Tribo-Rheometry: From Gap-Dependent Rheology to Tribology

We describe a new tribo-rheometry fixture that can be utilized with a commercial torsional rheometer in order to explore the coupled rheological and tribological properties of complex fluids and solid–liquid systems. The fixture is self-leveling and both the normal load and the sample gap can be monitored or controlled. At large gaps, the fixture imposes an approximately constant shear rate on the sample and the bulk viscometric properties of the fluid can be measured. However, as the gap between the plates is reduced, the measured viscosity function becomes gap-dependent. For gaps on the order of the surface roughness of the plates, the device is operated under a constant applied load and the tribological properties of the fluid–solid pair can then be measured. Using this new tribo-rheometer fixture it is possible to obtain tribological information over a wider range of sliding velocities than is typically possible using conventional devices such as pin-and-disk systems. The data can be represented in the form of a classical Stribeck diagram or, by using a dimensionless gap-dependent shift factor, it is possible to construct a more general friction map of the gap- and load-dependent effective viscosity (Luengo et al. Wear 200 (1996)). The capabilities of this system are illustrated using a number of different lubricant fluids, for a range of normal stresses and variations in surface properties such as the mean roughness.     

Relationship Between Ultralow Friction of Mesogenic-Like Fluids and Their Lateral Chain Length

Mesogenic fluids (MFs) may have a great potential for technical applications to increase the energy efficiency and to prevent wear. Aim of the present work was to study a homologous series of mesogenic-like fluids to evaluate the influence of the chemical structure on the tribological behavior. Rheological measurements were additionally performed to correlate the tribological properties with the viscosity and flow behavior. Raman spectroscopy and differential scanning calorimetry measurements were used to characterize the MFs. Furthermore, the surface topography of the wear scar was studied by profilometry. The results were summarized and hence an approach was made to describe the several mechanisms which may lead to the observed tribological results. This work shows that a better understanding of the tribological behavior of these MFs is essential to develop new lubricants.     

化学改质蓖麻油的流变性能和摩擦学特性研究

通过对天然蓖麻油进行化学改质，利用异构化反应增加分子侧碳链的长度，以提高其粘度指数和降低其倾点；考察了改质蓖麻油的流变性能和摩擦化学性能。实验结果表明，天然蓖麻油经化学改质后流变性能得到了较大改进，其抗磨减摩性能优于同粘度的矿物油，相当于季戊四醇酯或癸二酸二异辛酯。     

Conductive and Tribological Properties of Lithium-Based Ionic Liquids as Grease Base Oil

This article reports several conductive greases prepared by ionic liquids (ILs) that are synthesized by mixing lithium tetrafluoroborate (LiBF₄) or lithium bis(trifluoromethane-sulfonyl) imide (LiNTf₂) in diglyme (G2) and tetraglyme (G4) with appropriate weight ratios at room temperature (RT). The ILs have good solution in poly(ethylene glycol-ran-propylene glycol) monobutyl ether (PAG) and thus can be used as a base oil for preparing grease for steel–steel contacts. The electrical conductive properties of the grease prepared with the mixed oil of PAG plus ILs were evaluated using the DDSJ-308A conductivity tester, GEST-121 volume surface resistance tester, and HLY-200A circuit resistance tester. Combining the free volume with viscosity, the conductivity is inversely proportional to viscosity. The tribological properties were investigated using an MFT-R4000 reciprocating friction and wear tester. The results demonstrated that the prepared greases possess better conductive and tribological properties than the commercial grease with Cu powder as an additive.     

Monitoring of Transient Phenomena in Sliding Contact Application to Friction Brakes

This work focuses on the study of transient phenomena, in particular the non-uniformity and space–time variation of friction forces and surface temperature of a brake disc during stop-braking. Friction tests were conducted on a braking tribometer. The friction forces in the contact were measured using a 3D piezoelectric sensor, while the disc surface temperature was investigated by means of a high frequency fibre-optic two-colour pyrometer. An optical lap-top device was used to keep track of disc revolutions, and an original programme was written to plot the space–time variations of the measured parameters. This new original approach helps better understand the coupling between thermal and tribological phenomena occurring during braking.     

