Effect of hydrogenated DLC coating hardness on the tribological properties under water lubrication

Hydrogenated diamond-like carbon (DLC) coatings were deposited using unbalanced magnetron sputtering (UBM) equipment with different hardnesses. Effects of coating hardness on tribological properties were investigated with tribo-tests under water lubrication. Results showed that the wear volume increased rapidly during the initial running-in process, but remained nearly constant after the running-in process. The ball wear rate increased as the hardness of the DLC coating increased when metals (stainless steel and brass) were used as counter parts. In contrast, the UHMWPE ball wear rate was independent of the DLC hardness. TEM analysis and nano-indentation measurements were conducted of the transfer layer on the counter bodies’ contact surfaces. The transfer layer consisted mainly of Fe, O and C. The low friction of DLC coating is attributed to this low hardness transfer layer, which acts as a boundary-lubricating layer with low shear strength.     

Improvement of tribological characteristics under water lubrication of DLC-coatings by surface polishing

To achieve a hydraulic power system, it is important to control tribology because water has a lack of lubricity. Therefore, coated surface is necessary under water lubrication. Diamond-like-carbon (DLC)-coating is known as a useful material because of its high hardness and low friction, therefore it can be used as a coating durable for the water lubrication. Deposition methods of DLC-coating are developed in various ways. Particles called “droplets” are observed on the surface of DLC-coating depends on deposition methods and it can affect friction and wear properties. In this study, DLC-coating was prepared using a multi-cathode unbalanced magnetron sputtering (UBMS) system. The surface was polished with diamond slurry solution and aero lap to remove droplets. DLC-coatings were evaluated by tribo-tests before and after polishing. It is considered that some surface modification occurred. Moreover, the results of tribo-tests show that friction coefficients became lower and more stable than before polishing. Although partial delamination was observed after tribo-tests without polishing, no appreciable wear was observed after polishing.     

Comparative study of the tribological properties of various modified mild steels under boundary lubrication condition

AISI1045 steel was modified by laser heat-treatment and conventional heat treatment. The friction and wear behaviors of the steel specimens after various surface modifications sliding against SAE52100 steel under the lubrication of liquid paraffin containing sulphurized olefin were comparatively investigated on an Optimol SRV oscillating friction and wear tester. The worn surface morphologies of the modified steel specimens were analyzed using a scanning electron microscope. The elemental compositions and chemical states of some typical elements on the worn surfaces of the modified steel specimens were analyzed with an energy dispersive X-ray analyzer and X-ray photoelectron spectroscope, respectively. It was found that the laser heat-treated specimen showed the highest hardness and best wear-resistance. The laser heat-treated and conventionally heat treated AISI1045 steel specimens sliding against SAE52100 steel under the lubrication of liquid paraffin containing sulphurized olefin registered smaller friction coefficients than under the lubrication of liquid paraffin alone. This was partly attributed to the increased hardness of the modified specimens. The tribochemical reaction between the steel and the active elements in the additive was involved in the sliding of the modified steel specimens against SAE52100 steel ball under the boundary lubricating condition, with the formation of a surface protective film composed of various tribochemical products. This also contributed to improve the friction and wear behavior of the modified steel specimens. The steel specimens subject to different surface modifications showed differences in the wear mechanisms under the boundary lubricating condition as well. Namely, the tempered steel specimen was mainly characterized by plastic deformation and pitting, the quenched specimen by grooves and delaminating, and the laser heat-treated one by polishing and mild adhesion.     

