小Tabor数分形粗糙表面之间的黏着弹性接触分析

基于粗糙表面的分形描述和适用于小Tabor数微突体的黏着弹性接触理论,采用积分方法建立了小Tabor数分形粗糙表面之间的黏着弹性接触模型,获得弹性接触条件下的真实接触面积和载荷表达式,在此基础上采用单因素分析法分析表面粗糙度和材料性质对分离力的影响.结果表明,当分形维数增加时,粗糙表面单位面积上的微突体数目增加且高度减小,从而导致两表面间的分离力增大;当分形粗糙度参数增大时微突体高度增加,从而导致分离力减小;当材料弹性模量增加时黏着作用减弱,从而减小了分离力,而表面黏着能的增加会使分离力急剧增大.     

用侧向力显微镜对不同非晶态GeSbTe薄膜的摩擦性能研究

采用侧向力显微镜研究了磁控溅射方法制备的GeSbTe薄膜在大气环境中的纳米级摩擦性能,考虑了相对湿度、扫描速度及表面粗糙度对其摩擦性能的影响,对比不同成分的GeSbTe薄膜的摩擦特性.结果表明:在相对湿度较大时,扫描速度对针尖和GeSbTe薄膜之间的摩擦力影响很大;在其它条件相同、外加载荷较大时,同一载荷下的摩擦力与表面粗糙度呈线性关系,但在外加载荷较小的情况下,二者呈现非线性变化规律;相对湿度对Ge2Sb2Te5薄膜和针尖的粘附力影响较GeSb2Te4薄膜弱,且粘附力使得摩擦系数减小;在同一相对湿度下,由于薄膜成分的变化导致硬度不同,其对薄膜的摩擦性能也有一定影响.     

粘着行为中预压紧力和分离速度及相对湿度对分离力影响的实验研究

在改变粘着试验中接触副预压紧力、接触副分离速度以及环境相对湿度的条件下，利用表面力仪实时测量了新鲜解理云母接触副分离过程的分离力，并观察了接触后解理云母接触区的状态．结果表明：在保持弹性接触时，预压紧力对分离力影响不大，接触材料的表面能决定分离力的强度；当预压紧力超过临界值时，接触表面发生塑性变形，在一定范围内分离力随预压紧力的增加而增大；在干接触时分离速度与分离力无直接联系，但在存在大粘度中间层液体时，分离速度的增加可使分离力显著增大；分离力随相对湿度的增加而增大．     

考虑毛细作用微梁剥离的实验和力学模型

毛细力诱发的粘附现象在自然界和工农业生产中广泛存在，例如微机电系统、微纳米自组装、油气驱替等．本文系统研究了两根微梁的毛细粘附行为，包括梁剥离过程中液桥的形貌以及剥离力-位移变化规律．试验发现，微梁在毛细力作用下的剥离部分经历了液膜粘附和液滴粘附两个阶段．考虑两个阶段的液桥形状特征，分别建立系统的能量泛函，采用变分原理推导了考虑毛细力的微梁剥离的非线性微分方程和边界条件．基于Matlab 编程求解方程，得到了剥离力-位移曲线，理论计算与试验结果具有很好的一致性．另外，参数研究表明，接触角和表面张力系数对液膜粘附的微梁剥离影响显著，而对液滴粘附的剥离影响较小；微梁刚度对两个阶段的剥离都有明显影响．本文的试验结果和理论分析对于实际工程中微结构的定量设计具有一定参考价值．     

微梁与基底间的毛细粘附作用

在表面微型机械结构的制造过程中,强的毛细相互作用常常使得组成这些结构的微桥、微梁与基底粘附而导致失效。而在微尺度实验中,微桥与微梁又是微尺度材料常数和性能检测的常用的试件样式,如果实验中加载端与被检测的微尺度试件发生毛细粘附,将直接影响检测数据的准确性。本文应用微悬臂梁试件,讨论微梁与基底间的毛细粘附作用,并通过能量原理计算其粘附力的大小和试件几何尺寸、粘附面距离、粘附液体特性之间的关系。最后应用微散斑干涉,检测粘附平衡态时微桥和微梁的粘附力以及由毛细粘附所导致的弯曲变形,并与理论计算结果进行比较。     

热处理对GeSb2Te4薄膜微观结构及其摩擦性能的影响

利用射频溅射法制备了GeSb2Te4薄膜并对其进行热处理,分析热处理前后样品的结晶情况,用纳米硬度计测定硬度,利用静电力显微镜表征样品的表面电势,采用原子力显微镜观察薄膜表面形貌,利用侧向力显微镜对比考察了在考虑相对湿度的情况下,热处理前后GeSb2Te4薄膜的粘附力和摩擦性能.结果表明:经过退火的沉积态GeSb2Te4薄膜发生从非晶相到fcc亚稳相再到hex稳定相转变;粘附力与表面粗糙度之间没有明显的对应关系,但与样品表面自由能和表面电势有一定关系;在低载荷下GeSb2Te4薄膜的摩擦力很大程度上受粘附力支配,而在高载荷下的摩擦力受犁沟影响显著;经过340 ℃退火GeSb2Te4薄膜由于具有层状结构,呈现出一定的润滑作用.     