The tribological properties of several silahydrocarbons for use in space mechanisms

Silahydrocarbons are members of a relatively new class of liquid lubricants with great potential for use in space mechanisms. They are unimolecular species consisting of silicon, carbon, and hydrogen. They possess unique wear, viscosity, and volatility properties while retaining the ability to solubilise conventional additives. The tribological properties of several members of this class, including tri‐, tetra‐, and penta‐compounds, are presented. These properties include viscosity‐temperature, viscosity—pressure, vapour pressure, lubricant life, traction, and reciprocating and four‐ball wear rates. Lubricant lifetimes were determined using a vacuum ball bearing simulator, the spiral orbit tribometer. Wear was measured using a Cameron Plint reciprocating tribometer and wear rates with a vacuum four‐ball tribometer. Conventional viscometry was used for viscosity—temperature measurements and a Knudsen cell for vapour pressure. Thermogravimetric analysis was also used for volatility measurements. Pressure—viscosity coefficients (α—values) were estimated from elastohydrodynamic lubrication film thickness measurements. These properties are compared to those of existing state‐of‐the‐art space lubricants.     

Traction Characteristics of Siloxanes with Aryl and Cyclohexyl Branches

The molecular structures, rheological properties, and friction coefficients of several new siloxane-based polymers were studied to explore their traction characteristics. The molecular structures including branch content were established by nuclear magnetic resonance spectroscopy, while the molecular mass distributions were determined by gel permeation chromatography. Density, viscosity, elastohydrodynamic film formation, and friction were investigated over a temperature range of 303–398 K. Film thickness and friction measurements were studied under the conditions that are representative of boundary, mixed, and full-film lubrication regimes, aiming at maximizing traction performance and temperature stability by simultaneous optimization of the size and content of ring-shaped branch structures. This study provides quantitative insight into the effect of siloxane molecular structure on the tribological performance for traction drive applications such as continuously variable transmissions.     

Indirect monitoring of the dynamics at the contact interfaces of a pair of abrading titanium specimens

The novel method of using continuous tribo-electrification variations to monitor the dynamic tribological properties between metal films has been applied successfully [Y.P. Chang, A novel method of using continuous tribo-electrification variations for monitoring the tribological properties between the pure metal films, Wear, 262 (2007) 411–423]. The method was shown to produce clear and strong signals, superior to monitoring continuous friction coefficient variations. However, the above method was only shown to be suitable for the tested material pairs that were studied. In this paper, the method was improved and applied to monitoring the dynamic tribological properties between titanium oxide (TiO₂) films in the friction process. The experiment was conducted on a purposed-designed friction tester with a suitable measuring system. In order to investigate the tribological property of titanium (Ti) sliding against Ti with TiO₂ films in detail, the continuous variations of electrical contact resistance and friction coefficient were measured for monitoring the onset of film rupture between the TiO₂ films and the chemical reactions between the interfaces. Wear loss was measured by an accuracy balance and scan electron microscopy was used to observe the microstructures and material transfer. The experiments demonstrated that the novel method of using electrical contact resistance variations has great potential for monitoring the dynamic tribological properties and the chemical reactions of titanium specimens.     

Research on the physicochemical and tribological properties of nano-TiO2 in the aqueous rolling liquid

Aqueous rolling liquids with different concentrations of TiO₂ nano-particles are prepared. The dispersing mechanism of nano-TiO₂ in the rolling liquid is studied by FTIR and TEM. Wettability and viscosity of these rolling liquids is evaluated based on Dataphysics OCA50 contact angle measuring device and SYD-265C viscometer, respectively. New viscosity empirical formula that suitable for nano-TiO₂ aqueous rolling liquid is proposed. The tribological property is also investigated by the MM-WIA universal tribological tester. The results indicate that with the increase in the nano-particle’s addition, the contact angle gradually decreases, whereas the viscosity rises, which would contribute to ameliorating the frictional behaviour of aqueous suspensions. The aqueous suspension with 0.7 wt% TiO₂ possesses excellent friction reduction and anti-wear properties.     

合成基润滑油聚α-烯烃低温摩擦润滑性能研究

在低温条件下,润滑油黏度变大,流动性变差,添加剂活性降低,对润滑性能产生显著影响。为研究PAO润滑油的低温摩擦润滑性能,以不同黏度级别PAO基础油为研究对象,采用流变仪MCR302、SRV摩擦磨损试验机,研究PAO润滑油样及添加极压抗磨添加剂的油样在低温条件下的流变性能和磨损润滑性能。试验结果表明：在低温环境下,随着温度降低,PAO油样的黏度急剧增大,黏度越大的油样其受低温条件影响越明显;PAO油样在低温环境下,表现出明显的剪切稀化现象;低温环境使得极压抗磨剂添加剂的活性变低,添加剂并未表现出减摩抗磨作用。因此,低温试验条件对PAO基础油和添加剂的摩擦学性能产生显著影响,阻碍了基础油和润滑油添加剂减摩抗磨作用的发挥。     

