Softening and melting mechanisms of polyamides interfering with sliding stability under adhesive conditions

The thermal stability of polymers is a main issue when used as friction elements under dry sliding. Cast polyamide grades processed with either natrium or magnesium catalysors are slid on a small-scale and a large-scale test configuration to reveal the effect of softening or degradation on the sliding stability and to investigate possibilities for extrapolation of friction and wear rates between both testing scales. The combination of softening and afterwards transition into the glassy state is detrimental for the sliding stability of natrium catalysed polyamides, characterised by heavy noise during sliding. A transfer film formed under continuous softening also provides high friction. Melting during initial sliding is necessary for stabilisation in both friction and wear, and eventual softening of a molten film near the end of the test then not deteriorates the sliding stability. Softening of magnesium catalysed polyamides is favourable for the formation of a coherent transfer film resulting in more stable sliding than natrium catalysed polyamides. The differences in softening mechanisms of both polyamide grades is correlated to structural changes investigated by thermal analysis and Raman spectroscopy: the γ crystalline structure prevails in magnesium catalysed samples and the α crystalline structure is predominant in natrium catalysed samples. For internal oil lubricated polyamides, a time dependent degradation of the polyamide bulk deteriorates the supply of internal oil lubricant to the sliding interface, resulting in high friction and wear under overload conditions. As the degradation mechanisms during sliding are strongly correlated to the test set-up, extrapolation is only possible for friction in a limited application range, while wear rates cannot be extrapolated.     

Tribological behavior of pure and graphite‐filled polyimides under atmospheric conditions

As the use of common engineering plastics in tribological systems is limited to low sliding velocities and low loads because of creep and insufficient temperature resistance, there is increasing interest in application of high‐performance polymers such as polyimides, characterized by their ability to maintain favorable mechanical properties up to their melting point. However, for practical design, tribotesting remains necessary for determination of the material's performance under a given contact situation. In this article, two commercially available polyimides are tested at relatively high sliding velocities and contact pressures under atmospheric conditions of temperature and humidity. A consistent overview of tendencies in friction and wear for pure polyimides as a function of applied normal loads and sliding velocities is given. Addition of 15% by weight graphite powder as internal solid lubricant strongly influences friction and wear. Its behavior is compared with pure polyimide grades and differences are discussed in relation with experimental measured bulk‐temperatures. A linear temperature law is derived as a function of pv‐levels and a steady‐state condition is found at different temperature levels, in accordance with thermal conductivity of the polymer bulks. In case of graphite additives, a steady state in temperature coincides with the regime condition of wear rate.     

Calculation and significance of the maximum polymer surface temperature T* in reciprocating cylinder‐on‐plate sliding

Heat generation and surface temperature rise are the main parameters controlling friction and wear of polymers, while certain transitions in sliding behavior often are difficultly related to intrinsic polymer transition temperatures. The sliding temperature cannot be accurately measured through physical limitations in the contact interface, and different calculation methods are available. Some temperature models are reviewed and applied to the reciprocating sliding of polyimide (PI) and polyethylene terephthalate against a steel counterface. The bulk temperature and asperity flash temperature models cannot explain transitions in friction and wear for polymers under reciprocating sliding. The bulk temperature model provides too low temperatures, representing a long‐range temperature, while the asperity flash temperature model provides too high temperatures, not considering visco‐elastic deformation of the polymer surface. An experimental model for the maximum polymer surface temperature T* under reciprocating sliding is developed, considering the environmental temperature, gradual heating of the steel counterface, and additional heating of the polymer surface. The proposed temperature model is analytically validated by differential thermal analysis and thermogravimetric analysis. Transitions in tribological behavior are controlled by the maximum polymer surface temperature T*, coinciding with an endotherm reaction (PIs) or a glass transition and melting (polyethylene terephthalate). POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Role of internal additives in the friction and wear of carbon‐fiber‐reinforced polyimide