缅甸蟒蛇腹鳞表面的摩擦机理及摩擦各向异性研究

采用原子力显微镜观察缅甸蟒蛇腹鳞表面的微观结构,采用UMT-2型摩擦磨损试验机研究不同载荷及运动方向的腹鳞表面的宏观摩擦各向异性,建立了摩擦运动的接触模型,分析了腹鳞表面的磨损机理.结果表明:腹鳞表面的微观结构由指状微突体和板结构部分周期排列而成,其结构可用9个特征参数定量描述;腹鳞表面摩擦力由分子作用力、表面微突体的犁沟力、楔形作用力以及材料弹性滞后共同引起;腹鳞表面的摩擦系数在0.07左右并与运动方向有关,摩擦系数随载荷增加而减小;后向运动及左、右侧向运动时摩擦系数基本相等,比前向运动时高33%左右;腹鳞表面微突体不同方向上倾斜角度的差异是引起摩擦各向异性的主要原因.研究结果对仿生制造摩擦各向异性表面提供实验依据.     

孔隙率各向异性下饱和多孔弹性地基动力响应

由于不同的沉积条件和应力状态, 天然土体通常表现出一定的各向异性特征. 文章研究地基上表面受温度载荷和机械载荷时, 孔隙率各向异性参数变化对饱和多孔弹性地基热-水-力耦合动力响应问题的影响. 基于Lord-Shulman广义热弹性理论, 结合孔隙率各向异性基本假设, 建立了孔隙率各向异性饱和多孔弹性地基热-水-力耦合动力响应模型, 利用正则模态法推导出无量纲竖向位移、超孔隙水压力、竖向应力和温度分布的解析表达式并加以图示. 正则模态法是一种利用加权残差求得解析解的方法, 相较于其他方法能快速求解偏微分方程. 当孔隙率各向异性参数为1时, 可将该各向异性耦合动力响应模型退化为热-水-力耦合动力响应模型验证该地基模型的合理性. 着重分析了孔隙率各向异性参数变化对不同物理量的影响. 结果表明: 孔隙率各向异性参数变化对物理量均有一定影响. 在地基上表面受温度载荷作用时, 对超孔隙水压力和竖向应力影响最为明显; 在地基上表面受机械载荷作用时, 对超孔隙水压力和温度影响明显. 整体而言, 无论地基上表面受何种载荷, 随着各向异性参数增大, 峰值逐渐减小, 在地基深度增加方向峰值所在位置向靠近地基上表面方向移动.      

恒定大载荷划痕试验下紫铜的三维形貌及划痕硬度分析

采用圆锥形压头对紫铜进行划痕试验,并通过三维表面形貌仪获取划痕的三维形貌,研究正压力的变化对划痕沟槽所产生的影响. 结果表明:正压力的增大,使得划痕宽度和深度均线性增加,当正压力较大时,位错墙的形成使划痕深度出现周期性的波动,同时压头划刻过程伴有划痕两侧和前端的材料堆积现象,前端堆积高度和厚度、两侧堆积高度和宽度随着正压力的增加而线性增大. 通过“切削与塑性比”说明了压头对紫铜的刻划存在微犁耕和微切削两种变形机制,并且微切削机制在划刻过程中占主导地位. 磨损率随着载荷增加而线性增大,但划痕硬度不随载荷的变化而改变,约为0.77 GPa.      

轴对称热-力埋置载荷下半空间饱和孔隙介质的动力响应

基于修正的Biot热弹性本构理论,得到了饱和多孔介质热-力耦合的动力学控制方程.针对半空间孔隙介质在内置简谐热-力轴对称载荷作用下的动力问题,利用Hankel变换获得了响应的解析表达式,并利用Han-kel逆变换进行数值求解,分析了埋置深度、表面热边界条件等对响应的影响规律.结果表明:孔隙水压力在载荷作用处上方有负压出现;环向、径向及竖向应力在载荷作用处发生突变,且在载荷作用处上方均出现拉应力;在载荷作用处下方,孔隙水压力及竖向、径向和环向应力均随着深度的增大而减小.当内置热载荷仅设置温差,而无外热源输入时,温差对孔隙介质中的响应几乎不产生影响.当孔隙介质表面绝热时,孔隙水压力小于表面等温情况下的值.     

