Exploitable results
The aim of this CRAFT-Project was to develop a tool based on the fractal approach to describe the rod surface topography for design of hydraulic components achieving a high reliability in terms of friction efficiency/corrosion resistance and low leakage of hydraulic fluid. In the hydraulic component the research has been focused on the triplet rod surface/seal/lubricant where all the problems, of friction, corrosion and leakage are encountered. The achievements can be divided in two types : fractal approach as a direct issue and the other results as indirect issue (Know how on wetting, friction, wear, new surface treatment).
Our understanding in ftactal theory has been focused in the study of the correlation in one hand between the fractal dimension of the roughness of the rod and the some roughness parameters, on another hand the degree of correlation between the fractal dimension and some tribological properties of the interface rod/ lubricant/seal. The leakage quantity was used as a parameter of comparison. We have established some correlations that show that the area of contact ( bearing area), and volumetric aspect of roughness are correlated to the capacity of the surface to retain the lubricant as a function of the fractal dimension. An optimal value of fractal dimension for this tribological performance has been obtained.
The thickness of the lubricating film is critical. If the film is too thin, single asperities in the topography are able to penetrate it, with high friction and wear rate as a consequence. Correlations with the motifs of roughness (R, AR), the motifs of waviness (VV, AW) and the local slope of the boundary line have been studied. The result concerning the boundary line of the motifs show a very good correlation between the parameter W/AW, the slope of the boundary line and the fractal dimension. A second interesting correlation has been shown between the leakage and the parameter W/AW.
We have observed that the optimal fractal dimension for the friction force, is the same obtained for the tribological performance found for the bearing index and the good retention of the tin lubricant film.
In parallel, important fundamental knowledge was gained about the processes that are responsible for the wetting of surfaces and the spreading of hydraulic fluids. It has been shown that the microscopic morphology play the most important role in the wetting behaviour of hydraulic fluids tested on rod studied during this project. This result is very important as R confirms that fractal approach is the right tool. Big differences have also been found regarding the characteristics of some hydraulic fluids that are commonly used. One mineral oil and one green oil have been optimized by CONDAT during the course of the project.
The tribological behaviour of different possible sealing materials has been investigated and big differences have been found. Measured friction coefficients vary by more than one order of magnitude. The sealing materials show a high response to lubrication. Especially two new hydraulic fluids, which have been adapted during this project, exhibit excellent friction and wear reducing properties. All tribological results received with the tested sealing materials have been transferred to a data base and combined with the results of the physic-chemical characterization of the hydraulic fluids, which now allows for the first time a direct access to all necessary information for all project partners.
A surface treatment of piston-rod has been optimized during the project to obtain a high value of fractal dimension. It consists on a thermochemical treatment with an electrolytic passivation step before polishing that increase dramatically the corrosion resistance of the rod without significant loss in friction and wear behaviour compared with other thermochemical treatment. Such a treatment is considered to be very interesting for hydraulic jacks manufacturers because R can compete with the Nickel+Chromium surface coating used today for high corrosion resistant piston rods.
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