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Friction  2022, Vol. 10 Issue (2): 296-315    doi: 10.1007/s40544-021-0491-7
Research Article     
Differences in nano-topography and tribochemistry of ZDDP tribofilms from variations in contact configuration with steel and DLC surfaces
Lucija ČOGA1,Somayeh AKBARI2,Janez KOVAČ3,Mitjan KALIN1,*()
1 Laboratory for Tribology and Interface Nanotechnology, Faculty of Mechanical Engineering, University of Ljubljana, Ljubljana 1000, Slovenia
2 NanoSciTec GmbH, Munich 80687, Germany
3 Department of Surface Engineering and Optoelectronics, Jo?ef Stefan Institute, Ljubljana 1000, Slovenia
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In this work, we evaluated the effect of the counter-body material (the same or dissimilar) and contact configuration (moving or stationary body), at similar contact tribological conditions, on the tribochemical and nanotopography characteristics of adsorbed surface films. Zinc dialkyldithiophosphate (ZDDP), the best performing anti-wear additive, was used in self-mated steel/steel and DLC/DLC contacts, which were compared with mixed steel/DLC and DLC/steel contacts in 1-h and 6-h sliding tests. The macroscale (tribometer) and nanoscale (atomic force microscopy) friction, thickness, topography, and chemical (attenuated total reflection- Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy) properties of the tribofilms were studied. The results revealed unexpectedly large differences in all the studied tribofilm parameters; this is because all the tribofilms are completely different; this includes the chemical composition, which is known to have a crucial effect on the nano- and macro-scale tribological properties. These results clearly demonstrate that the surface material, additives, and common contact operating parameters, that is, pressure, velocity, and temperature, crucially affect the ZDDP tribofilm as well as the position of the moving or stationary surface within the contact, and the material of the moving/stationary bodies.

Key wordsdithiophosphate (ZDDP)      diamond-like carbon (DLC)      steel      tribofilm      tribochemistry      nano-friction      thickness      topography     
Received: 27 May 2020      Published: 17 January 2022
Fund:  Slovenian Research Agency ARRS(P2-0231)
Corresponding Authors: Mitjan KALIN     E-mail:
About author: Lucija ČOGA. She received her bachelor degree in chemistry from University of Ljubljana, Slovenia, in 2011 and her Ph.D. degree in physics in 2015 from University of Ljubljana. During her Ph.D. study, she was working as a young researcher at the Faculty of Mathematics and Physics in the research area of self-assembly of lipophilic nucleoside derivatives in thin surface films. Since 2016, she has been working in the Laboratory for Tribology and Interface Nanotechnology at the Faculty of Mechanical engineering in Ljubljana. Her research areas cover nanoscale interface phenomena, boundary films, and novel green-lubrication technologies. She specialized in characterization of surface films at nanoscale using different scanning probe microscopies, electron microscopy, and neutron reflectometry.|Somayeh AKBARI. She received her bachelor degree in physical chemistry from Tarbiat Moallem University, Iran, in 2010. After that, she studied nanoscience at the Universidad del País Vasco, Spain, from where she received her Master degree in 2012. In 2012, she received Marie Curie scholarship and joined the Laboratory for Tribology and Interface Nanotechnology as a Ph.D. student under the mentorship of Professor Mitjan Kalin. She finished her Ph.D. degree in 2016. During her Ph.D. study, she was working on reaction mechanisms of ZDDP additives in thermal films and tribofilms. Currently, she is employed as CEO of the NanoSciTech Company in Munich, Germany.|Janez KOVAČ. He received his Ph.D. degree in electronic vacuum technologies at the University of Maribor, Slovenia, in 2000. From 1991 to 1996, he was a researcher at the synchrotron light source Elettra in Trieste, Italy. From 1996 to 2003, he was a researcher at the Institute of Surface Engineering and Optoelectronics, and from 2003 to 2019, he is a Senior Research Associate at Jozef Stefan Institute, Ljubljana, Slovenia. He is currently a head of the Laboratory for Surface and Thin Film Analyses at Jozef Stefan Institute. In the period of 2013-2019, he was the president of the Slovenian Vacuum Society. The main fields of scientific interest of Janez Kovač are reactions at solid surfaces, thin films and multilayer structures, plasma physics, high resolution XPS, SIMS and AES depth profiling, and vacuum science and technology.|Mitjan KALIN. He received his Ph.D. degree from Faculty of Mechanical Engineering at University of Ljubljana, Slovenia, in 1999. After his post-doc research at National Institute of Standards and Technology (NIST, Gaithersburg, USA), he joined the University of Ljubljana in 2000 as an assistant professor, where he is now a full professor, head of the Laboratory for Tribology and Interface Nanotechnology, and head of the Chair for Tribology and Maintenance Technology. Currently he holds a position of a dean of the Faculty of Mechanical Engineering in a four-year term. His research areas cover the wear and friction mechanisms of advanced materials, nanoscale interface phenomena, and boundary films and contact engineering for novel green-lubrication technologies.
Cite this article:

