试样倾斜对球形压头微米划痕测试紫铜摩擦系数的影响

采用球形压头对紫铜进行微米划痕实验,在恒定正压力下研究了试样倾斜对测试摩擦系数的影响。结果表明:实验测得的名义摩擦系数与倾斜角线性相关,且±9°的差别会造成摩擦系数相差约14倍。通过建立球形压头与试样倾斜状态的位置关系模型,发现试样倾斜对粘着摩擦系数无影响,而犁沟摩擦系数随倾斜角线性变化。为了准确计算无倾斜条件下的真实摩擦系数,可通过平面接触力学模型实现名义摩擦系数与真实摩擦系数之间的转换,或者在试样表面的不同位置往复划刻,再取摩擦系数的平均值作为最终结果。     

冲击摩擦及其对弹性涂层冲蚀率的影响研究

将ChenDanian等人提出的弹性斜冲击摩擦模型进行了对称简化,用简化模型分析了粒子对弹性涂层冲击过程中摩擦系数的变化规律及其对涂层冲蚀率的影响;结论指出,在冲击接触的中间时段摩擦系数达到一个平台值(最大值),且有fmax=ctgα0,一次冲击全过程的平均值fp与试验值接近;随着初始入射角α0的增加,摩擦系数降低。将得到的摩擦系数值代入作者提出的冲蚀预估模型进行涂层冲蚀率的计算表明,在初始入射角α0=30°附近,弹性涂层的冲蚀率达到最大值,与实验结果吻合。     

寻求“四维破磨理论”联合攻关伙伴

钢球和矿石在球磨机中呈抛落和滑滚动状态,滑滚动部分占筒体填满量‘的比率称动态充填率刀动。其表面与水平面的夹角称动态安息角月。以有效内半径R。、转速n、刀动和月建立的x一。一}坐标系数学模型。按自由抛落和转速超临界转速n。受摩擦力影响(摩擦系数取qZ二0.25)做垂直减速和水平加速运动,从而算出各质点抛落规律及下落平均速度。。、抛落量Q、下落平均冲击速度:冲、滑滚动量E二刀动由厂o,为角速度)、相对滑滚动速度。对=。。+。。(*为回转平均速度)。研究Q、。冲、E和。对磨矿四要素变化与处理能力、能耗的关系就是“四维破磨理论,…     

CPR/NR和高性能填料对复合摩擦材料力学性能的影响

通过干法热压成型工艺制备性能优异的复合摩擦材料,研究了高性能填料以及改性酚醛树脂与丁腈橡胶质量比(CPR/NR)对复合摩擦材料性能的影响规律。对材料的摩擦磨损性能与力学性能进行了测试,借助热分析仪测试其耐热性能,并利用激光共聚焦显微镜、扫描电镜对表面形貌进行了观察和分析。结果表明,复合摩擦材料的密度、压缩强度、压缩模量、硬度随橡胶含量的减少而增加,冲击强度则呈相反的趋势。橡胶含量的减少,树脂比例的增加,使复合摩擦材料的耐热性得到提高,促进了第二接触面的形成,使摩擦系数与磨损率降低。高性能填料含量较低时,材料表面形成大且连续的第二接触面,第二接触面使摩擦系数、比磨损率降低,复合摩擦材料的主要磨损形式为粘着磨损与磨粒磨损;填料含量的增加会阻碍第二接触面的形成,使材料摩擦系数和比磨损率逐渐增大,材料的磨损形式由粘着磨损、磨粒磨损转变为磨粒磨损和疲劳磨损。     

R290与R410A旋转式压缩机滑片-活塞摩擦副特性的对比

为了模拟与压缩机工作状态尽量接近的实验条件,本文在密封的高压环境下测试了在R290/矿物油混合物组合下的滑片-活塞摩擦副的摩擦系数、咬合力和磨损量。测试样件为压缩机中的滑片与滚动活塞量产件,根据压缩机的实际运行工况设定了滑片与滚动活塞间相对滑动速度、负载及润滑油温度等实验条件范围,采用电子扫描镜检测实验后样件的表面形态,并与目前使用量较大的R410A/POE油混合物组合进行对比。结果表明,在R290/矿物油混合物组合下,滑片-活塞摩擦副的摩擦系数比R410A/POE油混合物组合的摩擦系数大0.05~0.10,但其咬合负载高出300 N,表明在R290/矿物油混合物组合下,压缩机的抗磨损能力增强,但摩擦功耗也同时增加,需进一步改进。     