Comparison of tribological characteristics between aluminum alloys and polytetrafluoroethylene composites journal bearings under mineral oil lubrication

This study examined the tribological behavior of journal bearings made from polytetrafluoroethylene (PTFE) composites and aluminum (Al) alloys. The PTFE composite journal bearings consisted of a steel backing with a thickness of 1.6 mm, a middle layer of sintered porous bronze with a thickness of 0.24～0.27 mm, and a surface layer of PTFE filled with fluorinated ethylene propylene (FEP) powder and carbon fibers with a thickness 0.06～0.14 mm. The other was an aluminum alloy journal bearing consisted of a steel backing with a thickness of 1.5 mm and a surface layer of an Al-6Sn-6Si alloy with a thickness 0.35～0.75 mm. A series of lubrication tests were performed using a journal bearing tester under various normal loads. The tribological properties for each journal bearing were evaluated by measuring the lubricant oil temperature and friction coefficient as a function of the applied normal load. In addition, the chemical compositions and microstructures of the journal bearing materials used in this study was analyzed by inductively coupled plasma (ICP), optical microscopy (OM), and scanning electron microscopy (SEM), respectively. The experimental results showed that the Al alloy journal bearings reduce the friction coefficient by 28 % compared to the PTFE composites bearings. In addition, the Al alloy journal bearing worked properly at the maximum load of ～ 8,000 N without adhesion. However, the PTFE composite journal bearings exhibited strong adhesion at the loads ranging from 6300 to 8000 N. This suggests that the Al alloy is a more promising material in journal bearings than PTFE composites.     

Effect of rubber on tribological behaviors of polyamide 66 under dry and water lubricated sliding

The friction and wear behaviors of polyamide 66 (PA 66) and rubber-filled PA 66 (PA 66/SEBS-g-MA) composites were investigated on a block-on-wheel model friction and wear tester under dry sliding and water lubricating conditions. In order to further understand the wear mechanisms, the worn surfaces and scraps of samples were analyzed by scanning electron microscopy (SEM) and differential scanning calorimeter (DSC). The experimental results indicated that the wear mass loss and the friction coefficient of PA 66 decreased with the addition of rubber particles. The friction coefficients of PA 66 and PA 66/SEBS-g-MA composites under water lubricating condition are lower than those under dry sliding condition, but the wear mass losses are higher than those under dry sliding condition. The main wear mechanisms under dry sliding condition are the plastic deformation and mechanical microploughing. Whereas the main wear mechanisms under water lubricating condition are the mechanical microploughing and abrasive wear.     

Comparing tribological behaviors of plasma nitrided and untreated bearing steel under lubrication with phosphor and sulfur-free organotungsten additive

The interactions and synergistic tribological effects between plasma nitrided bearing steel surface and phosphor and sulfur-free organotungsten lubricating additive compared with that of untreated steel were investigated in this paper. The tribological behaviors were examined on a four-ball friction and wear tester. The chemical characteristics of the tribofilms were analyzed by X-ray photoelectron spectroscopy (XPS). The results showed that the obvious synergistic effects of better tribological performance between nitrided surface and organotungsten additive were attributed to WN formed in the tribofilm and a stronger adsorption of organic carbon chains and higher contents of C and W element in the tribofilm.     

The tribological characteristics of JS material under lubrication of oil

A material composed of a steel backing, a sintered porous bronze middle layer and a layer of reinforced PTFE, which is named JS material, was prepared. The friction, wear and limiting PV values of this material under dry friction as well as the lubrication of number 20 mechanical oil were studied using a MPV-1500 friction tester. The worn surface of JS material and the transfer film formed on the counterface of carbon steel were investigated using scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). The results show that the friction, wear and limiting PV values of JS material can be greatly improved with the lubrication of oil. The results of SEM and EPMA analyses indicate that, under dry friction conditions, the solid lubricant PTFE and Pb easily transfer to the steel counterface and results in the reduction of friction and wear; while under the lubrication of oil, little transference of PTFE and Pb to the steel surface occurs and very small friction and wear are achieved. Analyses of frictional surfaces also suggest that the Pb filler gets enrichment on the rubbing surfaces, which is beneficial in increasing the adhesion of the transfer film with the steel surface.     

