Lubricants, Volume 7, Issue 4 (April 2019) – 10 articles

Diamond-like carbon is one of the most studied and used solid lubricants on the market. Despite this large use and its outstanding mechanical and tribological properties, there are still some unclear aspects related to its self-lubricant properties, and some drawbacks in the deposition methods. We deposited “soft” DLC films on Si(100), iron, and stainless steel substrates by PVD magnetron sputtering technique with a Cr/CrN adhesive interlayer. The DLC films were characterized from a chemical, mechanical, and tribological point of view. Our aim was to connect the coating chemical and mechanical characteristics to the different conditions used for the deposition, such as discharge power and substrate–target distance. We found a stronger sp³ dependence on the discharge power for DLC deposited closer to the target. The tribological results did not depend on the chosen substrate–target distance, but rather on the hardness of the substrate. This could be ascribed to the better mechanical coupling of soft DLC films on harder substrates. Full article

Deep eutectic ionic liquids (DES) possess similar properties to conventional ionic liquids (ILs). However, ILs cannot be considered as environmentally friendly compounds due to both its processing and synthesis, which could have significant polluting effects. On the contrary, deep eutectic solvents (DESs) can be biodegradable, non-toxic, and have a lower price than most ILs, making them potentially useful in a wide variety of advanced technological applications, such as tribology. On the other hand, graphene has recently been proposed as an extremely promising lubricant due to its combination of mechanical properties and chemical stability as well as its “green” character. In the present paper, graphene flakes (≈250 nm) have been used as an additive to DES composed of choline chloride (ChCl)-urea, ChCl-ethylene glycol, and ChCl-malic acid. According to the results, the addition of 1 wt% graphene reduces friction coefficient (COF) and, notably, prevents adhesive wear, reducing wear rate on steel-steel sliding contacts. Full article

Carbon nanodots (CDs) are extensively explored due to their low toxicity, excellent water solubility and biocompatibility. Particularly, fluorescent CDs have received ever-increasing attention. Nevertheless, only a few works have been published on measuring the tribological properties of doped CDs, especially Ga doped CDs (Ga@CDs) and nitrogen doped CDs (N@CDs), and comparing their tribological properties with those of pristine CDs. In this work we describe a simple one-pot synthesis of CDs and doped CDs, and examine their tribological properties as potential lubricants. It is suggested that doping of CDs with various elements can give them desired properties for anti-wear and extreme-pressure performances. Full article

A computationally lean model for the coarse-grained description of contact mechanics of hydrogels is proposed and characterized. It consists of a simple bead-spring model for the interaction within a chain, potentials describing the interaction between monomers and mold or confining walls, and a coarse-grained potential reflecting the solvent-mediated effective repulsion between non-bonded monomers. Moreover, crosslinking only takes place after the polymers have equilibrated in their mold. As such, the model is able to reflect the density, solvent quality, and the mold hydrophobicity that existed during the crosslinking of the polymers. Finally, such produced hydrogels are exposed to sinusoidal indenters. The simulations reveal a wavevector-dependent effective modulus $E^{*}\left(q\right)$ with the following properties: (i) stiffening under mechanical pressure, and a sensitivity of $E^{*}\left(q\right)$ on (ii) the degree of crosslinking at large wavelengths, (iii) the solvent quality, and (iv) the hydrophobicity of the mold in which the polymers were crosslinked. Finally, the simulations provide evidence that the elastic heterogeneity inherent to hydrogels can suffice to pin a compressed hydrogel to a microscopically frictionless wall that is undulated at a mesoscopic length scale. Although the model and simulations of this feasibility study are only two-dimensional, its generalization to three dimensions can be achieved in a straightforward fashion. Full article

Polymer-based materials show to be of increasing interest in replacing metal based materials in tribological applications due to their low weight, cost and easy manufacturability. To further reduce the environmental impact of these bearing materials recyclability is becoming more crucial, stimulating the need for high performing thermoplastic materials. In this study, polyphenylene sulfide (PPS) composites were prepared in an effort to enhance its tribological properties. Short carbon fibres (SCFs), graphene oxide (GO) and nano diamonds (NDs) as well as polytetrafluoroethylene (PTFE) were used as micro and nano reinforcements. The addition of SCFs especially decreased the linear coefficient of thermal expansions while enhancing the micro hardness and wettability of the polymer. Under water lubricated conditions, a decrease in friction up to 56% and a reduction of wear rate in the order of 10³ was observed by the addition of SCF. The reduction in friction and wear was further enhanced by the addition of NDs, providing a synergistic effect of the reinforcements in micro and nano scale. By testing the individual reinforcements under dry conditions, PTFE and SCFs were especially effective in reducing friction while the release and consequent abrasion of NDs and SCFs increased the wear under a higher contact pressure. Full article

