羰基铁磁流变液的摩擦性能研究

采用四球摩擦磨损试验机,测定了羰基铁磁流变液及其基础油的摩擦系数,磨斑直径,记录了摩擦系数随时间的变化曲线,考察了磁流变液中基础油类型和添加剂对其摩擦性能的影响,用扫描电子显微镜观察了钢球磨痕表面形貌,分析了摩擦磨损机理.结果表明:与基础油相比,同等试验条件下磁流变液的摩擦系数、磨斑直径增大 1～2 倍.基础油类型、触变剂以及抗磨减摩添加剂对磁流变液摩擦学性能影响显著,选取合适的种类可在一定程度内改善其摩擦性能.磁流变液的摩擦以微观切削为主,主要由磁性颗粒产生,随着载荷的增大,摩擦加剧,摩擦磨损机理也由微观切削向微切削与黏着磨损相结合过渡.     

碳含量对碳素钢磁场摩擦磨损性能的影响与作用机制研究

基于自制的HY-100销-盘式磁场摩擦磨损试验机，对比研究了有无外加磁场条件下，低碳钢、中碳钢和高碳钢的滑动干摩擦行为和其磨损特性，并使用扫描电子显微镜、金相显微镜等分析了摩擦微观表面与次表面，探讨了碳含量对碳素钢磁场摩擦的影响规律和作用机理. 试验结果显示:根据碳含量的不同，有无磁场条件下碳素钢摩擦系数的变化也不同，在外加磁场条件下，低碳钢摩擦系数降低，中碳、高碳钢摩擦系数升高；随着碳含量升高，磁场对其摩擦系数的影响越小. 外加磁场可以有效改善碳素钢的磨损性能，随着碳含量升高，磁场对其磨损率的改善程度越大. 分析其原因可能为铁碳竞争氧化，这既保证了铁氧化物的减磨效应，也减缓了铁氧化物积累后的剥落. EDS能谱分析结果表明高碳钢的氧铁比最低，和铁碳氧化竞争模型推论相一致.      

磁场作用下基础油和含磷酸三甲酚酯润滑油的摩擦磨损特性

在四球摩擦磨损试验机的摩擦区域外加磁场,考察了150SN基础油和添加磷酸三甲酚酯(TCP)润滑油在磁场作用下的摩擦磨损性能,用扫描电子显微镜(SEM)和X射线光电子能谱仪(XPS)分别分析了磨斑表面形貌及典型元素的化学状态,并对摩擦学机理进行了初步探讨.摩擦学试验结果表明:在磁场作用下,基础油和含TCP润滑油中钢球的磨斑直径均比无磁场时小,而两种油样的摩擦系数均比无磁场时大.XPS分析表明:磁场对润滑油摩擦学性能的影响与边界润滑膜的性质有关,磁场有利于TCP中P和O元素与金属表面的键合,促进了金属表面摩擦化学反应膜的形成,增强了含TCP润滑油的抗磨性能.     

中碳钢/不锈钢磁场摩擦中磨屑的行为和作用

以45钢销/302不锈钢盘摩擦副为研究对象，采用自制的销、盘摩擦磨损试验机，研究了直流磁场作用下磨屑在摩擦过程中的行为及其对摩擦磨损性能的作用. 为此分析了有、无磁场作用下磨屑在磨损面上的分布特点，利用扫描电镜观察了磨屑及45钢销磨损面的形貌，采用三维形貌仪表征了磨损面特征区域的相对高度. 与无磁场时的摩擦磨损情况相比，磁场作用下45钢销的磨损量有所增大，而摩擦系数稍有减小. 摩擦过程中出现了302不锈钢盘向45钢销的材料转移并形成了不连续的转移层，该转移层相对高度较大，承担了主要的摩擦磨损并趋于平滑. 磁场作用下45钢销磨损面吸附少量磨屑并使之细化和氧化，该吸附磨屑在一定程度上减小了摩擦副的摩擦系数，并阻碍试样之间的材料转移，从而增加了45钢销的磨损量.      

外加磁场对W-20Cu/45钢摩擦副干摩擦特性影响的实验研究

以W-20Cu销/45钢环摩擦副为研究对象,采用改进后的MPV-1500型摩擦磨损试验机,研究了直流磁场对W-20Cu干滑动摩擦磨损特性的影响.利用扫描电镜观察W-20Cu销磨损后的表面形貌、纵切面组织及磨屑形貌,采用三维形貌仪表征了磨损表面粗糙度.结果表明:随着磁场的施加和增大,W-20Cu销和45钢环的磨损率及摩擦系数均有减小趋势.磁场不仅降低了W-20Cu销摩擦面亚表层的变形程度,而且能吸附磨屑并使之细化.无外加磁场时,磨损机制主要为磨粒磨损;施加磁场后,W-20Cu销摩擦面趋于平滑,磨损机制变为磨粒磨损和氧化磨损的混合磨损.     

