Using Molecular Dynamics (MD) simulation, the current study determined the surface forces between iron oxide surfaces when immersed in methyl oleate. Condensed-phase Optimized Molecular Potentials for Atomistic Simulation Studies (COMPASS) force field was used to model the methyl oleate molecules. For the nano-confinement simulation, the iron oxide wall was modelled from its crystal structure. The nano-confinement simulation model was setup in a manner where the confined methyl oleate molecules were in contact with the bulk molecules surrounding each side of the iron oxide walls. Through the simulation, the load-separation gap profile was obtained by reducing the separation gap between the ferric oxide walls. When the separation gap was reduced from 2.75 nm to 1.88 nm, the load is shown to increase monotonically. Such increase in load bearing ability of the contact is observed to correspond to a more densely packed methyl oleate molecules, reflected by four well-formed layers across the separation gap. As the gap is dropped from 1.88 nm to 1.63 nm, the load instead reduces, indicating deteriorating load bearing ability of the contact. However, the load bearing ability of the contact is then shown to recover when the gap was further reduced till 1.38 nm. This oscillatory load trend is shown to be as a result of a layer of methyl oleate molecules being squeezed out of contact, corroborated by the density profile change where four well-formed layers were reduced to only three layers from 1.88 nm to 1.38 nm gap. This also indicates that the simulated contact exhibits structural forces, known as solvation forces. Thus, the MD simulation discussed in this study is demonstrated to be capable of providing a foundation to allow for a multi-scale simulation, integrating various force laws at different length scales, to study larger scale tribological contacts.

This paper proposes a hybrid (semi-analytic) solution for determining the contact footprint and subsurface stress field in a two-dimensional adhesive problem involving a multi-layered elastic solid loaded normally by a rigid indenter. The subsurface stress field is determined using a semi-analytic solution and the footprint using a fast converging iterative algorithm. The solid to be indented consists of a graded elasticity coating with exponential increase of decay of its shear modulus bonded on a homogeneously elastic substrate. By applying the Fourier Transform to the governing boundary value problem, we formulate expressions for the stresses and displacements induced by the application of line forces acting both normally and tangentially at the origin. The superposition principle is then used to generalize these expressions to the case of distributed normal pressure acting on the solid surface. A pair of coupled integral equations are further derived for the parabolic stamp problem which are easily solved using collocation methods.

In lubricated contact conjunctions film ruptures close to the exit boundary. This significantly affects the load carrying capacity and can lead to direct surface interactions. Nano-scale films (several molecular diameters of the lubricant) are no exception, a fact that has been observed using ellipsometry studies for ultra-thin film conjunctions representative for high storage capacity hard disk drives. Immediately beyond the film rupture an area of cavitation occurs and the continuity of flow condition is breached. It has been shown that for molecularly smooth surfaces solvation effect becomes dominant. This means that the contact exit is subject to discrete drainage of lubricant and may be devoid of a sufficient lubricant for film reformation to occur. This can be a stumbling block in an increasing quest to increase the data storage density of hard disk drives. Wear can become a problem as well as non-uniformity of free surface film at the inlet meniscus. It has been noted that peaks of lubricant can gather in some places, a phenomenon referred to as lubricant mogul. These localized piles of lubricant can exceed the nominally aimed for lubricant film thickness necessary for a given data storage level. This paper carries out an in-depth prediction of ultra thin film lubricant behavior through the contact. Hydrodynamic as well as near surface effects and intermolecular interactions responsible for the supply, formation, cavitation and reformation of thin films in the slider-disk conjunction have been considered.