Boundary Film Research Articles

A quantitative evaluation of inorganic boundary films formed in engine oils by TOF‐SIMS depth profiling with low energy Cs sputtering

Quantitative evaluation of lubrication boundary films was investigated by time‐of‐flight secondary ion mass spectrometry depth profiling with low energy Cs sputtering. Relative sensitivity correction coefficient (RSC) for calcium phosphate, calcium carbonate, or calcium sulfate was determined by measuring the pellets of the mixture of these compounds. The RSCs for the three calcium salts varied with measurement conditions such as sputter energy and primary ion species. By using the RSC determined in this investigation, the depth profiles of the boundary films formed in real engine oil were corrected to quantitative profiles. The difference of structures between two lubrication boundary films on the different substrate materials formed in the same engine oil was clarified.

Dual effects of TiSiO4 composite nanoparticles on dispersion stability and lubrication performance of vegetable oil‐in‐water emulsions

AbstractVegetable oil‐in‐water emulsions are potential metal‐working lubricants. These emulsions are susceptible to oil‐water phase separation due to the oil droplets' coalescence. In this study, we enhance the dispersion stability and lubricity of such emulsions with TiSiO4 nanoparticles (NPs). With 1 vol% soybean oil, stable emulsions are obtained with 0.25 to 0.75 wt% NPs, attributed to formation of shielding layers of NPs around the droplets. Lowest friction and wear are obtained without and with 0.25 wt% NPs, respectively. With increasing the NPs' mass fraction, NP‐shielded droplets become less attracted to metal surfaces. Association of NPs within boundary film is also found when NPs' mass fraction is above 0.5 wt%, inducing third body abrasive behaviour. Further increasing the NPs' mass fraction results in the formation of agglomerates of NPs in emulsions which increases vibration, friction, and wear. Findings suggest that a mass fraction of NPs exists at which stable emulsions with improved lubrication performance are obtained.

A numerical analysis and experimental investigation of three oil grooves sleeve bearing performance

PurposeThis paper aims to reveal the cavitation characteristics of three oil wedges sleeve bearing and set the theoretical and experimental basis for defining the oil film boundary condition.Design/methodology/approachComputational fluid dynamics model of three oil wedges sleeve bearings based on the Navier–Stokes equation is set using Fluent considering turbulent situation and two-phase flow theory. The cavitation characteristics of bearing is investigated by taking pictures of experiment.FindingsThe rupture region of oil film and the contours of air volume fraction increase distinctly with the increase of rotating speed and the decrease of input pressure. The critical rotating speed of cavitation occurrence and oil film pressure increases with the increase of input pressure. The change trend of experiment cavitation with the rotating speed and input pressure is consistent with theoretical cavitation in general.Originality/valueThe finite element model of three oil wedges sleeve bearings is established based on the Navier-Stokes calculation equation of the fluid, two-phase flow theory and turbulent model. Sleeve bearing is transparent, the pictures of cavitation can be easily taken by high-speed camera, the cavitation characteristics of bearing is studied by experiment. The cavitation performance of three oil wedges bearings is studied with the change of input pressure and rotating speed, the change trend is basically consistent for theory and experiment. The study on critical rotating speed of cavitation occurrence is benefit for defining the oil film boundary condition.

Carbon spheres as lubricant additives for improving tribological performance of polyetheretherketone

Polyetheretherketone (PEEK) has been increasingly used as sliding components in engineering. With the attempt to enhance the tribological performance of PEEK under oil lubrication, in the present work, carbon spheres were prepared and then added to lubricating oil as additives. Strikingly, compared with neat oil lubrication, the tribological tests manifest a notable reduction in coefficient of friction (23.5–33.2%) and wear rate (24.2%) under hybrid oil lubrication. Raman spectra, Fourier transform infrared spectra, and energy dispersive X-ray analyses indicate the presence of functional groups on carbon spheres, enabling the good interactions between carbon spheres and lubricating oil. The uniform mixture of carbon spheres and lubricating oil can fill in the gap of the surface irregularities and form a protective boundary film. Additionally, the carbon spheres can result in rolling movement to promote the lubricating performance, evidenced by scanning electron microscopy. This study proposes a simple and practicable approach to improving the tribological performance of PEEK.

