【作者】 王利军；

【导师】 郭楚文；

【作者基本信息】 中国矿业大学 ， 流体力学， 2008， 博士

【摘要】 由于纳米颗粒具有尺寸小、比表面积大、表面键形态不同于颗粒内部、表面原子配位不全及表面活性强等特性,在高技术新材料领域具有重要的研究和应用价值。正因如此,针对纳米微粒摩擦学性能的研究受到了广泛关注。业已发现,纳米颗粒作为润滑油添加剂通常表现出良好的抗磨性能、优异的极压性能和一定的减摩性能;此外,纳米材料在磨损表面的沉积可能对磨损表面起一定的修复作用。纳米磁性颗粒具有磁性材料的特性,将其作为润滑油的添加剂,具有良好的摩擦学性能,可以提高基础油的抗磨和承载能力,并且具备普通润滑剂不具备的一系列优点:如在磁场作用下良好的自密封性能,不发生泄漏,不产生污染等,有非常好的应用前景。目前针对磁性液体润滑的研究很少涉及磁场作用下纳米磁性液体润滑性能的研究,而关于变磁感应强度下磁性液体润滑性能的研究则更少,因此本文通过对MS-800四球试验机油杯的改进,在测试区域内产生可调磁场;利用改进后的油杯在四球机上测定了磁场作用下和无磁场作下时添加锰锌铁氧纳米磁性颗粒润滑油的摩擦学性能。四球机试验表明,纳米磁性颗粒可以明显改善基础油的摩擦学性能;另外通过对传统的旋转黏度计进行改进,测定了不同磁感应强度下纳米磁性液体的黏度,由于在磁场作用下磁粒将沿磁力线运动,这种运动会使悬浮粒子流动阻力增大,从而表现为黏度的增大,呈现了非牛顿特性;磁性液体黏度值随磁感应强度的增强而增大,是提高添加锰锌铁氧磁性颗粒润滑油的综合磨损值的主要要原因之一,综合磨损值最大可达基础液的1.43倍;其次纳米磁性颗粒中的Zn元素充当了极压剂作用,使润滑油的最大无卡绞负荷PB值提高26%,烧结负荷PD值提高100%。研究表明在所测定的试样中,质量分数为6%磁性流体的摩擦学性能最佳,相对于基础液而言磨斑直径降低了18.2%,综合磨损值提高了40%;然后通过对质量分数为6%的纳米磁性液体在不同磁感应强度下摩擦学性能的研究表明,对于质量分数为6%的纳米磁性液体而言,22mT为最佳磁感应强度。通过对磨痕表面的SEM(扫描电镜)以及EDS(能量色散谱仪)分析,对添加纳米磁性颗粒在不同润滑状态下的减摩机理进行了讨论,认为纳米粒子对油品摩擦学性能改善主要是以下三个方面的原因:首先,由于吸附、微电磁场作用,导致纳米粒子在摩擦表面富集,使表面润滑膜的厚度和强度增加;其次,部分吸附在摩擦表面的纳米粒子发生摩擦化学反应,形成新的润滑膜;最后,纳米粒子可以对摩擦表面进行优化,如抛光、修复表面纳米级微坑和表面微损伤等。对于纳米磁性液体而言,在磁场作用下可以加速磁性颗粒在磨损表面的沉积,加速了润滑膜的形成和对表面纳米微坑和表面微损伤的修复,从而起到了良好的润滑效果。最后本文对磁性液体润滑的径向滑动轴承问题进行了理论和数值研究,推导出了纳米磁性液体润滑作用下的Reynolds方程,并可扩展到其他非牛顿流体润滑问题,为进行基于磁性液体的滑动轴承流体动力润滑分析提供了理论依据。根据所推导得到的基于磁性液体润滑的广义雷诺方程,利用有限差分法,对磁性液体润滑轴径轴承的静特性进行了数值模拟,研究了不同浓度、间隙和磁感应强度对磁性液体轴承润滑性能的影响。数值研究表明,无磁场作用时,随着纳米磁性颗粒添加量的增加,有助于提高轴承的无量纲承载能力,最大增加了24.54%;在外加磁场作用下的磁性液体润滑轴承和无磁场作用时相比,油膜压力有所提高,摩擦学性能得到进一步改善;偏心率越大,轴承承载能力增加的幅度也越大;并且如果合理设计磁场分布,则完全可以达到零泄漏,这是普通轴承无法实现的,显示了磁性液体润滑轴承的优越性。数值模拟的结果与试验结果基本一致。更多还原

