DLC boundary and mixedd lubrication (P1)
Coatings based on diamond-like carbon (Diamond-Like Carbon, ie DLC) and other materials for tribological contacts in which iron predominates, usually have a significantly reduced reactivity with additives in oils. Despite accelerated research in this area in the last decade, the knowledge in this area is still highly inadequate, because unless various conditions and conflicting empirical results, says tribochemical mechanisms are not yet known very well. Problems and additional unknowns are introduced into the area is the fact that the chemical structure of the additives due to environmental restrictions and changes in the future is expected in the use of different additives than those at which they were already so limited studies conducted. The objective of research and study to determine the candidate tribochemical mechanisms in mixed and boundary lubrication regime on the non-conventional materials, especially DLC coatings and characterize the resulting films lubrication limit. To this end, we performed experiments under conditions of mixed and boundary lubrication in different model plants for studying tribological properties. Detailed analysis and detection of surface processes and tribological mechanisms will be studied with the help of established tribological instruments and various surface-sensitive analytical equipment.
1. 2003 -2007 Bachelor of Applied Chemistry in Tehran University,Tehran, Iran.
2. 2008-2011 Master of Physical Chemistry,Tarbiat moalem Tehran University, Tehran, Iran.
3. 2011-2012 Master of Nanoscience at the Universidad del País Vasco, Spain.
1."Michael" Nanocarriers Mimicking Transient-Binding Disordered Proteins (2013) ACS Macro Lett.
2. Design and Preparation of Single-Chain Nanocarriers Mimicking Disordered Proteins for Combined Delivery of Dermal Bioactive Cargos (2013) Macromolecular Rapid Communications.
3. Put as Front Cover of Macromol. Rapid Commun 21/2013.
Faculty of Mechanical Engineering
Laboratory for Tribology and Surface Nanotechnology (TINT)
University of Ljubljana (Slovenia)
Bogišičeva 8, 1000 Ljubljana, Slovenia
For more information contact Somayeh;
Supervisors: Prof. Mitjan Kalin
University of Leeds (UK): September -October 2013
Mercedes AMG (UK): October 2013
Modern mechanical systems operate under severe working conditions, such as high loads, high speeds, high temperatures and aggressive environment. In such conditions, this suggests that lubrication frequently corresponds to mixed or boundary regime, where direct contacts between the surfaces are predominant. Successful operation of tribological contacts and thus the whole system (power in moving parts is transmitted through the bearing contacts) under these small film thicknesses is therefore depending on the properties of the surface asperities and the ability to form wear-protective and low-shear tribochemical boundary nano-films between the surfaces, typically with additives and other in-situ forming tribochemical reaction products.
To respond to the increasing demand for fuel economy, in the recent years, low friction nonferrous coatings such as diamond-like carbon (DLC) coatings have become very popular for automotive tribo-components. DLC coatings have exceptional properties, e.g., high hardness and chemical inertness, providing favorable friction and wear properties. However, the problem is that most of the lubricants developed in the automotive industry so far are customized to form the tribofilm that will react to ferrous (steel) materials and until now no lubricants have been designed for the non-ferrous coatings such as DLC.
Zinc dialkyl dithiophosphates (ZDDP) is one of the most used industrial additives, which has been applied in many different kinds of engine and industrial lubricants. ZDDP reacts with the surfaces in the rubbing contacts to form a thin tribo chemical layer. It is shown that this layer acts as an antiwear, antioxidant and corrosion inhibitor under conditions of boundary lubrication. It has been recognised that utilisation of ZDDP additive in contact with iron-base alloyed surfaces (eg. steel), produces a tribofilm on the surfaces and it causes wear protection. ZDDP tribofilm has been known that is a two-layer pad-like structure with a gradient layer of iron and zinc (thio) phosphate below and a zinc polyphosphate film on top. Although there have been some studies on the ZDDP with different DLC coatings in recent years, the findings are quite different. In addition, there exist a lack of proposed chemical reaction mechanisms for ZDDP on DLC surfaces; hence, also there is a need to research.