非连续边界层对圆柱声学共鸣频率的影响研究

基于声学一阶微扰理论,建立了低压下由于气 固界面的温度阶跃和速度滑移带来的边界层不连续性对圆柱声学共鸣频率的扰动规律,进一步发展了圆柱声学共振频率的非连续边界层修正模型,计算了非连续边界层对4种惰性气体、不同声学模式和不同压力及温度的影响。分析研究显示,气体处于低压状态时,温度阶跃和速度滑移会使声学共振频率发生偏移,不引入声能损耗。在50 kPa时,非连续边界层对声学共鸣频率的影响可达10×10^-6,表明非连续边界层修正项对于10^-6不确定度水平的尖端声学共鸣测量,是一个重要的不确定性来源。     

Al-2.56Li-1.29Cu合金拉伸断裂行为的扫描电镜观察

在备有拉伸装置的扫描电镜下,观察了 Al—2.56 Li—1.29Cu 合金拉伸变形及断裂过程。观察结果表明,在固溶淬火及190℃时效状态下,主要析出相为δ’相。随着时效时间的延长,拉伸断口逐渐由韧窝穿晶型向沿晶突脊型及沿晶韧窝型过渡。在固溶状态下,当拉伸应力超过届服强度以后,出现平直滑移带及交叉滑移带;拉伸裂纹在晶内滑移带及晶界处萌生,并主要沿滑移带扩展。欠时效状态下,δ’相的析出量增多,使共面滑移加剧,交叉滑移带难于形成;裂纹除沿晶界扩展外,也有少部分沿滑移带扩展。在达到峰时效以后,交叉滑移带消失;裂纹在三角晶界处萌生并沿晶界扩展。     

表面爆炸处理后的ZGMn13Cr2钢的冲击磨损性能及硬化机理

对ZGMn13Cr2钢表面进行一次爆炸处理,测量其冲击磨损性能,观察不同硬化层的显微组织,分析其硬化机理。实验结果表明:经爆炸处理后的ZGMn13Cr2钢硬度随硬化深度的增加而降低,耐磨性提高了1.32～1.41倍,磨损形貌以犁沟、剥落坑为主,还有少量破碎坑;通过对不同硬化深度的实验用钢表面进行显微表征,发现硬化层内产生了大量的形变孪晶,爆炸表面具有高密度位错的畴界,距爆炸表面10 mm处有泰勒晶格以及距爆炸表面20 mm处出现大量平行的滑移带和滑移带交割,未观察到马氏体组织,仍以基体奥氏体为主,这说明一次爆炸处理的ZGMn13Cr2钢的硬化机理为孪晶硬化和位错硬化的复合机制。     

SiCw/LD2Al复合材料超塑变形协调机制的研究

为了研究金属基复合材料超塑变形时液相的作用,分析了金属基复合材料界面处的局部应力.当由滑移引起的界面处局部应力值高于外界施加于界面处应力值时,复合材料在含有液相的状态下超塑变形能力最大;反之,变形能力在不含液相的纯固相状态最大.因此,可以认为塑性变形过程中当有应力集中产生时,为了缓解应力集中,液相必需存在才能取得超塑性.当无应力集中时,超塑变形不需要液相的存在.上述理论与实验结果是一致的.     

SiCw／LD2A1复合材料超塑变形协调机制的研究

为了研究金属基复合材料超塑变形时液相的作用,分析了金属基复合材料界面处的局部应力,当由滑移引起的界面处局部分应力值高于界施加于同处应力值时,复合材料在含有液相的状态下超塑变形能力量大,反之,变形能力在不含液相的纯固相状态最大,因此,可以认为塑性变形过程中当有应力集中产生时,为了缓解应力集中,液相必需存在才能取得超塑性,当无应力集中时,超塑变形不需要液相的存在,上述理论与实验结果是一致的。     

Numerical study of a sphere descending along an inclined slope in a liquid

The descending process of a sphere rolling and/or sliding along an inclined slope in a liquid involves interactions between the hydrodynamic forces on the sphere and the contact forces between the sphere and the plane. In this study, the descending process of sphere in a liquid was examined using coupled LBM–DEM technique. The effects of slope angle, viscosity and friction coefficient on the movement of a sphere were investigated. Two distinct descending patterns were observed: (a) a stable rolling/sliding movement along the slope, and (b) a fluctuating pattern along the slope. Five dimensionless coefficients (Reynolds number (Re), drag coefficient, lift coefficient, moment coefficient and rolling coefficient) were used to analyze the observed processes. The vortex structure in the wake of the sphere gives a lift force to the sphere, which in turn controls the different descending patterns. It is found that the generation of a vortex is not only governed by Re, but also by particle rotation. Relationships between the forces/moments and the dimensionless coefficients are established.     

