Full Length ArticleCopper (II) oxide nanoparticles as additve in engine oil to increase the durability of piston-liner contact
Introduction
Worldwide concerned on an environmental issue keep increasing from time to time especially on the transportation [1], [2], [3] and energy sectors [4], [5]. Demand for the higher productivity and lower emission for both sectors have been an issue for many governments. There has been growing concern about the use of mineral oils as lubricants because of productivity and environmental issues [6], [7], [8], [9], [10], [11], [12]. The development of new lubricants can benefit many people because it may lower the cost of energy (cost saving), reduce waste and give positive environmental impact [13], [14], [15]. According to Abdullah et al. [16], and other researchers [17], [18], [19], [20], [21], [22] in recent years, many automotive studies have to use the nano-particles as an oil additive to improve the anti-wear property and also friction. In addition, there are other nano-particles that contribute to reducing tribological behaviour such as metal, metal oxide, metal sulphides, carbonate, borate organic material and more. Nano-particles that are effective as nano-lubricants not only depend on the type of nano-particle used, but also on the characteristics [23], [24].Therefore, according to researchers, the size range, shape and concentration of Cu nano-particles influence its friction reduction and anti-wear behaviour mostly in the range of 2–120 nm [16], [25], [26]. According to Koshy et al. [27], energy loss in a mechanical system can be reduced using lubricant by selecting a good combination of base oil and additive. Moreover, the conservation of material and energy becomes a global issue and the development of lubricants will be a world wide’s interest especially in environmental issues. Lubricants with solid nano-particles can reduce friction coefficient and increase the load carrying capacity of the lubricant fluid at the same time [28]. Nowadays, normal lubricants such as petroleum, coals or natural gases have been used, however, these sources are limited and will keep depleting due to high fuel consumption all over the world [29], [30], [31]. Idris et al. [32] stated that, when the piston reached the top dead center, the maximum friction coefficient occurs and the lubricant in minimal quantity. When the piston is at higher speed, the minimum friction coefficient will occur during the mid-stoke. In the development of a lubricant, the tribological characteristics of the oil need to be investigated using various experimental equipment and devices. Numerous studies have been carried out onthe effect of various inorganic nano-particles as lubricating oil additives on friction and wear. The nano-particle itself is good as it has self-repair function for the worn surface [33], in another word, it creates a protection layer, which is deposited at the contact area to improve tribological properties [26]. According to Zhang et al. [34], the interposed layer between two surfaces will improve the smoothness of the relative movement and prevent damage in a variety of materials in many forms such as gas, liquid and solid. The new variant of lubricant developed when the combination of any type of the nano-particle was mixed with the lubricating oil [35], [36].The study from Padgurskas et al. [36] shows that the combination of different nano-particles will result different additive effect as the lubricant. Most researchers stated that the size, concentration, shape, hardness influence tribological behaviour and play important roles in a lubricating oil as an additive [16], [25], [28], [35], [37]. In addition, nano-lubricant also can fill in and repair the worn surface with good environmentally friendly characteristics, but not all combinations improve friction and wear characteristics. The tribological effect of different combinations of nano-particles is better than base oil for reducing friction coefficient. However, according to Padgurskas et al. [36], copper nano-particles are more effective in mixed lubrication than full film lubrication and are the most active in combination or alone. Generally, friction and wear of material are considered as important properties in engineering practice. Recently, researchers reported on a comparison of various nano-particles such as [38], CuO [28], [33], [39], Zr [40], Fe, and Co [36], but copper is the most effective nano-particles in lubricating oil as it provides good friction reduction and anti-wearbehaviour. According to Padgurskas et al. [36], copper nano-particles have received the most significant attention because they are deposited on the friction surface, improve the tribological properties of the base oil and also display good anti-friction and wear reduction characteristics. The use of nano-particles as an additive in oil is because of some