Polyimide composites should function in sliding contacts under high temperatures, but the interference of carbon fibers with sliding mechanisms is difficult to predict: they often increase the coefficients of friction and act abrasively but show lubricating properties under other conditions. The friction and wear behavior of thermoplastic polyimides reinforced with short carbon fibers and filled with solid internal lubricant (polytetrafluoroethylene) or silicon oil was investigated in this study with a reciprocating cylinder‐on‐plate tester under 50 N at 0.3 m/s with steel counterfaces that were heated at 23–260°C. We concluded that polytetrafluoroethylene additives effectively reduced the coefficients of friction over the entire temperature range, especially under thermally controlled sliding conditions at 120°C, whereas the internal silicon oil increased the coefficients of friction. The wear rates of the fiber‐reinforced polyimide significantly decreased with respect to those of the thermoplastic polyimide, whereas additional fillers slightly increased the wear rates. We further analyzed the role of internal additives by considering the deformation and maximum polymer surface temperature during sliding. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Influence of solid lubricant reinforcement on wear behavior of Kevlar fabric composites

The friction and wear behavior of Kevlar fabric composites reinforced by PTFE or graphite powders was investigated using a Xuanwu‐III friction and wear tester at dry sliding condition, with the unfilled Kevlar fabric composite as a reference. The worn surfaces were analyzed by means of scanning electron microscope, and X‐ray photoelectron spectroscopy. It was found that PTFE or graphite as fillers could significantly improve the tribological behavior of the Kevlar fabric composites, and the Kevlar fabric composites filled with 20% PTFE exhibited the best antiwear and antifriction ability among all evaluated cases. The transfer films established with two lubricants in sliding wear of composites against metallic counterparts made contributions to reducing friction coefficient and wear rate of Kevlar fabric composites. In particular, FeF₂ generated in the sliding of Kevlar fabric composites filled with PTFE against counterpart pin improved the bonding strength between the transfer film and counterpart surface, which accounted for the lowest friction coefficient and wear rate of the Kevlar fabric composites filled with PTFE measured in the testing. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008.     

Tribological properties of PTFE‐filled thermoplastic polyimide at high load,velocity, and temperature

The effect of 20 wt% polytetrafluoroethylene (PTFE) fillers on the friction and wear properties of thermoplastic polyimides (TP) are investigated, under dry sliding in line contact against steel under 50 to 200 N, 0.3 to 1.2 m/s, and 60 to 260°C. Besides the lubricating mechanisms of PTFE based on mechanical shear, the thermal and tribophysical interactions in the sliding interface are considered in this research by using thermoanalytical measurements, Raman spectroscopy, and calculating the maximum polymer sliding temperature T*. The effect of hydrolysis of the TP bulk material, causing high friction at 100 to 140°C, is covered by PTFE. A transition at pv‐values 2.2 MPa m/s (T* = 120°C) is due to thermally controlled sliding of PTFE, while a transition at pv‐values 3.2 MPa m/s (T* = 180°C) remains controlled by degradation of the TP bulk material into monomer fractions. The reduced coefficient of friction in the presence of PTFE leads to smaller degradation and orientation of the molecular back‐bone and side‐chains within the TP structure. The formation of a homogeneously mixed transfer film is only observed at 180 to 260°C. The PTFE forms a fibrillar structure during wear at high sliding velocities, while they wear as separate particles at high normal loads. POLYM. COMPOS., 2009. © 2009 Society of Plastics Engineers     

The effect of PTFE on the friction and wear behavior of polymers in rolling-sliding contact

The effect of PTFE on the tribological behavior of polymers in rolling sliding contact has been investigated. The two most widely used polymers — nylon 66 and polyacetal—were used as the base material. Tests were conducted over a wide range of running conditions using a twin disc rolling-sliding test rig for both the unfilled materials and for the base materials filled with 20 wt% PTFE. The experimental results showed that the friction and wear performance of the PTFE filled polymers was superior to that of the unfilled polymers. In addition the surface cracking that was found in unfilled PA66 and was thought to be responsible for premature fracture of components such as gear teeth was suppressed by the PTFE. It is suggested that a combination of high surface temperature and high surface tensile stress, produced by friction, is required to initiate these cracks and that PTFE, by reducing friction, inhibits crack formation.     