Frequency-Modulation Atomic Force Microscopic Observation for Ultralow Frictional Solid–Liquid Interface of Diamond-Like Carbon in an Environmentally Friendly Oil

The structure of the low friction solid–liquid interface of the diamond-like carbons (DLCs) in fatty acid was investigated by controlled frequency-modulation atomic force microscopy (FM-AFM) to elucidate the low frictional mechanism that arises under boundary lubrication. We focused on two factors in particular: the effect of the DLC type and the frictional energy in the structure of the adsorbents at the interface. The FM-AFM results indicate that the ultralow frictional tetrahedral amorphous carbon–fatty-acid interface was composed of a thick boundary film with a high molecular density, which may support the sliding surfaces and thereby reduce the friction.

Laser damage of silica optical windows with random antireflective structured surfaces

High intensity nanosecond laser pulses induce damage on uncoated optics and antireflection (AR) coated optics with thin film boundaries. Structured surfaces with AR performance have shown higher damage resistance than conventional AR coatings. This work explores laser-induced damage threshold (LIDT) of random antireflective surface structures (rARSS), covering interfaces of planar fused silica substrates, using a 7 ± 1 nanosecond duration, single damaging pulse at 1064-nm wavelength. The rARSS treated windows were optimized for AR at 1064 nm, and transmittance of each treated interface was increased by 3.2% per ISO 13697. Incident fluence was controlled near LIDT for both entry and exit sides. Multiple locations were tested for each fluence setting in accordance with ISO 21254 1-on-1 LIDT testing. Double-sided and single-sided rARSS, orientation with both entrance-side and exit-side treatments, were tested to determine effects random structures have in damage induced by field enhancement on the exit side. Results show that rARSS on a single side of an optic have higher resistance to onset of laser damage when located at entry (structured) side. All tests resulting in damage of polished and single-sided rARSS oriented on exit side were ballistic on beam exit facet, showing surface cracks. Results from rARSS located on beam entry side (both single- and double-side treated) showed nonballistic damage and densification of rARSS features from localized melting or reflow, capping the rARSS but still preserving effective-gradient index and rARSS transmission enhancement. This suggests that the localized melting of the rARSS optic surface does not constitute damage optically, or at least not catastrophically within the prebulk damage regime. Therefore, it is possible to foresee the onset of true damage within an optical system via tracking transmission drop, and swap optics out before catastrophic system failure. The drop in transmission acts as an early warning system, making these rARSS optics an invaluable tool in high-energy laser development.

Boundary Lubrication Mechanisms for High-Performance Friction Modifiers.

We recently reported a new molecular heterocyclic friction modifier (FM) that exhibits excellent friction and wear reduction in the boundary lubrication regime. This paper explores the mechanisms by which friction reduction occurs with heterocyclic alkyl-cyclen FM molecules. We find that these chelating molecules adsorb onto (oxidized) steel surfaces far more tenaciously than conventional FMs such as simple alkylamines. Molecular dynamics simulations argue that the surface coverage of our heterocyclic FM molecules remains close to 100% even at 200 °C. This thermal stability allows the FMs to firmly anchor to the surface, allowing the hydrocarbon chains of the molecules to interact and trap base oil lubricantmolecules. This results in thicker boundary film thickness compared with conventional FMs, as shown by optical interferometry measurements.

