Tribological Investigation of Base Oil Supplemented with Zirconia Nanoparticles at Various Operating Temperatures ^†

Abstract

This study examines oil samples with zirconium dioxide (ZrO₂) nanoparticles in Group III base oil at different temperatures, revealing the effects of temperature and concentration on the tribological system. The samples contain 0.1% and 1% ZrO₂ nanoparticles, tested at 40–120 °C. The friction results showed that the nanoparticles increase the friction absolute integral values at all tested temperatures; however, static friction can be improved by 3–13%. The study demonstrates the wear-resistant effect of ZrO₂ nanoparticles. Significant wear reduction can be achieved even at low concentrations; wear volume can be reduced by 21–87% depending on the nanoparticle concentration and operating temperature. Scanning electron microscopy with EDX helped to identify wear types, the processes occurring on the surfaces, and the percentage of nanoparticles on the surface.

1. Introduction

Reducing vehicle emissions is a significant focus in the automotive industry, with increasing efforts towards achieving this goal. Selecting the right engine oil can significantly reduce emissions by minimizing friction between components, thus requiring less energy. Lubrication plays a fundamental role in mechanical systems, as it helps to reduce friction and wear, and contributes to heat dissipation, corrosion protection, sealing, and load distribution. Nanoparticles play a crucial role in motor oil development by reducing friction and protecting surfaces by forming a tribofilm layer that inhibits wear.

Numerous studies have reported on the effects and beneficial impacts of nanoparticles. Yuqing et al. utilized Cu and Fe nanoparticles in combination with MST measurements and dynamic simulations, resulting in a significant 57% reduction in friction [1]. Sanukrishna et al. added SiO₂ nanoparticles to an R134a refrigerant, reducing friction by 38% at a volume ratio of 0.6 and 29% at 0.4 [2]. Pin-on-disk tests showed a 6% friction reduction at the optimal concentration, and needle mass loss decreased by 86% compared to poured oil measurements. Zirconia nanoparticles are widely used in various tribological research areas, showing effective use as additives in lubricants to reduce friction and wear [3]. Thrush et al. experimented with ZrO₂ nanoparticles in PAO oil at different temperatures and concentrations, observing an increased tribofilm layer growth rate and enhanced robustness [4]. Fatima et al. comprehensively summarized various experiments and concluded that nanoparticles can provide beneficial effects at both low and high concentrations, depending on the type of base oil in which they are used [5].

This study aims to investigate the surface incorporation and tribological effects of 0.1% and 1% ZrO₂ nanoparticle samples at 40, 80, and 120 °C, providing insights into the temperature and concentration effects in the tribological system. The temperature ranges were selected due to the application temperature of an engine lubricant. The impact of nanoparticles can vary depending on their quantity; therefore, the studies were conducted with oil samples containing 0.1% and 1% ZrO₂ by mass.

2. Materials and Methods

During the experiments, ZrO₂ nanoparticles (Alfa Aesar, Ward Hill, MA, USA) were used. The nanoparticles had a size ranging from 15 to 25 nm and a purity greater than 99%. The ZrO₂ nanoparticles were incorporated into additive-free Group III base oil, allowing their effects on the base oil to be examined. The nanoparticles underwent surface activation using ethyl oleate before being mixed with the oil [6].

The tribology tests were performed using an Optimol SRV^®5 tribometer (Optimol Instruments, Munchen, Germany). A specific feature of the machine is its ability to monitor friction values per second, tracking both the friction absolute integral (FAI) and static friction (COF). FAI represents an integrated value obtained during reciprocating motion, while COF indicates the highest friction value over one stroke length. The measurements used the following two test specimens: a 10 mm diameter 100Cr6 grade steel ball; and a disc of the same material and quality (the assembly can be seen in Figure 1). The experiments involved oil samples with two concentrations of ZrO₂ nanoparticles, 0.1 wt% and 1 wt%, and were conducted at temperatures 40, 80, and 120 °C. The Group III base oil sample provided the reference values. During the measurements, each test ran under a 100 N load, with a 30 s initial running-in phase at 50 N to accommodate the running-in of the test specimens. Subsequently, the tests continued for 2 h under a 100 N load. At the end of the measurement, the contact pressure was 340–630 MPa, depending on the specific oil sample and temperature. The entire measurement series operated within a continuous oil circulation system at a flow rate of 225 mL/h. The ball movement involved a stroke length of 1 mm at a frequency of 50 Hz. Each oil sample underwent 4 measurements to calculate averages and deviations.