advantages and according to [16], their nanometre size allows them to enter the contact areas easily. Therefore, nano-particles are categorized as a new low friction technology and method to reduce wear. In the preparation of a lubricant, various types of nano-particles can be used such as polymer, metal also organic and inorganic materials [16]. Most researchers found that copper nano-particles influence and improve the friction coefficient and anti-wear performance of Syntium 800 SL 10W-30 engine oil mixture with CuO. The copper nano-particles size allow them to enter the contact area easily [16] and easier asperities interaction than that of bigger size [25]. The small size of nano-particles will improve the tribological behaviour more efficiently [36] and their shape, which is more nearly spherical inorganic fullerene-like (IF) particle, is likely to exhibit superior rolling, lower affinity to metal surface, decreased contact temperature, higher elasticity, and higher chemical resilience [25], and will also help in getting better tribological behaviour compared to that of only base oil. There are many factors of the nano-particles that as additive such as the concentration of the nano-particles in oil, the contact form of friction pairs and the lubricating oil [33]. Most researchers have discovered that Cu nano-particles can also improve the tribological properties at higher concentration [37] and copper is an active component in the mixture, even between two nano-particles in a biolubricant, because of higher friction without the participation of Cu [36]. According to Ettefaghi et al. [41], among the different methods which have been used for dispersing nano-particle inside the base oil, planetary ball mill had been determined as the most important method to stabilise nano-particles in engine oil, better than bath ultrasonic and probe ultrasonic. Many researchers reported that the addition of nano-particles in a lubricant or bio-lubricant will reduce friction and wear behaviour. Besides Copper (II) oxide, many types of nanoparticle, such as TiO2, Nano-Diamond, ZnO, ZrO2 and other additives can help to develop new lubricants [25], [37]. The concentration, shape and size of nano-particles also influence the lubricant performance. Nanoparticle can also be mixed with other additives, but the effect of the additive on friction and wear was not better. According to Padgurskas et al. [36], the mixing of additive is to investigate the most effective of the lubricant tribological behaviour. Therefore, Padgurskas et al. [36] stated that nano-particle showed reduced friction coefficient, and among all nano-particles, Cu was the active component of the mixture. The mechanisms of friction reduction and wear by nano-particles added to a lubricant have been reported as a colloidal effect, rolling effect, small size effect, protective film effect and third body effect [16], [25], [36]. Choi et al. [28] stated that nano-particles in lubricating oil can fill the scar and grooves of the friction surface as shown in Fig. 1. The physical film or protection layer forms above the nano-particles, however, it is present when the temperature and real contact pressure are high enough to cause a reaction between the material of the oil, surface and nano-particle itself. RSM, a statistical approach, used to construct a numerical mathematical model that predicts a response by considering the independent variables. Meanwhile, analysis of variation (ANOVA) routinely used to analyze the mathematical models by comparing their dissimilarities. RSM statistical approach chosen in this investigation rather than the artificial neural network, fractional factorial design or semi-empirical model development since an empirical model with high accuracy can be developed through this method [42]. Moreover, all interaction effect among the variable also not be neglected through this statistical approach. Meanwhile, the arbitrated R-squared value from ANOVA table provides the simplest way to evaluate the suitability of the developed empirical model [42].
The main focus of the nano-particle usage would be in the piston-cylinder contact, where a piston moving back and forth resulting in wear and tear on the inner wall of the cylinder liner [43], this will cause a reduction of durability between piston and cylinder, which can cause gas leaks, compression pressure is reduced and the energy produced is also reduced. The most important function of cylinder liner is to act as an excellence sliding partner for the piston or piston rings. In addition, to ensure that friction is low, good wear resistance, reduce oil consumption, the interaction between cylinder liner and piston must be optimized [44].The mechanical power loss in the engine accounts for about 15% of the total energy losses in the engine and half of this loss is caused by friction in the piston-liner system [45].