An investigation of the transfer film characteristics and the tribological behaviors of polyphenylene sulfide composites in sliding against tool steel

The effect of CuS, CuO, CaF₂, and ZnF₂ fillers in particulate sizes of 200–350 mesh on the friction and wear behavior of polyphenylene sulfide (PPS) was investigated. The filler proportion studied was 35 vol.%. The specimens which were prepared by compression molding were tested for friction and wear in a pin-on-disc configuration using hardened tool steel disc as the counterface. The morphologies of the worn surfaces and the transfer films were studied by scanning electron microscopy and the transfer films were analyzed by energy dispersive spectroscopy. The improvement in wear resistance with CuS was considerable and with CuO marginal, while the wear resistance decreased with CaF₂ and ZnF₂. The coefficient of friction increased significantly with CuO and decreased with CaF₂ and was not significantly affected by the other fillers. The changes in the friction and wear behaviors were analyzed in terms of the morphology and the composition of transfer films.     

Tribological behaviors of hierarchical porous PEEK composites with mesoporous titanium oxide whisker

Hierarchical porous PEEK self-lubricating composites were prepared by mold-leaching and vacuum melting process under high temperature. The tribological behaviors were investigated for the porous PEEK composite and the porous composite after incorporating micro-porogen (NaCl) and mesoporous titanium oxide whiskers. If only micro-porogen was incorporated, the lowest steady state specific wear rate was observed for PEEK composites filled with 30% NaCl. Based on this porous PEEK composite, the effects of mesoporous titanium oxide whiskers and non-perforated titanium oxide whiskers on the friction and wear properties of PEEK composites were studied. Results showed that nano-micro porous PEEK composites with 30 wt% micro-porogen and 5 wt% mesoporous titanium oxide whiskers reached the lowest friction coefficient and specific wear rate, which were recorded as 0.0194 and 2.135×10^–16 m³/Nm under the load of 200 N. Compared with 15 wt% carbon fiber-reinforced PEEK composite which is widely used in industry, the wear resistance of the designed hierarchical porous PEEK composite increased by 41 times, showing outstanding wear resistance.     

Effect of R600a on tribological behavior of sintered steel under starved lubrication

This paper describes the impact of the isobutane refrigerant on the wear performance of tribopairs at hermetic compressor bearings. The tribological behavior of 100Cr6 steel pin is investigated under starved lubrication condition in air and R600a environments when running against sintered steel which was treated with and without steam. EDS and SEM are carried out on pin and plate samples after wear tests. The results indicate that wear durability is lower for the tests with R600a than those with air. The adverse effect of R600a on wear rate is linked to the change in the viscosity and foaming characteristics of the oil in the presence of R600a as well as the lack of oxides.     

碳纤维/聚醚醚酮(CF/PEEK)复合材料摩擦磨损性能及抗摩擦静电特性研究

利用真空热压烧结技术制备了不同碳纤含量的碳纤维/聚醚醚酮(CF/PEEK)复合材料,采用热导率分析仪和热重测试仪对材料的热学性能进行表征,并利用多功能摩擦磨损试验机、三维形貌轮廓仪、扫描电子显微镜和摩擦静电计对材料的摩擦磨损性能和抗摩擦静电性能进行分析。分析结果表明：随着CF添加量的增加,复合材料摩擦因数、磨损率和摩擦静电电压先降低后升高,当CF添加量(质量分数)为20%时,摩擦因数、磨损率和摩擦静电电压达到最低,分别为0.247、5.6×10^-6 mm/(N·m)和3.3 V,证明此种方法制备的20%CF/PEEK材料具有优异的摩擦磨损性能和抗静电性能。CF/PEEK复合材料磨损机理以黏着磨损为主,并且伴随着轻微的磨粒磨损。     

