•The adhesion force at ∼15% humidity increases logarithmically with contact time.•The contact time plays a dominant role among the experimental parameters that have an influence on the adhesion force.•The adhesion forces with different normal loads and piezo velocities can be quantitatively obtained just by figuring out the length of contact time.•The adhesion force with repeated contacts at one location is in accordance with the contact time dependence.The influences of contact time, normal load, piezo velocity, and measurement number of times on the adhesion force between two silicon surfaces were studied with an atomic force microscope (AFM) at low humidity (17–15%). Results show that the adhesion force is time-dependent and increases logarithmically with contact time until saturation is reached, which is related with the growing size of a water bridge between them. The contact time plays a dominant role among these parameters. The adhesion forces with different normal loads and piezo velocities can be quantitatively obtained just by figuring out the length of contact time, provided that the contact time dependence is known. The time-dependent adhesion force with repeated contacts at one location usually increases first sharply and then slowly with measurement number of times until saturation is reached, which is in accordance with the contact time dependence. The behavior of the adhesion force with repeated contacts can be adjusted by the lengths of contact time and non-contact time. These results may help facilitate the anti-adhesion design of silicon-based microscale systems working under low humidity.

TiO2 particles were prepared by plasma electrolysis with a one-step method in aqueous solution. By adjusting the Ti(SO4)2 concentration, the phase and morphology of the synthesized particles were well controlled. When Ti(SO4)2 concentration was in the range of 0.1–0.3 mol L−1, TiO2 particles with anatase as the main phase were obtained. From transmission electron microscopy (TEM) and inverse fast Fourier transform (FFT) analysis results, the average grain size of the particles was found to be about 4.0–6.0 nm, which was in accordance with X-ray diffraction (XRD) results. Micrographs showed that the surface of the particles obtained in the low concentrations was rough and composed of tiny nano-structures (about 10 nm). With increase of the concentration, the main phase of the TiO2 particles was transformed from anatase to rutile and the grain size became bigger, while particle size became smaller and the surface of the particles was smoother. When the concentration was higher than 0.6 mol L−1, TiO2 particles with rutile as the main phase were synthesized. The average grain size of the anatase increased to 30.0–50.0 nm, and the rutile's size was about 60.0–75.0 nm. By measuring the current density–time curves, the effect of concentration on phase and morphology of the particles has been explained. The function of the cathode material and the generation of TiO2 particles have also been discussed, after analyzing the compositions, microstructure and weight of cathode material before and after plasma electrolysis.

Adhesion forces between a tipless cantilever and a silicon wafer were measured on an atomic force microscope (AFM) at two relative humidities (RH). Experimental results show that the behaviors of the adhesion force are largely influenced by the RH and measurement protocols. The capillary contribution to the measured adhesion force is time-dependent at low RH (31±1%) and time-independent at high RH (52±1%) due to different equilibrium times of water bridges. The time-dependent adhesion force increases logarithmically with contact time until saturation is reached. The effects of loading force, approach velocity, retraction velocity and measurement number on the adhesion force can be explained by the logarithmic contact time dependence of the adhesion force. However, the time-independent adhesion force decreases with dwell time, and increases with retraction velocity. No dependence of the adhesion force on loading force and approach velocity is found. The adhesion force by consecutively measurements on the same location shows fluctuations, and the trend is unpredictable.

•A novel rheometer with the maximum shear rate of 1000000/s has been fabricated.•The rheometer can measure lubricant viscosity and detect solid/liquid boundary slip.•The viscous shear resistance on PFPE coated surface is reduced by about 15–20%.•Possible boundary slip occurred and a slip length of order 10 μm was extrapolated.A novel rheometer with the maximum shear rate of 105 s−1 has been fabricated to measure lubricant viscosity and to detect solid/liquid boundary slip. The gap between two parallel plates is controlled in the range from 20 to 500 μm and the viscous shear torque is measured with a feed-backed laser reflection technique. Test results of the perfluoropolyethers (PFPE) coated sample indicated that the viscous shear resistance is reduced by about 15–20% compared with the bare plate, implying that possible boundary slip occurred at the liquid/solid interface and a slip length of order 10 μm was extrapolated.