Effect of several solid lubricants on the mechanical and tribological properties of phenolic resin‐based composites

The flake graphite, polytetrafluoroethylene, and molybdenum disulfide (MoS₂) filled phenolic resin‐based composites were prepared by hot press molding. The thermal, mechanical, and tribological properties of composites were studied systematically. The morphologies of the worn surfaces and the change of chemical compositions during the sliding process of the composites were analyzed by scanning electron microscopy and X‐ray photoelectron spectroscopy, respectively. It was found that the heat‐resisting performance and the hardness of the composites are less affected by solid lubricants, while the solid lubricants did harm to the flexural strength of the composites. The friction and wear behaviors of composites highly depended on the volume fractions of solid lubricants and the sliding conditions. The wear resistance increases and the coefficient of friction decreases when the filler load increases. In addition, the appropriate content of solid lubricants is beneficial to reducing the sensitivities of the composites to load and sliding speed. POLYM. COMPOS., 36:2203–2211, 2015. © 2014 Society of Plastics Engineers     

Effects of the combination of solid lubricants and short carbon fibers on the sliding performance of poly(ether imide) matrix composites

Solid lubricants, that is, graphite flakes and poly(tetrafluoroethylene) powders, were incorporated with short carbon fibers into a poly(ether imide) matrix to improve the tribological performance. Wear tests were performed with a polymer pin against a mild steel counterpart at a constant sliding speed of 1 m/s under various temperatures and contact pressures. Composites filled with equilibrium contents of solid lubricants and short carbon fibers, that is, 10 vol % of each filler, exhibited the lowest wear rate and friction coefficient. The relatively lower concentration of solid lubricants adversely affected the wear resistance, whereas the friction coefficient did not vary significantly in comparison with the friction coefficient of the composites filled with only short carbon fibers. The improved tribological behavior was attributed to more continuous and effective friction films formed on the material pairs during sliding. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 1428–1434, 2004     

Liquid Crystals in Tribology

Two decades ago, the literature dealing with the possible applications of low molar mass liquid crystals, also called monomer liquid crystals (MLCs), only included about 50 references. Today, thousands of papers, conference reports, books or book chapters and patents refer to the study and applications of MLCs as lubricants and lubricant additives and efforts are made to develop new commercial applications. The development of more efficient lubricants is of paramount technological and economic relevance as it is estimated that half the energy consumption is dissipated as friction. MLCs have shown their ability to form ordered boundary layers with good load-carrying capacity and to lower the friction coefficients, wear rates and contact temperature of sliding surfaces, thus contributing to increase the components service life and to save energy. This review includes the use of MLCs in lubrication, and dispersions of MLCs in conventional polymers (PDMLCs). Finally, new lubricating system composed of MLC blends with surfactants, ionic liquids or nanophases are considered.     

Friction and wear behavior of basalt‐fabric‐reinforced/solid‐lubricant‐filled phenolic composites

To improve the tribological properties of basalt‐fabric‐reinforced phenolic composites, solid lubricants of MoS₂ and graphite were incorporated, and the tribological properties of the resulting basalt‐fabric composites were investigated on a model ring‐on‐block test rig under dry sliding conditions. The effects of the filler content, load, and sliding time on the tribological behavior of the basalt‐fabric composites were systematically examined. The morphologies of the worn surfaces and transfer films formed on the counterpart steel rings were analyzed by means of scanning electron microscopy. The experimental results reveal that the incorporation of MoS₂ significantly decreased the friction coefficient, whereas the inclusion of graphite improved the wear resistance remarkably. The results also indicate that the filled basalt‐fabric composites seemed to be more suitable for friction materials serving under higher loads. The transfer films formed on the counterpart surfaces during the friction process made contributions to the reduction of the friction coefficient and wear rate of the basalt‐fabric composites. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Tribological behavior of Kevlar fabric composites filled with nanoparticles

The friction and wear characteristics of ZnO‐ or montmorillonite‐nanoparticle‐filled Kevlar fabric composites with different filler proportions when sliding against stainless steel pins under dry friction conditions were studied, with unfilled Kevlar fabric composites used as references. The worn surface and transfer film of Kevlar fabric composites were then examined with a scanning electron microscope. It was found that ZnO and montmorillonite as fillers could improve the tribological behavior of the Kevlar fabric composites with various applied loads, and the best antiwear property was obtained with the composites containing 5 wt % ZnO or montmorillonite. This indicated that these nanoparticles could prevent the destruction of Kevlar fabric composites during the friction process. The transfer film established by these nanoparticles during the sliding wear of the composites against their metallic counterpart made contributions to reducing the friction coefficient and wear rate of the Kevlar fabric composites measured in the test. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Friction and wear properties of polyimide matrix composites reinforced with short basalt fibers