During the evaluation, the following three microscopes were utilized: a Keyence VHX-1000 digital microscope (Keyence International, Mechlin, Belgium); a Leica DCM3D confocal microscope (Leica Camera AG, Wetzlar, Germany); and a Hitachi S-3400N scanning electron microscope (SEM, Hitachi Ltd., Chiyoda, Tokyo, Japan) equipped with an energy dispersive X-ray spectroscopy (EDX) accessory (Bruker Corporation, Billerica, MA, USA). The EDX operates on the principle of energy-dispersive X-ray spectroscopy, enabling the identification of surface materials and the creation of spectra.

The digital microscope facilitated the image capture of wear scars and allowed for various measurements, providing information on the wear scar diameter (WSD). The confocal microscope and its associated software determined the wear volume (WV) by generating three-dimensional images and calculating wear depth and volume numerically. The examinations were conducted using an EPI 20X-L lens (Nikon, Tokyo, Japan). Each image capture was performed over a 5 × 4 square area up to a depth of 26 μm, height of 30 μm, 20% image overlap, and a single image capture. The extent of wear was determined using the volume of the wear scar after testing on the applied test specimen.

The SEM provided insights into different types of wear patterns. Images were taken at the center and dead center of the disc wear scar. The highest relative velocity occurred at the center, highlighting the maximum effect of the oil, whereas the relative velocity was zero at the dead center, indicating the poorest lubrication properties. Analyzing these two points provided insight into the mechanism of the oil sample. SEM images were captured in scanning electron mode at 1000× magnification with a 20 keV accelerating voltage and a 10 mm working distance. The SEM also included an EDX module for elemental composition and quantitative analysis, facilitating the determination of the quantity of ZrO₂ nanoparticles integrated into the surface at both points.

3. Results

The FAI increased for the 0.1 wt% ZrO₂ oil sample compared to the reference Group III base oil at all tested temperatures, with an 11–30% increase depending on the temperature (see Figure 2). The 1% ZrO₂ oil sample showed a 1–2% increase, representing a minimal difference. The temperature did not affect the FAI value for this sample. Despite the rise in FAI values, variability decreased in all cases, indicating that the system proved more stable.

Static friction was examined as the average of the friction values measured at the two starting points per second during oscillation. Examining the static friction coefficient was necessary, as the worst lubrication properties appear at the two endpoints. Figure 3 illustrates the results of the static friction.

For the 0.1% ZrO₂ oil sample, the COF values increased by 9–29% at all tested temperatures, depending on the temperature (similar to the FAI values). In the case of the 1 wt% ZrO₂ oil sample, the COF values improved by 3–13%, depending on the temperature. Regarding COF, adding ZrO₂ nanoparticles minimized variability and increased the system’s stability.

The measured mean wear scar diameter (WSD) values are presented in Figure 4. At 40 °C, the measurements show a 4–7% decrease in wear diameter compared to the reference, with low and high zirconia concentrations applied. At 80 and 120 °C, the WSD increases slowly with temperature for the 0.1 wt% zirconia content, while a significant 23% decrease is noticeable for the 1 wt% oil sample.

The wear of the disc test specimen is characterized by its wear volume, as shown in Figure 5. The wear volume results demonstrate that the added ZrO₂ nanoparticles reduced disc wear at the tested temperatures for the samples of 0.1 wt% and 1 wt% ZrO₂. For the 0.1 wt% zirconia oil sample, the wear volume reduction ranged from 21–59%, while for the 1 wt% zirconia oil sample, the reduction was significantly higher, approximately 62–87%, depending on the temperature. Thus, the zirconia nanoparticles acted as effective wear-reducing additives.

