Full Length ArticleAnalyses of anti-wear and extreme pressure properties of castor oil with zinc oxide nano friction modifiers
Introduction
Friction, wear and lubrication are the important aspects of any process industries. Since ages, lubrication has been known to mankind in the form of animal fats, mineral oil, vegetable oil etc. Among them, mineral oil of various viscosities is most commonly used in heavy industries such as cement mills, open gear drives etc. The high demand of mineral oil as base lubricant due to the increased industrialisation has become a major concern for the lubricant manufacturers as well the end users. Furthermore, the mineral oil is also facing the problem of depletion and bio degradability issues [1]. Vegetable oil, which has been known to mankind for their good lubricant properties due of the presence of fatty acids [2], are commendable alternatives to mineral oil. These fatty acids are derived from triglyceride having functional polar group [2] which are beneficial in boundary lubrication regime because of the adherence capability of the lubricating film due to the presence of polar groups, but a base oil alone cannot provide adequate lubricity. Therefore, various friction modifiers are dispersed in the base oils to enhance the anti-wear and extreme pressure properties [[3], [4], [5], [6], [7], [8], [9]]. But it has been observed that micro size friction modifiers may increase the wear if they are added beyond certain concentrations [10]. Hence, the sizes of these friction modifiers play a major role in controlling friction. This made the researchers to think for small sized fiction modifiers (FM). It has been observed that the nano friction modifiers reduces wear coefficient by either of the following mechanisms or by a combination of such mechanisms: ball bearing [[11], [12], [13], [14]], formation of tribo film between the mating surfaces [[15], [16], [17], [18]], mending and polishing effect [19,20], third body effect [21], tribo chemical reactions [22]. It is a known that the area of contact is always stressed by the mechanical energy involved during the process which leads to the surface degradation and damage. Thus, the tribo film formed by adsorption or tribo-chemical reactions is an important aspect which prevents direct metal to metal contact, thus reducing wear. Rapoport et al. [21] discussed the role of WS2 nanoparticles in reducing friction. It was reported that WS2 behaved as a third body between the two mating parts and played a major role in controlling friction by eliminating surface contact between the two mating parts. Caixiang et al. used [22] rare earth metals such as ceria and calcium carbonate as lubricant additives and reported an increase of 40.25% in extreme pressure properties and 32% decrease in frictional coefficient when compared to the base oil. It was reported that formation of metal calcium, metal cerium and oxides film due to complex tribo-chemical reactions led to the filling of the surfaces and hence, prevented contact between the mating surfaces. Similar works of improvement in the tribological properties of the lubricant using metal oxide nanoparticles were also reported by other researchers [[23], [24], [25], [26]]. Wu et al. [27] investigated the tribological properties of oleic acid modified zinc oxide nanoparticles and reported an enhancement of tribological properties. Battez et al. [28] investigated and compared the tribological properties of copper oxide nanoparticles, zinc oxide nanoparticles and zirconium oxide nanoparticles as friction modifiers in poly alpha-olefin oil and concluded that there was a reduction in friction and wear in all the three types of nanoparticles but zinc oxide (0.5%) and zirconium oxide (0.5%) performed better as compared to copper oxide nanoparticles (2.0%). Trajano et al. [29] investigated the tribological properties of CuO and ZnO in soybean (144.72 cSt) and sunflower (151.2 cSt) oil and reported an increase in the wear (wear scar diameter) with the addition of the nanoparticles as compared to the lubricants without additives. Thus, it can be seen that the tribological behaviour of the oxide nano FM needs to be explored more with different base oils as the FM behaved differently in different base oils. The authors found it worth to investigate the role of zinc oxide nano FM systematically in a high viscosity vegetable oil i.e castor oil (242.81 cSt) to determine the optimum concentration of FM in the base oil to achieve anti-wear property capable to replace the existing commercial mineral oil, and also to study the plausible mechanisms responsible for enhancing the tribological properties of the base oil. The results obtained were compared with the previous published work of the authors [30] where they investigated the tribological properties of commercially available mineral oil (CMO), neat castor oil (NCO) and NCO with carbonaceous friction modifiers and mineral oil. This work will be helpful in developing new biodegradable nano lubricants.
Section snippets
Preparation of ZnO nano friction modifier (FM) based lubricant
Neat castor oil (NCO) and commercial mineral oil (CMO) (procured from local market, Chennai, India) has been chosen as base oil. These oils were used without further processing except the addition of friction modifiers at later stage in the present study. Laboratory analysis of the oils showed the presence of sulphur (4977 ppm) and phosphorus (4 ppm) in CMO. NCO has been chosen because of its high viscosity (242.81 cSt) and ease of availability. CMO (559.60 cSt) has been chosen as it is most
Results and discussions
The results in the present investigation has been compared with the results that was reported earlier [30] with carbonaceous FMs: graphite (GRT), multi wall carbon nanotubes (MWCNT) and graphene (GRPHN).
Conclusion
The following can be concluded from the present work:
An increasing trend in viscosity has been observed with the increase in concentration of ZnO nano friction modifiers. The coefficient of friction in case of castor oil samples is found to be less than the commercially available mineral oil but the coefficient of friction did not decrease further after a certain concentration of ZnO FM (0.1 wt% ZnO FM is the optimum in this case). The wear rate in the case of NCO+0.1 wt% ZnO is found to be the
Acknowledgement
The authors acknowledges the financial support from SRM Institute of Science and Technology, India under the Selective Excellence Project Grant No. 31.
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