This study focuses on the wear effects of nano-sized titania as a potential engine lubricant additive. Titanium dioxide nanoparticles have promising wear-reducing properties and significant tribological potential. Titania nanoparticles were homogenized in Group III automotive oil at five different concentrations (0.1; 0.2...0.5 wt%). The nanodoped oil samples were tested on a linear oscillating tribometer with oil circulation. Based on the tribological results, titania nanoparticles increased friction by 20–32% but can reduce the wear area by up to 32%. According to the confocal microscopic examination, wear volume can be reduced by up to 57% with titania nanoparticles. Titania nanoparticles improved the repeatability of tribological measurements. A scanning electron microscopy examination of the wear track revealed that the characteristic wear of the tribological system was abrasive, but a significant amount of adhesive wear was also observed. Energy dispersive X-ray spectroscopy analysis found that the nanoparticles fill the deeper trenches of the wear. The worn surface uniformly contains TiO2 particles and the quantified normalized titanium concentration was between 0.56 and 0.62%.
Figure 1 illustrates the evaluation of the measured friction coefficient integral average values of the investigated nano lubricant samples in the function of their titania concentration. An evident tendency can be defined from this bar chart: TiO2 nanoparticles homogenized into neat Group III base oil do not positively affect the lubricant regarding friction reduction. Each prepared sample presented a friction coefficient increase of 20 to 32%. However, it can also be observed that the measured error bars have reduced in each titania concentration, indicating the investigated nanoparticles' positive stabilizing effect.
Figure 1. Comparison of the Friction coefficient values of lubricant samples with various titania nanoparticle concentrations.
Figure 2 compares the measured and calculated Mean Wear Scar Diameter (MWSD) values in µm dimension. All the prepared nano lubricant samples decreased the measurable wear scar diameter values at each of the prepared TiO2 concentrations. The measured tendency shows that the samples with higher titania concentrations (especially 0.4 and 0.5 wt%) work the best from the investigated samples. However, the 0.2 wt% sample also provides similar wear reduction properties as the 0.5 wt% sample. It is also clear that the error bars decreased compared to the reference results. The sample, including 0.4 wt% titania nanoparticles, showed the lowest MWSD results with a decreasing value of 32%.
Figure 2. Comparison of lubricant samples' Mean wear scar diameter values with various titania nanoparticle concentrations.
The acquired digital microscope images of the worn disc surfaces with 0 and 0.4 wt% titania concentrations can be observed in Figure 3. The difference can clearly be defined: adding titania nanoparticles into the neat Group 3 base oil decreased the wear scar width. During a drastic wear process, the usually burned lubricant molecules (dark color on the surface) are removed by the connecting specimen and the worn surface becomes shinier. These shiny wear areas cannot be found in the wear scars in the case of titania-added samples, which leads to the deduction that the investigated TiO2 nanoparticle eliminated the establishment of the drastic deep wear grooves since no metal-shining areas can be seen in the wear scar image acquired with 0.4 wt% of titania containing nanolubricants.
Figure 3. Acquired digital microscope images of the worn surfaces on the disc specimens with 0% (a) and 0.4% (b) titania NP concentrations.
Figure 4 compares the nanolubricants to the wear volume generated during the tribological experiments. The bar chart clearly illustrates the differences between the lubricant samples with different titania concentrations and the positive antiwear effect of the titania nanoparticles. The tendencies according to wear volume and mean wear scar diameter are similar. The highest wear volume reduction was accomplished in the case of the 0.2 wt% titania containing oil sample with a decrease of 57%, but the 0.5 wt% sample also shows identical wear-decreasing properties.
Figure 4. Comparison of the Wear volume values of lubricant samples with various titania nanoparticle concentrations.
According to the acquired tribological results, the main influencing factors of the titania nanoparticles can be defined. Adding TiO2 nanoparticles into the neat Group III base oil significantly increases frictional losses2 nanoparticles into the neat Group III base oil increases the frictional losses significantly (at least 20% of friction coefficient increase). However, the wear reduction effect of the investigated oil samples is impressive: the measured MWSD values were reduced by 32% in the case of 0.4 wt% titania-containing lubricant and the calculated wear volume values were also dropped by 57% in the case of 0.2 and 0.5 wt% sample. Combining this information, the optimum concentration can be defined at 0.4 wt% because it provides the lowest possible friction coefficient and excellent wear reduction effect.