Analytical model for predicting friction in line contacts

This paper presents the development of an analytical model for the prediction of the friction coefficient in line contacts under thermal elastohydrodynamic lubrication (TEHL). A new theoretical equation is deduced for determining the friction coefficient, taking into account the rheology of common lubricants under TEHL. This approach also considers the heat generated and its penetration into the bulk of the contacting solids. Therefore, the increase in temperature and ensuing variations in the operating conditions are determined. In order to illustrate the use of the new model and verify its accuracy, an experimental stage is performed in a tribological test rig. The predictions of the proposed model are compared with the results obtained in the test rig and other data reported in the literature for diverse lubricants, showing a good agreement in every case. © 2015 The Authors Lubrication Science published by John Wiley & Sons Ltd.     

Tribological thermodynamic model of the viscosity,flow properties and lubricating performance of lubricating oils

Lubricating phenomena are described thermodynamically. Tribological parameters, in particular pressure and relative sliding speed, are related to thermodynamic properties of the tribological system such as thermodynamic potential, enthalpy and entropy. On this basis tribological thermodynamic models of the viscosity, flow properties and lubricating performance of materials are deduced. Changes of lubricating performance in a tribological system are determined by considering such tribological thermodynamic parameters. A tribological thermodynamic model of lubricating phenomena is therefore obtained which with the use of suitable thermodynamic data for optimum tribological parameters allows the prediction of the lubricating performance of tribological systems.     

抗磨添加剂对不同服役阶段润滑油摩擦学性能的影响

采用黏度测试仪测定新油及3种不同服役阶段润滑油的黏度,采用UMT-II摩擦磨损试验机考察其摩擦学性能,并同时考察3种在用润滑油添加抗磨添加剂后的摩擦学性能。研究结果表明:润滑油的黏度随着运行里程数的增加呈现先降后增的趋势;随润滑油运行里程数的增加,润滑油的摩擦因数增大,导致试验钢球的磨损量也增加;抗磨添加剂对不同服役阶段的润滑油的抗磨性能影响程度不同,在磨合磨损期和正常磨损期,加入抗磨添加剂后并不能改善润滑油的抗磨性能,而在异常磨损期,抗磨添加剂的加入可较好地改善润滑油的抗磨性能。     

Tribological characteristics of self-assembled monolayer with siloxane bonding to Si surface

The friction-reducing performance of adsorbed organic molecules on a solid surface is well known. To understand the tribological performance of organic molecules adsorbed on the surface, a well-defined molecular film on a very smooth Si wafer was used. This study examined the effect of the number of expected siloxane bonds of self-assembled monolayers (SAMs), as well as the effect of SAM alkyl chain length, on the tribological performance of SAMs. The performance of the SAMs used in this study corresponded with our observations of their molecular density on the surface and the degree of molecular orientation. Slight differences in these characteristics resulted in no significant differences in the SAMs’ tribological characteristics under a mild loading condition. However, the friction experiment under a severe condition apparently produced significant differences in tribological behavior. To confirm the effect of SAM molecular orientation on the tribological characteristics, the effect of temperature on OTS-SAM behavior was studied. The effect of molecular orientation on the tribological characteristics by changing temperature under the severe condition was also significant.     

Synthetic Lubricants in Hydrodynamic Journal Bearings: Experimental Results

Synthetic lubricants and additives have seen many major improvements in recent years. However, very little is known about the performance peculiarities of these new lubricants in actual machines. To fill this gap, a new full-scale hydro-dynamic journal bearing test rig has been constructed to evaluate the behavior of conventional and new bearing designs, synthetic lubricants and variations in operating parameters. This test rig’s bearing has diameter 180 mm with measuring capabilities including continuous film thickness and film pressure as well as temperature. The new machine was used to compare a number of synthetic lubricants to mineral based lubricants, finding that performance of the synthetic lubricants was superior to their mineral based counterparts of much higher viscosity grade. These tests showed that high viscosity index (VI) synthetic lubricants had higher viscosity in the region of maximum pressure and lower viscosity elsewhere in the bearing than similar mineral based lubricants. This reduction in viscosity in low pressure zones was found to produce a measurable reduction in friction and power loss in the bearing system. This paper provides comparative performance results of several formulations of current and future turbine oils from measurements of losses, oil film thickness, and temperature under a range of operating parameters. Lubricants tested include ISO VG68 and VG32 mineral based turbine oils (VG68 and VG32), ISO VG32 synthetic ester based oil (SE32), two ISO VG22 synthetic ester based oils (SE22 and SV22), and ISO VG15 synthetic ester based oil (SE15). It was found that SE32 and VG68 provided similar performance at lower speeds while SV22 provided similar performance to VG68 at the highest speed. Likewise, SE22 and SV22 provided similar performance to VG32 at low speeds while SE15 provided similar performance to VG32 at medium to higher speeds. Generally, the results demonstrate the potential for replacing mineral based lubricants with high performance synthetic lubricants of significantly lower viscosity grade without sacrificing bearing safety.     

