Ardian Morina

Tribofilms: aspects of formation, stability and removal

There has been much attention paid to the lubricant additives zinc dialkyldithiphosphate and molybdenum dialkyldithiocarbamate as the most commonly used antiwear/antioxidant and friction modifiers. The mechanism by which they function has been the subject of much research work. As a result of these efforts the tribofilms formed from the above additives are fully chemically characterized but understanding the physical properties and the dynamics of their formation, stability and removal is still not satisfactory and needs more research. This paper reviews the general characteristics of tribofilms formed from these additives in single component systems and also on their interactions as well as the current understanding of the dynamics of their formation. Experimental work is then presented alongside discussion of the literature to present a current status of understanding of the stability and the removal of tribofilms. The effect of temperature and additive interactions on the thickness of the steady state tribofilms, and consequently the effect on friction performance, is evaluated. The results of this study show that temperature and additive interactions play a significant role on the dynamic process of tribofilm formation as well as its chemical properties. This study also highlights the areas in which further development is needed to ensure progress in understanding of the tribofilms formation and removal processes.

Study of the ZDDP Antiwear Tribofilm Formed on the DLC Coating Using AFM and XPS Techniques

To meet the challenge of the increasing demand of fuel economy, in recent years low friction nonferrous coatings such as diamond-like carbon (DLC) coatings have become very popular for automotive tribo-components. The interaction of lubricant additives, which are designed for ferrous surfaces, with nonferrous coatings is an important issue for the automotive and lubricant industries. The aim of this paper is to establish a link between the evolution of antiwear zinc dialkyl dithiophosphate (ZDDP) tribofilm and the tribological performance of a DLC coating under boundary lubrication conditions. Experiments were performed in a pin-on-plate reciprocating tribotester to produce the tribofilm. Atomic force microscopy (AFM) was used to record high resolution topographical images of the ZDDP films while chemical analysis of the ZDDP tribofilms was performed using X-ray photoelectron spectroscopy (XPS). Results in this study show that the ZDDP tribofilm consists of short chain zinc pyrophosphate (Zn2P2O7) and zinc metaphosphate (ZnOP2O5), and it is formed along the raised portion of the initial marks of the DLC surface.

Synovial joint lubrication — does nature teach more effective engineering lubrication strategies?

Abstract Nature shows numerous examples of systems which show energy efficiency, elegance in their design and optimum use of materials. Biomimetics is an emerging field of research in engineering and successes have been documented in the diverse fields of robotics, mechanics, materials engineering, and many more. To date little biomimetics research has been directed towards tribology in terms of transferring technologies from biological systems into engineering applications. The potential for biomimicry has been recognized in terms of replicating natural lubricants but this system reviews the potential for mimicking the synovial joint as an efficient and durable tribological system for potential engineering systems. The use of materials and the integration of materials technology and fluid/surface interactions are central to the discussion.

Development of anti-icing materials by chemical tailoring of hydrophobic textured metallic surfaces

Ice on surfaces can have dramatic consequences for human activities. Over the last decades, the design of new materials with anti-icing properties has generated significant research efforts for the prevention of ice accretion. Here we investigate water freezing temperatures on untreated and negatively charged hydrophobic stainless steel surfaces and use these temperatures to evaluate icephobicity. Supercooled water microdroplets are deposited and undergo a slow controlled cooling until spontaneous freezing occurs. Textured hydrophobic stainless steel surfaces functionalized with anionic polyelectrolytes brushes display unexpectedly lower freezing temperatures, at least 7 °C lower than polished untreated steel. On the basis of the entropy reduction of the crystalline phase near a charged solid surface, we used a modification of the classical heterogeneous nucleation theory to explain the observed freezing temperatures lessening. Our results could help the design of new composite materials that more efficiently prevent ice formation.

Surface and tribological characteristics of tribofilms formed in the boundary lubrication regime with application to internal combustion engines

One of the biggest challenges in engine tribology is to formulate appropriate lubricants, which will increase fuel efficiency by reducing friction, yet still provide good wear resistance. The lubricant should also be formulated to limit particulate and gaseous exhaust emissions to the levels allowed by current regulations. In real lubricant formulations there can be 10–15 additives and the interactions between additives must be taken into account. The effects of eliminating the friction modifier and friction modifier plus anti-wear additive zinc dialkyl dithiophosphate (ZDDP) from the additive package of fully formulated lubricants on friction, wear and wear film forming characteristics have been examined. Tests have been conducted under lubricated wear conditions at bulk oil temperatures of 20, 50, and 100 °C using a reciprocating pin-on-plate tribometer. Boundary lubrication conditions were varied according to the value of starting lambda ratio. The wear film has been examined by Energy Dispersive X-ray analysis (EDX) and X-ray Photoelectron Spectroscopy (XPS). In order to investigate the morphology of the reaction films formed by the additive packages of these lubricants, Atomic Force Microscopy (AFM) was used. In this paper it has been shown that tribofilms, derived from ZDDP/surface interactions, affect friction, the extent of which is determined by tribological conditions. Detergent interactions with ZDDP enhance the complexity of the tribofilm and enrich the level of C in the film whilst affecting the friction and wear response. Through integration of tribological measurements and surface analysis, progress towards improving the nature of interactions is made and forms the focus of the paper.

