Tribological behaviour and wear mechanism of MoS2–Cr coatings sliding against various counterbody

MoS₂–Cr coatings with different Cr contents have been deposited on high speed steel substrates by closed field unbalanced magnetron (CFUBM) sputtering. The tribological properties of the coatings have been tested against different counterbodies under dry conditions using an oscillating friction and wear tester. The coating microstructures, mechanical properties and wear resistance vary according to the Cr metal-content. MoS₂ tribological properties are improved with a Cr metal dopant in the MoS₂ matrix. The optimum Cr content varies with different counterbodies. Showing especially good tribological properties were MoS₂–Cr8% coating sliding against either AISI 1045 steel or AA 6061 aluminum alloy, and MoS₂–Cr5% coating sliding against bronze. Enhanced tribological behavior included low wear depth on coating, low wear width on counterbody, low friction coefficients and long durability.     

Tribological performance of a DLC coating in combination with water-based lubricants

The tribological behaviour in water-based environments has been studied for a tungsten carbide-doped DLC coating (WC/C) deposited by physical vapour deposition (PVD) on bearing steel. Several tribological test equipments have been used to characterise the wear rate, coefficient of friction and resistance to seizure of the coated system, in comparison with uncoated bearing steel surfaces. It was observed that the wear was decreased and the coefficient of friction reduced in pin-on-disc measurements for poor lubricants. Further, the resistance to seizure in the four-ball method was improved by a factor of approximately three. Results from Reichert measurements showed a decreased wear rate and also a very pronounced running-in behaviour of the coating for some water-based lubricants. It has been shown that the performance of tribological systems with water-based lubricants can be significantly improved with this type of DLC coating.     

An Improved Tribological Polymer-Coating System for Metal Surfaces

Aromatic thermosetting polyester (ATSP)- and polytetrafluoroethylene (PTFE)-blended composites have been shown to exhibit improved tribological performance with low wear and low friction. In this article, pure ATSP films were coated on aluminum substrates and tested as a potential protective tribological coating. The tribological performance of this coating was tested against steel, using pure sliding sphere-on-disk experiments. A fluoroadditive powder (solid lubricant) was also added to further enhance the ATSP film wear and friction properties. For comparison, a commercially available PTFE-based coating was tested under the same conditions. Results show that the ATSP/fluoroadditive film has comparable coefficient of friction to the commercial coating, but significantly lower wear. Surface analysis techniques were employed to investigate the low-friction and low-wear mechanisms seen with the ATSP/fluoroadditive. Specifically TOF-SIMS depth-profiling showed that there is a high density of fluorine element within the wear track and penetrates well below the surface of the wear track.     

Influence of Polyfluo-Wax on the Friction and Wear Behavior of Polyimide/Epoxy Resin–Molybdenum Disulfide Bonded Solid Lubricant Coating

Polyimide/Epoxy resin–molybdenum disulfide bonded solid lubricant coatings (denoted as PI/EP-MoS₂) were prepared. The influence of polyfluo-wax (denoted as PFW) on the microhardness and friction and wear behavior of as-prepared PI/EP-MoS₂ lubricant coating was measured using a microhardness tester and a reciprocating ball-on-disc tribometer, respectively. The worn surfaces of the lubricant coatings were observed with a scanning electron microscope, and their wear rate was determined with a Micro XAM surface mapping microscope. Moreover, the transfer films formed on the counterpart steel ball surfaces were analyzed by X-ray photoelectron spectroscopy. Results indicate that the incorporation of a proper content of PFW filler is effective at improving the antifriction performance of the PI/EP-MoS₂ lubricant coating while maintaining better wear resistance. Moreover, the friction coefficient of the lubricant coating decreases with increasing content of PFW from 2 to 10%, and the one with a filler content over 6% PFW has a steady friction coefficient of 0.07. The improvement in the antifriction performance of the lubricant coating with the incorporation of the PFW filler is attributed to the excellent lubricity of homogeneously distributed PFW.     

