Tribological properties of onion like fullerenes and NiFe₂O₄ nanocomposites in reciprocating motion

Abstract

A novel way for a compressor to improve its coefficient of performance and enhance its reliability by employing nano-refrigeration lubricant oil was proposed. Onion like fullerenes (OLFs) and NiFe₂O₄ nano-composites, modified by Span 80, were dispersed in refrigeration oil KFR22 by solid grinding (SG). Morphologies of NiFe₂O₄ nano-composites were characterized by using transmission electron microscope (TEM). The tribological properties as a refrigeration lubricants additive were investigated using SRV (II), a multi-functional reciprocating friction and wear tester. The results showed that the friction coefficient was decreased from 0.15 to 0.04.

Keywords

refrigeration lubricants / friction coefficient / nano-composites / reciprocating motion / onion like fullerenes

Introduction

Friction loss and motion law of the friction pairs are modeled by compressor rotor dynamics and iterative calculation. It is found that the frictional loss occurs mainly in the crankshaft main bearings, the supplementary bearing, and the thrust bearing, which accounts for about half of the friction loss, while friction loss of slide and slide slot accounts for about 25.7% [¹]. It is necessary to analyze the friction characteristics of the sliding vane and the slide slot respectively. Cho et al. [²] analyzed the lubrication characteristics of the piston and the sliding vane by using the elastic fluid Reynolds equation, and the results showed that the increase of the thickness of the slide plate would increase the friction. Ito et al. [³] studied the sliding vane of the rolling rotor compressor under the mixed lubrication condition. It is concluded that the oil film thickness increased only when the exhaust gas and the contact force was reduced, and in the process of exhaust gas, the bearing capacity of oil film was very small and the solid contact had advantages, the elastic deformation was increased, while the design of the mixed lubrication state of the sliding vane and the sliding vane was solved after the analysis. Taking the current process level, production costs and practical application into consideration, the improvement in lubrication engineering was feasible and operable [⁴].

Improving the lubrication conditions, on the one hand, could optimize the mechanical structure design, and on the other hand, could modify lubricating engineering. The results showed that the addition of a small amount of nano particles into the lubricant could produce obvious anti-friction and anti-wear fraction. Wang et al. [⁵] examined the anti-friction and wear properties of nano Cu additive in different lubricating oils, and found that the nano material could reduce the adhesion wear and abrasive wear in the friction process. The CuO, ZrO₂ and ZnO nano particles were added to the (PAO₆) in different proportions, and the results showed that the nanocomposite had a good synergistic effect and a low friction effect [⁶]. Huang et al. [⁷] using scanning electron microscope (SEM), found that steel ball wear scar surface was smooth under the situation of lubricating lozenge shaped nano graphite based lubricant. Nano materials also could improve thermal and chemical properties, and could solve the problem of the mutual solubility of refrigerant and lubricant in the refrigeration system [⁸–¹²].

In this study, two kinds of nano materials were prepared, both of which were fit for refrigerant oil additives. Transition metal oxide NiFe₂O₄ has a low dielectric loss, oxidation resistance and corrosion resistance, while cage carbon cluster structure of onion-like fullerene is expected to show a rolling friction to replace the sliding friction of the ball bearing sliding friction. SRV (II) was used to simulate the friction between the sliding vane and sliding slot of the rolling rotor compressor. Nano-lubricating oil was observed to provide a reference for improving the lubrication passage of the sliding vane and the sliding slot of the rolling rotor compressor.