Combined effect of boundary layer formation and surface smoothing on friction and wear rate of lubricated point contacts during normal running-in processes

The combined effect of boundary layer formation and surface smoothing on friction and wear rate of metallic surfaces under lubricated point contact condition was investigated. The double trend of friction coefficient variations was revealed during running-in and sub-running-in processes. The evolution of surface topography was measured on-site using white-light interference profilometer and analyzed using bearing area curves. Comprehensive theoretical equations that explicitly express the contributions of boundary friction, adhesive friction and wear have been derived, and results obtained by these equations were compared with experimental observations. It is concluded that the theoretical models are quantitatively adequate to describe the combined effect of surface smoothing due to mechanical wear and formation of boundary films on the changes in friction and wear rate during normal running-in processes.

Study of oxidation processes in Mo/Be multilayers

The results of an investigation on oxidation processes in Mo/Be multilayer nanofilms are presented. The films annealed both in ambient atmosphere and in vacuum. The extreme ultraviolet (EUV) and X-ray reflectivity of the samples at 11.34 and at 0.154 nm respectively were measured before and after the treatment. No noticeable changes in film thicknesses and boundaries were observed during the annealing at temperatures up to 300°C. An oxidation mechanism of the nanofilms Mo/Be is established and the activation energy of the oxidation process is estimated to be 38 kJ/mol. To determine an absolute quantity of oxygen in the oxidized layers, a simple technique based on the EUV reflectivity data is proposed, and the range of its applicability is subsequently analysed.

Simultaneous measurement of infrared absorption coefficient of Carbon doped Al0.33Ga0.67As thin film and thermal boundary resistance between thin film and heavily Zn doped GaAs substrate using spectrally-resolved modulated photothermal infrared radiometry

In this paper, we investigated C doped Al0.33Ga0.67As thin film epitaxially grown on a Zn-doped GaAs substrate using spectrally resolved modulated photothermal infrared radiometry (SR-PTR). We assumed that thermal conductivity and diffusivity of the thin layer are known and estimate values of the infrared absorption coefficient of the thin layer and the thermal boundary resistance of the interface between the thin layer and the substrate. We found out that the thermal boundary resistance is two orders of magnitude greater than thermal boundary resistance of studied recently undoped AlGaAs/GaAs sample. We attribute this effect to formation of quasi 2-dimensional hole gas due to modulation doping. Finally, the infrared absorption coefficient of the thin layer decreases with increasing wavelength due to inter-valence band absorption. We found out that with increasing infrared absorption coefficient of the thin film, the sensitivity of the method for estimation of the infrared absorption coefficient increases, while the estimation error decreases.

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.

Tribological characterisation of epoxy–graphene–liquid filler composite coatings on steel under base oil external lubrication

ABSTRACTIn our earlier study, epoxy-based composites with graphene (10 wt-%) and in situ liquid fillers (base oil SN150 or perfluoropolyether at 10 wt-%) were found to provide low friction and highly wear durable as thin coatings on the steel substrate in dry interfacial state. In this present work, we have tested this composite in the presence of an external lubricant (base oil SN150). The lowest coefficient of friction was recorded as 0.04 and the lowest specific wear rate was measured as 9.8 × 10−7 mm3 Nm−1 for the composites without any failure of the coating up to 200,000 sliding cycles. It is shown that such polymeric coatings can be an excellent boundary film in both dry and lubricated conditions for various bearings.

Hot corrosion of borate melt and interface chemistry of borate-coated steel under tribological stimulation

Hot corrosion mechanism of Sodium Borate was investigated on mild-carbon steel at 800 °C by multiple surface characterizations. In addition, effects of tribological exposure on the interface chemistry were also examined in details by mean of ball-on-disk testing. The occurrence of interfacial reaction is characterized by the formation of a Sodium-rich film on the borate melt/oxide phase boundary. The basic dissolution reaction is accompanied by fluxing action of the protective scale in the case of static oxidation. External stressed shearing profoundly modifies the interface chemistry by establishing a hierarchical boundary film with remarkably low Oxygen density which renders improved oxidation resistance.