新型C/C-Cu复合滑动导电材料电摩擦磨损行为研究

采用无压熔渗工艺制备一种新型的具有自润滑耐磨性能的炭纤维整体织物/炭-铜(C/C-Cu)复合材料,在改装的MM-2000型环-块摩擦磨损试验机上考察其载流摩擦磨损性能,探讨外加直流电对摩擦磨损行为的影响并分析其作用机理.结果表明:电流方向决定了复合材料摩擦面磨屑层存在与否,对摩擦系数有较大影响.正接条件下复合材料的摩擦系数和磨损率高于负接条件下.电流强度对C/C-Cu复合材料磨损率影响较为显著.随电流强度增大,磨损率增大,磨损机制由犁削磨损向黏着磨损转变.摩擦系数随电流强度的增大先升高后降低.     

芳纶纤维织物摩擦磨损性能的研究

用ＭＭ－２００型摩擦磨损试验机考察了不同条件下芳纶纤维织物的摩擦学性能,并用扫描电子显微镜和Ｘ射线衍射仪对纤维织物的磨损表面、磨屑形貌及结晶性进行了观察和分析．结果表明：随着负荷的增大,织物的摩擦系数略有降低,磨损呈上升趋势;速度对织物的摩擦学性能影响不大;纤维束的挤压变形和磨屑的填充作用导致磨损表面被压实并变得光滑,从而使真实接触面积增大,摩擦条件改善．２５０℃处理后织物中的纤维强度降低,因而磨损增大．     

CVD TiC—TiN复合涂层球／Pb基固体润滑盘摩擦学性能的研究

本文对球(包括CVD TiC-TiN复合涂层球和GCr15钢球)盘(包括纯Pb与Pb-Sn-Cu电刷镀层盘)滑动摩擦副进行了摩擦与磨损试验。结果表明,真空下的摩擦系数比大气下的低,涂层球的摩擦系数比钢球的低,Pb-Sn-Cu涂层盘的摩擦系数比Pb涂层盘的低。磨损表面的扫描电镜观测发现,在CVD涂层破裂区的后沿形成了固体润滑转移膜,这有助于降低摩擦和磨损。磨损表面的俄歇电子能谱分析表明,在油润滑及干摩擦下,盘的磨痕处仍有可起润滑作用的固体润滑膜存在。这种膜含有Fe元素,表明它是摩擦表面作用后产生的。     

含纳微米硼酸盐和二烷基二硫代磷酸锌液体石蜡润滑下钢—钢体系的摩擦磨损性能研究

在四球试验机上考察了含纳微米硼酸盐及二烷基二硫代磷酸锌（ZDDP）复配添加剂的液体石蜡润滑下钢－钢摩擦副的摩擦学性能。采用X射线光电子能谱仪和扫描电子显微镜分析了复配体系的作用机理。结果表明：纳微米硼酸盐／ZDDP复配添加剂对钢－钢摩擦副的抗磨作用产生对抗效应，在摩擦过程中的某一阶段摩擦系数突然升高，磨损加剧；在试验初期，磨斑表面较为光滑，相应的边界润滑膜为物理和化学吸附膜；随着试验时间的延长，钢球磨斑表面吸附膜表面破裂，磨斑表面变得粗糙并形成微小磨屑碎片，相应的摩擦系数突然升高；随着试验时间的进一步延长，添加剂同钢球磨损表面发生摩擦化学反应，并生成含B、N、S和P等元素的摩擦化学反应膜，从而使摩擦系数波动减小。     

丝径对316L不锈钢丝摩擦磨损行为的影响

为探究金属橡胶微丝的最适直径,研究了不同载荷和速度条件下,金属橡胶不锈钢丝丝径对其小位移摩擦磨损行为影响的规律及机理,建立了磨损深度与丝径之间的定量关系来评定丝径对不锈钢丝摩擦磨损行为的影响. 结果表明:相同载荷、速度条件下,不同丝径实际接触面积的不同导致不锈钢丝的磨损深度随其丝径的增大而减小,且磨损深度随丝径的变化规律呈多项式曲线规律;而摩擦系数与其实际接触形貌和磨屑运动状态有关,不同的磨损状态导致了摩擦系数随丝径的增大而增大;探究表明改变载荷和速度并不影响丝径对不锈钢丝摩擦磨损行为的影响规律;但由于粗丝径试件间实际接触面积的稳定性,使得载荷和速度对粗丝试件的磨损深度、摩擦系数的影响要明显小于对细丝试件的影响.      

