基础油中金刚石纳米颗粒润滑性能和导热性能的分子动力学分析*

为了研究纳米颗粒对基础油液热导率的影响,在基础油蓖麻油酸中加入不同体积分数和粒径的纳米金刚石颗粒,采用LAMMPS和分子动力学的研究方法,对粒子数密度以及粒子的径向分布规律、导热系数进行研究。结果表明:加入纳米颗粒的纳米流体的热物理性质受到多方面的影响,其中包括纳米颗粒体积分数及粒径等;随纳米粒子体积分数的提高纳米流体的热导率呈近似线性增加,随着纳米粒子粒径的减小,纳米粒子润滑膜的承载能力增强。纳米润滑膜能承受很高的外界冲击力,这有助于减小两作用面之间的摩擦,减小表面磨损;加入纳米颗粒的润滑油会减小摩擦副之间的摩擦和增强散热,提高热导率。     

Tribological behaviour of polymeric coatings. Part I. Aramid particle‐reinforced epoxy nanocomposite systems

An epoxy‐based nanocomposite containing a low concentration of nanometric TiO₂ (4 vol. %), graphite powder (7.21 vol. %), and 2–14 vol. % aramid particles was developed as a coating material. The mechanical and tribological performance of the composites was investigated. The epoxy filled only with TiO₂ possessed significantly improved impact strength and flexural properties, whereas the further incorporation of graphite and aramid particles had a deleterious effect on most of the mechanical properties, except the modulus. The tribological behaviour of the composites was tested in sliding and fretting modes. Under sliding conditions, the addition of nanometric TiO₂ alone significantly improved the wear resistance and decreased the coefficient of friction compared to the neat epoxy. The sliding wear and friction behaviour was further enhanced with the incorporation of graphite and aramid particles. Contrary to the sliding wear behaviour, the fretting wear and friction behaviour was worse for the epoxy filled only with TiO₂, but was significantly enhanced by the incorporation of graphite and aramid particles. The optimum aramid particle content for sliding and fretting wear of the epoxy‐based nanocomposite was determined as 10 vol. %.     

A Pin-on-Disc Study Focusing on How Different Load Levels Affect the Concentration and Size Distribution of Airborne Wear Particles from the Disc Brake Materials

Airborne wear particles originating from disc brakes are one important contributor to the concentration of airborne particles in urban environments. It is therefore of interest to improve the knowledge of these particles. The purpose of this article is to investigate the concentration and size distribution of the airborne wear particles generated from the contact between a low-metallic pad material and a grey cast iron disc at different load levels. This is done on model level with a pin-on-disc machine that allows the cleanliness of the air surrounding the test specimens to be controlled, and thus the airborne portion of the wear particles to be studied separately. The concentration and size of airborne wear particles were measured online during testing with four particle instruments. In addition, airborne wear particles were collected on filters during the tests and afterward analysed using SEM. Trimodal size distributions with peaks around 280, 350 and 550 nm were registered during running-in for all load levels. After running-in bimodal size distributions with peaks around 350 and 550 nm were registered for all load levels with the exception of the highest load level where multimodal size distributions were registered. At the two highest load levels the concentration of ultrafine/fine particles showed an increase up to a factor hundred indicating a change in wear mechanism. SEM images show ultrafine, fine and coarse airborne wear particles.     

Microtribological studies of two-phase Al2O3–TiC ceramic at low contact pressure

The two-phase Al₂O₃–TiC ceramic (AlTiC) has many applications. One of the most common uses of AlTiC is for data recording heads where it is used as a bearing surface to support the magnetic sensing elements. This is one of the examples where the ceramic can be used in MEMS. Using Linear Tape Open (LTO) drive and metal particle (MP) tape media as the experimental platform; the wear of the AlTiC at very low loads and for very smooth surfaces has been studied.X-ray photoelectron spectroscopy (XPS), Auger electron Spectroscopy (AES) and Atomic Force Microscopy (AFM) were employed to analyse the AlTiC surface changes during wear at a variety of environmental conditions. Under all experimental conditions, the results showed the TiC phase of AlTiC to have been oxidized to form a surface layer. This gave rise to classical oxidational wear of that phase; with the delamination of the TiO₂ to form pullouts on the AlTiC surface and subsequent three-body abrasive wear particles were produced. The rate of oxidation of the TiC and hence the rate of production of the three-body wear particles increases with atmospheric water vapour content. In the experimental system chosen for this investigation, this results in an increase in differential wear, and hence pole tip recession of the magnetic metal poles of the recording heads. Pole tip recession was shown to correlate with increase in oxidation rate and also increase with atmospheric water vapour content.The wear of the Al₂O₃ phase was probably due to micro-adhesive wear with a wear rate much lower than that of the TiC phase.     

