Adhesion and friction studies of silicon surfaces processed using a microparticle-based method

A surface processing method that combines electrostatic deposition of microparticles and dry etching is utilized to modify the surface topography of silicon surfaces to reduce adhesion and friction force. Microscale adhesion and friction tests were conducted on flat (smooth) and processed silicon surfaces with a low elastic modulus thermoplastic rubber (Santoprene) probe that allowed a large enough contact area to observe the feature size effect. Both adhesion and friction force of the processed surfaces were reduced comparing to that of the flat surfaces.     

Express evaluation method of internal friction parameters in molding material briquettes

The purpose of this work is to develop an express method of evaluating the coefficient of internal friction and cohesive strength of molding material briquettes. Advantages and disadvantages of modern evaluation methods of internal friction parameters in particulates have been considered, and the need to improve them has been substantiated. An alternative method based on measuring the position of shear planes and the briquette cracking pressure in uniaxial compression strength tests has been proposed. The theoretical justification of the method has been provided, and examples of implementation have been given. The research results may be used in the process of educating students of technical specialties, as well as at research and design organizations.     

Measurement of the friction characteristics on materials surfaces using the pendulum microoscillations method

Free oscillations of a physical pendulum, with two balls placed on the surface of the silicon plate for the displacement of the balls substantially smaller than the radius of point of contact, were investigated experimentally and theoretically. The friction mechanisms resulting in the experimental dependence of the swing on time and the influence of surface elasticity were considered. An estimation of the coefficients of deformation and adhesive frictions for two plates of different surface qualities was carried out.     

Rubber friction on rough and smooth surfaces

Some rigorous contact mechanics is used in a dynamic stiffness approach to generate a new theory for hysteresis sliding friction on some ideal peak shapes. These were the two- and three-dimensional projections: cylinder, wedge, sphere and cone, configured singly or as a periodic array. The theory is then extended for a ‘single roughness order’ i.e. identical peaks arranged with a randomly distributed envelope. A simple algebraic expression is obtained that is closely linked to the rubber complex modulus, with friction bandwidths extending over several decades. Several other secondary effects are introduced: multiple roughness orders, adhesion, stick-slip behaviour, friction magnification from either moments or atmospheric pressure, as these influence the observed friction bandwidth and amplitude. The sliding friction theory and secondary effects are compared to the measurements of Grosch [K.A. Grosch, The relation between the friction and the visco-elastic properties of rubber, Proc. Roy. Soc. Lond. A 274 (1963) 21–39] and Barquins et al. [M. Barquins, A.D. Roberts, Rubber friction variation with rate and temperature: some new observations, J. Phys. D: Appl. Phys. 19 (1986) 547–563], and are able to account for the friction amplitude and bandwidth for both gum and carbon loaded rubbers.     

The adhesion and friction of smooth rubber surfaces

In an earlier study of adhesion between smooth rubber and rigid surfaces time effects were apparent. It now appears that under non-equilibrium conditions these effects largely determine the magnitude of the force required to separate adhering surfaces. This paper presents an optical study of contact area time effects between such surfaces and shows in a simple way how these optical observations may be used to predict the rate of rolling of a ball bearing on smooth rubber, the time taken to detach itself under gravity and its resilience when bouncing on smooth rubber. The friction when a rigid surface slides over smooth rubber under conditions where Schallamach waves are generated is also shown to be quantitatively related to their mutual adhesion.     

Interaction of anti-wear additives with friction surfaces

The effect of esters of phosphonic acids on frictional wear under diverse loads and sliding speeds has been investigated. The efficiency of such phosphonates as anti-wear additives depends on their structure and varies with the frictional modes. Principal groups of phosphonates have been grouped as esters which are ineffective under both high and low loads (esters with no chlorine atoms); esters which are effective under both high and low loads (CCl₃-group containing esters), and esters which are effective under either high or low loads. The interaction of phosphonates with metal (steel) has been studied. Dissimilarity in the efficiency of esters as lubricating oil additives is explained by their different adsorption and chemical actions.     

Analysis of friction contact between wavy surfaces

The objective of the present work is to study the tribological behaviour of two contact bodies coated with solid lubricant films. The suggested model relaxes two restrictions of the classical Hertz theory by introducing the friction effect and the elastic-plastic behaviour as irreversible effects.The analysis is based on the automated direct procedure developed by Mahmoud and Salamon and an extension of that model to deal with plastic flow and Coulomb friction between contact surfaces. The proposed model is applied to a typical example of a wavy surface pressed against a flat strata. Both the wavy surface and the flat strata are coated with a hard lubricant film. The history of the advancing contact and the initiation of the plastic flow are determined through the procedure of the model.     