Effect of Friction Modifiers and Antiwear Additives on the Tribological Performance of a Hydrogenated DLC Coating

There has been a lot of attention on the effect of lubricant additives on the friction at carbon coated surfaces. But only few papers have addressed the effect of additives on the durability of some diamondlike carbon DLC coatings. This paper presents a systematic study assessing the additive/additive and additive/surface interactions, and their influences on the durability of a low hydrogen-containing (15 at. % hydrogen) metal-free DLC coating (a-C:15H). In this study, lubricating oils containing a zinc dithiophosphate (ZDDP) antiwear additive and/or organomolybdenum friction modifiers (moly dimer and moly trimer) were used. Tribological tests were carried out in a pin-on-plate tribometer under boundary lubrication conditions. To understand the effect of additives, tribofilms formed on the wear tracks were analyzed using surface sensitive analytical techniques such as atomic force microscope, scanning electron microscopy, energy dispersive X-ray analysis, X-ray photoelectron spectroscopy, and Raman spectroscopy. Results showed that the ZDDP formed a zinc phosphate containing an ultrathin antiwear tribofilm, which offered excellent durability/wear protection to the a-C:15H coating. However, the antiwear performance of this additive was compromised when it was used with moly dimer or moly trimer. Surface analysis revealed that unlike steel surfaces, MoS 2 formed on the DLC surfaces had negligible influence on friction, while the low friction DLC wear debris had strong influence on friction. Abrasive wear was found to be the dominating wear mechanism in the cases when additives showed poor wear protection on the a-C:15H coating.

Tribological performance of an Al—Si alloy lubricated in the boundary regime with zinc dialkyldithiophosphate and molybdenum dithiocarbamate additives

The tribological performance of the cast iron/Al—Si alloy system lubricated in the boundary regime, using lubricants containing zinc dialkyldithiophosphate (ZDDP) and molybdenum dithiocarbamate (MoDTC) additives have been investigated. The results obtained are discussed in relation to the performance of these additives in cast iron/steel (ferrous)-based material systems. Tribological tests were conducted on a Plint reciprocating test machine (TE 77) in the contact conditions comparable to the conditions in the piston ring/cylinder liner system of the internal combustion engine. Surface sensitive analytical techniques such as environmental scanning electron microscopy and energy dispersive X-ray analysis have been used to determine the nature of the wear occurring as well as the chemical nature of the tribofilms formed on the Al—Si material. It has been shown that the MoDTC additive, added into a ZDDP-containing lubricant, improves the effectiveness of friction reduction in a completely ferrous system and a cast iron/Al—Si system. In terms of wear, an increase in MoDTC concentration (from 0.22 to 1.2 wt%) in the ZDDP-containing lubricant resulted in an antagonistic effect on the anti-wear performance of ferrous materials but improved the effect on Al—Si alloys. The tribofilm formation and wear mechanisms are discussed in relation to recent literature and it is shown that additives strongly affect both the processes.

Tribochemical Interactions of Friction Modifier and Antiwear Additives With CrN Coating Under Boundary Lubrication Conditions

In recent years, the optimized use of low friction nonferrous coatings under boundary lubrication conditions has become a challenge to meet the demands of improved fuel economy in automotive applications. This study presents the tribological performance of chromium nitride (CrN) coating using conventional friction modifier (moly dimer) and/or antiwear additive (zinc dialkyl dithiophosphate (ZDDP)) containing lubricants in a pin-on-plate tribometer. Using surface analysis techniques such as the atomic force microscope (AFM) and X-ray photoelectron spectroscopy (XPS), both topographical and chemical analyses of tribofilms were performed. This paper shows that ZDDP and moly dimer both give a positive effect for both low friction and antiwear performance in CrN/cast iron system. Both AFM and XPS analyses give evidence of the formation of ZDDP and moly dimer derived tribofilms on the CrN coating and thus support friction and wear results.

Tribological performance evaluation of a hydrogenated diamond-like carbon coating in sliding/rolling contact – effect of lubricant additives

In this article, a systematic study is carried out to assess the influence of lubricant additives on the durability of a hydrogenated diamond-like carbon (DLC) coating under mixed rolling/sliding contact is provided. Experiments were performed in a mini traction machine using a DLC (15 at.% hydrogen) coated disc on an uncoated steel ball at 50 per cent sliding/rolling ratio and in a motorized cylinder head (Ford Zetec) cam/follower test rig. Five different lubricants containing zinc dialkyldithiophosphate (ZDDP) and/or friction modifiers (Moly Dimer (MD)/Moly Trimer (MT)) were used. This study reveals that the lubricant additives have a strong influence on the coating durability while ZDDP additive effectively reduces wear to extremely low levels. The addition of the MT additives only has a marginal effect on wear, whereas the MD additive substantially reduces the performance. The mechanisms by which the additives affect the wear in this ultralow-wear regime are discussed.

A Semi-deterministic Wear Model Considering the Effect of Zinc Dialkyl Dithiophosphate Tribofilm

AbstractTribochemistry plays a very important role in the behaviour of systems in tribologically loaded contacts under boundary lubrication conditions. Previous works have mainly reported contact mechanics simulations for capturing the boundary lubrication regime, but the real mechanism in which tribofilms reduce wear is still unclear. In this paper, the wear prediction capabilities of a recently published mechanochemical simulation approach (Ghanbarzadeh et al. in Tribol Int, 2014) are tested. The wear model, which involves a time- and spatially dependent coefficient of wear, was tested for two additive concentrations and three temperatures at different times, and the predictions are validated against experimental results. The experiments were conducted using a mini-traction machine in a sliding/rolling condition, and the spacer layer interferometry method was used to measure the tribofilm thickness. Wear measurements have been taken using a white-light interferometry. Good agreement is seen between simulation and experiment in terms of tribofilm thickness and wear depth predictions.