PhDs in 2017
date: 21.12.2012
In 2017 two PhD theses defence took place within the Laboratory for Tribology and Interface Nanotechnology.
Dr. Blaž Brodnik Žugelj: Real contact area characteristics on the micro scale
In conventional engineering approach, contact conditions are calculated with the assumption of nominal contact area between two surfaces, which is greater than actual contact area. Consideration of such assumptions leads into calculations of milder contact conditions as they appear in real. In this PhD thesis we investigated the influence of engineering-relevant material and topographic properties on the actual behaviour within the contact of two nominally flat surfaces under static loading conditions. For this purpose, a novel test rig was developed, which enables the analysis of the contact on a submicron scale. The results showed that at the macro yield stress metals exhibit similar contact behaviour, while the influence of topographic properties is more prominent. On the contrary, polymer shows different contact behaviour than metals. In-depth experimental analysis of asperity deformations showed that the contact behaviour is strongly affected by the material and topographic changes of the contacting asperities. The obtained experimental results were compared to the predictions of well-established theoretical contact models. Due to the assumptions of theoretical contact models, large deviations between the predicted and actual contact behaviour were observed.
Dr. Kosta Simonović: Model for the Friction prediction of the Diamond like Carbon Coatings (DLC) in the Boundary Lubrication Conditions
This Ph.D. thesis studies individual and synergistic contributions of the Anti-Wear (Zinc dialkyldithiophosphate, ZDDP), Friction-Modifier (Glycerol isostearate, GIS) and Automotive additive package to the friction of the steel and DLC contacts across a broad range of boundary-lubrication contact conditions. A design-of-experiments was used to enable broad-range experimental matrix and Elastic-net regression technique was used to develop friction models and friction maps depending on the most relevant contact parameters, namely pressure, roughness, speed, and temperature. Moreover, obtained models are based on the experimental parameters and their interactions whose statistical significance has been undoubtedly proven. The major conclusion is that statistically identified interactions of the experimental parameters show which of the parameters interfere and compete in their influence on the coefficient of friction, hence, which of the investigated parameters must be simultaneously considered if proper conclusions are to be made. What is more, it is shown that single parameter investigations of the boundary lubrication regime do not reveal true properties of the contact/lubricant combination in the whole range of the boundary lubrication regime.