Comparative tribological behaviors of TiN, CrN and MoNCu nanocomposite coatings

The purpose of this study is to investigate comparative tribological behaviors of Cu-doped TiN, CrN, and MoN coatings under a wide range of dry sliding conditions. TiN and CrN coatings have been developed and used by industry in numerous tribological applications including, machining, manufacturing and transportation. In contrast, MoN has attracted very little attention as a tribological coating in the past, despite being much harder than both TiN and CrN. In this paper, we will mainly concentrate on the Cu-doped versions of these coatings whose tribological properties have not yet been fully explored. The results of this study have confirmed that the addition of Cu into TiN, CrN and MoN coatings has indeed modified the grain size and morphology, but had a beneficial effect only on the friction and wear behavior of MoN. The tribological behavior of CrN did not change much with the addition of Cu but that of TiN became worse after Cu additions. Raman spectroscopy technique was used to elucidate the structural and chemical natures of the oxide films forming on sliding surfaces of Cu-doped TiN, CrN and MoN films. The differences in the friction and wear behavior of Cu-doped TiN, CrN, and MoN is fully considered and a mechanistic explanation has been provided using the principles of a crystal chemical model that can relate the lubricity of complex oxides to their ionic potentials.     

High-speed imaging for online micrographs of polymer composites in tribological investigation

Studying wear process by effective means avoids undesirable situation such as frequent replacement of components, breakdown-maintenance and catastrophic accidents. In tribological investigations, dimensional changes due to wear processes are assessed using offline and online techniques. However, traditional postmortem analysis cannot display intermediate wear mechanisms. Visible textures as a direct measure of wear is a valuable evidence to understand tribological characteristics. In recent years, online-monitoring through computer vision has been used to study surface roughness. Nevertheless, online-monitoring for topographical and microstructural changes during wear could provide a clear insight of wear process. In this work, attempts are made to monitor rolling/sliding contact using a high-speed camera in conjunction with a microscope. Optical and imaging parameters were optimised to obtain clear images. Tests were conducted using polymer composite for understanding the contact surface topography of the wear process. Micrographs of the composite contact surface were acquired online using 35,000 fps and 0.375 ns shutterspeed. Periodical changes in wear mechanism are evident with resin back transfer, surface cracks and partial exposure of fibres. Thus, effective characterisation through online-monitoring gives a clear insight of the fundamental mechanisms behind wear process. The newly developed high speed imaging system stands as an effective tool in tribological investigations.     

The effect of surface chemistry in lubrication on the tribological behaviour of steel rollers under rolling-sliding contacts

Experiments were carried out on a wear test machine utilising a gearcam adapter to simulate line-contact lubrication. Due to the formation of a surface adsorption layer on the steel substrate, the tribological performance was detected by measuring the voltage for a lubricant with various additive concentrations. The roller wear rate was found to be strongly dependent upon both the rising rate of voltage in the wear process and the time period needed to create a positive voltage. The antiwear effect of differing additive concentrations was evaluated using the measurements of voltage for various operating conditions. An increase in rotational speed, with a lubricating oil with a low additive concentration, decreases the wear rate. A lubricating oil with a high additive concentration does little to decrease the wear rate at low rotational speeds.     

镍合金增强MoS2基自润滑复合材料的组织与摩擦学性能

本文以Ｎｉ－Ｃｒ高温合金粉的ＭｏＳ２粉为原料，用热压法制成了镍合金增强ＭｏＳ２基自润滑复合材料，借助Ｘ射线衍射仪和扫描电镜，分析了材料的组织结构；用ＳＲＶ高温摩擦磨损试验机考察也材料在室温和２５０℃时的摩擦学性能。试验结果表明，Ｎｉ－Ｃｒ合金易与ＭｏＳ２反应，生成结构复杂的Ｃｒ（Ｎ）Ｍｏ２Ｓ４相。该反应物在室温下表现出较好的转移润滑特性，但在２５０ ℃时明显谈判。因此，要保证复合材料在室温和２５０     

An investigation of tribological behaviors of dynamically loaded non-grooved and micro-grooved journal bearings

Tribological performances of non-grooved and micro-grooved journal bearings were studied under dynamic loading. Numerous experiments were performed using purpose-built test rig and then simulated using various numerical methods. Friction force, friction coefficient, shaft center orbit, and film thickness were determined experimentally and numerically. The experimental and numerical results were in good agreement and the friction forces progressively increased on plain and circumferential, herringbone, and transversally micro-grooved bearing. The results show that it is necessary to complete detailed investigation about the tribological properties of the micro-grooved journal bearing by taking their shape, depth and operating condition into account.     

