thermal Conductivity, Viscosity, and Tribological Characterization of Zirconia Nanofluids as a Function of Nanoparticle Concentration

Commercially prepared spherical zirconia (ZrO2) nanoparticles with an average particle diameter of 5 nm and functionalized with a hydrocarbon ligand were dispersed in a polyalphaolefin (PAO) base oil, producing a 10-nm stabilized zirconia nanofluid. The resulting nanofluid’s properties were characterized, including thermal conductivity and viscosity. A preliminary study of the nanofluid’s ability to generate protective tribofilms was conducted. Long-term stability was verified using dynamic light scattering (DLS) and average particle size was quantified as a function of time. A 10 wt% ZrO2 nanofluid maintained a 10-nm average particle size over 25 months, with no signs of agglomeration or sedimentation, and redispersion was not necessary before analysis. The tribological performance of several concentrations of ZrO2 nanofluids wasdisc characterized with a ball-on-disc tribometer. Tribofilm growth over time was collected via optical interference imaging. Friction data collected over time are also reported. Nanoparticle concentration was varied to understand its effect on tribofilm growth. The ZrO2 tribofilm morphology and chemistry were characterized using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and transmission electron microscopy (TEM).

Highly Dispersed Ag2S Nanoparticles: In Situ Synthesis, Size Control, and Modification to Mechanical and Tribological Properties towards Nanocomposite Coatings

A facile in situ synthesis approach and a size control strategy were established to obtain Ag2S nanoparticles in polyimide (PI) composite coatings. Such Ag2S nanoparticles in the composite coatings were characterized, and the effects of the as-obtained Ag2S nanoparticles of different sizes on the mechanical and tribological properties of the nanocomposite coatings were investigated. Results indicate that the in situ synthesized Ag2S nanoparticles exhibited good dispersibility and bimodal and multimodal size distribution in the nanocomposite coatings. The size of the Ag2S nanoparticles can be effectively controlled by adjusting the substituent alkyl chain length of single-source precursor, and these Ag2S nanoparticles exhibited superior improvement to mechanical and tribological properties of the nanocomposite coatings. More importantly, the Ag2S nanoparticles with the proper grain size and bimodal size distribution provided the optimal mechanical and tribological properties for the nanocomposite coatings, and the excellent tribological properties were attributed to their outstanding mechanical properties and strong ability to form a homogenous and stable protective tribofilm.

Tribological and Thermophysical Properties of Environmentally-Friendly Lubricants Based on Trimethylolpropane Trioleate with Hexagonal Boron Nitride Nanoparticles as an Additive

Dispersions based on hexagonal boron nitride, h-BN, nanoparticles, at 0.50, 0.75 and 1.0 wt.% mass concentrations, in an ester base oil composed mainly of trimethylolpropane trioleate, were investigated as potential nanolubricants. The stability of the dispersions was assessed to determine the reliability of the tribological, thermophysical and rheological measurements. Density and viscosity were measured from 278.15 to 373.15 K, while rheological behavior was analyzed at shear rates from 1 to 1000 s−1 at 283.15 K. Newtonian behavior was exhibited by all nanolubricants at the explored conditions, with the exception of the highest concentration at the lowest shear rates, where possible non-Newtonian behavior was observed. Tribological tests were performed under a normal load of 2.5 N. Wear was evaluated by means of a 3D profiler, scanning electron microscopy and confocal Raman microscopy. The best tribological performance was achieved by the 0.75 wt.% nanolubricant, with reductions of 25% in the friction coefficient, 9% in the scar width, 14% in the scar depth, and 22% of the transversal area, all with respect to the neat oil. It was observed that physical protective tribofilms are created between rubbing surfaces.

