4.1 Wear Scar Images of Balls
Micrographs of the wear scar formed on the balls for each of the formulations are shown in Figures 4 and 5. The addition of TiO2 led to reductions in the diameter of the wear scar.
Figures 4 and 5 reveal the wear scar maps of the tested ball sliding against the bearing steel balls. This shows that the wear degree of the samples containing TiO2 was obviously improved compared to that of the base oil. When the base oil was a lubricant, the worn surface of the ball presented black sediment, which confirmed that the main component was Fe resulting from ball-on ball milling. Moreover, the 0.01 wt.% and 0.025 wt.% samples also presented back sediment, but the wear radii were visibly reduced. In particular, the radius of the 0.075 wt.% oil sample in the stabilized state after rubbing was smaller than that of the other samples, showing the superior anti-wear performance of this sample.
To further investigate the wear mechanism, SEM images of worn surfaces of the lower balls lubricated by the base engine oil and the engine oil samples containing 0.01, 0.025, 0.050 and 0.075 wt.% TiO2 nanoparticles were investigated. The tests were performed at two different working temperatures, 23°C (RT) and with oil heated to 75°C.
Following the evaluation of the wear traces with SEM, radius differences of the circular wear trace are observed, and the radius difference is approximately 5-10%. This should not greatly influence the mechanism of wear (generation of wear residues, increase of the tribo-layer, etc.).
Figure 4 shows that for samples LS and L0, there was obvious plowing and some pits on the worn track, which were due to the second local rupture of debris during the sliding process; moreover, the friction of debris at the sliding interface clearly furrows on the worn surface.
Figure 4 (L2 and L3) shows that the worn surface presented finer mesh-like grooves, in agreement with the COF result in Figure 11. For the 0.05 wt.% sample, the worn surface was covered by fine grooves and detachments after the rolling test, as shown in Figure 4 (L1), and the mechanism of wear was dominated by microplowing. Furthermore, Figure 4 (L2 and L3) shows that the wear scratch was almost invisible; through a local zoom of the images in Figure 4, slight furrows of wear could be observed. This is mostly attributed to the increase in TiO2, which could reduce the wear of frictional interfaces. This indicates that TiO2 played a lubricating role and prevented wear in the rolling process. The above results are in agreement with the tribological results showing the decrease in the friction coefficient in Figure 11.
The wear scar diameter of each of the three bottom test balls was measured to determine the lubricity performance of the test lubricant. In general, the larger the wear scar diameter is, the more severe the wear, but we also consider the depth of the wear trace. The wear scar diameter was determined for each of the three fixed balls.
The temperature of the test lubricants was measured by a thermocouple attached to the four-ball tester to record the temperature changes throughout the duration of the experiment.
The base oil and nano-oils were tested at temperatures of 23°C and 75°C (close to the temperature service for engine oil). Increasing the temperature results in nanoparticle movement, which is associated with reducing the fluid resistance over the flow; therefore, the viscosity is reduced. With respect to viscosity, it is clear that either of the base oils or nano-oils are non-Newtonian fluids. However, the temperature of the contact point for the balls is also influenced by the sliding speed. A sliding speed of 0.80 ms−1 (calculated based on input parameters) was selected to provide the minimum and extreme heating due to sliding.
4.2 Wear Depth Scar on the Balls
The depth of the wear scar on the spheres was measured using an Alicona Inginite Focus G5 microscope. The surfaces were scanned with a microscope using 50x magnification, and the light source was coaxial with the eyepiece (lenses) and supplemented with a light ring. Scanning was performed using Image Field mode with a vertical resolution between 0.003 and 0.032 microns and a horizontal resolution of 2.13 microns. The duration of a scan was between 1.5 and 3 minutes. The average scan height was 0.150 mm. This gives a Vertical Dynamic of 150/0.032=4687.5.
The evaluation of the wear depth was performed by measuring the distance from the ideal circle, constructed using a fixed 6.35 mm beam with the Measure Circle function. The traces of the intersection between the scanned surface and the plane in which the depth measurement was performed were used to orient and position the ideal circle. The depth of wear (difference between the ideal circle and the trace on the sphere) was measured using the measure height step function or maximum distance. The 2D profiles of the worm surfaces for different lubricant oils after the wear test are shown in Figs. 6 and 7.
Compared with pure oil, TiO2 had a significant improvement in the wear surface. After the wear test at RT, only small groves could be observed, and the wear mechanism was mainly formed by microplowing. Although the wear depth of the 0.01 wt.% sample was much smoother than that of the base oil, microplowing still existed, with a corresponding wear depth of 1.2 µm – RT and 0.41 µm – 75°C. Moreover, the anti-wear property is improved with the amount of TiO2. This indicates that TiO2 in engine oil prevents the plowing wear that existed in the control sample.
Figure 8 shows a chart of the amount of ball wear, as measured using the Alicona Inginite Focus G5 microscope. Very little protection was provided by the base oil, but the addition of nanoparticles to the base oil significantly reduced wear.