不同条件下Al2O3基陶瓷材料的摩擦磨损性能研究

本文对ＡＩ２Ｏ３基陶瓷材料／４５＃钢摩擦副的摩擦系数与４５＃钢／４５＃风的摩擦系数作了对比滑动摩擦试验研究,观测分析了ＡＩ２Ｏ３基陶瓷材料磨痕形貌,并就干摩擦、油润滑状态下ＡＩ２Ｏ３基陶瓷材料的磨损机理进行了分析。结果表明：分别在干摩擦和２０＃机油润滑下,ＡＩ２Ｏ３基陶瓷材料／４５＃钢的摩擦系数均比４５＃钢自配副时的低。在干摩擦条件下,ＡＩ２Ｏ３基陶瓷材料的磨损机理是脆性微剥落和磨粒磨损,油润滑条     

Stochastic process of fracture in granular materials

By regarding the coefficient of particle friction of granular materials as a random variable distributed on the particle surface, the mechanism of particle sliding is interpreted as a stochastic process. Axial, shearing and volumetric strains are defined with regard to the deformation of a microscopic regular assembly of uniform spheres. For illustration, these strains are calculated for a uniform distribution of the coefficient of particle friction and applied to some triaxial test results reported elsewhere.     

Experiments and simulation of torque in Anton Paar powder cell

Torque measurements were performed with the powder cell mounted on an Anton Parr rheometer using glass beads of two different sizes and different impeller geometries. A discrete element method simulator was used to perform simulations of the experiments to compare calculated torque values with those of the experimental torque values. Experimental torques obtained with flat two-blade impeller helped to find the correct sliding friction coefficient between beads. Experimental torques obtained with the circular impeller allowed to obtain the sliding friction coefficient between the impeller and beads. To estimate the wall friction coefficient, it was necessary to activate particle shear on the wall in configurations in which the gap between the impeller and wall is comparable with the bead size. It was verified that at the low shear rates used in this paper, the particle restitution coefficient is not significant and, therefore, higher impeller rotational velocities should be used to calibrate this parameter. Also, the differences between the torque values and time series of different impeller shapes were attributed to the different shearing surfaces and to the formation of instantaneous high magnitude force chains.     

基于摩擦磨损实验的双盘直槽研磨方法的研具选材研究

双盘直槽（double-disc and linear-groove, DDLG）研磨方法是以1个平端面研磨盘和1个具有多条直沟槽的研磨盘为对磨研具,对圆柱滚子的滚动面进行精密加工的新方法。在加工过程中,圆柱滚子沿直沟槽连续供料,在2个研磨盘的摩擦力矩驱动下连续自转。研磨盘材料的选择是搭建双盘直槽研磨设备的基础。为了确定适用于双盘直槽研磨方法的研磨盘材料组合,基于摩擦磨损实验展开相关研究。首先,基于摩擦原理,分析了圆柱滚子的运动状态和研磨盘材料的摩擦特性对研磨效果的影响,并确定了研磨盘材料摩擦系数的筛选条件。然后,通过销-盘摩擦磨损实验测试了铸铁、45钢、黄铜、聚四氟乙烯（polytetrafluoroethylene, PTFE）、有机玻璃（polymethyl methacrylate, PMMA）、125%铸铁基固结磨料和125%树脂基固结磨料等多种备选材料在研磨条件下的摩擦系数、耐磨性和排屑性能。最后,搭建了双盘直槽研磨试验台,通过观察圆柱滚子的自转情况来验证基于摩擦磨损实验的研具选材方法的合理性。通过摩擦磨损实验测得,铸铁和45钢的滑动摩擦系数大,磨削效率高,但耐磨性差,适合用作大去除量场合的上研磨盘材料;有机玻璃的滑动摩擦系数大,耐磨性好,磨削效率高,适合用作小去除量场合的上研磨盘材料;聚四氟乙烯的滑动摩擦系数小,耐磨性好,可用作下研磨盘直沟槽材料;固结磨料的滑动摩擦系数变化大且易堵塞,不适合用作研磨盘材料。研究结果可为双盘直槽研磨设备的设计提供可行的研具选材依据。     