Experiment

Preparation of refrigerant oil

Nano particles have a large surface energy, which is easy to agglomerate. The inorganic nano particles are non oil soluble materials, which have a high hardness, and the wear and tear in the process of friction is easy to wear. Improvement in the dispersion and stability of nano particles in the lubricating oil is the first step for application of nano fluid [¹¹]. Because of the low dielectric constant of the refrigerant oil whose dispersion or aggregation in the system is small, it is necessary to improve the dielectric constant of the lubricating oil by adding surfactants. In this paper, non ionic surfactants sorbitol glycerin fatty acid ester (Span 80) is chosen to modify the surface of nano materials to weaken the interaction force between the interface for the stability of the dispersion in the KFR22 refrigerant oil. The basic parameters of basic oil are listed in Table 1, OLFs nanoparticles (purity≥99.5%), NiFe₂O₄ nanoparticles (particle size of 30 nm, purity≥99.5%). After surface modification of the NiFe₂O₄ nano refrigerant oil, the transmission electron microscope (TEM) scan of the NiFe₂O₄ nano refrigeration oil is shown in Fig. 1.

The nano powders are dispersed in the refrigerant oil by using the mechanical grinding and ultrasonic dispersion method. The base fluid is joined to Netzsch sand mill (MINICER 993.0490) in the experiment, followed by the slow addition of the nano powder and the dispersant to control the pressure. The temperature should not exceed the limit value in the adding process. At room temperature, the stable NiFe₂O₄ nano refrigerant oil can be obtained by using the ultrasonic wave oscillator for 30 min, and the suspended solution can be stably stored for several weeks under the condition of static setting.

The refrigeration oil is prepared by the above method. The dispersion stability is analyzed by the method of settlement. Taking into account the Nano lubricant move in circles in the actual operation of the process, it is less likely for settlement to occur.

Friction test

The analysis of the slide transverse force and corresponding deformation only take the main force into consideration and the main force is the lateral force out part of gas energy force, so longitudinal gas force on the skateboard and reaction force on the head can ignore. To simplify the calculation, regardless of the mixed lubrication, and assuming the skateboard and slide groove are rigid and the contact between them is linear, the SRV (II) is used to simulate the experiment.

The friction test is conducted on the SRV (II)-type high temperature friction and wear tester. The SRV (II)-type high temperature friction and wear tester with reciprocating motion can control the temperature, load, moving distance and frequency. In the SRV experiment, the contact form of linear contact friction pair is used to simulate the actual working condition. The result of reducing friction and wear resistance of NiFe₂O₄ nano refrigerant oil added with different contents of nano refrigerant is verified. In the linear contact friction test, the 45# steel is used as the upper and lower samples. The upper sample has a dimension of 22 mm × 15 mm while the lower one has a dimension of 24 mm × 7.88 mm.

The disposable oil supply mode is used to add a small amount of lubricating oil with a plastic dropper to cover the working surface. In the testing, the friction coefficient is recorded automatically by using the SRV tester. The tests are conducted in two groups. The variation of friction coefficient under different test temperatures is mainly tested in one group, whereas the anti-friction and anti-friction properties of the refrigerant oil in the process of friction test are tested by simulating the actual working conditions in another group. The specific parameters are tabulated in Table 2.

Results and analysis

Determination of experimental friction conditions (temperature)

The temperature starts at 40°C, acting under fixed load, and is increased by 20°C in every 5 min. The results of the variation of friction coefficients with temperature for nano-NiFe₂O₄ refrigeration lubricants are shown in Fig. 3. It can be seen from Fig. 3 that the friction coefficient of lubricating oil increases with the increase of temperature. The friction coefficient of the base lubricant has a minimum value of 0.136 at about 10 to15 min (temperature is 80°C). The friction coefficient of the lubricant oil with the addition of NiFe₂O₄ changes suddenly and begins to decrease at 15 to 16 min (at a temperature of 100°C), which explains the great influence on the friction coefficient of lubricant oil. At the appropriate contact temperature, the lubricating oil containing NiFe₂O₄ reacts chemically with the metal surface, and the film is formed under the catalysis of the metal, which can prevent the direct contact of the surface of the friction pair. In the experiment, when the temperature is too high, the boundary lubrication is difficult to maintain, and the friction coefficient increases rapidly. From the above analysis, it can be concluded that nano refrigeration oil can work normally when the temperature is not more than 100°C. As is known, the running environment of the compressor is 80°C. At this temperature, the nano refrigeration oil can run normally.