Abrasive wear behaviour of B4C particle reinforced Al2024 MMCs

In this study, the effects of volume fraction and particle size of boron carbide on the abrasive wear properties of B₄C particle reinforced aluminium alloy composites have been studied. For this purpose, a block-on-disc abrasion test apparatus was utilized where the samples slid against the abrasive suspension mixture at room conditions. The volume loss, specific wear rate and roughness of the samples have been evaluated. The effects of sliding time, particle content and particle size of B₄C particles on the abrasive wear properties of the composites have been investigated. The dominant wear mechanisms were identified using scanning electron microscopy. The results showed that the specific wear rate of composites decreased with increasing particle volume fraction. Furthermore, the specific wear rate decreased with increasing the size of particle for the composites containing the same amount of B₄C. Hence, it is deduced that aluminium alloy composites reinforced with larger B₄C particles are more effective against the abrasive suspension mixture than those reinforced with smaller B₄C particles.     

Fabrication of porous nanostructured TiO2 particles by an aerosol templating method

An aerosol templating method was applied to fabricate the spherical nanostructured TiO₂ particles containing both mesopores and macropores using two kinds of colloidal mixture such as polystyrene latex (PSL) particles and TiO₂ nanoparticles (P25), and PSL and a titanium hydroxybislactato (TC315). As the weight ratio of PSL/P25 increased from 0 to 1.30, morphology of the as-prepared particles changed from mesoporous particles to particles containing mesopores and macropores. As the furnace temperature decreased from 800 to 600 °C at the fixed process conditions, the increase of mesopore volume and specific surface area were determined. The TiO₂ particles fabricated from a mixture of TC315 and PSL were composed of lots of mesopores and a few macropores. The width of UV-absorption spectra of the porous particles synthesized from two colloidal mixtures decreased a little with respect to the increase of the weight ratio. The complete decomposition of p-xylene of 97.9 ppm was accomplished within 2 h under the illumination of UV-light by all the porous particles.     

Fretting Wear Study on Micro-Arc Oxidation TiO<Subscript>2</Subscript> Coating on TC4 Titanium Alloys in Simulated Body Fluid

Tribological properties of TiO₂ coatings synthesized by micro-arc oxidation (MAO) on the surface of TC4 titanium alloys were investigated at the fretting contact against 440C stainless steel in simulated body fluid (SBF). Fretting experiments were carried out by ball-on-flat contact at various loads for 1 h, with an amplitude of 100 μm and a frequency of 5 Hz. Results show that MAO TiO₂ coatings presented good tribological properties with lower friction coefficient in SBF. Less wear volume was observed for MAO TiO₂ coatings compared with that for TC4 alloy. At lower load, the wear mechanism of MAO TiO₂ coatings was dominated to abrasive wear. With an increase of normal load, however, fretting corrosion increased due to chemical reactions with SBF, and therefore, fretting fatigue coexisting with abrasive wear became the predominant mode.     

Nanoparticle suspension preparation using the arc spray nanoparticle synthesis system combined with ultrasonic vibration and rotating electrode

This study aims to investigate the use of a new nanoparticle preparation method, i.e., the arc spray nanoparticle synthesis system (ASNSS) combined with ultrasonic vibration and rotating electrode, to prepare TiO₂ nanoparticle suspension. For the proposed new method of nanoparticle suspension preparation, this study has designed four different process modes of experimentation for comparison, in order to obtain suspended nanoparticles with smaller particle size and relatively good dispersion. This study discusses the process modes with different settings of variables including peak current, pulse duration, breakdown voltage, temperature of the dielectric fluid and amplitude of ultrasonic vibration, in order to determine the better conditions for the preparation of TiO₂ nanoparticles suspension. The Transition Electron Microscope (TEM) image of the experiment result shows that TiO₂ nanoparticles prepared by the ultrasonic vibration assisted vacuum arc spray process has an average particle size of less than 10 nm.     