The scanning electron micrographs were taken with an accelerating voltage of 20 kV in secondary electron mode at a magnification 1000. Figure 5 shows scanning electron micrographs and titanium intensity images of the wear tracks of a disc tested using an oil sample containing 0.4 wt% (optimum) nano-sized titania. The SEM images were taken on the center line of the wear track of the disc in the direction of movement. Picture (a) shows an image taken at the center of the disc, where the relative speed between the test bodies is the highest and the formation of mixed friction is the most likely. Figure (b) shows an image taken in the middle of the dead center of the wear track of the disc. At this point, the relative speed is the lowest and reaches zero, so boundary layer friction is formed. Figures (c) and (d) show the intensity of the titanium element at the center of the wear mark (c) and the center of the dead center of the wear mark (d).
Figure 5. (a) SEM image taken at the center of the wear track. (b) SEM image was taken at the dead center of the wear track. (c) titanium intensity image in the center of the wear track belonging to (a). (d) titanium intensity image in the center of the dead center of the wear track belonging to (b).
Based on the SEM images (Figure 5 (a) and (b)), it can be established that the abrasion wear type is dominant on the entire wear track, the grooves of which are visible in the direction of movement. Traces of adhesive wear can be detected in both positions. However, it occurs in a higher proportion in the center of the wear track. EDX is a device suitable only for detecting elements. In the tribology system used during the tests, the titania nano additive is the only component that contains titanium. Therefore, it can be concluded that all titanium signals entering the detector originate from the titania nano additive. The figures showing titanium intensity indicate in red the areas from which titanium radiates with greater intensity. Red sites contain more titanium than green, blue and black areas. Comparing Figures 5 (a) and (c), as well as Figures 5 (b) and (d), it can be concluded that small amounts of titanium occur uniformly in all areas of the wear track. It is mainly found in significant quantities in the deepest grooves of wear marks. Based on the images, it can be established that the nano-sized titania additive fills the grooves of the wear track during its operation.
Figure 6 shows EDX spectra recorded at different points of the wear track of the disc. Figure 6 (a) shows the EDX spectrum taken at the center of the wear track of the disc, while Figure 6 (b) shows the spectrum taken at the dead center of the wear track of the disc.
Figure 6. The upper figure (a) shows the EDX spectrum taken at the center of the disk (belongs to Figures 5 (a) and 9 (c)). The lower figure (b) shows the EDX spectrum taken at the dead center of the disk (belongs to Figures 5 (b) and 9 (d)).
The EDX spectra (a) and (b) show similar results; from that, it can be concluded that the elemental composition of the worn surfaces is identical; it is independent of the position. The iron tip is the highest because it provides most of the disc's material. The primary alloying elements of the disc material are silicon and chromium. Carbon can also be found in the disc's material, but it was mainly incorporated into the surface from the lubricating oil during the measurement. The origin of the oxygen peak can come from several sources: the disc's steel material may contain it, from surface oxidation, from the titania nano additive. Since the origin of the oxygen is not clear, it is therefore not suitable for detecting TiO2. The signal of the titanium peak, marked in yellow in the spectrum, can only come from the presence of the titania additive, so it is used to determine the amount. Based on the intensity of the EDX signals, the titania content of the surface can be calculated; a summary is shown in Table 1.
Table 1. The table compares the titanium content of the unworn reference surface at the center of the wear track and the center of the dead center of the wear track.
Position on the disc |
Ti content [norm. wt%] |
reference |
0 |
middle point of the wear track |
0.62 |
middle point of the dead center |
0.56 |
From the quantification results, it can be concluded that a similar amount of titania nano-additives are found (settled in the valleys or embedded in the worn surface) in the entire area of the wear track. The unworn surface did not contain titania. The normalized titanium content of the center of the wear track is 0.62 wt%, while 0.56 wt% is found at the dead center.
It demonstrated the homogenization process of titanium dioxide nanoparticles to create a stable dispersion in Group III-based lubricants. Using the titania nanoparticles, five Group III-based lubricating oil samples were prepared (0.1; 0.2...0.5 wt% in concentration), suitable for tribology tests.
The tribological tests of the lubricating oil samples were carried out by the Department of Propulsion Technology, Széchenyi István University, Győr. Lubricating oils were tested on a ball-on-disc oscillating tribometer using continuous oil circulation. The friction was recorded during the measurements; then, worn surfaces were evaluated using a standard evaluation.
It can be stated that specific properties of the Group III lubricating oil doped with titanium dioxide nanoparticles have yet to be discovered and require further investigation.