The aim of the current research is to develop a gear transmission fluid based on water and plant extract. Up to now, mineral or synthetic oils are used as lubricants in most gear drive applications. These oils are made of fossil raw materials and are non-biodegradable. Lately, there have been some efforts made to develop lubricants as an alternative to conventional lubrication systems such as triglycerides from native oils or synthetic esters. These lubricants are more biodegradable than mineral oils but also show some difficulties during performance like saponification. Within a former research project, the company Carl Bechem GmbH and the Fraunhofer Institute for Process Engineering and Packaging IVV developed a cutting fluid based on water and plant extract. With a model sample of this fluid, which also contained gear typical additives, preliminary experimental investigations for a current research study were conducted. The results confirmed the general suitability of this water-based lubricant for gear drives under certain operating conditions. Using water as lubricants can lead to some improved characteristics due to the very high thermal conductivity and the reduced friction. This paper aims to point out the benefits of using water-based lubricants, share the results of some preliminary experimental investigations on a fluid sample based on water and plant extract, and discuss the challenges, which one has to overcome during the development of such new lubricants. Full article

The use of fuel mixtures of diesel and vegetable oils in diesel engines is a field of research due to the necessity of reducing pollution. Besides the properties required for the normal operation of diesel engines, other aspects that must be investigated are linked to the influence of these mixtures on piston ring–cylinder tribosystem behavior. Methods used for reducing the friction and wear on the engine cylinders, such as special surface machining, lubricant driving piston rings, etc., are well known. If the fuel mixture brings some improvement in this area, such as a reduction of the friction coefficient value, this can be a way to reduce the power lost by friction into the engine cylinders. In this paper, a methodology is presented based on artificial neural networks for analyzing the complex relationship between vegetable oil percentages in fuel mixtures, with the goal of finding an optimal proportion of vegetable oil corresponding to a minimum value of the friction coefficient. Regular methods were used for data acquisition, i.e., a pin-on-disk module mounted on a tribometer, and two types of vegetable oils were studied, namely sunflower and rapeseed oils. The obtained results show that for each type of vegetable oil there is an optimal proportion leading to the best tribological behavior. Full article

The principal factors that affect the characteristics of contact problem between cam and follower vary enormously during the operating cycle of this mechanism. This includes radius of curvature, surface velocities and applied load. It has been found over the last decades that the mechanism operates under an extremely thin film of lubricant. Any practical improvement in the level of film thickness that separates the contacted surfaces represents an essential step towards a satisfactory design of the system. In this paper a detailed numerical study is presented for the cam and follower (flat-faced) lubrication including the effect of introducing an axial modification (parabolic shape) of the cam depth on the levels of film thickness and pressure distribution. This is achieved based on a point contact model for a cam and flat-faced follower system. The results reveal that the cam form of modification has considerable consequences on the level of predicted film thickness and pressure distribution as well as surface deformation. Full article

The wear of tibial inserts in total knee arthroplasty (TKA) remains a major limitation of longevity. However, wear tests are expensive and time-consuming. Computational wear prediction using a finite-element (FE) model followed by validation through comparison with experimental data is effective for assessing new prosthetic designs or materials prior to functional testing and surgical implementation. In this study, the kinematics, volumetric wear, and wear depth of tibial inserts made of different materials (ultrahigh-molecular weight polyethylene (UHMWPE), polyetheretherketone (PEEK), and carbon fiber-reinforced PEEK (CFR–PEEK)) in TKA were evaluated by employing FE models and analysis. The differences among the materials were evaluated using adaptive wear modeling to predict the wear depth, volumetric wear, and kinematics under a gait loading condition. The volumetric wear and wear depth of the CFR–PEEK decreased by 87.4% and 61.3%, respectively, compared with those of the UHMWPE, whereas the PEEK exhibited increased volumetric wear and wear depth. These results suggest that CFR–PEEK is a good alternative to UHMWPE as a promising and suitable material for tibial inserts used in TKA. However, orthopedic research should be performed to evaluate the threshold conditions and appropriate applications for the newly developed and introduced biomaterial. Full article

Film thickness is the most important parameter of a lubricated contact. Its evaluation in a cam-follower contact is not easy due to the continuous variations of speed, load and geometry during the camshaft rotation. In this work, experimental apparatus with a system for film thickness and shape estimation using optical interferometry, is described. The basic principles of the interferometric techniques and the color spaces used to describe the color components of the fringes of the interference images are reported. Programs for calibration and image analysis, previously developed for point contacts, have been improved and specifically modified for line contacts. The essential steps of the calibration procedure are illustrated. Some experimental interference images obtained with both Hertzian and elastohydrodynamic lubricated cam-follower line contacts are analyzed. The results show program is capable of being used in very different conditions. The methodology developed seems to be promising for a quasi-automatic analysis of large numbers of interference images recorded during camshaft rotation. Full article