Tribological Properties of New MoS2 Nanoparticles Prepared by Seed-Assisted Solution Technique

Seed-assisted solution synthesis of hollow IF-MoS₂ nanoparticles allows independent control of particles size and MoS₂ slabs crystallinity. Variations of the reaction mixture composition influence the particle size in the range 50–150 nm. As demonstrated by Rietvelt refinement of the X-ray diffraction patterns, the sulfide crystallinity depends only on the post-treatment temperature (350–750 °C) and not on the particle size. The tribological properties of new MoS₂ nanoparticles prepared by seed-assisted solution technique were investigated and showed a strong decrease in the friction coefficient and wear compared with base oil. Small particles of 50–60-nm size showed the best results. The particle size above 100 nm is deleterious for the lubrication properties since it hinders particles penetration into the contact zone. MoS₂ slabs crystallinity had lesser influence on the lubrication efficiency. However, less-crystallized samples treated at 350 °C showed better lubrication, apparently because of easier exfoliation of the individual MoS₂ slabs, leading to more efficient formation of tribofilm.     

Study on the Synthesis and Tribological Property of Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> Based Magnetic Fluids

In this paper, Fe₃O₄ based magnetic fluids with different particle concentrations were prepared by the co-precipitation technique. The size of the Fe₃O₄ nanoparticles is about 13 nm and their shape is spherical. The tribological performances of the fluids with different concentration Fe₃O₄ nanoparticles were evaluated in a MMW-1A four-ball machine. The results show that the tribological performance of magnetic fluids with proper Fe₃O₄ nanoparticles can be improved significantly. The maximum nonseized load (P _B) has been increased by 38.4% compared with carrier liquid. The wear scar diameter has been reduced from 0.68 mm to 0.53 mm and the relative percentage in friction coefficient has decreased to 31.3%. The optimal concentration of the Fe₃O₄ nanoparticles in the carrier liquid is about 4 wt.%.     

A Study on the Sliding Wear of Hybrid PTFE/Kevlar Fabric/Phenolic Composites Filled with Nanoparticles of TiO2 and SiO2

TiO₂ and SiO₂ nanoparticles were introduced into hybrid polytetrafluoroethylene (PTFE)/Kevlar fabric/phenolic composites. The results showed the incorporation of TiO₂ nanoparticles can reduce the wear rate of the fabric/phenolic composite at elevated temperatures, although the wear of hybrid PTFE/Kevlar fabric/phenolic composite did not change much when TiO₂ or SiO₂ nanoparticles were used as filler. The wear behavior was explained in terms of morphology of transfer films and worn surfaces. There was a good correlation between the morphology of transfer film and wear results.     

A model and the methodology for determining wear particle generation rate and filter efficiency in a diesel engine using ferrography

A mathematical model was developed that describes the wear particle concentration as a function of time in a diesel engine. This model contains engine and lubrication system parameters that determine the concentration of wear particles in the engine sump. These variables are the oil system volume, oil flow rate, particle generation rate, filtering efficiency and the initial particle concentration. The model was employed to study the wear particle concentrations in the sump and the mass of particles in the filter for the Cummins VT-903 diesel engine. In addition, the model was used to develop a testing methodology for determining wear particle generation rates and filter efficiencies from used oil analysis. This testing methodology uses ferrography together with computer programs to yield accurate statistical information on the data as curve fitted to the model. The test set-up incorporated a remote-controlled sampling system that enabled the accurate and periodic taking of oil samples over an engine test approximately 5 h in duration.

Results of this research indicate that equilibrium wear particle concentrations increase with an increase in engine speed and load. The wear particle generation rate and filter efficiency as determined by the test methodology were found to decrease with an increase in engine speed and load. After oil and filter changes, the wear particle generation rate and filter efficiency continually increased with cumulative engine time up to approximately 11 h. The test methods used to obtain the results above were found to be repeatable to within ±15% and could conceivably be employed to determine wear parameters on other diesel engines as well as the effects that other engine variables such as lubricants, oil temperature, coolant temperature and engine components have on the wear parameters.     