Synthesis, characterization and tribological performance of two novel borate esters containing nitrogen as a multifunctional additive in synthetic ester base oil

For the development of national industry green environmental protection, the use of zinc dialkyl dithiophosphate (ZDDP) should be reduced in the process of mechanical lubrication, two novel borate esters, SS-(5-disulfanyl-1,3,4-thiadiazol-2-yl) O-(6-stearoyl-1,3,6,2-dioxazaborocan-2-yl) carbono(dithioperoxoate) (BNSD) and 6-stearoyl-1,3,6,2-dioxazaborocan-2-yl stearate (BNCD), were synthesized and characterized by FT-IR and TGA. Tribological performance of ZDDP, BNSD and BNCD as a multifunctional additive in synthetic ester base oil were investigated under different loads using a MR-S10 four-ball and MFT-3000 friction and wear test machine. A scanning electron microscope and laser scanning confocal microscope were used to observe worn scar surface morphology of the balls. The chemical compositions of worn surface were analyzed by X-ray photoelectron spectroscopy and the tribofilm of the worn surface was observed by transmission electron microscope. The TGA curves showed that BNSD and BNCD had a better thermal stability than ZDDP. BNSD exhibits excellent anti-wear and friction-reducing properties, the P B value is increase to 784 N when the optimized concentration is 0.3% the minimum WSD of BNSD-based lubricant is approximately 0.52 mm and decreased by 35% compared with the base oil. A protective tribofilm of the ball steel was analyzed by XPS and TEM, which revealed that B2O3 and Fe2B constitute the triboilm and about 5 nm. Therefore, BNSD could be a candidate as anti-wear and friction-reducing agent in lubricants instead of ZDDP.

Interaction of plasma functionalized TiO2 nanoparticles and ZDDP on friction and wear under boundary lubrication

Abstract Tribochemical interaction of zinc dialkyl dithiophosphate (ZDDP) with titanium dioxide nanoparticles and plasma functionalized TiO2, to ascertain their synergist effects on tribological performance of steel test pairs under boundary lubricated sliding conditions in a mineral base oil, was investigated. A pin on reciprocating flat test configuration was utilized to determine the tribological properties of the individual additives and their mixtures in the oil. Specifically, friction coefficients, wear losses, and electrical contact resistance (ECR) data from seven different formulations were determined. Tribofilms, generated during sliding, were subjected to extensive surface analysis using X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge structure spectroscopy (XANES) to determine the nature and the extent of tribochemical interactions. Results from these tribological tests, coupled with chemical characterization of the tribofilms, revealed that both ZDDP and TiO2 nanoparticles (by themselves) form very protective tribofilms on the rubbing surfaces. However, when used as a mix in the mineral oil, they surprisingly behave antagonistically hence resulting in severely increased wear. In contrast, TiO2 nanoparticles coated with boron rich plasma films showed relatively better anti-wear performance than when the uncoated ones were used singularly in the oil or as a mix with ZDDP.

Lubrication performance of graphene-containing oil on steel and DLC-coated surfaces

Abstract Several studies have successfully demonstrated the friction-reduction and anti-wear properties of graphene on the nano- and micro-scales, but our understanding of the tribological behaviour of graphene on the macro-scale remains very limited. Accordingly, this study presents the macro-tribological behaviour of a graphene-containing oil in the lubrication of steel/steel contacts and DLC/DLC contacts. We show that graphene platelets, as additives to the base oil, can, especially in the boundary-lubrication regime, decrease the friction in the DLC/DLC contacts by up to 50%, as well as in the steel/steel contacts by up to 44%. The predominant lubrication mechanism of the graphene platelets was shown to be the formation of a protective tribofilm for both types of surfaces. The effects of the concentration of graphene (0–5 wt.%) and the lubrication regimes on the formation of the tribofilm are also investigated and explained with a schematic 2-D contact model.

