Effect of Sliding Speed, Load, and Lubricant Viscosity on the Friction Behaviour of Soft Porous Lubrication

This study examines how sliding speed, applied load, and lubricant viscosity govern the friction behaviour of soft porous materials under dynamically evolving compression. Soft porous lubrication can generate load support while reducing friction by combining solid-matrix deformation with pore fluid pressurisation, but most existing theoretical models assume fixed compression and cannot capture the coupled response of real bearing systems. To address this gap, a modified Mini Traction Machine (MTM) was used, in which a cone-shaped upper specimen was paired with interchangeable soft porous samples on the lower disc, allowing the porous layer to compress and recover freely during operation rather than being held at a fixed thickness. Six porous materials — four open-cell polyurethane foams and two woven fibrous substrates, spanning a range of thicknesses and pore sizes — were tested dry and under water, glycerol (5% and 10%), and PEG 400 lubrication, across sliding speeds of 5-120 mm/s at constant load and normal loads of 1-7 N at constant speed. Dry friction coefficients ranged from approximately 0.1 (thin woven material) to 0.65 (thick open-pore foam), and results show that, contrary to predictions from steady-state, fixed-compression theory, friction did not consistently decrease with increasing sliding speed or applied load; for most materials the friction coefficient changed only marginally or even increased, indicating a self-regulating feedback loop in which rising pore pressure increases fluid lift, which in turn reduces compression and limits further pressure build-up. Introducing a liquid lubricant markedly reduced friction relative to dry contact, cutting the friction coefficient of the thinnest woven material from roughly 0.10 (dry) to as low as 0.02 with PEG 400 lubrication — a reduction of approximately 80% — although the benefit of increasing glycerol concentration plateaued beyond 5%. These findings demonstrate the limitations of fixed-compression theoretical models for soft porous lubrication and highlight the need for dynamic, compression-free testing protocols when selecting lubricant formulations and operating conditions for advanced low-friction, long-life bearing designs.