Lucija ČOGA,Somayeh AKBARI,Janez KOVAČ,Mitjan KALIN. Differences in nano-topography and tribochemistry of ZDDP tribofilms from variations in contact configuration with steel and DLC surfaces. Friction, 2022, 10(2): 296-315.

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Roughness, Ra (nm)5.1 ± 1.313.8 ± 0.46.8 ± 1.211.6 ± 1.5
Hardness (GPa)8.325 [29]
Thermal conductivity (W/(m·K))591-2
Specific heat (J/(kg·K))460970
Density (kg/m3)7,8502,340
Table 1 Surface roughness of bare steel surfaces and those coated with DLC.
Fig. 1 Schematic of ZDDP tribofilm formation during tribological test with reciprocating geometry using a stationary ball on a moving disc. The tests were performed at 100 °C and a normal load (FN) of 10 N.
Fig. 2 (a) Cross-section analysis along the selected line indicated on (b) the topographic image. The vertical difference between the substrate and tribofilm region is also indicated.
Fig. 3 COF for all four contact configurations in base oil and solution of ZDDP after (a) 1-h and (b) 6-h sliding. Arrows indicate the moving surface.
Fig. 4 Mean tribofilm thickness for all four contact configurations after (a) 1-h sliding and (b) 6-h sliding.
Fig. 5 Optical images of wear tracks on moving surfaces for all four contact configurations after cleaning with EDTA disodium salt solution. Profiles below the images were measured using scanning white-light interferometry across the cleaned region of the wear track.
Fig. 6 Topographic images of steel and DLC without any lubricating film.
Fig. 7 Topographic and corresponding LFM images of stationary steel and moving steel surfaces for the steel/steel contact configuration after 1-h and 6-h sliding.
Fig. 8 Topographic and corresponding LFM images of stationary DLC and moving steel surfaces for DLC/steel contact configuration after 1-h and 6-h sliding.
Fig. 9 Topographic and corresponding LFM images of stationary steel and moving DLC surfaces for steel/DLC contact configuration after 1-h and 6-h sliding.
Fig. 10 Topographic and corresponding LFM images of stationary and moving DLC surfaces for DLC/DLC contact configuration after 1-h and 6-h sliding. Full and dashed arrows indicate low- and high-friction regions, respectively.
Fig. 11 IR spectra of tribofilm on stationary and moving surfaces after 1-h and 6-h sliding for (a) steel/steel, (b) DLC/steel, (c) steel/DLC, and (d) DLC/DLC contact configuration.
Fig. 12 XPS spectra of O 1s signal for tribofilms formed on contacting surfaces (stationary and moving) of the DLC/steel, steel/DLC, and DLC/DLC contacts after 1-h and 6-h sliding.
Fig. 13 XPS spectra of S 2p signal for tribofilms formed on contacting surfaces (stationary and moving) of the DLC/steel, steel/DLC, and DLC/DLC contacts after 1-h and 6-h sliding.
Fig. 14 XPS spectra of P 2p signal for tribofilms formed on contacting surfaces (stationary and moving) of the DLC/steel, steel/DLC, and DLC/DLC contacts after 1-h and 6-h sliding.
DLC/steelStationary DLC1 h0.731.2
6 h0.421.1
Moving steel1 h0.290.8
6 h0.5010.6
steel/DLCStationary steel1 h0.231.2
6 h0.301.1
Moving DLC1 h0.260.7
6 h0.490.8
DLC/DLCStationary DLC1 h0.331.1
6 h0.490.8
Moving DLC1 h1.242.1
6 h0.401.0
Table 2 BO/NBO areal and P/Zn intensity ratios for the contacting surfaces in DLC/steel, steel/DLC, and DLC/DLC contacts.
Fig. 15 Schematics of ZDDP tribofilm formation on all contacting surfaces for all contact configurations.
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