PhDs in year 2013

date: 09.10.2014

In the year 2013, four PhD thesis defences took place within Laboratory of Tribology and interface nanotechnology.

tint-logoDr. Janez Kogovšek: Tribological effects of nanoparticles in lubricants

In the doctoral thesis we addressed the tribological effects and lubrication mechanisms of MoS2 nanotubes added to oil. We compared the effect of these nanoparticles to the effects of various solid lubricant particles and evaluated the influence of the properties of the particles on the reduction of friction and wear in the boundary lubrication regime. We have studied the lubrication mechanisms of the MoS2 nanotubes. We have determined the influence of the surface roughness, running-in and material of the contacting bodies on the lubrication with the MoS2 nanotubes in oil by investigating their behaviour in the steel and DLC-coated contacts in all lubrication regimes. We investigated and compared the formation of a tribofilm from the MoS2 nanotubes on the steel and DLC coated surfaces.

Dr. Aljaž Pogačnik: Effects of physical parameters on tribological properties of polymers for gears

The doctoral thesis analyses the effect of different physical parameters (normal load, sliding speed, temperature) on friction, wear and wear mechanisms of polymer materials, which are commonly used for gears. Wear maps and critical contact conditions were determined from extensive tribological investigation of 8 different material combinations of PA, POM and steel. With the analysis of theoretical contact temperatures, worn surfaces and changes in tribological mechanisms, several mechanical and thermal parameters are introduced in order to evaluate the effect of physical parameters on the crucial changes of the wear mechanism of polymer materials. The real contact area of polymer materials was determined using asperity-peak deformation measurements and also with the analysis of plastically deformed polymer surfaces.

Dr. Boris Kržan: Investigation of transmission operating with reduced quantity of liquid lubrication

In the thesis, the load-carrying capacity and surface fatigue life of standard case hardened gear set is compared with the gear system coated with tungsten doped diamond like carbon (W-DLC) protective film applied on the tooth flanks. Tests were conducted on laboratory four-square gear test rig with reduced oil level that corresponds to an immersion depth of three times the modulus of the gear. About 2-fold higher load-carrying capacity and 3-times longer pitting fatigue life was found for W-DLC coated gears in comparison with the standard case carburized gears lubricated with the high-quality conventional gear oil of the similar viscosity.

Dr. Rok Simič: Nanotribology of chemically and physically active additives on diamon-like carbon coatings

The doctoral thesis addresses the two options of lubrication of diamond-like carbon (DLC) coatings; by conventional lubrication by adsorption of polar molecules as chemically active additives or by innovative lubrication using nanoparticles as physically active additives. The effect of both additive types on friction and wear of DLC coatings was evaluated. Lubrication using nanoparticles proved to be inefficient, while the polar molecules decreased the wear of the DLC coatings substantially. The adsorption mechanisms of alcohols and fatty acids on the DLC surfaces were investigated using various surface analytical techniques. The tentative adsorption mechanisms on the DLC surfaces that include the environmental effect, the temperature effect and the effect of tribological rubbing were proposed.

Dr. Gabrijel Peršin: Fault detection and localization of mechanical drives based on data fusion techniques

The thesis focuses on reliable fault diagnosis of mechanical drives, while cosidering integration of several condition monitoring approaches. Partial decisions regarding machine condition, obtained independently from vibration and oil analysis techniques, are used within the fusion process based on the incidence table. The incidence table offers in-depth relations between faults and signatures from oil properties or vibration features, used for estimation of final fault probabilities. Analysis of oil parameters is based on trend change detection, followed by qualitative analysis, which reveals the nature of the ongoing change. Recognition of fault signatures in oil parameters is followed by estimation of fault probability. Vibration analysis is based on spectral kurtosis and filtering, used to extract fault-related non-stationary component from background noise. The component is used for extraction of diagnostic features, which are used for fault-related impact detection, based on k-means clustering and k-nearest neighbours classification. Experimental validation, which included oil contamination experiment, gear pitting, and bearing inner and outer race damage, proved the proposed approach to offer reliable estimation of fault probabilities, by fusion of partial probabilities obtained by oil and vibration analysis.