Nano-tribological characterisation of palm oil-based trimethylolpropane ester for application as boundary lubricant

Abstract To isolate shearing of boundary film from direct surface-to-surface asperity interactions, the study determines boundary lubrication properties of palm oil-based TMPE (PTMPE) using Lateral Force Microscopy coupled with fluid imaging. PTMPE is produced through a series of 3-step transesterification processes, converting palm oil (PO) into palm methyl ester (PME) and finally into PTMPE. It is shown that PME generates the lowest friction. However, using Eyring thermal activation energy approach, PME is shown to possess less desirable load bearing property, portraying a form of stiction or adhesive nature. Even though friction is higher, PTMPE exhibits better load bearing ability, demonstrating the onset of lubricant laminar flow due to increased hydrodynamic effect, which is not observed for the PO and PME measurements.

Microstructure evolution of wrought nickel based superalloy GH4037 in the semi-solid state

Study on semi-solid processing of superalloy is little reported due to the difficulties in fabricating semi-solid billets. In this manuscript, a novel method called semi-solid isothermal treatment of wrought superalloy (SSITWS) was firstly proposed in order to fabricate semi-solid billets of superalloy. The effects of isothermal temperature and soaking time on microstructure of Ni-based superalloy GH4037 in the semi-solid state were investigated. The results show that the near-globular microstructure of wrought GH4037 superalloy can be obtained by the semi-solid isothermal treatment. The average grain size increased and the roundness decreased with the prolonged soaking time at 1350 °C and 1360 °C. When the isothermal temperature was elevated to 1370 °C or 1380 °C, the microstructure evolution was complex due to the combination of breaking up mechanism and coarsening mechanism. At 1390 °C, the dendritic grains were obviously observed in the microstructure. With the increasing of isothermal temperature and soaking time, the liquid boundary film coarsening behavior occurred in the microstructure. With the increasing of soaking time, the size of intragranular liquid droplets increased, but the amount of them decreased. When the isothermal temperature was high, the intragranular liquid droplets disappeared. The coarsening rate constant at 1360 °C was lower compared to that at 1350 °C. However, when the isothermal temperature was higher than 1370 °C, the coarsening kinetics was not suitable for describing the microstructure evolution. The discrete carbide precipitates appeared in the grain boundaries. The distribution of Ti and Mo were higher in the grain boundaries than that in solid grains, and the distribution of other elements was homogenous in the grain boundaries and inside the grains.

Downward two phase flow experiment and general flow regime transition criteria for various pipe sizes

Flow regime map is often used in choosing constitutive correlations for the two-phase flow model. The related research mainly concentrates on the vertical upward and horizontal flow, while it is not sufficient in the vertical downward flow. Downward flow is very important as it is frequently encountered in the industrial applications. To enrich the downward flow research, an experiment is performed on a piping system with an inner diameter of 0.1524 m. Four different flow patterns (bubbly flow, cap bubbly flow, churn turbulent flow, and annular flow) are classified with the artificial neural network method. The probability density function (PDF) profile of each flow pattern is discussed. The proposed flow regime map is compared with the other experiments and the effect of the pipe size is discussed. The existing downward flow regime boundary criteria are assessed with the experiment results. It is found that these criteria cannot fit the experiment results well. A set of general boundary criteria are still needed. In this paper, the criteria for the boundary of the bubbly flow, the boundary between the cap bubbly flow and the slug flow, and the boundary of the falling film regime are proposed. They are verified with the experiments on different size pipes. A significant inlet effect on the flow regime boundary is found. The falling film boundary criterion proposed cannot be applied when a sparger is used to inject gas into the downward test section.