Microstructure and Tribological Properties of a HfB<Subscript>2</Subscript>-Containing Ni-Based Composite Coating Produced on a Pure Ti Substrate by Laser Cladding

A HfB₂-containing Ni-based composite coating was fabricated on Ti substrates by laser cladding, and its microstructure and tribological properties were evaluated during sliding against an AISI-52100 steel ball at different normal loads and sliding speeds. The morphologies of the worn surfaces were analyzed by scanning electron microscopy (SEM) and three-dimensional non-contact surface mapping. The results show that wear resistance of the pure Ti substrate and NiCrBSi coating greatly increased after laser cladding of the HfB₂-containing composite coating due to the formation of hard phases in the composite coating. The pure Ti substrate sliding against the AISI-52100 counterpart ball at room temperature displayed predominantly adhesive wear, abrasive wear, and severe plastic deformation, while the HfB₂-containing composite coating showed only mild abrasive wear and adhesive wear under the same conditions.     

Friction and Wear Behaviors of Several Polymers Sliding Against GCr15 and 316 Steel Under the Lubrication of Sea Water

Ocean tribology, a new research field of tribology, is currently being established and developed. The tribological behaviors of polyether ether ketone (PEEK), poly(phenyl p-hydroxybenzoate) (PHBA), polyimide (PI), and perfluoroethylene propylene copolymer (FEP) sliding against GCr15 and 316 steel rings under the lubrication of sea water were studied and compared with that under the lubrication of pure water. The results show that the friction and wear behaviors of a polymer under the lubrication of aqueous medium are not only related to the properties of polymer itself, but also to the corrosive effect and lubricating effect of the medium. When a polymer slid against GCr15 steel under sea water lubrication, the friction coefficient and wear rate of polymer were much larger than that under pure water lubrication because of indirect corrosive wear. However, when sliding against corrosion-resistant 316 steel, polymers PEEK, FEP, and PI exhibited lower coefficients of friction and wear rates under sea water lubrication, this was attributed to better lubricating effect of sea water as a result of the deposition of CaCO₃ and Mg(OH)₂ on the counterface. On the contrary, the friction coefficient and wear rate of PHBA sliding against 316 steel under sea water lubrication were larger than that under pure water lubrication, which may be related to the properties of PHBA itself.     

Tribological Behaviors of Self-lubricating Coating Prepared by Electrospark Deposition

Cu/h-BN self-lubricating coating was prepared on AISI1045 steel by electrospark deposition. The friction coefficient and wear rates were measured using the ball-on-disk method, and the tribological behaviors were discussed. Results showed that the friction coefficient decreased with an increase in sliding speed and load. The wear rate decreased with an increase in sliding speed and increased with an increase in load. The self-lubricating coating exhibited much lower friction coefficient and wear rate than the uncoated mild steel under the test condition. SEM micrographs show that the main wear mechanisms of the self-lubricating coating are abrasive wear and fatigue wear.     

A comparison of the tribological behaviour of steel/steel, steel/DLC and DLC/DLC contacts when lubricated with mineral and biodegradable oils

Diamond-like carbon (DLC) coatings, which can nowadays be applied to many highly loaded mechanical components, sometimes need to operate under lubricated conditions. It is reasonable to expect that in steel/DLC contacts, at least the steel counter body will behave according to conventional lubrication mechanisms and will interact with lubricants and additives in the contact. However, in DLC/DLC contacts, such mechanisms are still unclear. For example, the “inertness” of DLC coatings raises several questions about whether they are able to provide real boundary “lubrication” or whether they are just a “passive” member in these contacts. On the other hand, biodegradable oils, in particular vegetable base oils, possess a good lubricating ability, often much better than mineral or conventional synthetic oils as a result of the large amount of un-saturated and polar components that can promote the lubricity of DLC coatings. Accordingly, in this study, we present the results of experiments under severe boundary-lubrication conditions during reciprocating sliding. We look at the effect of the type of mating surfaces - steel/DLC, DLC/DLC and steel/steel - and the type of oil on the tribological performance of DLC coatings. We compare the wear and friction behaviours of two types of DLC coatings, i.e., a “pure” non-doped a-C:H DLC coating (denoted as a-DLC) and a WC-containing multilayer coating (denoted as W-DLC) tested with a mineral oil and a biodegradable vegetable oil. These oils, which have very different chemical compositions, were used as base oils and also with mild AW and strong EP additives. Among other things, the results confirm the following: (1) coating/coating lubricated contacts can resemble metal-lubrication mechanisms; (2) additives reduce wear in coating/coating contacts by up to 80%; (3) better wear and friction performance are obtained with oils that contain large amounts of polar and un-saturated molecules; (4) a coating/coating combination generally results in less wear than a steel/coating combination.     