The strong focusing and field enhancement effects of a metal nanofinger surrounded by multiple concentric rings are investigated through both COMSOL Multiphysics and finite-difference time-domain (FDTD) simulations. The aspect ratio of the nanofinger is the main parameter determining the full width at half maximum (FWHM) and the strong local field enhancement. The optimal values of the aspect ratio for the maximal enhancement and minimal FWHM are close to 1.8 and 3.0, respectively. Furthermore, the optimal aspect ratio of maximal field enhancement intensity decreases linearly with the incident wavelength, and the optimal aspect ratio of minimal FWHM increases linearly with the metal film thickness. The nanofinger fabricated with the focused ion beam method has a small conical angle, which results in a higher field enhancement and smaller focal spot size than straight sidewall finger. However, the shorter finger defect deteriorates FWHM and field enhancement because of the bias from the optimal aspect ratio value.

The transient behavior of cavitation phenomenon in textured thrust bearings with oil lubricant was experimentally investigated. During experiments, the cavitation phenomenon in different surface textures was observed directly with a high-speed camera. It is shown that cavitation shape and area change according to texture patterns. The bubbles within cavitation zones increase with running time in the beginning and gradually reach a steady state, meaning that a transient period is needed to achieve equilibrium. In addition, the bubble composition was analyzed under the temperature of 35 °C and pressure of 30 kPa. The results show that oil vaporization plays a minor role in the formation of bubbles.

In mixed lubrication, wear is inevitable under the sliding condition due to the existence of the asperity contacts that bear partial normal load. This study proposes a numerical approach to predict the wear process of a cylinder sliding over a ring disk in mixed lubrication. Elastohydrodynamic lubrication of the rough contact, which is a line contact at the beginning of the sliding and gradually becomes a flatten cylinder/disk contact with the wear progress, is simulated with the three-dimensional finite element method. The elastic–plastic asperity contact model is adopted to calculate the asperity contact pressure. Based on the asperity contact load, the cylinder wear is computed with the Archard’s wear law. The wear process is simulated step by step, starting from an initial line contact configuration. In each calculation step, the cylinder geometry profile is updated, and the balance of the externally applied load with the elastohydrodynamic and the asperity contact loads is achieved. Variations of the cylinder geometry profile, the lubricant film thickness, and the friction coefficient are obtained for the whole rubbing process. Reasonable agreements on the changes of the wear scar width and the friction coefficient during the rubbing process between the simulation and the experiment results are obtained.

A complete understanding of the mechanism of boundary lubrication is a goal that scientists have been striving to achieve over the past century. Although this complicated process has been far from fully revealed, a general picture and its influencing factors have been elucidated, not only at the macroscopic scale but also at the nanoscale, which is sufficiently clear to provide effective instructions for a lubrication design in engineering and even to efficiently control the boundary lubrication properties. Herein, we provide a review on the main advances, especially the breakthroughs in uncovering the mysterious but useful process of boundary lubrication by adsorption film. Despite the existence of an enormous amount of knowledge, albeit unsystematic, acquired in this area, in the present review, an effort was made to clarify the mainline of leading perspectives and methodologies in revealing the fundamental problems inherent to boundary lubrication. The main content of this review includes the formation of boundary film, the effects of boundary film on the adhesion and friction of rough surfaces, the behavior of adsorption film in boundary lubrication, boundary lubrication at the nanoscale, and the active control of boundary lubrication, generally sequenced based on the real history of our understanding of this process over the past century, incorporated by related modern concepts and prospects.

This paper proposes a new eddy current method, named equivalent unit method (EUM), for the thickness measurement of the top copper film of multilayer interconnects in the chemical mechanical polishing (CMP) process, which is an important step in the integrated circuit (IC) manufacturing. The influence of the underneath circuit layers on the eddy current is modeled and treated as an equivalent film thickness. By subtracting this equivalent film component, the accuracy of the thickness measurement of the top copper layer with an eddy current sensor is improved and the absolute error is 3 nm for sampler measurement.