Ni nanodot-patterned surfaces for adhesion and friction reduction

Surface nano-patterning with Ni nanodot arrays was investigated for adhesion and friction reduction of contacting interfaces. Self-assembled anodized aluminum oxide (AAO) templates in conjunction with thermal evaporation was used to fabricate nano-patterned surfaces with ordered Ni nanodot arrays on Si substrates. Surface morphology of the Ni nanodot-patterned surfaces (NDPSs) was characterized by scanning electron microscopy (SEM). Adhesion and friction studies on a Ni NDPS and a baseline smooth Si(100) surface were conducted using a TriboIndenter employing a diamond tip with 100 μm nominal radius of curvature. The results show that the ordered Ni nanodot-patterning reduced the adhesion forces and coefficients of friction up to 92 and 83%, respectively, compared to those of the smooth silicon surface. Surprisingly, the nanoscale multi-asperity contact between the diamond tip and inhomogeneous Ni NDPSs under low loads follows a continuum contact mechanics model.     

Rigidity and damping characteristics of hydrodynamic sliding bearing with deformable working surfaces

The model of a hydrodynamic sliding bearing has been developed that takes into consideration the effect of the deformation of sliding surfaces on the bearing characteristics. The deformations of the sliding surfaces are determined when solving the problem of elastohydrodynamic contact of the journal and bearing with account for the pressure in the lubricating film. Variation in the clearance size at the deformation of the bearing and shaft surfaces is found by iterations when solving jointly the problems of lubricant flow and working surface deformation. Elastic deformations of the working surfaces are calculated using a two-dimensional boundary element model and a three-dimensional finite element model of the shaft and bearing. The method of finite elements is applied to calculate the parameters of lubricant flow in the bearing based on the solution of Reynolds equation in the disturbed form. The rigidity and damping characteristics of the sliding bearing with the deformable surfaces are compared to those of the bearing with the rigid surfaces; the results of the two-dimensional model of bearing deformation are compared to those of the three-dimensional one.     

Mechanisms of elastic contact and friction between rough surfaces

The mechanisms of elastic contact and friction between two rough surfaces were analysed, assuming that the surface asperities were spherical, at least near their summits, and that they contacted elastically. It was found that the real contact area and the number of contact spots are approximately proportional to the load, whereas the mean area of contact spots and the mean pressure at the contact areas are almost independent of load. The frictional force F is almost equal to sA_r, where s is the shearing strength at the contact area and A_r is the real contact area. The experimental results using Pyrex glass specimens agreed within experimental limits with the theoretical results.     

摩擦离合器的热力参数设计

研究表明,摩擦离合器的热力学参数是影响其摩擦磨损性有的决定因素,应用以临界摩擦温度为设计准则,以有限元方法为计算手段的摩擦离合器的热力参数的设计方法,给出摩擦电磁离合器的极限摩耗功能线。     

Computer simulation of the friction of rough surfaces

A program for the computer simulation of the friction of two rough surfaces has been developed. The program makes it possible to predict the coefficient of friction and limiting contact pressures, which restrict the range of normal wear, for friction units with and without fairly soft coatings. The computer model has been tested using a comparison of the results of numerical simulation with the experimental data. The results can be used to predict the tribological characteristics of rough surfaces.     

The method of assessment of the of friction contact fatigue parameters of technological ceramics and crystals

Two formulas were derived earlier using the theory of wear of I.V. Kraguelskii to calculate the wear resistance of friction pairs. To use them it was necessary to determine the friction contact fatigue parameters. The present paper deals with the method of experimental calculated assessment of these parameters and the results of its validation using wear resistant nanostructured alloyed zirconium dioxide crystals and engineering ceramics.     

Hydrodynamical modelling of lubricant friction between rough surfaces

The steady Couette flow of a Newtonian fluid between two plates, one of them a plane, the other one provided with riblets aligned perpendicular to the flow direction, is taken as a model for lubricant friction with wall roughness. In cases where the amplitude of the riblets is small compared to the riblet spacing, Reynolds lubrication approximation leads to an explicit solution. In contrast to this, a treatment of the full hydrodynamic equations is required for higher amplitudes. Under creeping flow conditions, an analytical treatment of the Stokes equations based on complex function theory allows for a reduction of the problem to the solving of ordinary differential and integral equations for functions of one variable. After this problem reduction, the resulting equations are solved by Fourier analysis and computer algebra.The resulting streamline patterns of the flow reveal the formation of vortices under certain conditions. Since these vortices act like a kind of fluid roller bearings, their influence on the drag force and material transport of the lubricant is studied.     

