Dr. Kalin’s areas of research are wear and friction mechanisms of advanced materials, nanoscale interface phenomena, as well as boundary films for novel green lubrication technologies. He has published over 140 peer-reviewed journal papers, 10 book chapters, 2 books and holds 11 patents, including USA and EU patents. He has been active as a member of Editorial boards of 8 international journals, including Associate Editor of the ASME Journal of Tribology (2006-2012) and being founding Associate Editor of Frontiers in Mechanical Engineering – Tribology section (2018). Since 2012 he serves as the Editor-in-chief of Lubrication Science (Wiley). He has led over 35 large, typically 3-year projects, mostly international and colaborated with industrial partners in over 130 R&D projects. He has got several awards, including a prestigious ASME Burt L. Newkirk Award (2006), Fellow of STLE (2012), the two highest Slovenian state awards, namely Zois Prize (2006) and Zois Award (2015) and the Top 10 scientific achievements at University of Ljubljana (2014). Since 2017 he is a Deputy Chair of International Tribology Council that embraces tribology societies world-wide. Since 2010 he is the Head of the Laboratory for Tribology and Interface Nanotechnology and the Chair for Tribology and Maintenance Technology at University of Ljubljana. Since 2013 he acts as a coordinator of a joint European Master Programme on Tribology of surfaces and interfaces – TRIBOS, run in cooperation of four renown Universities sponsored by European Comission. In 2007-2011 he served as a Vice-dean for research and international affairs and in 2013-2017 as a Vice-dean for master and doctoral studies at the Faculty of Mechanical Engineering. Since 2017 he is a Dean of Faculty of Mechanical engineering.
SUBMICRON SCALE EXPERIMENTAL AND THEORETICAL INVESTIGATION OF THE MULTI-ASPERITY CONTACTS FOR REAL ENGINEERING SURFACES WITH VARIOUS TOPOGRAPHIC AND MATERIAL PROPERTIES
M. KALIN*, B. Brodnik Žugelj
University of Ljubljana, Faculty of Mechanical Engineering, Ljubljana, Slovenia
Keywords: surface topography, material properties, experimental in-situ analyses, modelling, real contact area, asperity deformation.
In conventional engineering approach, contact conditions are typically 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. Actual conditions within engineering contacts strongly depend on load, topographic parameters and material properties of contacting surfaces.
In this study we analysed how relevant engineering roughnesses (Ra = 0.1, 0.5 and 1.0 µm) and different materials (various steels, aluminium alloy and POM polymer) influence the actual behaviour of multi-asperity contacts in the full engineering loading range up to the material’s macro yield stress. Contact pressure, real contact area and deformation of the asperities were simultaneously measured experimentally in the newly developed test rig, which enables direct and in-situ investigation of the real contact area with submicron resolution.
The experimental results showed that the roughness strongly affects the deformation behaviour of the asperities and the real contact area that ranges from 10 to 20 % of nominal contact area at yield stress, depending on the roughness. Surprisingly, at the same Ra roughness and normalised pressure, metals exhibit quite similar type of contact behaviour despite the wide range of material properties covered in this study. On the other hand, polymer behaves very differently.
Moreover, the multi-asperity experimental results were also compared to the most common elastic-plastic statistical models to provide information that is seriously lacking today, especially at the sub-micron scale, which is studied in this work. Experimental results and the statistical models shows good agreement for the smoothest surface under all loads, while for rough surfaces and loads above ≈0.3∙Y the models overestimate the real contact area for several-times. It is shown that for the real engineering surfaces, the models applicability is expected in a loading range up to about 0.5·Y for smooth surfaces, but this limit decreases for rough surfaces.