Mathematical modeling of magnetorheological fluids

Magnetorheological (MR) fluids are a class of smart materials whose rheological properties may be rapidly modified by the application of a magnetic field. These materials typically consist of micron-sized ferrous particles dispersed in a fluid. In the present paper, we consider the full system of equations as well as the Clausius-Duhem inequality for moving isotropic MR fluids in an electro-magnetic field. We present the material constitutive relations for a non-Newtonian incompressible MR fluid. To illustrate the validity of the constitutive relations, the flow of a MR fluid between two parallel fixed plates under the influence of a constant magnetic field perpendicular to the flow direction is considered.Received: 14 July 2003, Accepted: 18 May 2004, Published online: 22 February 2005 Correspondence to: A. Dorfmann     

High-flux magnetorheology at elevated temperatures

Commercial applications of magnetorheological (MR) fluids often require operation at elevated temperatures as a result of surrounding environmental conditions or intense localized viscous heating. Previous experimental investigations of thermal effects on MR fluids have reported significant reductions in the magnetorheological stress with increasing temperature, exceeding the predictions made by considering the thermal variations in the individual physical properties of the fluid and solid constituents of a typical MR fluid. In the low-flux regime, designers of MR fluid actuators can alleviate this thermal reduction in stress by increasing the applied magnetic field strength. However, this is not possible in the high-flux regime because of magnetic saturation, and it becomes necessary to explore and understand the intrinsic limitations of the fluid at elevated temperature. We describe a new magnetorheological fixture, which was designed as an accessory to a commercial torsional shear rheometer, capable of applying magnetic flux densities up to 1 T and controlling the sample temperature up to 150°C. During the design of the instrument, close attention was given to the uniformity of the magnetic field applied to the sample by using numerical simulations. Incorporation of a custom-built magnetic flux sensor which matches the environmental capabilities of the fixture enables in situ measurement of the local magnetic field at each temperature. The numerical results are also validated by spatially resolved measurements of the local magnetic field. Finally, we explore the ability of a shift factor between fluid magnetization and yield strength to describe the measured variation in the MR fluid response at elevated temperatures.     

Nonlinear viscoelastic properties of MR fluids under large-amplitude-oscillatory-shear

Nonlinear viscoelastic properties of the MR fluid, MRF-132LD, under large-amplitude oscillatory shear were investigated. This was accomplished by carrying out the experiments under the amplitude sweep mode and the frequency sweep mode, using a rheometer with parallel-plate geometry. Investigations under the influence of various magnetic field strength and temperatures were also conducted. MR fluids behave as nonlinear viscoelastic or viscoplastic materials when they are subjected to large-amplitude shear, where the storage modulus decreases rapidly with increasing strain amplitude. Hence, MR fluid behaviour ranges from predominantly elastic at small strain amplitudes to viscous at high strain amplitudes. Large-amplitude oscillatory shear measurements with frequency sweep mode reveal that the storage modulus is independent of oscillation frequency and approaches plateau values at low frequencies. With increasing frequency, the storage modulus shows a decreasing trend before increasing again. This trend may be explained by micro-structural variation. In addition, the storage modulus increases gradually with increasing field strength but it shows a slightly decreasing trend with temperature.     

Study of magnetorheological fluids at high shear rates

The tunable rheological properties of magnetorheological (MR) materials at high shear rates are studied using a piston-driven flow-mode-type rheometer. The proposed method provides measurement of the apparent viscosity and yield stress of MR fluids for a shear rate range of 50 to 40,000 s⁻¹. The rheological properties of a commercial MR fluid, as well as a newly developed MR polymeric gel, and a ferrofluid-based MR fluid are investigated. The results for apparent viscosity and dynamic and static shear stresses under different applied magnetic fields are reported.     

