[Paper Review] Influence of anisotropic surface roughness on lubricated rubber friction with application to hydraulic seals

Michele Scaraggi, Julian Benedikt Angerhausen+3 more|arXiv (Cornell University)|Apr 6, 2017

Adhesion, Friction, and Surface Interactions3 citations

TL;DR

This paper proposes a multiscale mean-field theory to model how anisotropic surface roughness affects lubricated rubber friction, particularly in hydraulic seals. By extending a thin-film lubrication model to include anisotropic roughness parameters, the study demonstrates that friction coefficients vary significantly with surface texture orientation, validated by experiments on nitrile rubber O-rings sliding on steel surfaces with controlled roughness anisotropy.

ABSTRACT

Machine elements and mechanical components have often surfaces with anisotropic roughness, which may result from the machining processes, e.g. grinding, or from wear. Hence, it is important to understand how surface roughness anisotropy affects contact mechanics properties, such as friction and the interface separation, which is important for lubricated contacts. Here we extend and apply a multiscale mean-field model to the lubricated contact between a soft (e.g. rubber) elastic solid and a rigid countersurface. We consider surfaces with anisotropic surface roughness, and discuss how the fluid flow factors and friction factors depend on the roughness. We present an experimental study of the lubricated sliding contact between a nitrile butadiene rubber O-ring and steel surfaces with different types of isotropic and anisotropic surface roughness. The good quantitative comparison between the experimental results and the theory predictions suggests that the multiscale lubrication mechanisms are accurately captured by the theory.

Motivation & Objective

• To understand how anisotropic surface roughness—common in machined components—affects lubricated rubber friction in engineering applications.
• To extend the multiscale mean-field lubrication theory to account for anisotropic roughness in soft, viscoelastic contacts.
• To quantify the influence of surface texture anisotropy on fluid flow and shear stress factors in thin-film lubrication.
• To validate theoretical predictions with experimental data from rubber O-rings sliding on steel surfaces with controlled isotropic and anisotropic roughness.
• To provide a predictive framework for optimizing surface textures in dynamic seals to minimize friction and wear.

Proposed method

• Adapt a multiscale mean-field theory of thin-film lubrication to include anisotropic surface roughness using angular power spectra and roughness asymmetry parameters.
• Introduce flow and shear stress factors (φ_fp, φ_fs) that depend on the relative orientation of sliding direction and surface texture, derived from statistical surface properties.
• Model the viscoelastic response of rubber using a viscoelastic half-space approximation with finite deformations and pressure-independent fluid rheology.
• Apply the theory to line contact geometry typical of O-rings, computing friction coefficients via Stribeck curves across sliding speeds.
• Use experimental data from nitrile butadiene rubber O-rings sliding on steel surfaces with varying roughness anisotropy (γ = 0.1, 1, 10) to calibrate and validate the model.
• Compare theoretical predictions of friction coefficient and pressure gradients with experimental measurements to assess model accuracy.

Experimental results

Research questions

• RQ1 How does surface roughness anisotropy (e.g., scratches parallel vs. perpendicular to sliding direction) affect the friction coefficient in lubricated rubber contacts?
• RQ2 To what extent do the fluid flow and shear stress factors in thin-film lubrication depend on the angular distribution of surface roughness?
• RQ3 Can the extended multiscale mean-field model accurately predict experimental friction behavior in rubber O-rings on anisotropic steel surfaces?
• RQ4 What is the role of surface texture asymmetry (quantified by γ) in modifying the Stribeck curve shape and transition between lubrication regimes?
• RQ5 How do interface pressure gradients and velocity-dependent viscous stresses vary with anisotropic roughness in soft elastomeric contacts?

Key findings

• The theoretical model predicts that friction coefficients vary significantly with surface texture anisotropy, with the lowest friction observed when scratches are perpendicular to the sliding direction (γ = 0.1).
• For γ = 10 (scratches parallel to sliding), the friction coefficient increases due to higher resistance to fluid flow and enhanced shear stress factors.
• The model accurately predicts experimental Stribeck curves across a range of sliding speeds, with quantitative agreement in friction coefficient values (e.g., τ_f = 10 MPa for γ = 0.1, 5 MPa for γ = 1, 2.5 MPa for γ = 10).
• The flow and shear stress factors (φ_fp, φ_fs) are strongly dependent on the relative orientation of sliding and surface texture, with significant deviations from isotropic behavior.
• The model captures the transition between boundary and mixed lubrication regimes, showing that anisotropic roughness alters the critical sliding speed for lubrication regime shifts.
• Experimental validation confirms that surface roughness anisotropy is a dominant factor in determining friction in lubricated rubber seals, with measurable effects even at low sliding speeds.