Tribological studies of epoxy and its composite coatings on steel in dry and lubricated sliding

Industrial lubricants are invariably used with additives (with high sulfur and phosphorous contents) for tribological performance enhancement. However, these additives are environmentally very harmful. Hence, there is an urgent need to find alternate solutions for enhancing the tribological performance of lubricants and components without the use of harmful additives. The objective of this work is to investigate the feasibility of using polymer composite coatings in enhancing the tribological properties of steel surfaces in dry and base oil lubricated conditions. Pure epoxy and its composite (with 10?wt-% of graphene or graphite powder) films were coated onto steel substrates and tested under dry and base oil lubricated conditions. Friction and wear experiments were conducted on a ball on cylinder tribometer between polymer/composite coated cylindrical steel surface (shaft) and an uncoated steel ball as the counterface. Tests were conducted at various normal loads and speeds. In dry condition at 3 N load and 0.63?m s^??1 sliding speed, the wear life of epoxy was increased by five times and coefficient of friction was nearly the same (0.18) on inclusion of graphene nanoparticle. In lubricated case, epoxy/graphene composite coating performed eight times and more than five times better than pure epoxy and epoxy/graphite respectively.     

火焰喷涂PA1010/n-SiO2复合涂层干摩擦磨损性能

利用火焰喷涂法制备PA1010/n-SiO2复合涂层,并采用均匀试验设计方法研究涂层在干摩擦条件下同GCr15 钢环配副时的摩擦学性能;利用SPSS 12.0统计软件对试验结果进行回归分析, 建立涂层摩擦系数和磨损质量损失同pv值 (摩擦载荷与摩擦速度的乘积)相关性的数学模型;利用示差扫描量热仪(Differential scanning calorimetry, DSC)和扫描电子显微镜(Scanning electron microscope, SEM)对复合涂层的热性能和磨损表面形貌进行分析。结果表明,n-SiO2的加入能明显提高涂层的结晶性能、耐磨性能。当n-SiO2含量为1.5%时,复合涂层摩擦磨损性能最佳,在试验条件下磨损质量损失降低近4倍,摩擦因数降低23％,跑合期缩短44％,复合涂层与GCr15钢环对磨时的磨损机理主要为疲劳磨损和轻微的粘附 磨损。     

Tribological behaviour of MW PACVD diamond coatings in vibrating contacts

Six diamond coatings were produced by microwave plasma-assisted chemical vapour deposition (MW PACVD) on polished SiC substrates. Process gas pressure and process gas composition were varied systematically, resulting in different coating morphologies, roughnesses and grain sizes. Friction and wear were investigated for unlubricated vibrating contact conditions in air at room temperature. Tribological tests on diamond coatings were performed in a ball-on-disc configuration, with steel balls (100Cr6) and alumina balls acting as counterbodies. For comparison of the tribological performance of the diamond coatings, standard tests were performed with a stroke of 200 μn, a load of 10 N, and a frequency of 20 Hz. The effect of relative humidity (RH) was investigated by testing in dry (3% RH), in normal (50% RH), and in moist air (100% RH), respectively To investigate running-in effects and wear propagation, the test duration (10⁴, 10⁵, and 1.2·10⁶ sliding cycles) was varied. For steel/diamond pairings, high wear at the steel ball and high friction were measured, influenced by roughness and grain size of the diamond coating and by relative humidity. For Al₂O₃ against diamond, a pronounced running-in of friction and wear was found. High friction and high wear at the beginning of each test are followed by a stationary phase with extremely low friction coefficients (⩽ 0.05) and with wear rates below the limit of resolution. This running-in depends on grain size and roughness of the coating, on relative humidity, as well as on the operational parameters. Surface analysis using SEM, EDX, LRS and AFM was conducted, to investigate tribologically induced surface changes.     

Friction and Wear Behaviors of TiN Coatings under Dry and Vacuum Conditions

In this study, pure TiN coatings were applied on AISI 52100 steel surfaces with a magnetron sputterer at different direct current (DC) powers and N₂ flows to develop a wear-resistant coating for sliding tribopairs working in atmospheric and vacuum conditions. Wear and friction characteristics of these coatings were investigated with a tribometer under both vacuum and atmospheric conditions. DC power and N₂ flow affected the coating thickness and structure. TiN coatings showed different wear and friction characteristics under ambient and vacuum conditions due to the oxidation level of the surface.     

