Friction Characteristics of Nanoscale Sliding Contacts between Multi-Asperity Tips and Textured Surfaces

Nanoscale sliding contacts of smooth surfaces or between a single asperity and a smooth surface have been widely investigated by molecular dynamics simulations, while there are few studies on the sliding contacts between two rough surfaces. Actually, the friction of two rough surfaces considering interactions between more asperities should be more realistic. By using multiscale method, friction characteristics of two dimensional nanoscale sliding contacts between rigid multi-asperity tips and elastic textured surfaces are investigated. Four nanoscale textured surfaces with different texture shapes are designed, and six multi-asperity tips composed of cylindrical asperities with different radii are used to slide on the textured surfaces. Friction forces are compared for different tips, and effects of the asperity radii on the friction characteristics are investigated. Average friction forces for all the cases are listed and compared, and effects of texture shapes of the textured surfaces are discussed. The results show that textured surface II has a better structure to reduce friction forces. The multi-asperity tips composed of asperities with R=20r0 （r0=0.227 7 nm） or R=30r0 get higher friction forces compared with other cases, and more atoms of the textured surfaces are taken away by these two tips, which are harmful to reduce friction or wear. For the case of R=10ro, friction forces are also high due to large contact areas, but the sliding processes are stable and few atoms are taken away by the tip. The proposed research considers interactions between more asperities to make the model approach to the real sliding contact problems. The results will help to vary or even control friction characteristics by textured surfaces, or provide references to the design of textured surfaces.

Effects of different characteristic surfaces at initial stage of frost growth

The effects of surface energy on phase change of water vapor at initial stage of frost growth were studied to find an effective method of restraining frost growth.The mechanism of restraining frost growth by low energy surface(bigger contact angle) was analyzed based on crystal growth theory.Then,the phase change of water vapor and the process of frost growth on the copper and wax energy surfaces were observed using microscope.The results indicate that it is difficult for wax surface(low energy surface),on which there are still water droplets at 100 s,to form critical embryo,so frost growth can be restrained in a way.Water formation,droplet growth,ice formation and dendritic ice growth processes happen on both surfaces,ordinally.But the ice beads,with larger average diameter and sparse distribution on the wax surface,form later(at about 300 s) than that on the copper surface,and the dendritic ice also appears later.All of these support that ice crystal formation and dendritic crystal growth at initial stage of frost growth can be retarded on the low energy surface.

Simulation Analysis on Quasistatic Characteristics of Multistage Face Gears with Double Crown Surfac

The multistage face gears are the core component of the planetary gear train,which is symmetrically meshed by multiple single-stage face gear and multiple cylindrical gears.However,it is difficult to synchronize the contact between face gears and cylindrical gear due to the tooth number differences.Thus,the interference problems between two stages and big stress concentration are obvious.The crown surface configuration technology and structure design were introduced to optimize the meshing condition.To improve the double crown design feasibility and meshing stability,it is necessary to establish a reasonable multistage face gears pair simulation model to evaluate various influence factors on the contact condition and quasi-static characteristics of multistage face gears structure.The relevant scatter plots are fitted by comparing the contact results with different crown quantities and friction coefficients to intuitively obtain the variation regularity of maximum deformation,maximum strain,maximum stress and maximum strain energy.The natural frequency and mode coefficient are also determined by modal analysis under these two conditions.All the above mentioned studies verified the contact conjugate properties of double crown surface configuration were suitable.The results can provide a foundation for structure optimization and transmission reliability of multistage face gears.

Three-Dimensional Conjugate Tooth Surface Design and Contact Analysis of Harmonic Drive with Double-Circular-Arc Tooth Profle

A three-dimensional conjugate tooth surface design method for Harmonic Drive with a double-circular-arc tooth profle is proposed. The radial deformation function of the fexspline (FS), obtained through Finite Element (FE) analysis, is incorporated into the kinematics model. By analyzing the FS tooth enveloping process, the optimization of the overlapping conjugate tooth profle is achieved. By utilizing the hobbing process, the three-dimensional machinable tooth surface of FS can be acquired. Utilizing the coning deformation of the FS, simulations are conducted to analyze the multi-section assembly and meshing motion of the machinable tooth surface. The FE method is utilized to analyze and compare the loaded contact characteristics. Results demonstrate that the proposed design method can achieve an internal gear pair consisting of a circular spline with a spur gear tooth surface and the FS with a machinable tooth surface. With the rated torque, approximately 24% of the FS teeth are engaged in meshing, and more than 4/5 of the tooth surface in the axial direction carries the load. The contact patterns, maximum contact pressure, and transmission error of the machinable tooth surface are 227.2%, 40.67%, and 71.24% of those on the spur gear tooth surface, respectively. It clearly demonstrates exceptional transmission performance.

