Progress and Challenges in Lubrication: Green Tribology

In this work, studies were performed on oxidative stability by determining the transmittance spectra, the components and trichomatic coordinates, and the color differences for grape seed oils (GSO) subjected to a forced oxidation treatment at temperatures of 100 °C and 120 °C, for 4, 8 and 10 h. For this purpose, a constant airflow of 30 L/min was used. GSO was also subjected to flammability tests on a heated cylindrical surface to determine the lowest temperature at which this oil ignites, correlated with the highest temperature at which the oil does not ignite. According to the results, these temperatures are 475 °C and 470 °C, respectively. At these temperatures, the tested oils were darker in color than the reference oil, with the L* parameter having lower values (91.53 and 89.56, respectively). In addition, the correlation coefficients between the evaluated parameters were significant (p ≤ 0.05). Full article

The total loss lubrication system that is typical of chainsaws is responsible for a massive dispersion in the agro-forestry environment of highly impactful pollutants, mostly of fossil origin, often well known as carcinogenic substances, which, in addition to presenting a risk to the environment, represent an important risk factor for human health, especially for chainsaw users. During its use, the chain lubricant is dispersed from the guide bar tip in the form of droplets and aerosol, or it is adsorbed on wood residues and sawdust. Then, it is subjected to drift, settles on the ground and vegetation, and can hit the operators, who, after prolonged exposures, can suffer both irritation of the respiratory tract and dermal absorption. Such a risk factor is often amplified by the widespread use of less-expensive, sometimes illegal alternatives, such as exhausted motor oils. To mitigate said negative effects, a process has been in progress for several years that is aimed at replacing conventional lubricants with synthetic or biobased oils with increasing biodegradability. As a contribution to this process, a study has been started on the possibility of using refined olive pomace oil (ROPO) as a base stock for the formulation of a totally biodegradable chainsaw lubricant. On purpose, to improve its properties of viscosity and adhesivity, such an oil was added with a biodegradable thickening agent, obtaining four formulations with different viscosity. After a lab test and a preliminary cutting test on firewood, the formulation with 2% of thickener resulted in being the best, and 3.0 g kg⁻¹ of tert-butylhydroquinone (TBHQ), a food-grade antioxidant, was then added to form the final formulation (F₂) to be compared, in the subsequent four test sessions, to a biodegradable commercial chain lubricant (S_B). The tests were carried out without changing the chainsaw setting, on different wood species, both in forest and, with the aim of increasing the repeatability of tests conditions and comparability of results, at a fixed point. The fluids’ performances were mainly evaluated based both on the operators’ opinions and on the measurements of the chain–bar temperatures and of saw chain wear related to a predefined number of cuts. As to the destiny of the fluid dispersed during cutting, the overall dispersion was assessed by considering the average working time, the consumption of chain lubricant, and the forest area cut down daily. Eventually, the amounts of inhalable and respirable dust particles as vectors of oil residues were quantified by means of personal air samplers worn by the operators and analyzed to determine any differences in the concentration of metallic elements. The test results evidenced chain temperatures that were 0.5, 4.9, and 12.5 °C higher with F₂ relating to S_B, respectively, in the cutting of trunks of fresh Pinus, Eucalyptus, and dry Pinus. They were accompanied by chain weight losses of 89.5% and 35% higher with F₂ relating to S_B, respectively, in cutting tests of Turkey oak and Poplar. Such a greater wear, however, apparently did not affect the saw chain’s cutting efficiency with F₂, since the operators declared that they did not notice any difference between the performances of the two fluids at the time of comparison. The effects of higher wear on the chain lifetime, any deriving risks for the operator’s safety, and the possibility to reduce the wear levels observed with F₂ will be explored in a further study, e.g., through different settings of the lubricating system of the chainsaw. The results of the analyses of the air-sampled dust residues that were evidenced with F₂ showed lower concentrations of respirable and inhalable particles and of some metallic elements (Al, Mg, and Ca) than those with S_B. This behavior probably depends on the different interaction between sawdust and the two fluids, which differ according to their chemical–physical characteristics (different viscosity, composition, and additives). However, it represents a positive factor in favor of the use of the ROPO-based lubricant, emphasized by the total biodegradability of its residues that are possibly contained in the dust inhaled by the operators. Full article

As nanotechnology has developed, some nano-additives have been employed to improve the performance of lubricants. The mechanisms of nano-additives still need to be investigated. The wear characteristics of Fe₃O₄ nano-additive lubricant were investigated in this study. Different diameters of Fe₃O₄ nanoparticles were mixed in basic oil using an ultrasonic mixer. The new lubricant was used for analytical tests at room temperature. The results showed that nano-lubricants with 20 nm nanoparticles increase the oil film strength. The coefficient of friction was reduced when 20 nm diameter 8 wt% Fe₃O₄ nanoparticles were mixed with lubricants. The effect of surfactants and nanoparticles in the base oil was measured using numerical simulation methods. The adsorption capacity of the lubricants was significantly improved by Fe₃O₄ nanoparticles, particularly when looking at the small relative atomic mass of the metal. The 8 wt% Fe₃O₄ lubricant exhibited optimal tribological properties when applied in micro-rolling tests. The results showed that the surface quality of the rolled samples was significantly improved, and the rolling force was dramatically reduced. At the same time, the shapes of the samples were effectively controlled in the rolling process. Therefore, Fe₃O₄ nanoparticles can improve the friction and wear characteristics of lubricants. Full article

The tribological properties of engine oils for heavy-duty trucks are evaluated, taking into consideration their variation during operation. After testing new oils or oils after a 2500 km and 5000 km run, there were no essential differences in their tribological properties at lower loads, but at higher loads and longer durations of operation, significant differences were found, including increased friction losses and the reduced surface wear protection ability of the oils. There are two main reasons for this reduced ability of the tested oils to form a boundary lubrication layer: the consuming of the functional additives and the aging of the oil, i.e., oxidation and an increase in acidity. Research data show a close relationship between the increasing acidity and surface wear. Full article