The tendency of smooth surfaces to stick spontaneously to each other is becoming a serious problem, with: a) the increasing quality in surface finish for many components and systems, b) on miniaturization in mechanical components, and c) in demanded precision of positioning of parts in high-end equipment machines and systems. Surfaces tend to be made smoother in order to gain flatness or in order to fulfill the need for more precise and reproducible positioning of parts. Adhesion or even sticking of the surfaces is a major showstopper for these applications. There are several measures that can be taken in order to reduce spontaneous adhesion. Quantification of the effectiveness of the chosen solution is most often done using an AFM with probes varying from 1 nm to 8 micron of contact diameter. A serious disadvantage in measuring adhesion by sharp tips is the wear of the tips. Sharp tips wear easily, resulting in undefined contact areas. When the real area of contact is not well defined, the quantification of the adhesion force is not significant. In the current study results of AFM measurements from literature with different tip diameters of colloidal probes are compared with measurements we performed using AFM cantilevers with a plateau tip and using probes from large spheres using an alternative setup (UNAT). These methods give results that are in good agreement with values found in literature. Large contacting surface enhance the quality of the measured adhesion values. Another part of the study deals with a deliberately roughening of smooth surfaces to minimize (spontaneous) adhesion. Good agreement has been found with existing results. For the use of larger surfaces it is important that the surfaces to be tested are extremely clean. Particles on smooth surface do influence the measurements quite easily. Especially for larger areas, the possibility of encountering particles on the surface are more likely, when particles are present. For the measurements in this study a lot of care has been taken therefore to remove contamination: particles as well as contamination of organic origin.

Nanometer-thick liquid lubricant films are useful for the lubrication of miniaturized mechanical systems, such as hard disk drives or microelectromechanical systems. However, there are no established methods for measuring the mechanical properties of such thin films, which can be an obstacle to the optimal design of lubrication systems. We previously developed a highly sensitive method for measuring shear forces, which we called the fiber-wobbling method (FWM). In the FWM, we used a ball-ended optical fiber to probe shear and we measured the friction force acting on the probe tip by detecting the deflection of the fiber. By this means we succeeded in measuring friction forces in nanometer-thick liquid lubricant films. However, we could not evaluate the mechanical properties of the films quantitatively because the contact area between the probe tip and lubricant film was unknown. Here, we developed a method for measuring the contact area during shearing of nanometer-thick liquid lubricant films in the FWM.

The mechanical behavior of eukaryotic cell components is critical to cell mobility, division, signaling, and proliferation. Cell components are in such harmony with each other that often mechanical testing of individual components produces less meaningful results than testing performed within living cells. Moreover, cell components are constantly undergoing growth and remodeling, normally through polymerization and depolymerization. An appropriate experimental and modeling framework is therefore essential for cell mechanics. In this study, a special atomic force microscope (AFM) with controlled adhesion area of the cell to a substrate was used to probe the mechanical response of cells to an external force. This experimental set up enables direct control of the applied force and precise measurement of the cell penetration by the AFM tip. A modeling framework that combines the displacement field with the applied force/stress obtained with the AFM is developed to estimate the mechanical properties of each cell component. The tribological relevance of this research is the viscoelastic deformation of cell components under different contact loads.

Conventional friction force microscopes (FFMs) had the disadvantage of low force sensitivity due to mechanical interference between torsion caused by friction force and deflection by normal force. In order to overcome disadvantage, we developed a dual-axis micro-mechanical probe, which measures the lateral force by the double cantilever and the vertical force by the torsion beam. However, the calibration method of the lateral force has not been established. In this study, we present a new calibration method using a step-structure.

Wear at the nanoscale is a key limitation of conventional silicon and silicon nitride atomic force microscope (AFM) probe tips. Tip degradation and contamination induced by tip-sample interactions can result in decreased resolution and uncertainty in AFM measurements. Prediction and control of the wear behavior is challenging since there is no rigorous theory for the wear of a <100 nm asperity. However, ultrananocrystalline diamond (UNCD) and diamond-like carbon (DLC) are potentially ideal materials for AFM probe applications because of their high stiffness and hardness, low surface roughness, low macroscale friction coefficient and wear, and chemical inertness. The nanoscale adhesion and wear behavior of UNCD, DLC, silicon, and silicon nitride AFM probes have been characterized through systematic AFM wear tests and characterization of the corresponding nanoscale modification of the tips through transmission electron microscopy (TEM) imaging, AFM-based adhesion measurements, and AFM-based blind reconstruction of the tip shape. Our results demonstrate that significant reductions in the nanoscale wear can be achieved through the use of these carbon-based materials. We will discuss how the nanoscale wear behavior of the tips can be linked to their intrinsic materials properties through consideration of the mechanics and physics of nanoscale contacts.