Effect of transfer layer on dry sliding wear behaviour of cast Al-based composites synthesized by addition of TiO2 and MoO3

Two types of composites have been developed by solidification processing by addition of 3, 4, and 5 wt% powders of oxide—TiO₂ and MoO₃, to molten Al-5 wt% Mg alloy. The oxide particles react with the molten alloy resulting in alumina and releasing alloying elements of Ti or Mo. Dry sliding wear behaviour of pins of cast composite, fabricated by solidification of melt-particle slurry in mold, has been determined by pin-on-disc wear tests carried out conventionally and while removing wear debris by camel brush. The accumulated volume loss in composites increases linearly with increasing sliding distance and the wear rate increases more or less linearly with increasing load. Increasing particle content decreases wear rate at a given load. The accumulated volume loss is considerably higher when wear debris is removed by camel brush during dry sliding wear. The nature of the wear debris has been confirmed to be oxidative. The relatively brighter compacted oxide transfer layer could be observed in the SEM micrograph of worn pin surfaces of the composites developed by addition of MoO₃ and TiO₂ respectively. Since the accumulated volume loss in wear is relatively more when the wear debris is removed during dry sliding wear test it may be inferred that wear debris is more beneficial for wear resistance through formation of transfer layer rather than its harmful role in enhancing volume loss through three body wear. At higher loads, the oxide debris are expected to get better compacted to form transfer layer, spread over a larger area of the sliding surface and thus, their removal causes a larger wear compared to that without removal of wear debris. However, a larger cover of transfer layer at higher load does not necessarily imply reduced accumulated volume loss because the wearing process is more aggravated at higher load. Apart from adhesion, micro-cutting and abrasion, the transfer layer also flakes off during dry sliding wear as indicated by the presence of chunky sheet of oxides in wear debris.     

Tribochemical Transformation of Nano TiO2 to Ilmenite on the Surface of Wearing Steel Parts: Antiwear Action of Nano TiO2 as an Additive in Engine Oil

TiO₂ nanoparticles of average size about 20–30 nm were hydrothermally synthesized from TiCl₄ under mild acidic conditions. The nanoparticles were mixed with dispersant and base oil to give a partially transparent concentrate with 1.5 wt% of Ti content. The concentrate was dispersed in hexane and base oil to characterize, respectively, by transmission electron microscopy (TEM) and dynamic light scattering (DLS). The concentrate was diluted with base oil to a parts per million level of Ti containing dispersion blends that were evaluated for wear and friction control performance. Nano TiO₂ containing fully formulated oil blend showed excellent load-bearing capability in Swingung, Reibung, Verschleiβ (SRV; oscillation, friction, wear) tests. Four-ball test results show that the wear scar diameter was considerably reduced to 0.30 mm for TiO₂-added blend compared to neat base oil (0.60 mm). The performance of TiO₂-added blend was comparable to secondary zinc dialkyl dithiophospate (ZDDP)-added blend under identical condition. Raman spectra of the worn surface on the tested ball revealed the presence of ilmenite (FeTiO₃) and no deposits of pure TiO₂.     

Effects of supply of fine oxide particles onto rubbing steel surfaces on severe–mild wear transition and oxide film formation

This study is the first to show a quantitative condition required for the establishment of severe–mild wear transition with sliding distance, by studying the effects of supply of Fe₂O₃ particles onto rubbing steel surfaces on the transition and oxide film formation process. The supply of fine Fe₂O₃ particles was found to accelerate the wear transition, and the sliding distance at which the transition occurs was found to increase with particle diameter and applied load. Oxide films are produced on the rubbing surfaces by sintering of the supplied Fe₂O₃ oxide particles. At the severe–mild wear transition, the relative area of oxide films is the same for all diameters of supplied Fe₂O₃ particles. This finding suggests that the transition occurs when the relative area of oxide films reaches a specific value, which is proportional to the area of real contact.     