To understand the results, 1000× magnified images of the wear tracks on the discs were made using a scanning electron microscope at the center and dead point of wear. Additionally, quantitative analyses were conducted on the elemental composition of the surface. Figure 6 illustrates the 1000× magnified images taken at the center of the wear track for the 0.1% ZrO₂ content at 40 °C (a), 80 °C (b), and 120 °C (c).

Adding 0.1% ZrO₂ nanoparticles to Group III base oil resulted in various wear tracks on the surface at three different temperatures. At 40 °C, abrasive wear was dominated by minimal fatigue wear. A relatively high amount of debris was observed in the wear track grooves. At 80 °C, oxidative wear tracks appeared along with abrasive wear. At 120 °C, oxidative, abrasive, and significant fatigue wear tracks were seen on the surface. Similar results were observed in the images taken at the dead point.

The SEM images of the wear tracks, taken from the center and dead point of the measurements with the 1% ZrO₂ oil sample, show that a high zirconia content layer builds up on the surface at low temperatures. Additionally, plastic deformation and abrasive wear are present on the surface. This layer becomes less rough as the temperature increases; the built-up layer on the surface decreases and the surface undergoes more significant plastic deformation. It is hypothesized that dynamic recrystallization can be induced by plastic deformation and high frictional temperatures [7]. Figure 7 illustrates the built-up layer and its evolution at 40 °C, 80 °C, and 120 °C.

Examining the surface’s elemental composition provides essential information for uncovering the ongoing processes. The EDX spectra (Figure 8) and the quantification of the percentage of materials on the surface provide this information. Figure 7 shows the spectra at the dead point of the oil sample with 1 wt% ZrO₂ content run at 40 °C. The spectra show the disc’s main components and the following alloying elements: iron, chromium, silicon, oxygen, and carbon.

The spectra and quantification tables can infer processes occurring on the surface. Atomic percent quantities were used during the examinations. In the case of the 0.1% samples, a large amount of oxygen was found on the surface, suggesting the formation of an oxide layer. The Zr content of the surfaces is shown in

Table 1. Zr content was detected on the wear track as a function of concentration and temperature.

NP concentration [wt%]	0.1	0.1	0.1	1	1	1
Temperature [°C]	40	80	120	40	80	120
Zr content [norm.at%]	0.42	0.23	0.65	2.51	1.24	0.49

, with analysis performed in the middle of the disc.

The ZrO₂ content found on the surface in measurements performed with a 0.1% oil sample shows an increasing trend with the temperature rising. Due to the thickness of the ZrO₂-reinforced tribofilm on the surface, the equipment could not detect Fe content in certain areas; this influenced the obtained percentage value of ZrO₂. In the measurements performed with a 1% NP oil sample, it was observed that increasing the temperature decreased the ZrO₂ content in the tribofilm. At the 40 °C temperature tested disk, 2.51 norm.at% of zirconium element can be found on the surface. At 120 °C, the zirconium content dropped to 0.49 norm.at%.

4. Conclusions

From the conducted tribological examinations and evaluation of the measurements, the following conclusions can be drawn:

• The 0.1 wt% ZrO₂ oil sample did not improve friction (9–30% increase) compared to Group III. Ball wear scar diameter decreased by 4% at 40 °C but increased by 2–4% otherwise. Disc wear volume decreased by 21–59% depending on temperature. The lubricant protected against fatigue at low temperatures, but higher temperatures introduced abrasive, fatigue, and oxidative wear;
• The 1 wt% ZrO₂ sample improved every parameter compared to the reference Group III, contributing to the stability of ZrO₂ nanoparticles and the oil sample at all three temperatures. Static friction was reduced by 3–13%, ball wear was reduced by 7–17%, and disc WV was decreased by 62–87% compared to the reference. Wear tracks showed a surface layer rich in zirconium forming at low temperatures, plastically deformed as temperatures increased. The COF decreased at all tested temperatures, likely due to the surface structure of this layer, which retained oil in its grooves;
• The current results provide insight into nanoparticle application in base oils. Base oils form the foundation of various oils available in the industry; therefore, depending on the application, the addition of nanoparticles can enhance the performance of Group III base oils.