Effect of High Temperature Annealing on Characteristics of 4H Silicon Carbide MESFET

For very high temperature annealing (1620℃) after ion implantation for 4H silicon carbide (4H SiC),the residual components of Al and O in the alundum furnace impact seriously on the surface of material,which yields the derivation of SiOC.This causes a significant degradation of the 4H SiC surface characteristics according to the results of surface composition analysis.As validity,Ni/SiC ohmic contact measurement illustrates a higher specific contact resistance than the normal value by a factor of 2～3.Consequently the MESFET fabricated with this kind of 4H SiC material results in a degraded I V output performance compared with that of normal 4H SiC MESFET.

Adsorption characteristics of droplets applied on non-smooth leaf surface of typical crops

To further understand the adsorption characteristics of different-type leaf surfaces adsorbing pesticide droplets and reveal the adsorption mechanism of pesticide droplets on non-smooth leaves,non-smooth leaves of 12 kinds of typical target plants were investigated in this study.The parameters of surface morphological characteristics were measured,which include contact angle between leaves and water droplets,water holding capacity surface tension,polar component,dispersion component and other quantitative range of indicators and variation by modern means.The relationships between the indicators and water holding capacity were investigated respectively.The experimental results show that the number of trichomes,epidermal wax,morphology and distribution characteristics have influences on adsorption characteristics of the water droplets.There is a negative correlation between free energy of obverse side and the water holding capacity(R=−0.447)while the free energy of reverse side and the water holding capacity show a positive correlation(R=0.212).Also,there is a negative correlation between polar component of obverse side and the water holding capacity(R=−0.357)while the polar component of reverse side and the water holding capacity in plant leaves show a positive correlation(R=0.149).The research can provide a scientific theory for reasonable spraying of pesticide in the agricultural production,and can be a reference for the development of pesticide adjuvants and bionic pesticides.

隧道围岩-衬砌接触面剪切特性模拟研究

探究隧道围岩-衬砌接触面在不同法向应力及不同粗糙度条件下的剪切特性规律,对进一步研究隧道围岩与衬砌间的相互作用及岩体稳定性具有重要的理论和工程意义。为考虑粗糙度条件,利用分形理论,通过改变分形公式中的分形维数D及分形粗糙度G生成5种不同接触面,基于二维颗粒流程序(PFC^(2D)),建立接触面数值剪切模型,尺寸为150 mm×150 mm。开展同一接触面在不同法向应力下,数值模拟剪切试验及不同接触面在同一法向应力下的数值模拟试验;将数值剪切试验与室内剪切试验获得的剪切应力-剪切位移曲线进行对比,验证数值剪切模拟的可靠性。研究结果表明:(1)数值模型剪切破坏产生的裂隙主要集中于中间接触面处,破坏后的应力主要集中于接触面粗糙度最大处,破坏裂纹可分为剪切裂纹和张拉裂纹,其中张拉裂纹占主要优势;(2)剪切应力-剪切位移过程曲线存在爬坡、啃断、滑移三大阶段,具有明显的峰值强度和残余强度,法向位移随着剪切位移的增加而增大,产生剪胀效应;(3)随着法向应力增加,剪切和张拉裂纹数量增加,破坏现象越明显,峰值强度和残余强度增大,剪胀效应减小;(4)随着粗糙起伏度增大,即分形维数D的增大或者分形粗糙度G的减小,破坏裂纹数量增加,其中张拉裂纹增加较明显,峰值强度和残余强度增大,剪胀效应增大。