Rheological properties and dispersion stability of magnetorheological (MR) suspensions

In the present article, the rheological responses and dispersion stability of magnetorheological (MR) fluids were investigated experimentally. Suspensions of magnetite and carbonyl iron particles were prepared as model MR fluids. Under an external magnetic field (H ₀) and a steady shear flow, the yield stress depends upon H ₀ ^3/2. The Yield stress depended on the volume fraction of the particle (φ) linearly only at low concentration and increased faster at high fraction. Rheological behavior of MR fluids subjected to a small-strain oscillatory shear flow was investigated as a function of the strain amplitude, frequency, and the external magnetic field. In order to improve the stability of MR fluid, ferromagnetic Co-γ-Fe₂O₃ and CrO₂ particles were added as the stabilizing and thickening agent in the carbonyl iron suspension. Such needle-like particles seem to play a role in the steric repulsion between the relatively large carbonyl iron particles, resulting in improved stability against rapid sedimentation of dense iron particles. Furthermore, the additive-containing MR suspensions exhibited larger yield stress, especially at higher magnetic field strength. Received: 4 April 2000 Accepted: 6 November 2000     

Static yield stress of ferrofluid-based magnetorheological fluids

In this work, the yield stress of ferrofluid-based magnetorheological fluids (F-MRF) was investigated. The fluids are composed of a ferrofluid as the liquid carrier and micro-sized iron particles as magnetic particles. The physical and magnetorheological properties of the F-MRF have been investigated and compared with a commercial mineral oil-based MR fluid. With the addition of a ferrofluid, the anti-sedimentation property of the commercial MR fluids could be significantly improved. The static yield stress of the F-MRF samples with four different weight fractions (ϕ) of micro-sized iron particles were measured using three different testing modes under various magnetic fields. The effects of weight fraction, magnetic strength, and test mode on the yielding stress have been systematically studied. Finally, a scaling relation, , was proposed for the yield stress modeling of the F-MRF system.     

Magnetorheology in an aging, yield stress matrix fluid

Field-induced static and dynamic yield stresses are explored for magnetorheological (MR) suspensions in an aging, yield stress matrix fluid composed of an aqueous dispersion of Laponite? clay. Using a custom-built magnetorheometry fixture, the MR response is studied for magnetic field strengths up to 1?T and magnetic particle concentrations up to 30?v%. The yield stress of the matrix fluid, which serves to inhibit sedimentation of dispersed carbonyl iron magnetic microparticles, is found to have a negligible effect on the field-induced static yield stress for sufficient applied fields, and good agreement is observed between field-induced static and dynamic yield stresses for all but the lowest field strengths and particle concentrations. These results, which generally imply a dominance of inter-particle dipolar interactions over the matrix fluid yield stress, are analyzed by considering a dimensionless magnetic yield parameter that quantifies the balance of stresses on particles. By characterizing the applied magnetic field in terms of the average particle magnetization, a rheological master curve is generated for the field-induced static yield stress that indicates a concentration–magnetization superposition. The results presented herein will provide guidance to formulators of MR fluids and designers of MR devices who require a field-induced static yield stress and a dispersion that is essentially indefinitely stable to sedimentation.     

Rheometry on magnetorheological (MR) fluids

Test fixtures of a commercial concentric cylinder rheometer (Physica Rheolab MC 20) were modified to enable measurements under magnetic inductions up to 0.5 Tesla in a shear rate range of 0.1 up to 1000 s ^–1 and temperatures 0° to 150°C. In the 2 x90°-cups only two 90° sectors of the stationary part of the double concentric cylinder arrangement are submitted to the magnetic field which is created outside the test tools by an electromagnet. A prototype of a 360°-cup contains the electromagnet within the cup and avoids the correction necessary for the sector geometry. Measurements are shown for a carbonyl iron MR fluid and two nano MR fluids. An encouraging comparison of the viscosity function and MR effect (shear stress changes due to the field) measured by using the various cups is presented. The detailed investigation of the magnetic field distribution in the tools yields a distinct radial field gradient and also stray fields that make the quantification of the effective field in the gap difficult. The change of the field when the gap is filled with MR fluid is addressed. MR effects up to 13 000 Pa have been found, the limited torque range of the rheometer making it necessary to use relatively small gap dimensions which introduce errors due to edge effects. Shear rates up to 40000 s^–1 as typical for the application in dampers were investigated by a piston-driven capillary rheometer making use of a thermostated rectangular slit with superimposed magnetic field. A satisfactory agreement of the magnetorheological data with the concentric cylinder results is found in the overlapping shear rate range.     

Two-layer flow of magnetic fluids

The plane two-layer flow of viscous incompressible fluids with differentmagnetic properties between two horizontal rigid planes in a nonuniform traveling magnetic field is investigated. An arbitrary nonuniform periodic traveling magnetic field generates wavelike changes in the interface between the media and fluid flows with nonzero flow rates. From the given magnetic field the interface shape and the velocities, pressures and mean flow rates of the fluids are calculated. The cases of a cosine magnetic force and of the magnetic field created by ferromagnetic cylinders moving in a uniform magnetic field are considered. The effect of various parameters on the mean flow rates of the fluids is investigated.