Wear resistant low friction coatings for machine elements

University dissertation from Uppsala : Acta Universitatis Upsaliensis

Abstract: By the introduction of machine elements made of light metals, e.g. Al or Mg alloys, which are coated with a material providing high wear resistance and low friction coefficient, both economical and environmental benefits can be gained. A high resistance against wear results in increased lifetime and lower costs for spare parts and maintenance. This also means fewer production stops and less machine downtime. Moreover, a vehicle of lower weight and with reduced friction losses will consume less fuel.The light metal alloys display low hardness and elastic modulus and in addition they have a high tendency to stick to the countersurface in sliding contact. Hence, to be used in tribologically demanding applications, they must be coated with a material providing low friction and wear. Due to the thin and brittle nature of the available coatings an intermediate load-carrying layer has to be introduced to protect the base material from large deformations and the brittle surface coating from cracking.In this thesis both experiments and theoretical simulations has shown that the load-carrying layer between the soft and compliant base material and the thin brittle coating should have high elastic modulus and hardness as well as a for the contact situation sufficient thickness.A number of vapour deposited coatings have been investigated regarding their tribological behaviour with special emphasis on the mechanisms providing low friction. When sliding against steel, it was found that carbon-rich coatings show much lower friction and wear of the countersurface than nitrogen-rich coatings. The explanation is that steel has a tendency to stick to nitrogen-rich coatings, leading to steel against steel contact, but not to carbon-rich coatings. Another explanation is that material is transferred from a carbon-rich coating to the steel and this gives an easily sheared contact with low friction coefficient.

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