Three different ionic liquids (ILs), 1-octyl-3-methylimidazolium tetrafluoroborate ([OMIm]BF4), 1-octyl-3-methylimidazolium hexafluorophosphate ([OMIm]PF6) and 1-decyl-3-methylimidazolium hexafluorophosphate ([DMIm]PF6), were used as additives in the base ester propylene carbonate (PC) for the lubrication of AISI 4340 steel surfaces. Ball-on-disk friction tests were done under different electrical potentials to investigate the synergetic effect of IL concentration and electrical potential on lubrication performance, and electrochemical and ellipsometric tests were conducted to explore the adsorption of IL additives at different potentials. The friction reduction and anti-wear performance of the tested three IL/PC solutions illustrated similar dependence on electrical potential. In the potential range from −0.6 to +0.6 V, friction coefficient increases rapidly. When the potential is more negative than −0.6 V, friction coefficient is at the lower level of about 0.13. When the potential is greater than +0.6 V, friction coefficient is at the higher level of about 0.2. The electrochemical test results show that [DMIm]PF6/PC solution is the lowest in corrosion against AISI 4340 steel among the three tested lubricants. The wear of steel surface in 0.5 mM [DMIm]PF6/PC solution is reduced when electrical potential is shifted to −1.0 V comparing with that at open-circuit potential. The potential-dependent friction and wear behaviors are explained in terms of the variation of the adsorbed ion species and the surface concentration of the adsorbed ions under different additive concentration and electrical potential conditions.

The objective of this study is to correlate the mass and structure of the adsorbed sodium dodecyl sulfate (SDS) on stainless steel surface in aqueous solution with the boundary lubrication behavior, especially the stick–slip phenomenon of the ZrO2/stainless steel friction pair. Surface tension measurement, quartz crystal microbalance (QCM) technique and ball-on-flat friction test were performed in this study. The adsorption isotherm of SDS on stainless steel surface in SDS aqueous solution was measured by QCM, and four-stage adsorption behavior was found at the solid/liquid interface. As the SDS concentration increases, the mass of the adsorbed SDS molecules increases, while the structure of the adsorbed layer changes from monomers to hemimicelles. Tribological properties of ZrO2/stainless steel friction pair in SDS aqueous solution were verified with an UMT-3 tester. Boundary lubrication was emphasized as water-based lubrication easily fails in hydrodynamic lubrication due to its low viscosity. Stick–slip phenomenon was observed in the boundary lubrication of the adsorbed SDS film under certain loading and sliding conditions. Both the static and kinetic friction coefficients were analyzed with respect to the SDS concentration and sliding velocity. The stick–slip phenomenon is related to both of the adsorbed mass and adsorption structure of the SDS boundary film.

In aqueous solutions, sodium dodecyl sulfate (SDS) surfactant can form a boundary film on metal surfaces to provide lubrication for sliding surfaces in contact. Previous studies have demonstrated that the boundary lubrication of SDS film can be inhibited or enhanced substantially by changing the surface potential of the rubbing metal surfaces. In this study, the SDS surfactant was added to a non-aqueous base fluid, propylene carbonate (PC), and the boundary lubrication behaviors of the solution for stainless steels were investigated under different potential conditions. Friction measurement, electrochemical impedance spectroscopy, cyclic voltammetry, and electrochemical quartz crystal microbalance techniques were employed to investigate the lubricating performance and adsorption film of the sodium dodecyl sulfate (SDS) film on two kinds of steels (AISI 316L, AISI 440C) in propylene carbonate (PC) solution. Similar to aqueous SDS solutions, the lubricating performance of the SDS/PC solution depends upon the electrode potential within the potential range from −1.5 to +1.5 V versus Ag/AgCl, which suggests the potential-dependent reversible change in the adsorbed film. When the potential is positive, both friction and wear of the tested stainless steels are relatively lower due to the presence of the adsorbed SDS film. As the potential is shifted to the negative regime, the DS chains in the adsorbed film are replaced by the PC molecules gradually, and friction coefficient increases by 100 % or more, depending on the load condition and the hardness of the stainless steels.