Material removal characteristics of 20CrMnTi steel in single grit cutting

The single grit cutting test was regarded as an important approach for understanding a complex grinding process, and revealed the connection between grinding wheel state, grinding parameters, and the surface integrity of workpieces. To investigate the formation mechanisms and morphology of grinding chips of 20CrMnTi steel, experiments on the high-speed grinding of a single grit on the surface of 20CrMnTi steel were performed by the finite element method. Combining this with the analysis of scratch morphology and the use of the finite element method, the authors showed that the process included sliding friction, ploughing, and produced fragmented, and continuous chips which changed with the cutting depth. Moreover, the distributions of physical quantities including stress, strain, heat, and temperature fields in the three areas subjected to cutting deformation were analyzed in greater detail. Moreover, combined with the deduced theoretical formulae, finite element simulation tests were conducted to analyze the undeformed chip thickness, shear angle, chip shape, strain in the formation process of chips from a single grit grain, and its changes with grinding parameters and the shape of abrasive grains.     

Shearing of fibrillar adhesive microstructure: friction and shear-related changes in pull-off force.

To characterize the effect of shearing on function of fibrillar adhesive microstructure, friction and shear-related changes in pull-off force of a biomimetic polyvinylsiloxane mushroom-shaped fibrillar adhesive microstructure were studied. In contrast to a control flat surface, which exhibited pronounced stick-slip motion accompanied with high friction, the fibrillar microstructure demonstrated a stable and smooth sliding with a friction coefficient approximately four times lower. The structured contact also manifested zero pull-off force in a sheared state, while the flat surface exhibited highly scattered and unreliable pull-off force when affected by contact shearing. It appears that the fibrillar microstructure can be used in applications where a total attachment force should be generated in a binary on/off state and, most surprisingly, is suitable to stabilize and minimize elastomer friction.     

Tribological behaviour and acoustic emissions of alumina, silicon nitride and SAE52100 under dry sliding

The friction, wear and acoustic emission behaviour of various combinations of alumina, silicon nitride, and SAE52100 steel, operating under dry sliding conditions, was investigated. A designed ball-on-flat-disc type of tribometer was used to conduct these experiments. This apparatus, equipped with a force sensor, using silicon strain gauges, measured simultaneously the normal load and friction force. Both forces were used to determine the real-time value of the dynamic coefficient of friction. The AE signal arising from the interaction of the surfaces in dynamic contact was also detected and a data acquisition system was used to gather this signal as well as the outputs from the force sensor, at high frequency. The effects of test duration, sliding speed and normal load on the above mentioned tribological parameters were evaluated. The interest of this study further extended to assess the correlations that may exist between the integrated rms acoustic signal (AE) and the friction mechanisms, wear volume, friction work as well as the material removal power. Under the specific conditions of the present experiments, no consistent relation was found between the variations of AE and corresponding dynamic coefficient of friction (COF) as function of time. The variation of COF and wear rate, obtained considering a fixed total sliding distance of 500 m, as function of a range of sliding speed (0.05–2.5 m/s) and normal load (5–40 N) are presented. It was found that the test duration has an important impact on wear results of the experiments conducted at different sliding speeds and fixed travelling distance. More expected behaviour was observed when the relationships between the AE and wear volume, friction work, and material removal power were investigated considering the data obtained at different loadings and fixed sliding speed. Some models representing interesting relationships which could be used for predicting tribological properties in the case of practical applications, similar to the tribo-systems investigated in this study, are proposed.     

Sensitivity of numerical parameters on DEM predictions of sediment transport

The current work presents a sensitivity study of selected numerical parameters on the large-eddy simulation–discrete element method predictions of sediment transport in a unidirectional open turbulent channel. The sensitivity of the particle friction factor, restitution coefficient, and spring stiffness used in the soft sphere collision model is tested in the three regimes of sediment transport comprising of essentially no motion, bed load, and suspended load flow regimes. The simulations are run for 10?s using base values of the parameters of interest (reference calculation), then one parameter is changed at a time and the corresponding change in quantitative result is observed for the following 15?s where averaged results in the last 10?s are compared to the reference calculation. The sensitivity analysis shows that the underpredicted sediment transport in the suspended load regime can be bridged by moderately decreasing the friction factor of the particles from 0.6 to 0.25. The impact of the same change in bed load regime is not as significant. Both the coefficient of restitution and the particle stiffness show less significant to negligible impact as compared to the friction factor.