毛细管进样-激光解吸电离气溶胶飞行时间质谱仪检测超细纳米颗粒物化学成分

采用毛细管大气压进样接口，结合激光多光子解吸、电离等技术，自行研制了毛细管进样-激光解吸电离气溶胶飞行时间质谱仪，用于在线测量超细纳米颗粒物的化学成分。应用商品化的气溶胶发生器和自制的离子诱导成核反应器，在实验室内分别产生了KCl、NaCl、NaI/NaBr混合物和Air/H₂O/SO₂混合气体成核反应的粒径小于100 nm的超细纳米颗粒物，并通过质谱仪获得了它们的化学成分信息。结果表明，相对于具有较大粒径的颗粒物，超细纳米颗粒物在激光多光子电离过程中更容易完全原子化电离，但通过改善激光的聚焦条件可以获得超细纳米颗粒物成分的分子信息。实验测量了毛细管大气压进样接口传输超细纳米颗粒物的效率，为0.33%，与国际同行采用类似大气压进样接口传输气体分子的效率一致。     

Some observations on two-body abrasive wear

Pin-on-disc-type two-body abrasion tests were carried out on five metals with seven particle sizes over a range of loads, lengths of travel and sliding speeds. The familiar results that two-body abrasive wear is proportional to load and to distance travelled were confirmed. The “size effect”, in which particles below about 100 μm produce progressively less wear, was shown to be independent of load, sliding speed and prior cold working. Increasing the sliding speed from 1 to about 100 mm s^?1 produced an increase in wear resistance of about 50% for AISI 1020 steel. An increase in velocity above 100 mm s^?1 had little effect on the wear resistance. Plots of the wear resistance against the hardness of the annealed metal showed significant deviations from the linear relationship reported in the literature. The result is influenced by both sliding velocity and particle size.     

Influence of the countermaterial on the dry sliding friction and wear behaviour of low temperature carburized AISI316L steel

Low temperature carburising (LTC) allows a significant hardness increase, with a consequent increase in wear resistance, without deteriorating corrosion behaviour. However, wear resistance strongly depends on contact conditions, therefore this work focuses on the dry sliding behaviour of LTC-treated AISI316L austenitic stainless steel against several countermaterials (AISI316L, LTC-treated AISI316L, hard chromium or plasma-sprayed Al₂O₃–TiO₂). LTC produced a hardened surface layer (C-supersaturated expanded austenite), which improved corrosion resistance in NaCl 3.5% and increased wear resistance, to an extent which depends on both normal load and countermaterial. The best results were obtained when at least one of the contacting bodies was LTC-treated, because this condition led to mild tribo-oxidative wear. However, LTC did not improve the behaviour in terms of friction.     

Dry sliding wear properties of unhybrid and hybrid Al alloy based nanocomposites

Abstract

Nanosize B₄C and/or MoS₂ particles reinforced AA2219 alloy composites were prepared using the stir casting process. The wear properties were evaluated for several speed (3.14–5.65 m s^?1), load (10–50 N) and distance (0–2500 m) conditions. The nanoparticles dispersion, density, wear resistance, morphology of the worn surface and loose wear debris were discussed in detail. The wear resistance improvement results by nanoparticle addition correspond well with the hardness. Between the nanocomposites, hybrid composites show significantly higher wear resistance for all load, speed and sliding distance conditions. The better wear resistance is attributed to the matrix strengthening by nanoparticles and the lubricant-rich tribolayer controlled wear in the hybrid composites. The intensity of abrasive, oxidation and delamination wear mechanisms decide the wear rate at any particular wear testing condition.     

Tribological Behavior of PTFE Composites Filled with PEEK and Nano-Al2O3

In this study, the tribological properties of polytetrafluoroethylene (PTFE) composites filled with polyetheretherketone (PEEK) and nano-Al₂O₃ particles were studied using a block-on-ring wear tester. The tribological performance of the composites was affected by the experimental parameters (sliding speed, normal load, and environmental temperature) and the composites achieved a high-speed sliding friction state. The results showed that the PEEK and nano-Al₂O₃ particles significantly improved the wear resistance of the PTFE composites. In addition, the nano-Al₂O₃ particles increased the hardness of the composites and enhanced the mechanical properties to enable applications in a wider range of industrial fields. The effects of the sliding speed and normal load on the tribological properties were more significant than that of the environmental temperature. In addition, the entire wear process was divided into three stages (the initial wear stage, severe wear transition stage, and ultralow stable wear stage), according to the evolution of the tribological characteristics (wear rate, morphology of the worn surface and transfer film, and wear debris morphology).