•An eddy current measurement system for Cu thickness in CMP was established.•The nano-scale sensitivity under large lift-off distance was reached by optimizing.•The differential method for signal processing avoids the drift of system.A fast, accurate and in situ method for determination of interconnect Cu film thickness in the range from 200 to 500 nm is a crucial requirement in the stress free polishing (SFP) process which is a novel semiconductor manufacture technique. This paper presents an in situ Cu film thickness measurement system with an optimized eddy current transducer for this purpose. The fabricated transducer has been integrated into a chemical mechanical polishing (CMP) platform for the purpose of testing. Measurement tests for both the Cu film with uniform thickness and the Cu film with interconnects are conducted. The CMP experiment results show that the resolution of the transducer can achieve a few nanometers under 3 mm lift-off distance. With the increase in the density of the underlying Cu interconnects, the measured thickness value of the overlaid Cu film is also increased. By using an on-line differential signal processing method, the drift of the measurement system is removed. The repeatability and instability errors are less than 1.6%. The system can meet the technical requirements for advanced CMP processes.

Wear phenomena in microelectromechanical systems have been systematically studied in recent years. Novel models are needed to quantitatively describe the wear process at microscales. In this research, a bulk-fabricated microtribotester was used to study the relatively stable wear stage of Si-MEMS devices. Before tested in controlled atmospheres, the rubbing surfaces of some microtribotesters were modified with AgCuTi, NiW, Pt or W coatings by the magnetron sputtering deposition. Test conditions were carefully selected to avoid severe wear during the test period. The major feature of the worn surfaces was the formation of bearing islands. A modified Archard wear model, taking the property of the initial surface morphologies into account, was proposed to analyze the factors affecting the size of such islands. It is found that the surface energy is as important as the material strength to determine the wear process. The contour contact ratio about 20 % at the stable wear stage for silicon devices is an intrinsic feature of the tested samples and could not be changed by the working conditions.

Photochemical machining (PCM) was utilized to fabricate microtextures on carbon steel surfaces in this study. A series of experiments were carried out in the two stages of this process, photolithography and wet chemical etching. In the first stage, the influences of photolithography parameters, including spin coating speed, exposure time and development time, on the patterning of photoresist films were investigated, and the optimum process parameters were found. In the second stage, through a trial and error approach, it has been found that the mixture solution of HNO3:H3PO4:H2O = 8.5%:59.5%:32% (mass percentage) is a suitable etchant for the wet chemical etching of carbon steel. Based on the optimum results, the microtextures of circles and right triangles with different sizes were fabricated on the end faces of carbon steel discs. The variation of the end face profiles and etching depths of the microtextures with the etching time was studied. A prediction model of the geometry of the fabricated microtextures was proposed. Its accuracy was verified by comparison with additional experiment results, and its application scope was also discussed. It makes the precise control of microtexture profiles possible, and facilitates the use of precise microtexture profiles in the hydrodynamic lubrication analysis of textured surfaces.

When a textured ring rotates relatively against the other texture-free ring in a parallel thrust bearing, cavitation of liquid lubricant may occur in the divergent zones of the dimples or grooves on the textured surface due to local pressure drops. The Reynolds and Jakobsson–Floberg–Olsson (JFO) models are two widely used cavitation models in hydrodynamic lubrication theory, where the former lacks mass conservation while the latter enforces it. In order to investigate the applicability of the two models to the hydrodynamic lubrication analysis of parallel thrust bearings with surface textures, comparison between experiment and simulation results has been carried out on parallel thrust bearings in terms of cavitation zone morphology in a groove, friction coefficient, and bearing clearance. The results have shown that the observed cavitation morphology in steady state is more similar to the prediction from the JFO model than that from the Reynolds model.