SRV curves of constant temperature and constant load

SRV curve of lubricating oil with single Span 80

Span 80 is a kind of low molecular weight nonionic surfactant, whose requirement of temperature and shear force is not high. Considering the fact that the viscosity of Span 80 is relatively large, and that it has a great influence on the viscosity of the base oil, Span 80 is analyzed separately. The results are depicted in Fig. 4. By adding Span 80 into the lubricating oil, the friction coefficient decreases quickly to the lowest point in the first two minutes and then increases gradually to a steady-state in the later running-in period. Compared with the experiment using lubricating oil, using Span 80 decreases the friction coefficient more rapidly in the initial stage of the test. The results show that the addition of Span 80 can change the friction performance of the lubricating oil, but the reduction friction coefficient is not very high. Because it has good oil solubility and it is more prone to emulsification, it makes the friction coefficient change slowly, even though there is only a slight loss in friction coefficient.

SRV curve of NiFe₂O₄/OLFs lubricating oil

Figure 5 is the SRV curve of the experiment of NiFe₂O₄/OLFs lubricating oil. From Fig. 5, it can be seen that the friction coefficient of NiFe₂O₄/OLFs is significantly lower than that of pure oil. In the running period of 3–20 min, the change range of friction coefficient is 0.04–0.06 smaller than that of pure oil whose friction coefficient is 0.135–0.145, with a maximum standard deviation of 0.0035. This provides a better friction reduction behavior, an improvement of 64.3% compared to pure oil when the concentration of NiFe₂O₄ is 100 ppm and the concentration of OLFs is 60 ppm. This indicates that under the condition of constant temperature and constant load, the friction performance of NiFe₂O₄/OLFs with surface modification is better than that of pure oil, and can shorten the time needed for running. Moreover, the addition of NiFe₂O₄/OLFs does not have a running failure to the lubricating oil. This shows that the mixed additive retains the anti-wear and anti-friction effect of NiFe₂O₄, improves the friction stability of lubricating oil, and overcomes the defects of the instability of the friction performance of the lubricating oil as a single additive.

Experimental analysis

In this study, the friction coefficient that is concerned is the instantaneous friction coefficient. The experimental results show that temperature has a great influence on the friction coefficient of the nano refrigerant oil. The friction coefficient of the nano refrigerant oil first decreases then increases with the increase of temperature. When the temperature is too high, the boundary lubrication is difficult to maintain, and the friction coefficient is not stable. Span 80 mainly modifies nano powder by surface coating to affect the friction and wear properties of pure oil together with nano powder. In the process of friction, nano particles that are dispersed uniformly in lubricating oil play a role in filling with the concave convex friction surface, which reduces the direct contact of the friction pair. Some scholars believe that, due to the high local temperature of the friction surface, nano particles are likely to be in a melting, semi melting or sintering state, thereby forming a layer of nano film, while nano film is different from the ordinary film whose toughness and bending strength are much higher than the ordinary film. After the addition of NiFe₂O₄/OLFs, the nano refrigeration oil shows good friction stability and reliability. OLFs has a unique spatial reticulated structure and performance, which is covered with NiFe₂O₄ nano particles, and thus playing a supporting role of the “ball bearing.” It should be pointed out that NiFe₂O₄/OLFs as additive is effective to overcome the creep problem in reciprocating mechanism, and can eventually reduce the mechanical losses of the compressor.

Conclusions

In this paper, a novel way for a compressor to improve its coefficient of performance, enhance reliability by employing nano-refrigeration lubricant oil was proposed. It can be concluded that temperature has a great influence on the friction coefficient of the nano refrigerant oil. The experimental results show that the boundary lubrication is difficult to maintain when the temperature is above 90°C. The additive, NiFe₂O₄/OLFs, improves the stability of lubricating oil and has a better lubricating property. The friction coefficient is decreased from 0.15 to 0.04.

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