In a probe-based ferroelectric recording system a protective coating of lubricant provides low friction and a low wear rate as well as allows the continuous contact of the probe tip with the storage medium. SO5, a member of the commercially available electric connector lubricants polyphenyl ether (PPE), was experimentally proven effective as a lubricant in supporting this probe/media interface and has enabled the demonstration of the probe-based ferroelectric recording technology. The unique stability of SO5 lubricant in an electric field and in the presence of electrons makes this liquid particularly suitable for ferroelectric recording storage devices. A broad range of analytical techniques were used to identify this lubricant as a mixture of alkylated diphenyl ethers having varying molecular weight. A thin layer of this lubricant was uniformly coated on the probe media surface through a dip-lubing procedure. Its thermal stability and wettability was investigated in terms of the requirements for the successful application of this unique ferroelectric recording technology. The impact of this lubricant film on the read/write capability, durability and tribological performance was also explored.

The unique capabilities resulting from combining a scanning tunneling microscope (STM) and a quartz crystal microbalance (QCM) have been used to characterize the heating and wear at the interface of a tungsten tip and Indium substrate with a change in the contact characteristics of the interface occurring for sufficient sliding speeds. The advantage of this system is the ability to probe heat rise from an asperity contact which will aide in developing a more complete understanding of the complex issue of heat generated via friction.

We achieved a viscoelastic measurement of monolayer lubricant films using the fiber wobbling method, which is the highly sensitive shear force measuring method that we have developed. In order to make the measurement possible, we attained the accurate determination of nanometer-sized gap widths between sliding surfaces. In addition, we developed a new method to align the parallelism between the sliding surfaces. We observed the differences in the mechanical responses of sheared lubricant films due to their adsorptivity to the substrate surfaces.

This paper reviews the application to tribology research of the techniques of high-vacuum surface science, in particular their integration into high-vacuum tribometers. The state of the art, the varied research interests, and the rationale for requiring such research instruments are discussed. The authors have developed a new type of high-vacuum tribometer that is aimed to the detection and characterization of charged-particle triboemission. This tribometer has been successfully used to count triboemitted charges as channel-electron-multiplier pulses and their energy distribution from different material pairs (i.e., ceramics and semiconductors when scratched by a diamond pin, and ceramics-on-same-ceramics). Instrument enhancement is ongoing to include simultaneous under-vacuum photon counting and surface work-function measurement by Kelvin-probe. The enhanced facility will enable one-of-a-kind experiments by characterizations and correlations of electron and photon emission, and of surface property evolution dynamics during sliding contact.

Biopolymers produced from renewable and inexpensive natural resources, such as natural oils, have drawn considerable attention over the past decade, due to their low cost, ready availability, environmental compatibility, and their inherent biodegradability. In this study, the micro/nanotribological wear behavior of biopolymers of different crosslinking densities prepared from low saturated soy-bean oil (LSS) by Rh-catalyzed isomerization with divinyl benzene and polystyrene are evaluated and compared. Microtribological measurements were performed using a ball-on-flat reciprocating microtribometer using two different probes — 1.2 mm radius Si 3 N 4 spherical probe and a 100 micron radius conical diamond probe with 90° cone angle. Nanoscale tests were done using a DLC coated antimony (n) doped silicon probe of radius ∼200 nm. Wear volumes were estimated from groove geometry using Atomic Force Microscope (AFM) and adhesive and abrasive wear coefficients were evaluated for the materials. Elastic modulus and hardness information were evaluated using tensile test and microhardness tests respectively. Correlations between cross-linking density and wear behavior were observed. These results provide some insight into the wear behavior of biorenewable materials.

A new rig to test journal fluid film bearings was designed, manufacture and installed at NASA Glenn Research Center in Cleveland, Ohio, USA. This rig can apply total radial loads to 133,000 N (30,000 lbs) and can rotate the shaft to speeds to 13,000 RPM. The test bearing has a diameter of 68 mm and is 38 mm long. Two such bearings are used to support the total load. The shaft is also supported by two fluid film bearings. The rig is well instrumented for measuring oil flow, oil inlet and outlet temperatures, bearing sleeve temperatures along its circumference and the oil temperatures in oil supply pockets. The shaft position is monitored by proximity probes. Vibration levels at four locations are also displayed and recorded. Preliminary tests of wave bearings at 8,000 RPM and loads to 20,000 N (4,500 lbs) show a good correlation between the test and prediction data.