Interactions between the lubricant and the lubricated component surface can be physical and/or chemical in nature. However the friction and the wear performance in the boundary lubrication regime are controlled mainly with the chemical reactivity of lubricant additives, which form tribofilms in the contacting surfaces. The characterization of these reactive boundary layers is done commonly by XPS, AES, and SIMS. However, these techniques require ultrahigh vacuum. However, in an ultra-high vacuum (UHV) system surfaces must be cleaned with pure solvents and be quite free of oils, which were used in tribocontact. Whereas, DLC surface is very low reactive with additives, therefore, when pure solvents were used for cleaning maybe some tribochemical material that were on the surface get removed. Hence, it is better to use techniques that do not work in UHV, such as FTIR or Raman. Because of these problems, the evidence of chemical changes at the surfaces due to additive/oil action under boundary lubrication is quite difficult to obtain and current understanding of the DLC lubrication is limited, and no complete mechanism has been reported so far. Some of the reasons may also be the lack of relevant surface evidence.
Accordingly, in the proposed work, we investigated lubrication mechanisms of DLC surfaces at nano level, by using high purity and well-characterised coatings and ZDDP additive, which would able us to study relations between the physical- chemical and structural properties of coatings and lubricants with the formation of nano-tribochemical films by using techniques that do not require UHV conditions. Moreover, these techniques not used sufficiently in the past represent a novel approach by itself. The results will enable an improvement in the performance of the DLC-lubricated systems and introduction of new DLC coatings and oil formulations into automotive and lubricant industries.
Methods of research
The research focused on the interactions of DLC coatings with ZDDP additives, and a comparison with steel (100Cr6). First, we have blended these additives in different concentrations with mineral base oil and then applied on the surfaces of our samples at different temperatures. After a certain time and the ultrasonic cleaning of the samples, they have been investigated with the relevant chemical characterization techniques, in order to determine the interaction of DLC and steel surfaces with these additives.
For the understanding of the fundamental principles of reaction on the DLC surfaces in tribological contacts, we did tests in static and dynamic conditions. In static condition, we determine the thermal chemical interactions of DLC surface with the additives without any interference from mechanical and tribological effects. This approach of static reactivity tests was very useful for complementing the tribological results and explaining the tribochemical mechanisms.
After static tests, we performed tribological tests with by using a tribometer with a ball-on-disc reciprocating test geometry, at different temperatures, constant pressure and velocity, corresponding to boundary lubrication regime. In tribological tests, ZDDP additives with different concentrations were mixed with the base mineral oil, which served as a lubricant in contact with the contact pairs of DLCs/steel and steel/steel. All tests were repeated several times to ensure statistic evaluation and reproducibility. All tests were evaluated by the amount of wear and friction coefficient evolution and with its steady state value. Wear was measured by conventional methods using an optical microscope and subsequent numerical analysis and calculations. After the tribometer tests, surfaces were cleaned in the ultrasonic bath with cleaning solvents for a certain time and then sample surfaces were further investigated before and after tribotests using sensitive chemical characterization techniques such as FTIR, but Raman, XPS and others were complementary these results accordingly, to study nanotribological processed in boundary lubrication regime. With these analyses, we explored the reactivity of ZDDP additive on DLC coatings. Finally, we proposed lubrication mechanisms of ZDDP additive on DLCs coatings, which is of great value for the friction and wear phenomena.
- Obtained new information of boundary lubrication films on DLC surfaces with new chemical analytical techniques, which had not been explored before for this purpose.
- Determined the effect of different temperatures on the reactivity of ZDDP on the DLC surfaces and proposed the temperatures for the formation of effective boundary lubrication films with ZDDP additive.
- Understood the effect of different concentrations of ZDDP on the formation of the tribofilms and boundary lubrication mechanisms for DLC surfaces.
- Proposed some new reaction mechanism of the ZDDP additive on DLC surfaces. This proposed models abled us for an introduction of new DLC coatings and oil formulations into automotive and lubricant industries.
- The results enabled us for an improvement in the performance of the DLC-lubricated systems.