青铜-石墨热喷涂层在干摩擦和水润滑条件下的摩擦磨损性能

在MM-200摩擦磨损试验机上研究了青铜-石墨热喷涂层在干摩擦和水润滑条件下的摩擦磨损性能,采用扫描电镜(SEM)对磨损表面形貌进行了观测和采用X射线能谱分析(XPS)分析了涂层成分。结果表明,在水润滑条件下涂层摩擦因数和磨损率均低于干摩擦条件下;在水润滑条件下磨损机制为轻微磨粒磨损和犁削磨损,在干摩擦下主要是较为严重的粘着磨损和犁削。这是由于水润滑降低了摩擦副界面温度,提高了石墨润滑膜的韧性,改善了润滑效果,从而阻止了粘着磨损的发生,水还促进了钢偶件表面致密氧化膜的形成,从而减轻磨损。因此水润滑对涂层磨损性能有较大影响。     

Tribological Behaviors of Different Diamond-Like Carbon Coatings on Nitrided Mild Steel Lubricated With Benzotriazole-Containing Borate Esters

Three synthesized benzotriazole-containing borate esters were separately added into poly-alpha-olefin (PAO) as additives, using molybdenum dithiocarbamate (MoDTC) as the comparison. The friction and wear behavior of Ti-DLC and Ti/Al-DLC coating on nitrided AISI-1045 steel sliding against AISI 52100 steel under the lubrication of PAO containing various additives was evaluated using a reciprocating ball-on-disk friction and wear tester. The morphology and chemical feature of the worn surfaces of the DLC coatings were observed and analyzed using a three dimensional (3D) surface profiler, a scanning electron microscope (SEM), and an X-ray photoelectron spectroscope (XPS). Results show that the three kinds of benzotriazole-containing borate esters as additives in PAO had much better tribological properties than MoDTC; the wear resistance of Ti/Al-DLC coating was better than Ti-DLC coating.     

The tribological characteristics of TiCN coating at elevated temperatures

The tribological behaviour of TiCN coating prepared by unbalanced magnetron sputtering is studied in this work. The substrates made from austenitic steel were coated by TiCN coatings during one deposition. The measurements were provided by high temperature tribometer (pin-on-disc, CSM Instruments) allowing measuring the dependency of friction coefficient on cycles (sliding distance) up to 500 °C. The evolution of the friction coefficient with the cycles was measured under different conditions, such as temperature or sliding speed and the wear rate of the ball and coating were evaluated. The 100Cr6 balls and the Si₃N₄ ceramic balls were used as counter-parts. The former were used at temperatures up to 200 °C, the latter up to 500 °C. The wear tracks were examined by optical methods and SEM. The surface oxidation at elevated temperatures and profile elements composition of the wear track were also measured.The experiments have shown considerable dependency of TiCN tribological parameters on temperature. Rise in temperature increased both friction coefficient and the wear rate of the coating in case of 100Cr6 balls. The main wear mechanism was a mild wear at temperatures up to 200 °C; fracture and delamination were dominating wear mechanisms at temperatures from 300 to 500 °C.     

Ways of improving the tribological properties of steel gas-flame coatings

The paper reports the results of study of the structure and properties of steel gas-flame coatings. The effect of the formation of an abnormally great amount of retained austenite during the deposition of coatings from steel wire is found. Techniques for implementing the “austenite effect” to improve the wear resistance of the coatings are proposed. A method for introducing a solid lubricant into the coating during its deposition is developed. A possibility of enhancing the tribological properties of the coatings for friction members is shown. It is based on thermochemical treatment (nitrocarburizing) and tribomodification of the friction surfaces during running-in with a lubricant containing nanosized modifiers.     