In this work, we have compared the friction behavior of two different materials (a) mica and (b) ultra-high molecular weight polyethylene (UHMWPE) at two length scales. The friction experiments were carried out at the nanoscale with an atomic force microscope (AFM) and at the microscale, with a custom-built microtribometer. The material interface (Si 3 N 4 probe) and the environmental conditions (RH < 10%) were kept the same at both the scales. The friction data obtained were analyzed for dependence on normal load or contact area, based on which, a coefficient of friction has been reported or an appropriate contact mechanics theory was applied and an interfacial shear strength value was calculated for the material pair. Friction between a silicon nitride and UHMWPE interface resulted in contact area dependence at both the length scales, for the applied load ranges of our experiment. Friction between silicon nitride and mica at the nanoscale showed an initial nonlinearity and then exhibited damage and linearity with normal load beyond certain loads. At the microscale, the mica-silicon nitride interface resulted in a linear friction behavior.

The feature sizes on Integrated Circuits (ICs) continue to decrease to provide higher device densities and smaller chip designs. To accomplish this, current fabrication and processing technology must be advanced to achieve these goals. In particular, Chemical Mechanical Polishing (CMP), which is used for planarization of wafers and logic circuit components during IC fabrication, can cause severe surface damage to components in the form of delamination or distortion of surface features. CMP utilizes polishing particles suspended between a polymeric pad and the substrate to be polished. To control the process with higher precision the fundamentals of friction between CMP surfaces need to be analyzed. To investigate the friction contributions of the polishing particles in the CMP process, individual CMP abrasive particles are modeled by a silica atomic force microscope (AFM) probe with a radius of curvature on the order of 200 nm that is utilized in a scanning probe microscope (SPM). Lateral forces are measured that occur in simulated polishing of silica substrates and polyurethane pad material in a liquid environment. Results are obtained as a function of pH and environment and are compared with macroscopic friction results obtained using a high precision tribometer with a glass ball.

In this study the friction and wear behavior of medical grade ultra-high molecular weight polyethylene (UHMWPE) (GUR 1050 resin) were evaluated as a function of polymer crystallinity. Crystallinity was controlled by heating UHMWPE samples to a temperature above its melting point and varying the hold time and cooling rates. Degree of crystallinity of the samples was evaluated using differential scanning calorimetry (DSC). Quantitative friction experiments were conducted at two different scales. A custom-made microtribometer with commercially available spherical Si 3 N 4 probes in dry conditions was used to test friction at the microscale. An atomic force microscope with commercially available Si 3 N 4 probes under dry conditions was used for nanoscale experiments. A higher degree of crystallinity in the UHMWPE resulted in lower friction force and an increase in scratch resistance at both scales. Reciprocating wear tests preformed using the tribometer show that higher crystallinity also results in lower friction, as well as lower wear depth and width.

Using a coarse-grained molecular dynamics simulation based on the bead-spring polymer model, we reproduced the film distribution of molecularly thin lubricant films with polar end groups coated on the disk surface and quantified the film surface morphology using a molecular-probe scanning method. We found that the film surface morphology changed periodically with increasing the film thickness. The monolayer of a polar lubricant that entirely covers the solid surface provides a flat lubricant surface by exposing its nonpolar backbone outside of the monolayer. By increasing film thickness, the end beads aggregate to make clusters, and bulges form on the lubricant surface accompanying an increase in surface roughness. The bulges continue to grow up even though the averaged film thickness reaches or exceeds the bilayer thickness. With further increases in film thickness, the clusters start to be uniformly distributed in the lateral direction to clearly form a third layer. As for the formation of fourth-fifth layers, the process is basically the same as that for the second-third layers. Through our calculations of the intermolecular potential field and the intermolecular force field, these values are found to change periodically and are synchronized with the formation of molecule aggregations, which explains the mechanism of forming the layered structure that is inherent to a polar lubricant.

Small (1 mm diameter) probe tips are used on sub-micron accuracy scanning coordinate measuring machines. One of the larger contributions to measurement uncertainty is micron-scale probe tip wear. Wear was generated on ruby and silicon nitride probe tips under precisely controlled dry sliding conditions on ground, carburized and hardened steel surfaces. Volumetric wear was measured using an interferometric microscope. Wear volume data was then expressed in terms of Archard wear coefficients. The measured wear rate of silicon nitride sliding on steel was approximately twice that of ruby.

This research studies the effects of microstructures of poly(vinylidene fluoride) (PVDF) on its adhesion force with the probe of an atomic force microscope (AFM). The adhesion problem is currently a bottle neck for the development of micro-electro-mechanical systems (MEMS). Understanding the surface adhesion mechanisms is an important step to advance MEMS technology. In the present work, we fabricated PVDF thinfilms using spin casting and in situ corona poling methods. The microstructure was thus changed from α to the mixture of β and γ phases. Surface forces were then evaluated using an AFM. It was found that the adhesion forces between the AFM probe and the polymer surfaces were affected by microstructures. This paper discusses details of molecular structures. We propose a surface molecular structural model in order to achieve the level of adhesion forces desired.

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.