考虑结合面接触刚度的机床床身动态特性分析

床身结合面刚度是影响机床动态特性,进而影响机床加工精度的重要因素之一。为了获得精确的机床动态特性参数,运用接触刚度预测模型识别出了床身结合面的接触刚度并建立了等效动力学模型。首先,建立接触刚度预测模型;其次,通过预测模型识别出两种材料(铸铁和矿物复合材料)床身结合面的接触刚度;然后,利用结合面接触特性参数,基于弹簧单元约束建立有限元模型;最后,对比有限元模态频率和试验模态频率,发现基于弹簧单元约束的有限元模态频率与试验模态频率一致性较好,且该模型对铸铁和矿物复合材料均有效。研究表明考虑结合面接触刚度的机床床身有限元模型更具合理性,可以为分析机床动力学特性和优化机床结构提供理论依据。

新型矿用EMI降尘剂对煤尘的润湿特性研究

随着矿井机械化开采程度的提高,煤尘产生量也随之增大。针对煤矿井下作业环境中的粉尘污染日益严重的问题,通过测定含不同质量分数绿色高效生物降尘剂(EMI)溶液的表面张力,并以不同粒径煤尘与EMI溶液的接触角、煤尘润湿速度、煤尘吸湿量及保水性为指标,研究了不同质量分数的EMI溶液对不同粒径煤尘的润湿效果。实验结果表明:随着溶液中EMI降尘剂的质量分数增大,煤尘的接触角逐渐减小,煤尘完全沉降所需时间逐渐下降,吸湿量逐渐增大,保水性逐渐增强,且失水后煤样的板结性较大。通过分析得出EMI润湿煤尘的作用机理:降低水的表面张力;增加溶液润湿性;减少水分流失;提高溶液的抗蒸发性。

基于无网格方法的机械结合面接触分析

典型的无网格方法采用移动最小二乘函数(moving least squares,MLS)作为近似函数,但由于MLS不具备Kronecker delta函数性质,本质边界施加困难。LRPIM是采用径向基点插值形函数的无网格方法,本质边界条件无需特殊处理,可以直接施加,在保持高精度的前提下提高计算效率。将LRPIM应用于机械结合面接触问题的计算。根据位移连续条件推导了含接触特性的线性互补方程,建立了基于LRPIM的计算模型,采用线性互补算法利用数值积分计算了几种典型的接触问题,得到了接触面压力分布和接触变形,分析了插值函数形状参数和积分域尺寸对计算结果的影响。研究结果表明,插值函数形状参数α_(c)对接触力的影响较小,而形状参数q取-0.5~1.2时有较好的收敛效果;积分域无量纲尺寸a_(qx)、a_(qy)大于1.5时计算结果开始收敛,大于2.5时出现发散现象,取值2.1时收敛效果最佳。将计算结果与已有结果进行比较,表明本研究方法有较高的求解精度。

刮研表面法向接触特性研究

刮研对改善结合面接触特性具有显著效果,为深入研究刮研表面的法向接触特性,基于三维形貌仪得到不同精度的刮研和磨削表面形貌,并基于W-M函数模拟得到微织构形貌。逆向建模得到刮研表面、磨削表面和微织构表面,建立法向接触仿真模型。进行非线性接触仿真分析,并对比分析不同载荷作用下不同精度等级刮研表面、磨削表面和微织构表面的接触特性。分析结果表明,在载荷作用下,刮研、磨削以及模拟微织构表面首先在高点或微凸体发生接触并产生弹性变形,随着载荷的增加逐渐出现塑性变形,同时接触应力、形变以及接触面积也随之增大;当载荷为80 N时,Ⅰ级精度刮研表面真实接触面积大于精磨削表面,小于微织构表面,Ⅰ级精度刮研表面接触特性优于精磨削表面;在相同载荷下,Ⅰ级精度刮研表面总变形量小于精磨削和模拟微织构表面,其接触刚度大于精磨削和模拟微织构表面,刮研加工能够有效改善结合面接触特性。

Experiment and constitutive modelling of creep deformation in the frozen silt-concrete interface