Effects of hydrophilic/hydrophobic surfaces on polymer‐complexation kinetics

ABSTRACTTo establish hydrophilic/hydrophobic effects on polymer‐complexation kinetics, chitosan resins were prepared by radical copolymerization of methacryloyl‐modified chitosans with 2‐hydroxyethyl methacrylate and/or styrene monomers. The primary particles of the gelled copolymers in the suspensions were nicely solidified by basic coagulation bath to form globular beads. Degree of the hydrophilic/hydrophobic properties was controlled by the copolymerization ratio, and each ion‐adsorption kinetics was evaluated by batch method. The initial rate of adsorption in hydrophilic chitosans was higher than that in hydrophobic chitosans, which was attributed to less thickness of the boundary film. We quantitatively evaluated the rate‐determining steps by the initial‐rate analysis, in which the viscosity of the suspension contributes to the film‐diffusion kinetic resistance. The role of hydrophilic/hydrophobic properties of the resins is reasonably rationalized to the length of the boundary films by considering long‐range interactions between the ion and surface. Our results are robust evidence for the ion‐surface interactions. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46493.

ToF-SIMS investigation of tribochemical surface reaction films built up by specially added cooling lubricants

Cooling lubricants based on polyalphaolefin (PAO) are used in finishing processes for metallic components, but their influence on the formation of lifespan increasing tribochemical surface reaction films (boundary films) is poorly understood. The binding characteristics of different additives during finishing processes have been investigated using time-of-flight secondary ion mass spectrometry (ToF-SIMS). Steel substrates heated to selected temperatures between 150 and 700 °C were dipped in four different cooling lubricants based on PAO to study fundamental binding processes. The boundary films built up on the steel surfaces were analyzed using ToF-SIMS via acquiring surface spectra as well as depth profiles. The results of these studies reveal the temperatures at which additives bind to steel surfaces and provide information regarding the thickness of the boundary films formed during the finishing. They also show that the temperature at which the boundary film formation starts is at around 400 to 450 °C. The starting temperature, as well as the thickness of the boundary films formed, depends on the composition of the cooling lubricants and on the concentration of additives. Thus, an increase in additive concentration leads not only to delayed starting temperatures, but also to greater film thicknesses.

Dual properties of a hydrogen oxidation Ni-catalyst entrapped within a polymer promote self-defense against oxygen

The Ni(P2N2)2 catalysts are among the most efficient non-noble-metal based molecular catalysts for H2 cycling. However, these catalysts are O2 sensitive and lack long term stability under operating conditions. Here, we show that in a redox silent polymer matrix the catalyst is dispersed into two functionally different reaction layers. Close to the electrode surface is the “active” layer where the catalyst oxidizes H2 and exchanges electrons with the electrode generating a current. At the outer film boundary, insulation of the catalyst from the electrode forms a “protection” layer in which H2 is used by the catalyst to convert O2 to H2O, thereby providing the “active” layer with a barrier against O2. This simple but efficient polymer-based electrode design solves one of the biggest limitations of these otherwise very efficient catalysts enhancing its stability for catalytic H2 oxidation as well as O2 tolerance.

Significance of an in-situ generated boundary film on tribocorrosion behavior of polymer-metal sliding pair

Polymer composites have a high potential for applications as tribo-materials exposed to sea water owing to their self-lubrication characteristic and high chemical stability. In the present work, tribological behaviors of polyetheretherketone (PEEK) composites rubbing with stainless steel in sea water were explored using a pin-on-disc tribometer integrated with a potentiostat for electrochemical control. It was demonstrated that further adding 5 vol% hexagonal boron nitride (h-BN) nanoparticles into PEEK reinforced with short carbon fibers (SCF) significantly enhanced the wear resistance. Moreover, the stainless steel exhibited significantly enhanced tribocorrosion resistance when rubbing with the hybrid nanocomposite, in comparison to the sliding against PEEK filled only with SCF. Nanostructures of the boundary films formed on the steel surface were comprehensively investigated. It was manifested that tribo-chemistry products of h-BN, i.e. H3BO3 and B2O3, were arrayed in a closely packed boundary film. It seems that inclusion of layer-structured H3BO3 and B2O3 improved the resilience of the boundary film. The continuous boundary film covering the steel surface provided a lubrication effect and strengthened the passivation layer. A new route for enhancing simultaneously tribological and corrosion resistance of polymer-metal pairs by controlling in-situ tribo-chemistry was thus proposed.