Short basalt fiber (BF) reinforced polyimide (PI) composites were fabricated by means of compression‐molding technique. The friction and wear properties of the resulting composites sliding against GCr15 steel were investigated on a model ring‐on‐block test rig under dry sliding conditions. The morphologies of the worn surfaces and the transfer films that formed on the counterpart steel rings were analyzed by means of scanning electron microscopy. The influence of the short BF content, load, and sliding speed on the tribological behavior of the PI composites was examined. Experimental results revealed that the low incorporation of BFs could improve the tribological behavior of the PI composites remarkably. The friction coefficient and wear rate decreased with increases in the sliding speed and load, respectively. The transfer film that formed on the counterpart surface during the friction process made contributions to reducing the friction coefficient and wear rate of the BF‐reinforced PI composites. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Tribological behavior of poly(ether ether ketone) composites filled with potassium titanate whiskers sliding in different media

The friction and wear properties of poly (ether ether ketone) (PEEK) composites filled with potassium titanate whiskers (PTWs) under alkali, water, and dry conditions were investigated. The wear mechanisms in different lubrication situations were studied on the basis of examinations of the worn and counterpart surfaces with scanning electron microscopy and optical microscopy. The results showed that PTWs could obviously increase the wear resistance and reduce the friction coefficient of the PEEK composites under dry sliding conditions. Only when the PTW content was greater than 35 wt % did the wear resistance and friction coefficient deteriorate. Sliding in water caused increases in the wear rate and friction coefficient of the PEEK composites, and the PTW‐filled PEEK composites showed the highest friction coefficient and wear rate under this lubrication condition. On the contrary, sliding in an alkaline solution, the PTW‐filled PEEK composites showed the lowest friction coefficient and almost the same level of wear resistance as that found under the dry condition. Furrows and abrasive wear were the main mechanisms for the PTW‐filled PEEK composites sliding in water. The transfer onto the counterpart rings was significantly hindered with sliding under water and alkali conditions. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Effect of Interfacial Layers on Wear Behavior of a Dental Glass-Ceramic

Wear studies on a glass-ceramic dental restorative material were performed under two types of lubrication conditions with distilled water. In one set of experiments, the contact interface was continuously flushed with fresh water to remove the wear debris. In a second set of experiments, the water was not replaced during the testing period. Flushing resulted in a lower friction coefficient and more than twice the wear volume compared with the nonflushing condition. Examination of the worn surfaces in SEM confirmed the presence of an interfacial layer at the sliding interface for the nonflushing experiments. These results suggest that the specific conditions at the sliding interfaces and particularly the nature of interfacial layers must be considered when evaluating the tribological performance of ceramics and especially when the wear data from different studies are compared.     

A synergistic effect of nano‐Tio2 and graphite on the tribological performance of epoxy matrix composites

The influence of incorporated 300 nm TiO₂ (4 vol %), graphite (7 vol %), or combination of both fillers on the tribological performance of an epoxy resin was studied under various sliding load (10–40 N) and velocity conditions (0.2–3.0 m/s). Mechanical measurements indicated that the incorporation of TiO₂ significantly enhanced the flexural and impact strength of the neat epoxy and the graphite including epoxy. Tribological tests were conducted with a cylinder‐on‐flat testing rig. The incorporation of nano‐TiO₂ significantly improved the wear resistance of the neat epoxy under mild sliding conditions; however, this effect was markedly diminished under severe sliding conditions (high velocity and normal load). Nano‐TiO₂ reduced the coefficient of friction only under severe sliding conditions. Graphite showed a beneficial effect in reducing the wear rate and the coefficient of friction of the neat epoxy. Compared to the nano‐TiO₂‐filled epoxy, the graphite‐filled epoxy showed more stable wear performance with the variation of the sliding conditions, especially the normal load. A synergistic effect was found for the combination of nano‐TiO₂ and graphite, which led to the lowest wear rate and coefficient of friction under the whole investigated conditions. The synergistic effect was attributed to the effective transfer films formed on sliding pair surfaces and the reinforcing effect of the nanoparticles. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 2391–2400, 2006     

Polymer–Metal Interfacial Friction Characteristics under Ultrasonic Plasticizing Conditions: A United-Atom Molecular Dynamics Study

