Information about the interactions between lubricants and DLC coatings is scarce, despite there having been many studies over the years. In this investigation we used ToF-SIMS, XPS and contact-angle analyses to examine the adsorption ability and mechanisms with respect to two oiliness additives, i.e., hexadecanol and hexadecanoic acid, on an a-C:H coating. In addition, we analyzed the resistance of the adsorbed films to external influences like solvent cleaning. The results show that both molecules adsorb onto surface oxides and hydroxides present on the initial DLC surface and shield these structures with their hydrocarbon tails. This makes the surfaces less polar, which is manifested in a smaller polar component of the surface energy. We also showed that ultrasonic cleaning in heptane has no significant effect on the quantity of adsorbed molecules or on their chemical state. This not only shows the relatively strong adsorption of these molecules, but also provides useful information for future experimental work. Of the two examined molecules, the acid showed a greater adsorption ability than the alcohol, which explains some of the previously reported better tribological properties in the case of the acid with respect to the alcohol.
COBISS.SI-ID: 14399003
This work focuses on the ZDDP concentration (1, 5 and 20 wt%) to form a ZDDP film on surfaces during static thermal tests at 150 °C. Silicon-doped and hydrogenated DLC coatings, as well as steel as reference, were studied using Attenuated Total Reflection-Fourier Transform Infrared (ATR-FTIR) spectroscopy and X-ray Photoelectron Spectroscopy (XPS). The results show that, on the three surfaces, the structure of the ZDDP thermal film consists of identical groups of pyrophosphate and zinc oxide, while the sulphuric groups are dissimilar. On the steel surface, the sulphuric part consists of a mixture of organic sulphide and sulphohydryl groups, but on H-DLC and Si-DLC only organic sulphide groups are found; there are no sulphohydryl groups. Moreover, both ATR-FTIR and XPS show that different concentrations of ZDDP do not affect the final chemical structure of the ZDDP thermal film on any of the studied surfaces. In addition, the XPS results show that the thickness of the thermal film is linear with the concentration for the whole range from 1 to 20 wt%, supporting also its uniform chemical structure. These thicknesses further show that the reactivity of the ZDDP film is higher on the steel surface than on the DLC coatings.
COBISS.SI-ID: 14627099
Neutron reflectometry (NR) has proven to be an important analytical tool for investigations of the adsorption in boundary lubrication for both simple and complex additives by using either deuterated additives or the base carrier lubricant. NR enables in-situ measurements of the adsorption during surface exposure to additivated lubricants, and at the same time provides quantitative parameters for the adsorbed layer with sub-nanometer precision. In this work, NR was successfully used to determine the thickness and the density of the adsorbed layers of various lubricating additives (oiliness additives and ZDDP) on different DLC coatings. First, the influence of the coating structure and the presence of the doping element on the adsorption of two simple oiliness additives, i.e., hexadecanol and hexadecanoic acid, are presented. Second, the interaction ability of the ZDDP additive on a hydrogenated DLC (a-C:H) coating was measured as a function of the temperature and exposure time. Finally, an in-house-developed tribological rig was implemented in the neutron beam reflectometer to investigate the ZDDP film’s growth and removal. The results show the unique ability of NR to provide direct evidence of the adsorption of additives on DLC coatings as a function of several relevant engineering parameters, such as time, temperature, and rubbing.
COBISS.SI-ID: 23235677
In this work we have investigated whether ZDDP boundary tribofilm maintain the same characteristics when different contact configurations of dissimilar surfaces, i.e. steel and DLC, are used, and how these characteristics change in self-mated steel and DLC contacts compared to combined dissimilar ones. Macro-scale (tribometer) as well as nano-scale (AFM) friction, film thickness and topography, and chemical (FTIR and XPS) tribofilm properties, were studied. Steel/steel, DLC/steel, steel/DLC and DLC/DLC contact configurations were used in 1-hour and 6-hours sliding tests. Results show unexpectedly large differences, since all the tribofilms are completely different, especially in terms of the chemical composition, which is known to have crucial effect on macro and nano scale tribological properties. These results thus clearly show that not only surface material crucially affects the ZDDP film tribochemistry, but also the position of the moving or stationary surface within the contact.
COBISS.SI-ID: 45678932
Organic Friction Modifiers (OFMs) are additives that work through adsorption of the polar headgroup to the surface. Despite numerous literature, there are no studies to investigate the adsorption of OFM from a commercial lubricating oil. In order to develop new, better and more environmentally friendly additives, the understanding of both adsorption and wetting is of key importance. In collaboration with TOTAL company (France), we performed adsorption measurements for simple OFM using a quartz microbalance (QCM). Although it has become obvious that wettability is an important characteristic in tribology, there is still no information on how additives and their different molecular structure change the wetting behaviour of oil, if at all. Studies have reported that the most commonly used static contact angle to describe the wettability of oil on steel might be inappropriate parameter. In this work, we studied the influence of the molecular structure of some simple OFMs (fatty acids, alcohol and amine) on their adsorption from lubricating oil and how the adsorbed film affects the static and dynamic wettability of the oil. By evaluating the wettability with the parameters of static and dynamic wetting (advancing and receding contact angle and the contact angle hysteresis CAH), we wanted to determine, which of them is the most appropriate for evaluating wetting behaviour of oil on steel.
COBISS.SI-ID: 12345678