Latest PhD defences

date: 31.01.2020

In December 2019 and January 2020 two PhD thesis defences took place within the Laboratory for Tribology and Interface Nanotechnology.

Dr. Maja Kus: Influence of Dynamic Wetting on Friction

majadrIn the PhD thesis we have studied the effect of different organic friction modifiers on wetting with oil and consequently on elasto-hydrodynamic friction reduction. The theoretical background shows that, currently, the role of wettability in tribological applications is still not fully understood, especially with regard to wettability with lubricating oils, which exhibit the so-called spreading wetting behaviour in contact with most metals. In this work, we have evaluated the wettability of steel with oil at 25 °C and 100 °C using static and dynamic wetting parameters. The results show that the dynamic parameters are more suitable for wetting characterization in real tribological applications than the static ones. The results of surface energies and Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) spectroscopy show that all tested additives adsorbed to the steel surface at both 25 °C and 100 °C, and that the adsorbed film significantly increases the oleophobicity of the surface. The Quartz Crystal Microbalance with Dissipation monitoring (QCM-D) results confirm that the tested additives adsorbed from oil on steel at both 25 °C and 100 °C, and that the adsorption is governed by the polarity of the additive. Variations in the molecular structure of the additive (number of polar groups, alkyl chain length, polarity of the functional head-group, saturation) also affects oleophobicity, the trends are the same at 25 °C and 100 °C, this influence being most evident with dynamic wetting parameters. The results of tribological tests in the elasto-hydrodynamic (EHD) lubrication regime at 25 °C and 100 °C show that the tested additives reduce the friction coefficient even under when the surfaces are completely separated by the lubrication film. The coefficient of friction decreases with increasing oleophobicity of the adsorbed film (larger advancing and receding contact angles). Only with the addition of organic friction modifiers to the base oil that have the ability to adsorb to the steel surface, we have reduced the coefficient of friction up to 12.4 % at 25 °C and up to 22.2 % at 100 °C, which presents an important technological contribution.


Dr. Ervin Strmčnik: Influential parameters on operating of water orbital hydraulic motor

ervindrIn this PhD thesis we studied the influential parameters with respect to the operation of an orbital hydraulic motor, the main function of which is the conversion of hydraulic energy into mechanical work. The total efficiency, which depends on the volumetric and hydraulic-mechanical efficiency, was selected as the main criterion for the above-mentioned conversion of energy. Oil and water test rigs were developed and built for the two different working fluids. Research activities were split into two parts. In the first part, we tried to change the design parameters and reduce the internal leakage, which would lead to an increase of the volumetric efficiency of the hydraulic motor. In the second part, we focused on the contact engineering and tribological improvements, which could lead to better hydraulic-mechanical characteristics. The result shows that the design parameters of the hydraulic motor play a very important role. The total efficiency increased up to 5 % when the hole size in the valve plate was changed. The result of a basic hydraulic test shows that the labyrinth sealing with the corresponding annular grooves can significantly reduce the internal leakage. Within the tribological tests we investigated the influence of surface roughness, surface hardness, load, and lubricant on the coefficient of friction, wear loss and wear coefficient in two contacts, i.e., (1) steel/steel and (2) DLC/steel. Considering the coefficient of friction and the wear loss, very promising tribological behaviour was observed for the SS/DLC in water. The result of the tribological test gave us the motivation for further investigations related to the DLC coating, deployed on the outer floating ring of the hydraulic motor. The maximum total efficiency of the modified hydraulic motor was 23 %. A combination of DLC, steel and water represents a promising solution for environmental sensitive applications, where oil should be replaced by water.