【Abstract】 The synthesis and application of nanometer-sized particles have received considerable attention in recent years because of their different physical and chemical properties from those of the bulk materials or individual molecules. When nanometer-sized particles mixed with lubricating oil, the nanometer particles can improve the lubricating oil’s abilities of extreme pressure, anti-wear, reduce frictional resistance and repair the friction surface. Research results show that the magnetic fluid is really a new kind of lubricant that is reliable and economical. And there is no leakage and pollution in the process of lubrication under the effect of magnetic field.However, few experimental investigations on the tribological properties of lubricating oils with nano-ferromagnetic particles have been performed under the effect of magntic field. In this paper a controllable and variable magnetic field was got by improving the oil cup of the MS-800 four-ball tester. By this improved four-ball tester, the load capacity of Mn_0.78Zn_0.22Fe₂O₄ magnetic fluid was tested with the magnetic field and without the magnetic field. The results show that, tribological characteristics of the lubricant adding Mn_0.78Zn_0.22Fe₂O₄ nanoparticles improve considerably with the effect of magnetic field. First, the viscosity of magnetic fluids increase with the increasing of magnetic field,the comprehensive load capacity is improved greatly, especially the composite wear value of Mn_0.78Zn_0.22Fe₂O₄ magnetic fluid is 1.43 times of the base oil. Then, Zn element contented in the Mn_0.78Zn_0.22Fe₂O₄ nanoparticles improve the load carrying capacity PB and PD of lubricant, and 26% increasing of PB, 100% increasing of PD is tested. Moreover, the 46# turbine oil doped with 6wt% Mn_0.78Zn_0.22Fe₂O₄ nanoparticles show the best tribological properties among the tested oil samples. When the mass percentage is 6wt%, it is found that 22mT magnetic induction is the optimum magnetic induction.In addition, more investigations were performed by using Scanning Electron Microscope （SEM） and Energy Dispersive Spectroscopy （EDS） to interpret the possible mechanisms of lubrication and wear with the nanoparticle. It was found that: First, at the effect of short force such as adsorption, faintness electron magnetism field. The thickness and intensity of oil films will be increased which will improve the tribological properties of magnetic fluid. Secondly, new oil film was built because of tribochemical reaction concerned with nanoparticles ernriched in surface under the effect of friction and wear. Thirdly, the worn surface was optimized by nanoparticles, for instance filling the tiny hole and crackle on the surface. Under the effect of magnetic field Mn_0.78Zn_0.22Fe₂O₄ particles are easily deposited and adsorbed on the worn surface, which then formed a protection film due to the shearing effect. This film can smooth and repair the wear scar then improve the wear resistance, load carrying capacity and antifriction ability of Mn_0.78Zn_0.22Fe₂O₄ magnetic fluid.At last, theoretical analysis and numerical study of the lubrication performance for magnetic fluid journal bearings are presented. A general Reynolds equation based on magnetic fluid model is obtained, which can be easily extended to other non-Newtonian fluids and this equation can provide theoretical basis for hydrodynamic analysis of magnetic fluid journal bearings. For the case of static loaded magnetic journal bearings, the influence of magnetic fluid effects on the lubrication performance is studied under various eccentricity ratios, magnetic intensity and concentration. The numerical results show that: with the increasing of concentration, the bearing capacity is obviously increased; the increase magnitude is larger when the eccentricity ratio is large. Under the effect of magnetic field, the bearing capacity increasing with the increasing of magnetic field intensity. When the eccentricity is small, the side leakage is highly decreased. It can be completely eliminate by appropriately designing the bearing geometry and the magnetic field which can’t be existed in normal journal bearings. Under the effect of magnetic field little change of the attitude angle is obtained.更多还原