A running-in process is usually intentionally employed as an effective way to make friction pairs in macro-scale machinery more suitable for heavy load conditions. For silicon-based microelectromechanical systems (MEMS) with rubbing parts, however, even a short-time period of running-in under air or nitrogen environment can induce severe damage on rubbing surfaces, which prohibits any possibility of continuing operation. In this article, we demonstrate a new running-in process for MEMS devices, which can effectively reduce undesirable initial wear and thus generate an endurable bearing surface. The new running-in process consists of two steps of tapping treatment. A home-made microfriction test system has been developed to evaluate the effectiveness of this method. Through the tapping treatments, bearing spots on the rubbing sidewalls are restricted on several dominant locations, and the spreading of wear scar often observed in the test conditions without the tapping treatments is eliminated. After the running-in process, the life-time of the tested silicon MEMS devices has been extended in dry nitrogen test environment from 1 min to more than 1 h.

Wear is one of main obstacles to expand functions of microelectromechanical systems (MEMS). While the wear mechanism of surface-fabricated MEMS devices has been extensively investigated, the characteristics of bulk-fabricated Si-MEMS devices have hardly anywhere been reported. In this research, wear tests of bulk-fabricated Si-MEMS devices were carried out on a composite microtribology test system using out-of-chip loading and driving mechanisms. Wear evolution and the effects of initial topology, driving frequency and external normal load have been investigated over the running time from 5 s to 120 min. A transition from adhesive wear to abrasive wear has been observed, which is conceptually analogs to the wear process in surface-fabricated devices. The results of the experimental study indicate that plastic deformation of the silicon substrate down to nano-size and migration of the wear debris are the main characteristics of wear. When the normal load is >56 μN, wear particles can be grinded into pie or mud shape. At driving frequency higher than 2,000 Hz, the agglomerated wear debris can be thrown out of the contact zone, while at 100 Hz it remains on the rubbing surface and causes evolutionary damages. The energy consumption during wear has been estimated by tests at elevated temperatures, which is helpful to understand the poor wear resistance and the mechanical dominant mechanism of wear.

Adsorption of sodium dodecyl sulfate (SDS) surfactant on the surface of gold or graphite in aqueous solutions has received extensive attention in the past. However, few studies have been done on the adsorption/desorption of SDS surfactant at surfaces of engineering materials as well as on their influence on friction behavior. In this article, quartz crystal microbalance (QCM), electrochemical spectroscopy, atomic force microscopy (AFM), lateral force microscopy (LFM), and ball-on-disc friction test have been jointly used to investigate the effects of electrode potential on adsorption and desorption of SDS surfactant, surfactant aggregate morphology on stainless steel surfaces, nanoscale and macroscale tribological behavior in dilute SDS aqueous solutions. Experiment results have shown that DS− anions adsorb on the surface of the stainless steel electrode and form stripe-shaped aggregates at the open circuit potential (+0.03 V vs. SCE), which corresponds to a low friction coefficient. Under the negative potential of −0.4 V versus SCE, the adsorbed aggregates of DS− anions are removed from the stainless steel surface, resulting in a high friction coefficient. By adjusting the electrode potential of stainless steel, both of the surfactant adsorption and tribological property can be controlled in a significant range.

The transfer of perfluoropolyether (PFPE) lubricant from the disk surface to the slider as a function of head-disk clearance has been investigated experimentally. The effects of lubricant thickness, bonding ratio, molecular polarity, and main chain stiffness on the lubricant transfer rate and the critical clearance below which lubricant transfer gets much enhanced are clarified. The critical clearance can be effectively reduced by decreasing the lubricant thickness or increasing the number of polar hydroxyl end-groups per lubricant molecule. Increasing the film bonding ratio or using lubricants with stiffer backbone can significantly decrease the lubricant transfer rate especially below the critical clearance. The results are discussed in terms of the effective disjoining pressure and its slope with respect to the film thickness.