Influence of counter surface topography on the tribological behavior of carbon-filled PPS composites in water

This study is aimed at investigating the influence of counter surfaces׳ topography on tribological behavior of several carbon-filled polyphenylene sulfide (PPS) composites in water lubricated contacts. The results of this study showed significant increase in wear rate of pure, graphite and/or multi-walled carbon nanotubes filled PPS composites with increase in mean slope of profile along the sliding direction (Δα_y). This is while SCF filled PPS composites exhibited 1–3 orders of magnitude lower wear rate with little dependence on counter surface roughness characteristics. Among the roughness parameters studied, R_pk and lay orientation played a more significant role in friction, and R_pk and Δα_y were found to correlate best with the wear rate of the composites not containing SCF in their matrices.     

Fretting wear behaviors of nanometer Al2O3 and SiO2 reinforced PEEK composites

The influence of diameter and content of Al₂O₃ particles on the tribological behaviors under fretting wear mode was investigated. The surface of PEEK composite and steel ball were examined by SEM and EDS, to identify the topography of wear scar and analyze the distribution of chemical elements in the friction counterparts, respectively. It can be found that the filling of Al₂O₃ powder improves the fretting wear resistance of PEEK composite. With the increase of Al₂O₃ diameter, the area of wear scar on specimen increases first and decreases afterward. However, the wear of composites increases monotonically with increasing Al₂O₃ content. Although the filling of 10 wt.% and 200 nm PTFE powder in PEEK makes the lowest wear of all specimens, no synergistic effect was found when Al₂O₃ and PTFE were filled into PEEK composite together. For the friction pair of PEEK composite and steel ball, abrasive wear and adhesive wear dominate the fretting wear mechanism during fretting. Thermal effect plays a very important role during fretting; thus the property of temperature resistance for polymer material would affect the wear degree on the surface of wear scar.     

Effect of carbon nanotube addition on the tribological behavior of carbon/carbon composites

Carbon nanotube composite coatings were applied onto carbon/carbon composites to improve wear properties. Carbon nanotubes have been prepared by catalytic pyrolysis of hydrocarbons. The nanotube slurry was prepared by addition of phenolic resin and solvent to infiltrate into C/C composites. The nanotube added composites were then carbonized in a nitrogen atmosphere. Ball-on-disc type wear tests were performed to evaluate the tribological properties of the carbon nanotube added carbon composites. The result showed that addition of nanotube has the potential to increase the wear resistance of carbon composites. Changes in Raman spectra, morphology and surface damage were studied to explain observed wear behavior.     

Effect of compatibilizer on the transfer and tribological behaviors of PA46/HDPE polyblends

The effect of compatibilizer HDPE-g-MAH of different contents on the transfer and tribological behaviors of PA46/HDPE polyblends was investigated, using a ring-on-block wear tester. All the polyblends showed friction as low as HDPE of 0.20. The wear of the polyblends was reduced when the compatibilizer ranged from 1 to 5 wt.%, showing a minimum value as 1/7 of that of PA46 when the concentration of the compatibilizer was 5 wt.%. Scanning electronic microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) analyses were used to examine the worn surface and the physical state of the acid-eroded transfer films on the counterface. It was found that the physical bonding between PA46 and HDPE phases became stronger due to the better compatibility of the polyblends. The strengthened physical bonding might decrease the peel-back process of the dispersed HDPE particles from the polyblend, enhance the adhesion between the transfer film and the counterface, and ultimately decreased the wear. The formation of a transfer film on the counterface was hardly detectable, which was the primary reason for the relatively high wear of the polyblend with excess compatibilizer.