The wear rate of the lower balls at RT was somewhere 70 x 103 µm3 for the base oil, gradually reaching up to 10 x 103 µm3 for the concentration of 0.075 wt.% TiO2. At a temperature of 75°C, the wear decreased to 33 x 103 µm3 for the base oil, and then with the addition of wt% TiO2 nanoparticles, it decreased to 5 x 103 µm3. The measurements of the wear depths produced on the balls were accurate and repeatable with the help of the Alicona Inginite Focus G5 microscope.
In the current study, the authors tried to avoid overloading the tribocouple due to a high risk of layer deformation and change in the wear mechanism. Considering the wear rates of the balls studied by the authors (33 – 70 x 103 µm3) at RT, it can be concluded that the wear of the balls is a few orders of magnitude larger for balls lubricated at 75°C. This can be explained by the fact that the connecting rods are always in contact, and therefore, the phenomenon of continuous overheating occurs.
No transfer film was observed on the balls at a slip velocity of 0.80 ms−1. However, the tests were accompanied by vibrations and unwanted noise.
4.3 Friction property of lubricating oils with TiO2 additives
The tribological performance of engine oil (Motul 5100 4T10 W-30) with TiO2 additive loading as a lubricant additive is shown in Figure 10. The coefficient of friction (COF) of engine oil with different additive TiO2 amounts was measured at an applied load of 10 N at the arm of the tribometer, 396 ±4 N normal load applied on balls and rotation 1200 rpm, as presented in Figures 2-3. Relative fluctuations in the friction response of base engine oil were observed in comparison to the additive response, and the COF was found to increase with time in the initial stage during the running period.
The coefficients of friction were remarkably reduced by the addition of TiO2 nanoparticles to the base lubricant. At a normal applied force of 396 N, the coefficient of friction of the nanolubricant was reduced by approximately 60% of the COF value for the base lubricant at RT and by approximately 80% at a temperature of 75°C.
Clearly, the nanolubricant with more TiO2 nanoparticles had the best coefficient of friction (Figure 9). These results indicate that the TiO2 nanoparticles decrease the ball-to-ball friction contact compared to the base lubricant.
The medium lowest COF of 0.01 was obtained by the oil sample with 0.075 wt.% TiO2 under a 75°C lubricant temperature. Moreover, Figure 10 shows the influence of particle concentration on the COF of oil suspensions, indicating that the average COF was influenced by the TiO2 concentration. The average COF obviously fell from 0.112 to 0.05 in the range of 0.01 wt.% to 0.075 wt.% TiO2, reflecting that the addition of nanoparticle lubricants straightened the sliding response when stabilizing additive amounts below 0.025 wt.% TiO2. However, the average COF showed a slightly decreasing trend from 0.010 – 0.088 in the range of 0.010 wt.% to 0.025 wt.% at RT. On the other hand, at 75°C, the average COF obviously fell from 0.095 to 0.015 in the range of 0.01 wt.% to 0.075 wt.% TiO2. Furthermore, the COF tended to be stable after rubbing, and the corresponding average COF in the stable period for higher concentrations of TiO2 presented a lower antifrictional property, as shown in Figures 9 and 10. The relevant tribological mechanism at RT is due to TiO2 particles at higher concentrations accumulating in the inlet of the ball-on-ball contact area, which causes an insufficient supply of lubricant and starvation in the contact area.
The running-in period is of great significance to the regulation of tribological performance to a certain extent. Reducing the running-in period is beneficial to improving the antifrictional property. The formation of a boundary lubrication film is the main reason for the stability of the friction coefficient. The coefficients of friction stabilized in the second part of the test time. The rubbing period of nano-oil with a 0.1 wt.% concentration lasted longer than that of the others with a time of 780 s. In addition, it is noteworthy that the rubbing time obviously decreased with increasing concentration. The 0.05 and 0.075 wt.% samples had the shortest rubbing time in terms of friction properties.
The friction coefficient was calculated according to IP-239 and is expressed as follows:
4.4 Flash Temperature Parameter
The flash temperature parameter is a unique number that gives us indications of the critical flash temperature above which the lubricant used will be out of use 
For working conditions in the four-ball tribometer, the flash temperature parameter is:
A flash temperature parameter (FTP) was calculated for all of the experimental conditions according to Eq. 2. In this equation, F is the normal load in kilograms and d is the mean wear scar diameter in millimeters at the particular load. A detailed explanation of the parameter is given by Lane [16, 17].
High values for the flash temperature parameter indicate that the lubricant shows good performance with a reduced possibility of lubricant breakdown .
Figure 11 shows the plot of TiO2 percentage vs. flash temperature parameter (FTP) for different testing temperatures, more exactly room temperature RT and 75°C. From the figure, it can be seen that the maximum and minimum FTPs were obtained from 0.075 wt.% contaminated lubricant and pure lubricant, respectively. The maximum FTP value means that good lubricating performance occurred, indicating a lower possibility of lubricant film breakdown. This phenomenon has also been observed by other researchers . This seems to indicate that TiO2 nanoadditives are a potential anti-wear additive for lubricating oil. The 0.075 wt.% TiO2 in this investigation improved the lubricant performance based on the higher value of FTP observed compared with pure lubricant. The graphs also show the effect of temperature on the FTP of lubricants.