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 stick-slip contact problem is investigated here when at least one of the contacting bodies contains inhomogeneous inclusions. Cylindrical inclusions with parallel axes are considered. The Eshelby’s equivalent inclusion method is used to solve the problem numerically. Interactions between close inclusions are taken into account in the numerical procedure, as well as the coupling between normal and tangential behaviours. It is found that the presence of heterogeneities in the vicinity of the surface contact affects the contact pressure, distribution and subsequently the distribution of shear traction and slip at the interface. The effects of the fiber orientation from the direction of the tangential load and different material properties would be presented.

In recent years, significant academic and industrial research and development has explored novel methods of creating green and environmentally friendly materials for commercial applications. Natural fibers offer the potential to develop lower cost products with better performance, sustainability, and renewability characteristics than traditional materials, particularly in the automotive industry. In this respect, natural fiber reinforced polymer composites have emerged as an environmentally friendly and cost-effective option to synthetic fiber reinforced composites. Hence, in this study, a review of the tribological behavior of natural fiber reinforced polymer composites has been undertaken to better understand their usability for various automotive applications.

The paper investigates the forces that can contribute to the attachment and detachment of the tip of a synthetic fiber to a substrate in normal atmosphere. The contact is subjected to a number of competing forces, which together with the localized contact deflection can promote equilibrium. The pull-off force is predicted as a function of the angle of approach of the fiber toward a substrate and an initial fiber preload.

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.

In this study, we measured temperature dependence of viscoelastic properties of PFPE lubricant that was confined in nanometer-sized gap widths. In the viscoelastic measurement, we used the fiber wobbling method, which is the highly sensitive shear force measuring method that we have originally developed. Our experimental results showed that the temperature dependence of the lubricant’s viscosity was declined at the gap width of less than 5 nm. Since the elasticity also appeared at the gap width of 5 nm, the reduced temperature dependence is considered to be caused by the repression of molecular mobility due to the solidification of the confined lubricant.

Friction tests for two types of carbon-carbon (C-C) composites were performed using a sub-scale aircraft brake dynamometer for 100 stops. Worn surface profiles were measured using a surface profilometer. Roughness and waviness parameters from the profilometry measurements were compared to weight loss, thickness loss, effectiveness, stop time, and torque for each C-C composite type. The results show that when roughness is analyzed, for an increase in torque and effectiveness and a decrease in stop time, there is a decrease in skewness and high spot count; and an increase in maximum valley depth, mean spacing of local peaks, and mean arithmetic wavelength. For waviness profiles, for an increase in torque and effectiveness and a decrease in stop time, there is a decrease in skewness; and an increase in high spot count, maximum valley depth, mean spacing of local peaks, and mean arithmetic wavelength. Furthermore, there are differences in both friction performance and surface topography between the CC composite types which can be attributed to the differences in fiber orientation and microstructure at the wear surfaces.

Articular cartilage is an avascular soft tissue that covers the ends of the bones in diarthrodial joints. Since there are no blood vessels, chondrocytes rely on solute diffusion to receive nutrients and remove waste products. Diffusive transport is thus essential to the operation and maintenance of cartilage tissue. Cartilage is composed mainly of interstitial water and extracellular matrix, which consists of collagen fibers and proteoglycan. The distribution of collagen fibers and proteoglycan impart an inhomogeneous and anisotropic microstructure to the cartilage.

The authors have reported on results from a sliding test, which was conducted using a ball-on-disk type point-contact sliding test apparatus under gradually increased loading. Under slow sliding conditions where formation of an oil film is not expected, friction-resistant properties of grease are better than those of base oil. This result indicates that the soap fibers in grease work to protect the rubbing surface. Grease with long soap fibers shows better frictional resistance properties and offers greater protection against friction at the rubbing surface than grease with short soap fibers. In this paper, we examined factors that influence different frictional resistance properties according to soap fiber structures in the grease. Evaluation results of the friction-resistant properties of soap fibers alone, and the evaluation results of conditions of entraining grease into the rubbing surface area, show that friction-resistant properties vary according to the soap fiber structure in grease as a result of the conditions of introducing grease into the rubbing surface area.

In the head disk interface (HDI) of a magnetic recording system, lubrication caused by a monolayer thick lubricant film is necessary to achieve stable relative motion between a magnetic disk and a magnetic head. Viscoelastic properties of lubricant films should be clarified for improvement of lubrication performance, however, measurement methods have not been established yet. In this study, we present a new method precisely detecting the starting point of asperity contact of sliding solid surfaces in order to measure viscoelastic properties of the molecularly thin lubricant film on the magnetic disk.

The behavior of a nano-scale cylindrical body (e.g. a fiber), lying on a substrate and acted upon by a combination of normal and tangential forces, is the subject of this investigation. As the scale decreases to the nano level, adhesion becomes an important issue in this contact problem. Thus this investigation treats the two-dimensional plane strain elastic deformation of both the cylinder and the substrate during a rolling/sliding motion, including the effect of adhesion using the Maugis model. For the initiation of sliding, the Mindlin approach is used, whereas for rolling, the Carter approach is utilized. Each case is modified for nano-scale effects by including the effect of adhesion on the contact area and by using the adhesion theory of friction for the friction stress. Analytical results are given for the normal and tangential loading problems, including the initiation of sliding and rolling in terms of dimensionless quantities representing adhesion, cylinder size, and applied forces.