To ensure the long-term safety and stability of bridge pile foundations in permafrost regions,it is necessary to investigate the rheological effects on the pile tip and pile side bearing capacities.The creep characteristics of the pile-frozen soil interface are critical for determining the long-term stability of permafrost pile foundations.This study utilized a self-developed large stress-controlled shear apparatus to investigate the shear creep characteristics of the frozen silt-concrete interface,and examined the influence of freezing temperatures(−1,−2,and−5°C),contact surface roughness(0,0.60,0.75,and 1.15 mm),normal stress(50,100,and 150 kPa),and shear stress on the creep characteristics of the contact surface.By incorporating the contact surface’s creep behavior and development trends,we established a creep constitutive model for the frozen silt-concrete interface based on the Nishihara model,introducing nonlinear elements and a damage factor.The results revealed significant creep effects on the frozen silt-concrete interface under constant load,with creep displacement at approximately 2-15 times the instantaneous displacement and a failure creep displacement ranging from 6 to 8 mm.Under different experimental conditions,the creep characteristics of the frozen silt-concrete interface varied.A larger roughness,lower freezing temperatures,and higher normal stresses resulted in a longer sample attenuation creep time,a lower steady-state creep rate,higher long-term creep strength,and stronger creep stability.Building upon the Nishihara model,we considered the influence of shear stress and time on the viscoelastic viscosity coefficient and introduced a damage factor to the viscoplasticity.The improved model effectively described the entire creep process of the frozen silt-concrete interface.The results provide theoretical support for the interaction between pile and soil in permafrost regions.

A Modified Elastoplastic Contact Stiffness Model Considering Continuous Smooth Contact Characteristics and Substrate Deformation

The contact stiffness of the joint surface directly affects the static and dynamic mechanical behavior,and accuracy of the machine tool.A new elastoplastic contact stiffness model is proposed by considering continuous and smooth contact characteristics and substrate deformation.First,the interpolation interval of cubic Hermite polynomials is improved to meet the continuous and smooth change of contact parameters during asperity deformation.Then,the micro-contact mechanism considering substrate deformation is explored by establishing an asperity-substrate system model.Furthermore,combined with statistical principles,a new contact stiffness model of the joint surface is established.Finally,the correctness of the built model is verified by comparing with experimental data and different contact models.The simulation results show that the model changes continuously and smoothly in the three deformation regions.The substrate deformation mainly affects the asperities in the elastic contact stage.The smoother is the surface,the more significant is the influence of substrate deformation.

基于改进分形方法的齿面磨损故障直齿轮啮合特性分析

考虑实际的轮齿表面微观形貌,建立了轮齿间相互作用的三维分形接触模型。结合轮齿承载接触分析方法,考虑轮齿间摩擦力的影响,建立了直齿轮副的时变啮合刚度模型,并通过有限元方法进行了验证。结合Archard磨损模型,考虑齿面粗糙度和摩擦力的影响,分析了齿面磨损故障状态下直齿轮副的啮合特性和磨损深度,同时分析了摩擦力和分形参数对时变啮合刚度和磨损深度的影响。摩擦力导致节线处的时变啮合刚度发生突变。当直齿轮副处于双齿啮合时,摩擦因数的增加和分形维数的降低导致磨损深度的增加和时变啮合刚度的降低。时变啮合刚度随分形维数的变化呈线性变化。分形维数对时变啮合刚度和磨损深度的影响随着磨损周期的增加更为明显,超过了摩擦因数对其的影响。

翅片表面融霜水滞留机理及其影响因素

为研究空气源热泵除霜过程中翅片表面融霜水的滞留现象,理论分析了滞留融霜水形成的原因,构建了结霜-融霜实验平台,对具有不同表面特性的4种翅片试样进行了结霜-融霜实验,分析了翅片表面特性、结霜程度和融霜温度对融霜水滞留的影响。结果表明,各表面的滞留融霜水分布特性有显著区别:融霜水在亲水表面铺展形成薄薄的水膜,而在超疏水表面凝聚成较小的球形水珠,且分布稀疏。滞留水量随着接触角滞后的减小和接触角的增大而减小,超疏水表面的滞留水量比亲水表面减少了79.82%。普通铝表面的滞留水量及其分布受结霜程度影响,而超疏水表面则不受其影响。改变融霜温度对翅片表面的融霜水滞留几乎没有影响。