Section snippets
Experimental setup
The main properties of the nano-particles, lubricant and specimen used in the experiment are listed in Table 1. The concentrations of nano-particle were selected to be between 0.005% to 0.03%, that was stated as the optimum concentration for nano-particles in previous studies [25], [37], [41], [46], [47]. In the present research, CuO nano-particles (0.005% and 0.01%) on weight basis were selected as the tribological behaviour modifier. CuO nano-particles were weighed using a precision
Results and discussion
Statistical analysis was carried out on the data obtained from the Box-Benkhen approach using a statistical software [51], [52], [53]. Table 2shows the data obtained from the experimental results. CuO nano-particles had low frictionforexperiment9 (90 N, 200 rpm, 0.005%) and the highest coefficient of friction was for experiment 5 (20 N, 200 rpm, 0.005%). The specific wear rate (material loss) obtained was lower in experiment 3 (55 N, 250 rpm, 0.005%), 9 (90 N, 200 rpm, 0.005%) and 12 (55 N,
Effect of parameter on Tribology: Coefficient of friction
Three-dimensional response surface plots and contour plots representing the influence of load, speed and concentration on the COF of the lubricant containing nano-particles are shown in Fig. 7, where the coefficient of friction of the nano-lubricant reduced as the load and speed increased (Fig. 7(a)). The load had a more significant effect than speed. The load and concentration plots in Fig. 7(b) show that as the load increased the COF went up slightly. Meanwhile, the COF increased to a higher
Effect of parameter on Tribology: Wear rate due to load
The response surface and contour plots in Fig. 8 show the effect of the different combination of parameters on the wear rate of the lubricant. The load-speed interaction in Fig. 8(a) show that the wear rate was lowered from 20 to 0 with increasing load, however, it increased for loads from 70 N to 90 N toward the maximum wear rate. A load around 60 N to 70 N resulted in a minimum wear reduction. Meanwhile, the wear rate following increasing speed showed less significant impact due to increased
Wear mechanism
When CuO nano-particles were used in the lubricant for this research, the presence of the copper layer always exhibited daubed corner (Fig. 9, Fig. 10), which clearly confirms the formation of copper layer. The EDX spectra of the scar surface for all figures confirm the presence of the corresponding chemical elements when the friction pairs were operated using the selected lubricant. The figures also present the elements in the surface of the specimen using EDX. Moreover, the figures above (EDX
Friction characteristics
The coefficient of friction presents an energy loss caused by friction. Therefore, it is found that syntium oil with CuO nano-particles can reduce energy loss in mechanical lubricant. When used at high load, the friction coefficient decreased, which showed the load is the important parameter in the experiment, However, the speed of sliding and the concentration of the nano-particles also influence friction. According to Padgurskas et al. [36], nano-particles are most efficient at boundary and
Optimisation of tribological behaviour
The main advantage of using the response surface methodology (RSM) is that the response or the yield can be optimized by controlling the parameters used. The tribological behaviour performance of the lubricant used in this experiment not only depend on the properties of the lubricant, but also on the sliding condition. In most cases [27], [58], [59], [60], [61], [62], [63], [64], [65], [66], [67], [68], [69], [70], [71], the target of optimisation is to find a set of parameter condition that
Conclusion
In the present study, copper (II) oxide nano-particle was dispersed in SAE10W-30 to reduce wear and friction on a piston skirt. It can be concluded that:
- i.
The coefficient of friction and wear rate were affected significantly by the linear load and quadratic load for both models. The optimized and predicted values of the parameters are allowing the lower response of COF and wear rate to occur. For improving the lubricant, the optimal values were 75.152 N load, 291.3360 rpm speed and at 0.0086 wt%
Acknowledgement
The authors would like to express their deep gratitude to Universiti Malaysia Pahang (www.ump.edu.my) through research grant RDU160352, RDU160319 and RDU160357 and Malaysia Ministry of Higher Education through research grant RDU130129, RDU140125 and RDU160152 for providing laboratory facilities and financial support.
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