Tribological Properties of Lubricating Films on the Al-Si Alloy Surface via Laser Surface Texturing

Dimpled textures were prepared by using a pulse solid laser on the surface of Al-Si alloy. The combination of laser surface texturing (LST) and MoS₂ solid lubricant as well as their tribological properties were investigated in this article. The obtained friction and wear data were critically analyzed to investigate how the parameters of texture influence the tribological performance of Al-Si alloy. Furthermore, morphological investigations of the transfer layers on the worn surfaces were performed and the wear mechanisms are discussed. The results show that the combination of LST and solid lubricant improves the tribological characteristics of Al-Si alloy. The friction coefficient of Al-Si alloy: steel friction pairs can be reduced to 0.15 under dry friction. The lubrication mechanism is attributed to a synergetic effect of providing solid lubricant and traps wear debris in the dimples. It was found that the optimum density of structure was 37% for excellent tribological properties.     

Tribochemical interactions of Si-doped DLC film against steel in sliding contact

This study concerns the effects of tribochemical interactions at the interface of Si-DLC (silicon-doped diamond-like carbon) film and steel ball in sliding contact on tribological properties of the film. The Si-DLC film was over-coated on pure DLC coating by radio frequency plasma-assisted chemical vapor deposition (r.f. PACVD) with different Si concentration. Friction tests against steel ball using a reciprocating type tribotester were performed in ambient environment. X-Ray photoelectron spectroscopy (XPS) and auger electron spectroscopy (AES) were used to study the chemical characteristics and elemental composition of the films and mating balls after tests. Results showed a darkgray film consisting of carbon, oxygen and silicon on the worn steel ball surface with different thickness. On the contrary, such film was not observed on the surface of the ball slid against pure DLC coating. The oxidation of Si-DLC surface and steel ball was also found at particular regions of contact area. This demonstrates that tribochemical interactions occurred at the contact area of Si-DLC and steel ball during sliding to form a tribofilm (so called transfer film) on the ball specimen. While the pure DLC coating exhibited high coefficient of friction (∼0.06), the Si-DLC film showed a significant lower coefficient of friction (∼0.022) with the presence of tribofilm on mating ball surface. However, the Si-DLC film possesses a very high wear rate in comparison with the pure DLC. It was found that the tribochemical interactions strongly affected tribological properties of the Si-DLC film in sliding against steel.     

Microstructure of PTFE-Based Polymer Blends and Their Tribological Behaviors Under Aqueous Environment

Four polytetrafluoroethylene-based polymer blends (PTFE blends) with polyimide (PI), polyether ether ketone (PEEK), poly(phenyl p-hydroxybenzoate) (PHBA), and perfluoroethylene propylene copolymer (FEP) were prepared by compression molding and follow-up sintering. Their microstructure was observed by scanning electron microscope. And the tribological behaviors of PTFE blends sliding against 316 steel under pure water and sea water lubrication were comparatively evaluated using block-on-ring tribology test rig. The worn surface of counterpart was examined by X-ray photoelectron spectroscopy. The results showed that by blending with the four polymers, PTFE exhibited the transformed microstructure and improved wear resistance. Compared with FEP, rigid polymers PI, PHBA, and PEEK can enhance the wear resistance of PTFE greatly because they can effectively improve the load-carrying capacity of PTFE matrix and can more efficiently prevent the crystalline bands of PTFE from being pulled out. However, because of the weak inhibition on the pulling out of PTFE crystalline bands, FEP cannot enhance the wear resistance of PTFE as significantly as other polymers. In addition, the friction coefficients and wear rates of PTFE and its blends were lower under the lubrication of sea water than under the lubrication of pure water, which was ascribed to more excellent lubricating effect of sea water originating from the deposition of CaCO₃ and Mg(OH)₂ onto the sliding surfaces.     

Tribological investigation of low friction duplex surface treatment based on modified molybdenum disulphide coating

Abstract

One of the numerous possibilities to decrease friction and wear of sliding surfaces consists of the application of two-step (so called duplex) technologies. A novel duplex technology of gas sulphonitriding of a 41CrAlMo7 steel with subsequent deposition of a low friction MoS₂(Ti,W) coating by means of non-reactive magnetron deposition is described in the present paper. A pin and vee block geometry and a relevant Falex apparatus were chosen for implementation of a linear friction contact under heavy loadings. For comparison, five other treatments of machine parts applied in the industry were used in the paper as well. As a result of the tests, only the specimens after the proposed novel duplex treatment based on sulphonitriding and a low friction MoS₂(Ti,W) coating deposition outlasted the full load of 3500 N during 6 min without any failure from all the six different treatments investigated in the work.