PEGylated carbon black as lubricant nanoadditive with enhanced dispersion stability and tribological performance

Abstract Poly(ethylene glycol) (PEG) chains were covalently grafted onto carbon black (CB) using a PEG-terminated trimethoxysilane, yielding PEGylated CB additives (CB-SiPEG) with different SiPEG contents. Unlike pristine CB, CB-SiPEG was readily dispersed in PEG200 base oil, with CB-SiPEG hydrodynamic particle sizes being considerably smaller than those of pristine CB. At optimal SiPEG content, friction coefficient and specific wear rate in ball-on-disk tribometer tests were reduced by 35% and 67% respectively, with 1% wt. CB-SiPEG compared to PEG200 alone. In contrast, pristine CB, alone or with unbound SiPEG, did not show friction-reducing or antiwear capacity. Disk analysis supports CB-SiPEG surface deposition forming a protective tribofilm. Hydrodynamic particle size and lubricating performance of CB-SiPEG/PEG200 blends were preserved after 6 months, indicating high dispersion stability.

Microstructure and properties of WC-17Co cermets prepared using different processing routes

WC-17Co cermets were prepared using three different processing routes. An as-ball milled powder mixture (BMPM) and pre-solid solution treatment powder (PSTP) were used as feedstock. Sinter-hot isostatic pressing (SHIP) and high-velocity oxy-fuel (HVOF) thermal spraying were used as consolidation methods. The microstructure and properties of the WC-17Co cermets prepared using the BMPM-SHIP, PSTP-SHIP, and PSTP-HVOF routes were characterized and systematically compared. Solid-solution strengthening of the Co binder phase accounted for the higher hardness and lower fracture toughness (KIC) of the cermets produced using the PSTP-SHIP and PSTP-HVOF routes compared with those of the BMPM-SHIP-produced cermet. Moreover, the SHIP method yielded WC grains featuring an equilibrium truncated triangular prism morphology, which were faceted compared to the round-edged WC grains prepared using HVOF. Owing to the formation of a dense protective tribofilm on its surface, the cermet prepared using the BMPM-SHIP route exhibited the lowest wear rate of 0.13 × 10−7 mm3/(Nm), while the wear rates of the cermets prepared using PSTP-SHIP and PSPT-HVOF were 0.24 × 10−7 and 0.96 × 10−7 mm3/(Nm), respectively.

Unveiling oil-additive/surface hierarchy at real ring-liner contact

Zinc-dialkyldithiophosphate (ZDDP) additive in oil has been extensively reported to form a protective tribofilm between the moving components of engine for preventing wear. In this study, surface and interface characterizations were conducted on real piston ring/cylinder liner contacts, the interacting mechanism between oil-additive and solid surfaces under realistic mechanical and thermal conditions was predicted according to the identification of the chemical composition and structure of the tribofilm. The results demonstrate the interesting laminated hierarchy of tribofilm. This tribofilm is composed mainly of dense carbon precipitation at the top region and a thinner polyphosphate intermediate layer in contact with deformed iron oxide substrate on the cylinder liner. The depletion of polyphosphate compounds and competitive growth of solid carbon in term of lubricant's degradation was speculated. These significant findings help manufacturer to understand possible tribochemical processes occurring in the real tribosystem from the practical view, and in particular inspire researchers to develop the new strategy to improve capability and efficiency of oil-additive at sliding contacts.

Effect of temperature on the friction and wear properties of graphene nano-platelets as lubricant additive on Al-25Si alloy

The development in automotive field requires a new class of lubricant additive that can be environment friendly and stands against wear and reduce the energy dissipation by enhancing friction properties. The present study reports the friction and wear behaviour of polyalphaolefin base oil with and without the addition of 0.5 wt% graphene nano-platelets. The formulations were tested on hypereutectic Al-25Si alloy against steel ball under dry and lubricated conditions. Tribological tests were conducted from room temperature to 250 °C with an applied load of 5 N at 100 rpm. The normal load was varied from 5–20 N at 200 °C and also, sliding distance tests were carried out upto 1000 m in step of 250 m at 200 °C with an applied load of 5 N. Coefficient of friction and wear rate were significantly reduced due to the adsorption of GNP between the tribopair. Moreover, the results noticeably prove that GNP particles in PAO oil easily form protective tribofilm to prevent the direct contact between the tribopair surfaces.