The friction of sliding surfaces carrying boundary layers

Conventional boundary films are thought of as ``solid' layers in which shear strength is directly proportional to local contact pressure. However, recent studies suggest both that the properties of adsorbed or deposited surface films may be much more com- plex than this and that the details of surface topography cannot be ignored in determining the overall coefficient of friction of real engineering contacts. In this contribution we examine both these effects.     

A numerical model of friction between rough surfaces

This paper describes a computational method to calculate the friction force between two rough surfaces. In the model used, friction results from forces developed during elastic deformation and shear resistance of adhesive junctions at the contact areas. Contacts occur between asperities and have arbitrary orientations with respect to the surfaces. The size and slope of each contact area depend on external loads, mechanical properties and topographies of surfaces. Contact force distribution is computed by iterating the relationship between contact parameters, external loads, and surface topographies until the sum of normal components of contact forces equals the normal load. The corresponding sum of tangential components of contact forces constitutes the friction force. To calculate elastic deformation in three dimensions, we use the method of influence coefficients and its adaptation to shear forces to account for sliding friction. Analysis presented in Appendix A gives approximate limits within which influence coefficients developed for flat elastic half-space can apply to rough surfaces. Use of the method of residual correction and a successive grid refinement helped rectify the periodicity error introduced by the FFT technique that was used to solve for asperity pressures. The proposed method, when applied to the classical problem of a sphere on a half-space as a benchmark, showed good agreement with previous results. Calculations show how friction changes with surface roughness and also demonstrate the method's efficiency.     

Measurement of static friction coefficient between flat surfaces

An experimental device, called the centrifugal friction apparatus (CFA), was developed which is capable of measuring the static coefficient of friction, μ, between flat surfaces with very low normal forces. Experiments were performed with the CFA using the polished surface of n-type arsenic doped silicon wafers to investigate static frictional behavior under differing light load conditions. For loads less than 100 mgf, the average μ and the standard deviation were found to increase as the normal load decreased. For loads between 100 mgf and 1.1 gf μ was found to be a constant in the range of 0.30−0.40. Results suggest that despite extreme smoothness the real area of contact is still significantly smaller than the apparent area of contact and μ is still a function of load. Tests with solid particles between the silicon mating surfaces showed that under low load conditions ( <0.9 Pa), the change in μ was negligible for both abrasive particles and for solid lubricants. At the higher loads used ( ≈475–575 Pa) the expected changes in μ were starting to occur for both types of solid particles.     

微变形反射镜主要性能测试研究

微变形反射镜(MEMS-DM)是用于自适应光学中波前校正的重要元件。测试实验中对37单元微变形反射镜的光学影响函数矩阵进行了推导和全面测量,从而验证了其叠加性。由光学影响函数推导出了微变形反射镜的控制电压矩阵,利用电压矩阵校正了变形镜的初始面形。最后,对微变形镜校正波前畸变能力进行了测量和评估,得出关于优化微变形镜设计相关方面的一些结论。     

Magnetization of friction surfaces and wear particles by tribological processes

The relationship between magnetization of a friction surface and wear phenomena is discussed for the case of rubbing of nickel on nickel. Experiments were performed by sliding-friction tests of the pin-on-block type. Magnetization of the friction surface was estimated by measuring changes in the magnetic flux density with a Tesla meter, and the effects of lubrication and various normal loads on changes in the magnetic flux density were examined. Also, the magnetic flux density of wear particles was examined for single particles and particle aggregates. The magnetization of the friction surface was found to be related to surface damage by transfer (adhesion). Individual wear particles and aggregates of fine wear particles were all magnetized in one direction by tribological action, showing that wear particles and transfer particles on the friction surface are the principal source of magnetization.     

Surface characterization and adhesion and friction properties of hydrophobic leaf surfaces

Super-hydrophobic surfaces as well as low adhesion and friction are desirable for various industrial applications. Certain plant leaves are known to be hydrophobic in nature. These leaves are hydrophobic due to the presence of microbumps and a thin wax film on the surface of the leaf. The purpose of this study is to fully characterize the leaf surface and to separate out the effects of the microbumps and the wax on the hydrophobicity. Furthermore, the adhesion and friction properties of the leaves, with and without wax, are studied. Using an optical profiler and an atomic/friction force microscope (AFM/FFM), measurements on the hydrophobic leaves, both with and without wax, were made to fully characterize the leaf surface. Using a model that predicts contact angle as a function of roughness, the roughness factor for the hydrophobic leaves has been calculated, which is used to calculate the contact angle for a flat leaf surface. It is shown that both the microbumps and the wax play an equally important role in the hydrophobic nature as well as adhesion and friction of the leaf. This study will be useful in developing super-hydrophobic surfaces.