Influence of atomic force microscope (AFM) probe shape on adhesion force measured in humidity environment

In micro-manipulation, the adhesion force has very important influence on behaviors of micro-objects. Here, a theoretical study on the effects of humidity on the adhesion force is presented between atomic force microscope （AFM） tips and substrate. The analysis shows that the precise tip geometry plays a critical role on humidity depen- dence of the adhesion force, which is the dominant factor in manipulating micro-objects in AFM experiments. For a blunt （paraboloid） tip, the adhesion force versus humidity curves tends to the apparent contrast （peak-to-valley corrugation） with a broad range. This paper demonstrates that the abrupt change of the adhesion force has high correla- tion with probe curvatures, which is mediated by coordinates of solid-liquid-vapor contact lines （triple point） on the probe profiles. The study provides insights for further under- standing nanoscale adhesion forces and the way to choose probe shapes in manipulating micro-objects in AFM experiments.     

Analysis of humidity-dependent adhesion between a probe tip and a surface

Adhesive forces commonly exhibit a monotonic increase or a maximum with increasing relative humidity. However, anomalous behavior has been reported. Here, a numerical model of adhesive forces, comprised mainly of capillary and van der Waals forces, between a tip and a surface is established. It is described by a power law that considers the geometry, the liquid bridge wetting radius, the contact angle, and the separation distance. Capillary forces (sum of surface tension and Laplace pressure) and van der Waals forces are calculated. The latter cannot be neglected in the adhesion even at high humidity. Decrease in adhesion with increasing relative humidity can be attributed to a blunt tip shape, which is validated by experimental data. Specifically, the decrease in adhesion is attributed primarily to a transition from a rounded to a blunt tip shape. Structuring objects at the micro- or nanoscale can either increase or decrease adhesion as a function of relative humidity. This has a wide range of applications in robotic manipulation and can provide a better understanding of adhesion mechanisms in atomic force microscopy in ambient air.     

MEASUREMENT OF ADHESION FORCES IN AIR WITH THE VIBRATION METHOD

The vibration method represents a practical method for the measurement of adhesion forces and adhesion force distributions. This method causes sinusoidally altemating stresses and yields detachment and contact forces between particles and substrate of the same order of magnitude. Alternating contact forces of the vibration method can cause an adhesion force intensification through flattening of asperities. The measuring principle of the vibration method and the analysis of experimental results are described in the article. Normal adhesion forces (pull-off forces) are measured using the vibration method and the colloidal probe technique. The results of both methods show good agreement for small particle sizes. The influence of the detachment force direction is shown by comparing tangential and normal adhesion forces measured using particle reentrainment in a turbulent air flow and the vibration method, respectively. The surface roughness of the substrate and the relative humidity are shown to significantly influence the measured adhesion forces. For the calculation of the adhesion forces, an approach by Rabinovich was combined with approximations of plastic micro asperity flattening. The Rabinovich approach accounts for roughness effects on the van der Waals force by incorporating the rms roughness of the interacting surfaces. rms-values of the particles and substrates were measured with atomic force microscopy at different scanning areas.     

Nano-tribological study on a super-hydrophobic film formed on rough aluminium substrates

A novel super-hydrophobic stearic acid (STA) film with a water contact angle of 166^o was prepared by chemical adsorption on aluminum wafer coated with polyethyleneimine (PEI) film. The micro-tribological behavior of the super-hydrophobic STA monolayer was compared with that of the polished and PEI-coated Al surfaces. The effect of relative humidity on the adhesion and friction was investigated as well. It was found that the STA monolayer showed decreased friction, while the adhesive force was greatly decreased by increasing the surface roughness of the Al wafer to reduce the contact area between the atomic force microscope (AFM) tip and the sample surface to be tested. Thus the friction and adhesion of the Al wafer was effectively decreased by generating the STA monolayer, which indicated that it could be feasible and rational to prepare a surface with good adhesion resistance and lubricity by properly controlling the surface morphology and the chemical composition. Both the adhesion and friction decreased as the relative humidity was lowered from 65% to 10%, though the decrease extent became insignificant for the STA monolayer. The project supported by the National Natural Science Foundation of China (50375151, 50323007, 10225209) and the Chinese Academy of Sciences (KJCX-SW-L2)     

