A Simple Mechanistic Model for Friction of Rough Partially Lubricated Surfaces

Abstract

We report experimental measurements of friction between an aluminum alloy sliding over steel with various lubricant densities. Using the topography scans of the surfaces as input, we calculate the real contact area using the boundary element method and the dynamic friction coefficient by means of a simple mechanistic model. Partial lubrication of the surfaces is accounted for by a random deposition model of oil droplets. Our approach reproduces the qualitative trends of a decrease of the macroscopic friction coefficient with applied pressure, due to a larger fraction of the micro-contacts being lubricated for larger loads. This approach relates direct measurements of surface topography to realistic distributions of lubricant, suggesting possible model extensions towards quantitative predictions.

Multiscale Modeling of Rough Contact Interfaceswith Trapped Compressive Liquid Pockets

“…Therefore, it is reasonable to assume that our current model underestimates the lubricant's contribution. Nevertheless, the simplifications applied in our model successfully capture the experimental trend, which was previously unattainable [14]. Fig.…”

“…Two methodologies are adopted for this calculation. Initially, a method described in [14], which excludes lubricant spreading and fluid pressure, is utilized. Conversely, the fully-coupled approach outlined in Algorithm 2 is employed, capturing the impact of lubricant pressure within trapped pockets.…”

“…16 Comparison of friction coefficients obtained in the experiments with the developed model (Algorithm 3). The blue line represents the friction coefficient assessed by excluding lubricated contacts from the total dry contact area [14]. The simulations utilized the lubricant properties (κ 1 and P * ) of mineral oil from Ref.…”

“…However, for small lubricant densities and small liquidsolid adhesive forces, the lubricant may form droplets, some of which may become trapped in between dry contacts, and which then can carry a portion of the contact load. The challenge of trapped fluid has been explored in various contexts, including crack propagation [11], porous media [12,13], and sheet metal forming [14]. A classical solution proposed by Kuznetsov [15] provides an analytical approach to the contact problem between a smooth elastic surface and a rigid periodic surface in the presence of a compressible fluid.…”

“…With BEM the problem dimensions are reduced from volume to surface, significantly reducing computational complexity. Costagliola et al [14] developed a BEM model for a 3-dimensional rough surface of aluminum, measured by confocal microscopy, to elucidate the reduction of friction in partially lubricated surfaces during aluminum deep-drawing. This model focused on solving normal dry contact while modeling partial lubrication by neglecting the fluid's contribution to the pressure field.…”