# Synergistic Effects of CuO and ZnO Nanoadditives on Friction and Wear in Automotive Base Oil +
## 2 Materials and Methods  
For this investigation, a Group III base oil with a kinematic viscosity of 6 cSt at 100 \({}^{\circ}\)C (supplied by MOL-LUB Ltd., Almasfuzit\(\ddot{\rm{u}}\)t, Hungary) was selected as the base lubricant due to its widespread use in automotive engine oils. Spherical cupric oxide (CuO) and zinc oxide (ZnO) nanoparticles were employed as additives to enhance the tribological performance of the base oil. The CuO nanoparticles had an average particle size ranging from 30 to 50 nm and a purity of 99%, while the ZnO nanoparticles had an average particle size of approximately 50 nm and a purity of 97%. The bulk Vickers hardness of CuO is approximately 130-200 HV, while ZnO exhibits a significantly higher hardness of about 1000-1200 HV. It should be noted that at the nanoscale, these values can increase due to grain refinement and surface effects. Both types of nanoparticles were procured from a certified supplier. CuO and ZnO nanoparticles were surface-modified using oleic acid as a surfactant to improve their dispersion stability in hydrocarbon-based lubricants. The surface modification procedure with oleic acid was previously validated on SiO\({}_{2}\) nanoparticles in a similar tribological system, as described in [21], and the same method was applied here for CuO and ZnO nanoparticles. The preparation of the tested lubricant began by measuring the right amount of pre-surface-modified nanopowders into a beaker. A total of 6 mL of toluene was added to the dry nanopowder, and the mixture was stirred using a magnetic mixer for 1 min (200 rpm) at room temperature to reduce the size of larger bulks in the powder. Toluene helps to separate the individual nanoparticles from each other. Then, a proper amount of Group III 6 cSt base oil was added to the beaker to reach the desired 0.3 wt% nanoparticle concentration. This mixture was then mechanically stirred (1000 rpm) using a magnetic stirrer for 20 h under ambient conditions to allow complete evaporation of toluene, facilitating surface interaction between the nanoparticles and the surfactant. Final homogenization to ensure the proper initial dispersion for the tribotests was performed using an ultrasonic homogenizer at 50 \({}^{\circ}\)C for 15 min to eliminate residual agglomerates and achieve a stable and homogeneous nanolubricant suitable for tribological evaluation. Five lubricant formulations were prepared for comparative testing:  
* Pure Group III base oil (6 cSt) as the reference lubricant. * Base oil with 0.3 wt% CuO nanoparticles. * Base oil with 0.1 wt% CuO and 0.2 wt% ZnO. * Base oil with 0.3 wt% ZnO nanoparticles. The lubricant formulation steps are shown in Table 1.  
Each formulation was evaluated using a tribological testing protocol on an Optimol SRV\({}^{\otimes}\)5 tribometer (Optimol Instruments GmbH, Munich, Germany), operated in the Tribological Laboratory of the Department of Propulsion Technology, Szechenyi Istvan University, Gyor, Hungary. The tribometer was configured in ball-on-disk mode in compliance with ISO 19291:2016 standards [22]. The ball specimens, made of 100 Cr6 bearing steel (1.3505), were 10 mm in diameter, polished to an Ra value of 0.025 \(\pm\) 0.005 \(\upmu\)m, and exhibited a hardness of 60 \(\pm\) 2 HRC. The disk specimens (24 mm diameter \(\times\) 6.9 mm thickness) underwent vacuum arc remelting, spheroidizing annealing, and subsequent grinding and lapping to reach a surface roughness of Ra = 0.035-0.05 \(\upmu\)m and hardness of 62 \(\pm\) 1 HRC. Before testing, all specimens were ultrasonically cleaned at 50 \({}^{\circ}\)C for 15 min using brake disk cleaner to eliminate surface contaminants and ensure identical initial conditions. The tribological test cycle consisted of three sequential stages:  
* preheating at 100 \({}^{\circ}\)C under a 50 N preload,
* a 30 s run-in phase at 50 N to promote tribofilm formation, and
* a 2 h steady-state friction test at 150 N.  
Sinusoidal oscillation motion was applied (1 mm stroke at 50 Hz), and the temperature was maintained at 100 \({}^{\circ}\)C throughout.