Lubrication mechanisms of C‐MoS2‐Fe2O3 (Fe3O4) nano‐composite lubricants at the rubbing interfaces of non‐copper coated solid wires against the contact tube

AbstractGraphite‐MoS2‐Fe2O3 (Fe3O4) nano‐composite lubricating coatings were prepared on the surfaces of non‐copper coated solid wires by a mechanical coating technique. The tribological behaviours of graphite‐MoS2‐Fe2O3 (Fe3O4) coatings at the rubbing interfaces of welding wires against the contact tube were investigated. The results demonstrate that the lubricating properties of graphite‐Fe3O4 coatings outperform the lubricating properties of graphite‐Fe2O3 coatings. The anti‐wear performance of the contact tube is strengthened with increasing nano‐MoS2 contents. Layers of protective tribofilms are formed at the rubbing interfaces of welding wires against a contact tube by tribochemical reaction among lubricants. The tribofilms are composed of FeO, MoO3 and FeMoO4 with excellent lubricating properties. They can avoid direct contact of welding wires against the contact tube, thus decreasing contact tube wear. With the transition of the contact tube wear from mild to severe, the dominant wear mechanisms of contact tube change from fatigue peeling and oxidative wear to abrasive wear and arc ablation.

Complex Behavior of Nano-Scale Tribo-Ceramic Films in Adaptive PVD Coatings under Extreme Tribological Conditions.

Experimental investigations of nano-scale spatio-temporal effects that occur on the friction surface under extreme tribological stimuli, in combination with thermodynamic modeling of the self-organization process, are presented in this paper. The study was performed on adaptive PVD (physical vapor deposited) coatings represented by the TiAlCrSiYN/TiAlCrN nano-multilayer PVD coating. A detailed analysis of the worn surface was conducted using scanning electron microscopy and energy dispersive spectroscopy (SEM/EDS), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Auger electron spectroscopy (AES) methods. It was demonstrated that the coating studied exhibits a very fast adaptive response to the extreme external stimuli through the formation of an increased amount of protective surface tribo-films at the very beginning of the running-in stage of wear. Analysis performed on the friction surface indicates that all of the tribo-film formation processes occur in the nanoscopic scale. The tribo-films form as thermal barrier tribo-ceramics with a complex composition and very low thermal conductivity under high operating temperatures, thus demonstrating reduced friction which results in low cutting forces and wear values. This process presents an opportunity for the surface layer to attain a strong non-equilibrium state. This leads to the stabilization of the exchanging interactions between the tool and environment at a low wear level. This effect is the consequence of the synergistic behavior of complex matter represented by the dynamically formed nano-scale tribo-film layer.

Friction and wear behaviours of in situ reduced graphene oxide reinforced zirconia ceramic

The tribological behaviour of zirconia composites reinforced by in situ reduced graphene oxide (IrGO) was investigated by a rotating ball-on-plate configuration at room temperature, and was followed by a comparison with composites reinforced by pre-reduced graphene oxide(rGO). The results indicate that both the friction and wear resistance of ceramics increase with the incorporation of graphene oxide (GO). IrGO is better for enhancing the tribological performance of zirconia than rGO. The wear rate decreases by up to one order of magnitude (from 2.33 × 10−5 mm3·N−1·m−1 to 4.66 × 10−6 mm3·N−1·m−1) with 0.5 wt% GO. A protective tribofilm containing reduced graphene oxide forms at wear surfaces of ceramics, and the protruding-out of rGO is more pronounced. The main wear mechanism changes from severe delamination to plastic deformation and micro-cracking with increasing GO content. The analysis of wear tracks for ceramics with GO additives by Raman mapping reveals a decrease in the tetragonal-monoclinic phase transformation after sliding wear, which is regarded as the intrinsic reason for the improved wear resistance.