Understanding the properties of polymer–metal interfacial friction is critical for accurate prototype design and process control in polymer-based advanced manufacturing. The transient polymer–metal interfacial friction characteristics are investigated using united-atom molecular dynamics in this study, which is under the boundary conditions of single sliding friction (SSF) and reciprocating sliding friction (RSF). It reflects the polymer–metal interaction under the conditions of initial compaction and ultrasonic vibration, so that the heat generation mechanism of ultrasonic plasticization microinjection molding (UPMIM) is explored. The contact mechanics, polymer segment rearrangement, and frictional energy transfer features of polymer–metal interface friction are investigated. The results reveal that, in both SSF and RSF modes, the sliding rate has a considerable impact on the dynamic response of the interfacial friction force, where the amplitude has a response time of about 0.6 ns to the friction. The high frequency movement of the polymer segment caused by dynamic interfacial friction may result in the formation of a new coupled interface. Frictional energy transfer is mainly characterized by dihedral and kinetic energy transitions in polymer chains. Our findings also show that the ultrasonic amplitude has a greater impact on polymer–metal interfacial friction heating than the frequency, as much as it does under ultrasonic plasticizing circumstances on the homogeneous polymer–polymer interface. Even if there are differences in thermophysical properties at the heterointerface, transient heating will still cause heat accumulation at the interface with a temperature difference of around 35 K.     

Impact of fillers and counterface topography on wear behavior of PTFE polymers for ultrasonic motor

Aimed to study the effects of reinforcing or functional fillers on mechanical and tribological properties of PTFE‐based friction materials of ultrasonic motor, carbon fibers reinforced PTFE composites modified with different functional fillers with differences in dimension, size, and hardness are fabricated. The tribological performances of PTFE‐based friction materials are comparatively investigated under different sliding velocities and normal loads on different surface morphologies, respectively. The experimental results reveal that nano‐SiO₂ shows excellent performance in improving friction stabilities and wear resistance in different operating conditions. It is believed the silica‐based tribofilms, higher deformation resistance, and bearing capacity play a key role in improving friction stabilities. Furthermore, the results also show that the surface topography plays an important role in wear properties. The lower wear rate (sliding against with the disordered surface) is believed to be attributed to wear debris easy‐store characteristic of the topography, which promotes transfer films formation and decreases the wear rate effectively. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44835.     

Development of Fully Bio‐Based Lubricants from Agro‐Industrial Residues under Environmentally Friendly Processes

Biolubricants are becoming interesting alternatives to mineral lubricants. Despite their advantages, development of lubricants from vegetable oils may compete with food production, turning their use impractical due to socio‐economic aspects. Here, cardanol is used as raw material in the synthesis of novel biolubricants under environmentally friendly conditions. These compounds are characterized by NMR and Fourier transform infrared spectroscopy. Thermal‐oxidative studies show the elevated stability and higher onset oxidative temperatures. Tribological analyses performed under high‐frequency linear‐oscillation motion indicate lower coefficients of friction and wear rates compared to a synthetic oil. Therefore, cardanol, under simple, fast, and sustainable processes can be transformed into valuable alternatives for petroleum‐based lubricants. Practical Applications: The novel cardanol‐based biolubricants presented in this work show interesting chemical and lubricity properties superior to standard synthetic oils, which make them potential substitutes for the current oil‐based products used as lubricants. Additionally, the ecofriendly methodologies employed reduce significantly the reaction time as well as eliminating the need for catalysts or solvents, making these processes viable alternatives for the traditional methods reported in literature for the synthesis of this class of compounds.     

纳米氟化镧对生物质燃油碳烟颗粒流润滑特性影响

为探索极端乏油工况下碳烟颗粒在发动机摩擦副界面的功效及润滑油添加剂的作用,本文采用往复摩擦磨损试验机研究了纳米氟化镧（nano-LaF₃）颗粒对往复滑动条件下生物质燃油碳烟（BS）颗粒流润滑性能的影响。借助拉曼光谱仪、3D激光扫描显微镜、X射线光电子能谱仪等仪器探讨了滑动条件下nano-LaF₃对BS颗粒流润滑特性的影响作用机理。结果表明：当nano-LaF₃的添加质量分数大于20%时能显著改善往复滑动条件下BS颗粒的抗磨减摩性能,并且随着nano-LaF₃添加浓度的增大,摩擦表面碳烟石墨化程度和石墨微晶尺寸均增大。nano-LaF₃在含BS颗粒流润滑的摩擦界面形成了LaF₃摩擦膜以及含镧化合物、碳氧化合物及铁氧化合物的化学复合反应膜,同时nano-LaF₃会加大诱导碳烟的石墨化作用,从而增强了BS颗粒流润滑的减摩性。