Structural and compositional changes of tribolayer material induced by unlubricated sliding of aluminum: Experiments and computer simulation .

摘要

Sliding is a process that can drive material far from equilibrium. Commonly, it involves material transfer, mechanical mixing and formation of a tribolayer (or tribomaterial) that controls both friction and wear. Tribomaterial characteristics can be revealed by analyzing wear debris because it detaches directly from the tribolayer. The present dissertation describes tribological behavior of aluminum and characteristics of the tribolayer in terms of both structural and chemical changes during sliding.; Sliding tests were carried out with a pin-on-disk tribometer. Post-characterization included structural, chemical and thermal analysis. Experimental results showed that tribological properties of Al and tribolayer formation depend strongly on test environment, especially the relative humidity. Measurements demonstrate that wear debris contains a large amount of oxygen, but not in the form of a simple oxide. Debris characterization revealed that the tribomaterial consists partly of a highly strained aluminum-oxygen solid solution and partly of hydroxylated material (amorphous). The dependence of frictional behavior on the test environment, in particular the relative humidity, was explained by the formation and removal of soft hydroxylated aluminum.; An experimental technique for high sliding velocity with high pressure between two metals has been developed. Aluminium alloy/stainless steel and pure aluminium/pure copper tribopairs have been studied. SEM and TEM revealed a clearly delineated layer of nanocrystalline material at the aluminum surface. Mixing also occurs at a very fine, possibly atomic, scale.; Quantum mechanical ab initio simulations were performed to complement the experimental results, in particular for an Al-O solid solution. Using a GGA pseudopotential, the relaxed structure and the system free energy were compared. The simulations indicate that tetrahedral sites in aluminum are favored for oxygen in solid solution.; Non Equilibrium Molecular Dynamics simulations using a Lennard-Jones potential were conducted to explore the origin and mechanism of nanostructure formation and mixing at the sliding interface. Sliding results showed shear instability at the sliding interface and generation of eddies and vorticity. It was inferred that nanostructure formation is caused by vorticity. The mixed layer grows parabolically with respect to sliding time and approximately linearly with sliding velocity. The growth behavior is reminiscent of diffusion.