Tribological Influences of CuO Into 3Y-TZP Ceramic Composite in Conformal Contact

The aim of the study was to investigate the friction and wear phenomena of 3 mol % yttria-stabilized tetragonal zirconia polycrystals (3Y-TZP) ceramics with the inclusion of copper oxide (CuO) in large area conformal contact geometry. The pin-on-disk tribometer was used to conduct the dry sliding test using CuO/3Y-TZP as pin and alumina as counter surface. The coefficient of friction (μ) for CuO-added 3Y-TZP was decreased by ∼38% compared to pure 3Y-TZP due to formation of protective tribo film to the substrate. In addition, the experiments also showed that the specific wear rate (k) was reduced by ∼54% with the inclusion of CuO in to 3Y-TZP matrix. The different phases of the zirconia, copper, and yttria as well as the phase transformation before and after sliding test were identified by X-ray diffraction (XRD) analysis. Field emission scanning electron microscopy (FESEM) and energy dispersive X-ray (EDS) analysis revealed the existence of CuO in the patchy layers in the worn-out surface of the tested CuO/3Y-TZP sample leading to lower coefficient of friction and improve the wear resistance against alumina counterface in conformal contact geometry. Severe wear mechanism was the dominating factor due to the local plastic deformation of the large number of asperities since the pair of contact was conformal.

Solvent-free ionic nanofluids based on graphene oxide-silica hybrid as high-performance lubricating additive

Solvent-free ionic nanofluids (NFs) of graphene oxide-silica hybrid nanocores (GO@SiO2) were synthesized for the first time via functionalizing in succession the nanocores with a covalently charged corona and an ionically tethered oligomeric canopy. Being intrinsically one-component materials, GO@SiO2 NFs display a liquid-like behavior in the absence of any solvent and show excellent dispersion stability in polyethylene glycol (PEG) base oil. Adding even small amount of GO@SiO2 NFs improved dramatically the lubrication performance of PEG base oil. It was revealed that GO@SiO2 NFs played an important role in physicochemical actions on the friction interface. This work provided direct evidence that a protective tribofilm grew on the sliding surfaces when molecular species of GO@SiO2 NFs were fed continuously onto the contact zone. In particular, graphitized GO and SiO2 nanoparticles digested in the tribofilm enhanced its lubricity and robustness. It is expected that solvent-free ionic GO@SiO2 NFs open up new opportunities for developing advanced lubricating nanomaterials.

Nanoscale Generation of Robust Solid Films from Liquid-Dispersed Nanoparticles via in Situ Atomic Force Microscopy: Growth Kinetics and Nanomechanical Properties

Inorganic metal-oxide nanoparticles have been identified as candidate materials for replacing zinc dialkyldithiophosphate (ZDDP) antiwear additives in automotive lubricants due to their ability to form protective antiwear tribofilms. However, the nanoscale mechanisms that control the growth and properties of these tribofilms remain unknown. Here, we present the results of an in situ study of the kinetics of nanoparticle tribofilm growth using colloidal-probe atomic force microscopy (AFM). We report on the nucleation and growth of antiwear tribofilms formed with a dispersion of a novel zirconia (ZrO2) nanoparticle additive in low-viscosity oil. Tribofilms in this study are generated in situ at the lubricated nanoscale contact of the colloidal-probe AFM, which helps elucidate the effects of localized contact parameters on tribofilm nucleation and growth. The results strongly support that ZrO2 nanoparticle tribofilms grow through a process of stress-activated tribosintering, after removal of organic ligands that attached to the nanoparticles. In contrast to ZDDP-derived tribofilms, ZrO2 tribofilm growth occurs across temperatures from -25 to 100 °C. Nanomechanical properties of the resulting tribofilms are found to approach values for single-crystal and conventionally sintered macroscale structures, suggesting the potential for robust wear protection across a broader range of conditions than those where ZDDP is effective.