In this article experimental results have been presented on response characteristics of the potential-controlled friction of ZrO2/stainless steel sliding contacts in sodium dodecyl sulfate (SDS) aqueous solutions. Two methods for modifying interfacial potential, by using an electrochemistry station and a signal generator, respectively, of the ball-on-disk contacts, are described firstly. Then friction tests under steady and dynamic potential conditions are reported. From the steady potential experiment result, a potential range in which friction coefficient varies with potential quasi-linearly has been found. The potential range is within the electrochemical window of the system, and neither hydrogen nor oxygen evolution happens. When interfacial potential is modulated within the potential range in the form of a triangular or a sinusoidal wave, by using either an electrochemistry station or a signal generator, friction coefficient varies in the same form between lower and higher levels, as long as the frequency of the applied potentials is lower than a break frequency. When the interfacial potential is changed abruptly from the open circuit potential down to a negative value, a response time ranging between 0.2 and 1.5 s, depending on the magnitude of the potential and the SDS concentration, is observed for friction coefficient to increase from the lower level of about 0.1 to the higher level of about 0.45. When the interfacial potential is elevated suddenly to 0 V by shorting or to a small positive value, friction coefficient can recover from the higher level to the lower level within a short time, 0.5–2 s, depending on the SDS concentration, which is much shorter than the recovery time of friction in the case of just switching-off the voltage. At last, effect of solution temperature on the response time of friction to stepwise changes in potential is also presented.

Ball-on-plate sliding friction experiments were designed and performed to show the possibility of local friction control by electrochemical methods. By partitioning a metal plate into charged and uncharged zones, the friction coefficient in these zones can be differential when an external voltage is applied during rubbing. It is also possible to achieve differential friction at different locations of contact by arranging the position of the auxiliary electrode on which the range of electrochemical effects on friction depends. The morphological differences in the worn surfaces between the different zones are given, together with a discussion on the principles of the electrochemical control of friction.

The unprecedentedly-high mechanical strength and the intrinsically lubricating property make graphene an ideal candidate for atomically-thin solid lubricant. Despite its high potential, graphene was often found to get worn in micro- or macroscale friction tests in ambient conditions. To explore the detailed wear process and evaluate its impact on lubricating performance, we performed tribological experiments on monolayer graphene using a macroscale glass lens under normal loads down to 50 μN. Our ambient experiments show that there existed a critical normal load, below which wear of graphene was undetectable (with friction coefficient on the order of 10−2). Beyond the load threshold, graphene started to become worn and friction slowly increased. In contrast to the abrupt change in nanoscale tests, friction increase due to graphene wear was gradual at the macroscale and it only became apparent (with friction coefficient on the order of 10−1) when graphene damage was substantial. This suggests that a partially worn graphene can remain effectively lubricious at the macroscale. Our comparative experiments also demonstrate that humidity can noticeably minimize graphene wear and substantially extend lubrication life time. The study provides new insights on macroscale wear characteristics of graphene, which is crucial for mechanical applications of this atomically-thin lubricant.

•Sensitivity analysis of metal film thickness measurement of eddy current method.•Sensitivity of an eddy current sensor is not proportional to its Q factor.•Sensitivity can be improved by reducing the resistance of the sensor coil.•Multi-thread wire is better than the single wire for getting higher sensitivity.Measurement of nano-scale copper film thickness is of great importance in the semiconductor industry. The eddy current method is used for the purpose due to its non-destructive and fast dynamic response features. In this paper, an equivalent circuit model is used to get the relationship between the measurement sensitivity and sensor parameters. It is found that the internal resistance of an eddy current sensor plays a primary role in the improvement of the measurement sensitivity beside of the Q factor of the sensor. A simple experimental setup is established and a series of Cu films with the thicknesses ranging from 20 nm to 350 nm are prepared as test samples. Test results indicate that the sensitivity of an optimized sensor made of a lower resistant multi-wire Cu line has better sensitivity than that wound with a higher resistant single Cu wire under large lift-off.