In the global design of an hydraulic jack or a shock absorber, the tribological problems lie on one hand in the definition of the surface of the piston rod in terms of hardness, scale of roughness and morphology, on another hand in the definition of adequate fluid, seals and guiding elements with respect to such surface, both definitions ensuring that the system will demonstrate the lowest possible level of fluid leakage, together with the lowest and most stable values of friction coefficient and wear rate.
Abrasion and corrosion resistance being needed as well as tightness and smooth friction, two surface treatments are presently used: hard chromium plating (with ou without nickel underlayer depending on the corrosion resistance required), and oxynitrocarburizing. The latest involved in one of its recent development a first stage of ferritic nitriding nitrocarburizing is molten salt with a controlled catalyst, followed by an oxidizing sequence and a further step of mechanical finishing to adapt the surface roughness to specific operating requirements.
This article investigates the compared frictional behavior of oxynitrocarburized and hard chromium plated steel components, sliding against seal and guiding materials in lubricated conditions. Experiments are made using model test rig and actual hydraulic systems. Comparative friction and wear data are presented. A discussion based on a new approach of surface roughness description is then proposed. This innovative approach enables to provide a unique quantitative parameter to describe the functional properties of a surface sliding in lubricated conditions.