OBRABOTKAMETALLOV Vol. 26 No. 4 2024 TECHNOLOGY Results and Discussion In this study, the stability of corn oil-based CuO/Al2O3 hybrid nanofl uids was investigated by visual observation. After 96 hours of sample preparation, it was observed that the CuO/Al2O3 hybrid nanofl uids in the ratio of 1:1.5 were most stable at the weight fraction of 0.4 and 1.6. Increasing the concentration led to increased aggregation and hence decreased stability. The stability test results of corn oil-based CuO/Al2O3 hybrid nanofl uids for diff erent weight concentrations are shown in Fig. 3. Fig. 3. Results of a 96-hour sedimentation test Fig. 4, a shows the viscosity results using diff erent concentrations of CuO/Al2O3 hybrid nanofl uids. The viscosity can be increased by using hybrid nanoparticles in addition to the base oil with a particle concentration of 0.4–2.4 wt. %. The CuO/Al2O3 hybrid nanofl uid becomes more viscous as a result of increasing the particle concentration in the liquid. The viscosity of the solution decreased with increasing temperature. As a result of the decrease in intermolecular cohesion between particles at higher temperatures, the viscosity becomes less signifi cant. Fig. 4, b shows the eff ect of temperature on thermal conductivity. This can be achieved by increasing the nanoparticle content (weight percent) in CuO/Al2O3 hybrid nanofl uids and temperature. Fig. 4, b also shows that the thermal conductivity of the material was improved by adding nanoparticles due to Brownian motion and the huge surface area of nanofl uids. Fig. 4, c shows that the experimental values of specifi c heat capacity of CuO/Al2O3 HCF increase with the increase of particle concentration (in percent by weight). The specifi c heat capacity increases with the increase of nanoparticle concentration and temperature. At the nanoparticle concentration of 2.4 wt. %, the specifi c heat capacity of CuO/Al2O3 hybrid cutting fl uid is 11.86 % higher than that of the base oil. This is probably due to the high stability of HCF. The thermal conductivity and specifi c heat capacity of HCFs were improved, which enables it to dissipate heat more effi ciently. The tribological properties of hybrid nanofl uids can be evaluated using the pin-on-disk friction test. By mixing copper oxide and aluminum oxide hybrid nanoparticles with corn oil, a thin tribofi lm is formed between the pin and the disk. The fi lm thickness and the obtained result become larger with the increase of the amount of nanoparticles to a certain ratio. Fig. 4, d shows the decreasing pattern of the friction coeffi cient up to 1.6 wt. %, and then the increasing pattern of the friction coeffi cient after 1.6 wt. % is observed. After increasing to 1.6 wt. %, the friction coeffi cient is proportional to the content of nanoparticles, which indicates that the lubricating properties of the developed cutting fl uid are eventually reduced. The reason is the agglomeration of nanoparticles observed as a result of sedimentation. The minimum friction coeffi cient of 0.124 is observed at a content of 1.6 wt. % of CuO and Al2O3 in hybrid nanofl uids.
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