以汽油和甲醇为燃料时小型二冲程发动机摩擦特性比较

采用往复振动机模拟小型二冲程发动机运转工况，实验研究汽油和甲醇为燃料时发动机气缸和活塞环间的摩擦特性，并比较分别使用润滑油新油、润滑油老化油、润滑油新油和老化油的混合油作为润滑油时气缸和活塞环间的摩擦特性。结果表明，以甲醇为燃料时的摩擦因数和磨损量均小于以汽油为燃料时的摩擦因数和磨损量，特别是使用添加了润滑油新油的燃料时的摩擦因数和磨损量最小。通过黏度和热重（TG）分析，探讨甲醇燃料改善气缸和活塞环间的摩擦特性的原因，结果表明，甲醇燃料具有较高的黏度和较低的摩擦因数，因而以甲醇为燃料时可以降低磨损     

Effect of saliva viscosity on tribological behaviour of tooth enamel

Employing a special device simulating conditions of the masticatory process, two series tests were conducted involving studies of the tribological behaviour of enamel subjected to two- and three-body tests at different saliva viscosities.The studies showed no significant influences of saliva viscosity on the mean of the friction coefficient. However, below 2.08 mPa s the coefficient was unstable, i.e. scatter was observed. Both higher wear rate and higher scatter were observed for viscosities below of 1.68 mPa s. Comparing the wear results of the three- and two-body tests, no significant differences were obtained for viscosities above 1.68 mPa s, although for lower viscosities the wear rate in the presence of abrasive particles was higher than that obtained with pure saliva.Two regions with different tribological behaviour of enamel were observed in relation to saliva viscosity; the region with stable and low values of the tribological parameters beyond some threshold value, and the other with higher mean values of tribological parameters and their scattering. The author suggests that different concentrations of proteins are possible at the same low saliva viscosity, thus there are qualitatively different interactions between enamel surface and saliva components resulting in different lubrication regimes.     

Viscoelasticity and solution viscosity of perfluoropolyether lubricants

Viscoelastic characteristics including the viscosity, storage modulus, and loss modulus of perfluoropolyether (PFPE) lubricants with reactive endgroups are examined via a rotational rheometer. Both temperature and molecular weight effects on rheological properties are extensively analyzed via the Rouse theory in polymer dynamics. The modified Cole–Cole plots for storage modulus vs. loss modulus exhibits the presence of microstructures due to the interaction among PFPE endgroups. To examine the solvent effect during the dip-coating process, we examined the intrinsic viscosity, or Huggins coefficient for dilute PFPE solutions, which depends on the aspect ratio of PFPE cluster and potential energy among PFPE molecules. We also correlated the viscoelastic data with the tribological data.     

Rheological and Tribological Characterization of a New Acylated Chitosan–Based Biodegradable Lubricating Grease: A Comparative Study with Traditional Lithium and Calcium Greases

This work compares the thermal, rheological, and tribological properties of a new gel-like biodegradable formulation, prepared using an acylated chitosan thickener and castor oil, with properties exhibited by two conventional greases thickened with lithium and calcium soaps, respectively, taken as benchmarks. Thermogravimetric (TGA), rheological (small-amplitude oscillatory shear [SAOS], rheodestruction, and viscous flow) and tribological (friction and wear analysis) tests, as well as roll-stability measurements were carried out to characterize the three grease samples. In addition, infrared spectroscopy and differential scanning calorimetry (DSC) were used to chemically characterize the acylated chitosan thickener agent. From a thermogravimetric point of view, the new formulation displayed better thermal resistance than the calcium and lithium lubricating greases. The evolution of the linear viscoelasticity functions with frequency and viscosity values in the shear rate and temperature ranges studied were similar to those obtained with the commercial lubricating greases. However, the linear viscoelasticity functions of the biodegradable formulation were slightly more affected by temperature. The mechanical stability behavior and recovery of the rheological functions found in the biodegradable formulation were also better than that exhibited by the calcium-based grease. However, the friction coefficient measured at low rotational speed is slightly higher than that obtained with the benchmarks, with similar or lower values obtained at a high rotational speed. Resulting wear marks obtained after the frictional tests using the acylated chitosan–based grease were larger than those obtained with the commercial greases.