Fundamental friction phenomena and applied studies on tribological surfaces

This thesis is based on two different projects, one more focused on applied research and one on more basic research. The first project examines the po- tential of nitriding as an alternative to case hardening in tribologically loaded components while the second project involves micro scale studies of the relations between roughness, transfer and friction between metals. The first project consists of an evaluation of the tribological properties of nitrided steels. The aim is to increase the understanding of the wear and fric- tion behavior of different nitrided steels in relation to the choice of steel grade, microstructure, thickness and composition of the compound layer, among other parameters. This study is a part of a bigger project called Surf- Nit, which primary objective is to optimize the nitriding process in order to increase the use of nitrided steels for applications like high stress compo- nents. Today case hardening is the standard heat treatment for these applica- tions but nitriding is both more environmentally friendly and less time con- suming. In the present study, the steel grade with the highest content of ni- tride-forming elements and highest hardness showed the best wear re- sistance, regardless of the composition of the compound layer. Further, steels of a given grade but with different phase compositions of the compound layer showed differences in their wear behaviour. It was also shown that nitrided steels with ε-phase in the compound layer acted more brittle than those containing Υ’. The goal of the more basic project is to increase the understanding of the mechanism behind sliding friction. The main focus has been the relation between friction and material transfer. A better understanding can be of help when developing new tribological materials, for example wear resistant components that can operate without lubrication. It could also enable specif- ic recommendations for surface finishes to avoid material transfer and be of help when trying to make more realistic tribological models. Scratch tests have been performed on samples with different surface roughness and dif- ferent surface composition. It was shown that nano scale topography had a bigger impact on both material transfer and friction compared to micro scale topography. Experiments both in air and in situ in a SEM have been per- formed to determine the effect of presence of air on the friction and material transfer. The experiments in air resulted in more material transfer and higher friction than those performed in vacuum. Both lubricated and unlubricated contacts have been studied in order to see how surfaces otherwise separated by boundary lubrication will be affected if the lubrication fails.