ENERGY DISSIPATION IN TAPPING MODE ATOMIC FORCE MICROSCOPY

原子力显微镜有多种成像模式,其中轻敲模式是最为常用的扫描方式.轻敲模式能获取样品表面形貌的高度信息和相位信息,其中相位信息具有更多的价值,如能反映样品的表面能、弹性、亲疏水性等.依据振动力学理论,相位与振动系统的能量耗散有关.探针样品间的能量耗散对于理解轻敲模式下原子力显微镜的成像机理至关重要,样品特性和测量环境会影响能量耗散.本文在不考虑毛细力影响下,基于JKR接触模型,给出了探针样品相互作用下的加卸载曲线,结合原子力显微镜力曲线实验,给出了探针-样品分离失稳点的位置,从而计算一个完整接触分离过程的能量耗散,进而讨论考虑表面粗糙度对能量耗散的影响.在轻敲模式下考虑毛细力影响,通过特征时间对比,证明挤出效应是液桥生成的主导因素,在等容条件下,用数值方法计算了不同相对湿度对能量耗散的影响.通过一维振子模型,简要说明原子力显微镜相位像与样品表面能、杨氏模量、表面粗糙度、相对湿度之间的关系.分析表明,表面粗糙度和环境湿度均会引起相位的变化,进而认为它们是引起赝像的因素.     

海上浮油定位浮标的力学分析与尺寸优化

海面发生原油泄漏后,采用海上浮油跟踪定位浮标锁定油污飘逸轨迹,实现对溢油的跟踪定位. 定位浮标的定位能力受到浮标内部的吸油材料对原油的吸附能力以及海洋风的影响,吸附能力越强则定位浮标与油污结合越紧密,定位效果越好. 首先,从吸油材料受力分析出发,通过室内实验,采用杠杆放大原理对吸油材料受到的风载荷进行测量,并研究风载荷随着吸油圆盘直径的变化趋势,得到了风载荷随着圆盘直径的增大呈线性增大;对吸油材料受到的黏附力随着吸油圆盘直径的变化趋势进行研究,表明黏附力与圆盘直径呈二次方程关系,随着圆盘直径的增大,黏附力会急剧增大. 其次,对黏附力的组成进行分析,研究认为黏附力由分子间作用力、液体表面张力及剪切黏性力组成,对这3 种作用力的大小进行测量计算对比,发现分子间作用力为主要作用力,液体表面张力及剪切黏性力比分子间作用力低1~2 个数量级. 最后,基于浮标正常工作时黏附力应该大于风载荷的原则,将实验得到的风载荷和黏附力随着圆盘直径变化规律推广到大风速和大圆盘直径下,研究了恶劣海况条件下圆盘直径变化对浮标受力的影响,得到了适应恶劣海况条件的圆盘最小直径为38 cm.      

倾斜仿生刚毛的设计?制备及黏附性能研究

依据JKR理论,从几何仿生角度出发设计并制备了2种具有斜截面的倾斜人工刚毛.理论计算表明:刚毛支杆倾斜20°,斜截面为45°时,刚毛具有较为理想的黏附性能,且具有这种结构的刚毛可便利地调控吸附和脱附.利用微摩擦试验机测试了人工刚毛表面的黏附性能,结果表明:在无滑移的平面接触下,刚毛阵列的黏附力随法向载荷增加而增大,当法向载荷超过一定值后,黏附力增加缓慢并趋于饱和;法向载荷为30mN时,刚毛无滑移卸载所产生黏附强度约为2.4kPa,略高于逆向滑移后卸载所产生的黏附强度2.2kPa,而正向滑移后卸载所产生的最大黏附强度较逆向滑移后卸载有明显增加,达3.1kPa,增加了近41%;短距离的滑移可增加黏附力,但随着滑移距离加大,黏附力随之下降.     

Adhesion model of side contact for an extensible elastic fiber

For accurately predicted adhesion laws of fibrillar structures contribute to the rational design of high-performance biomimetic adhesives, an adhesion model is proposed to study the directional adhesion behavior of an extensible elastic fiber that contacts a rigid smooth surface with its side surface under the coupling effect of normal and shear forces, based on the extensible Euler Bernoulli beam theory and the surface energy concept. The deformed configuration of the fiber is obtained analytically, and on the basis of this result, the detachment mode and the normal pull-off force of the fiber for a given shear force are predicted directly. It is also found that, due to the extensibility of the fiber, there exists a maximum normal pull-off force (MNPF) when an optimal shear force is applied. The MNPF will be enhanced by increasing the axial stiffness, and reduced by increasing the bending stiffness. In addition, generating an optimal pre-tension in the adhered part of the fiber will maximize the MNPF. The derived adhesion law is expected to contribute to the optimal design and applications of single-level fibrillar adhesives.