基于细观数值试验的非饱和土石混合体力学特性研究

从土石混合体细观结构出发,融合细观结构模型生成技术、主-从接触面模型及非饱和土渗流与强度理论,建立非饱和土石混合体的细观数值模拟方法。通过与非饱和土石混合体室内试验结果进行对比,验证所建立的细观数值模拟方法的可行性和合理性。利用该细观模拟方法,分析土-石界面接触特性、含石量及饱和度等因素对非饱和土石混合体力学特性与破坏机制的影响。结果表明:(1)非饱和土石混合体在低围压下表现出明显的剪胀性,且受含石量和饱和度影响显著;在较高围压下基本上表现为剪缩变形,随含石量的增大其剪缩变形减小,饱和度对剪缩性的影响较小。(2)土石混合体的峰值强度和变形模量随土-石界面摩擦因数的增大呈非线性增长,在界面摩擦因数大于0.6以后,两者基本趋于稳定值。(3)含石量越大,非饱和土石混合体的峰值强度和变形模量越大,应变硬化特征更为显著,在含石量增加到58%后峰值强度和变形模量趋于稳定值。在低围压下剪胀变形随含石量的增加而增大;在较高围压时,剪缩变形随含石量的增大而减小。(4)饱和度越大,基质吸力越小,非饱和土石混合体的峰值强度越低,但变形模量变化不大。

考虑两粗糙面分形特性的接触模型

基于M-B分形接触模型,通过构建接触系数建立了考虑两粗糙面分形特征的接触模型。分析结果表明,修正模型能较好地与实验数据相吻合,并适用于更大的载荷,修正模型综合考虑两粗糙面的特性,其计算结果与实际接触情况更相符,为摩擦磨损预测、磨粒分析以及不同粗糙度表面的接触分析提供了参考。

二维自适应粗糙表面表征参数相关性研究

采用自适应粗糙表面表征的弹塑性接触模型,能在确保良好计算精度的同时,有效地减少接触计算时间。利用随机函数和傅立叶变换技术生成了一系列不同粗糙度、不同相关长度的随机粗糙表面轮廓,研究粗糙表面轮廓的坡度参数、峰顶曲率参数随轮廓相关长度的变化规律,并提出自适应阈值与峰顶曲率参数的比值参数δ-,为不同粗糙表面轮廓的自适应阈值的选择建立了依据。最后,分析了数值生成的自适应粗糙表面与一刚性平面的弹塑性接触情况。结果显示,当比值参数δ-<1.3×10-6 mm2,不同粗糙度、不同相关长度的自适应粗糙表面轮廓与刚性平面的接触计算,均可获得较佳精度。

粘土与结构接触界面剪切特性的试验研究

对DSJ-2型电动四联等应变直剪仪进行改装,在改装后的剪切仪器上进行了粘土自身、粘土与光滑及粗糙混凝土、粘土与平行及垂直纹理木材4个含水率共80个试样的剪切试验。结果得到了不同含水率不同正应力下土与不同结构接触界面的剪切应力应变曲线。研究结果表明,粘土与结构接触界面的抗剪强度、摩擦角、粘聚力等力学参数与土体含水率、接触材料类型及接触材料粗糙度密切相关。

修正G-W模型研究结合面微观接触特性

G-W模型在模拟结合面微观接触时发现,当微凸体接触点数0<n<1时,两表面会产生接触这一不合理现象,提出修正G-W模型研究低载荷下机械结合面的微观接触特性。通过测量表面微观形貌特征和功率谱函数,获得结合面的微观形貌参数,研究表面粗糙度与接触力、微凸体接触数以及真实接触面积间的关系,分析结果表明:不同表面粗糙度下中心线间距与微凸体接触个数和接触力呈对数关系,间距越大,微凸体接触数目越少,接触力越小;在相同间距下,表面粗糙度越大,微凸体接触数目越少,接触力越大;不同表面粗糙度下微凸体接触个数与接触面积和接触力之间呈线性关系,微凸体的接触数目越多,表面接触面积和接触力越大;相同接触数目下,表面粗糙度越大,真实接触面积和接触力也越大;当接触变形量较小时,修正的G-W模型与Hertz接触模型吻合较好,接触变形与接触力之间呈对数递增,而与真实接触面积间近似呈线性递增关系。