3.1. Friction and wear behaviors
The relationship between the concentration of different additives in lubricating oil and wear under extreme pressure conditions was tested by four ball friction and wear tester. The amount of wear was expressed by the wear diameter of the ball and the experimental results were shown in Fig. 3. The wear spots diameter of lubricating oil with different additives first decreases and then increases with the increase of the concentration. When the concentration was 0.4wt%, the minimum wear spot of oil containing T304 additive was 520 µm, and when the concentration was 0.8wt%, the minimum wear spot of ZDDP was 490 µm. The wear loss of the lubricating oil with PN additive was relatively small, and it reached the minimum value of 310 µm when the concentration was 1wt%.
Figure 4(a) and Fig. 4(b) represented the time-dependent curves of COF and temperature of different additives, respectively. The experimental load was 392 N, and the speed is 1200 r/min. The concentration of these three additives was 1wt%. It could be seen from Fig. 4 that the COF of ZDDP was stable above 0.1, and it had the largest COF. The COF of T304 rose with time and fluctuated greatly. The COF of additive PN was the lowest among the three additives, and after quickly reaching stability, it always remained at about 0.75. After 3600 s four ball test, the temperature of lubricating oil with ZDDP additive was the highest, reaching 66oC, and the temperature with T304 additive was 61oC. The experimental temperature of lubricating oil with additive PN was the lowest, which was 58oC. This showed that under low-load and high-speed conditions, the synthetic PN additive had a better anti-friction effect than the commonly used ZDDP and T304 additives. During the use process, it would increase the transmission efficiency of the gear set and reduce the working temperature of the gear.
Figure 5 showed the changing curve of COF and wear amount in the four-ball experiment of lubricating oil with different additives under different loads. The experimental conditions were as follows: rotating speed is 1200 r/min; running time is 3600s; the additive concentration was 1wt%. In Fig. 5(a), the average COF rose sharply with the load from 98 N to 392 N. When the load was greater than 392 N, the COF of lubricating oil containing ZDDP and T304 slowly rose and the COF of lubricating oil containing PN additives had a slow downward trend. Overall, the COF of lubricating oil containing PN additives was the smallest, and it changed relatively smoothly as the load increased. Fig. F(b) showed change of wear spot diameter with load. It was clearly that the wear spots increased with the load, and the wear loss of the lubricating oil containing PN additive was smaller than that of the other two additives. The higher the load was, the greater the difference between the wear scar of the PN additive and the other two additives were. Therefore, it could be said that PN additive would exert excellent anti-friction and anti-wear performance under extreme pressure conditions.
3.2. SRV testing
To study the effect of experimental temperature on the tribological properties of PN additives in MVI-500 base oil, the SRV testing machine was used to test the oil with 1wt% ZDDP, T304 and PN additives, respectively. The contact mode of friction pair was line contact movement mode. The experimental conditions were shown as Table 3 and the results were shown as Fig. 6. It could be seen from the curve of COF changing with temperature in the figure that the lubricating oil containing PN additive showed excellent anti-friction performance at high temperature. As the temperature increased, the COF of lubricants containing ZDDP and T304 additives increased significantly, while the COF of lubricants containing PN additives decreased first, then slowly increased, and the minimum COF was 0.066 at 70 and 80oC. At the high temperature of 120 oC, the COF of PN additive was 0.073, while ZDDP and T304 were 0.104 and 0.112, respectively. The COF of PN additive at high temperature was 30% lower than traditional additives. Therefore, PN additive could effectively improve the antifriction performance of lubricants and the transmission efficiency of gears and other transmission components under high temperature conditions.
Table 3
The RSV tests experimental conditions
Items
|
parameter
|
Frequency /Hz
|
50
|
Amplitude /mm
|
1.5
|
load /N
|
400
|
Time /min
|
40
|
temperature / oC
|
50–120
|
Heating rate / oC /5min
|
10
|
3.3. SEM and EDS analysis
Under the condition of 392 N load, after 3600 s four ball test, the friction interface of steel balls was seriously worn. And the SEM images of worn surface with different additives and the EDS analysis of worn surface were shown in Fig. 7. When the lubricant with PN additive was used for lubricating, the wear surface showed high temperature adhesion marks, and there was slight abrasive wear on the surface (Fig. 7a, b). After the extreme pressure test, the friction interface lubricated with ZDDP additive showed severe adhesive wear, and a large number of hard oxidized and high-temperature recast particles appeared, resulting in deep furrow at the friction interface (Fig. 7d, e). On the surface of the wear spot, the elements of P, S and Zn were detected by EDS. This was because ZDDP took place thermal decomposition reaction to generate phosphate during friction, which was chemically adsorbed to the friction interface to reduce friction and wear resistance, and the S in ZDDP reacted with Fe to form FeS, which had a certain anti-wear effect. The worn surface lubricated with T304 additive showed adhesive wear and there was shear tearing on the surface under high load condition (Fig. 7g, h). The ketone structure of phosphite ester and Fe formed a complex compound, which produced a friction reaction film attached to the friction interface, thus improving the performance of gear oil under extreme pressure. So, there was much P in the EDS graph (Fig. 7i). While the content of P and S elements on the wear spot containing PN additive was very low, which could be speculated that the anti-friction and anti-wear mechanism of PN additive on the surface of iron-based metal was mostly related to Mannich base containing benzotriazole.
3.4. XPS analysis
XPS elements analysis of wear area under PN additive lubrication was done with an X-ray photoelectron spectroscopy (Thermo Fisher ESCALAB Xi+, Thermo Fisher Technology Co., Ltd., America). Figure 8 the figure of XPS analysis of surface wear area with PN additive lubrication. The curve-fitted C 1s region was shown in Fig. 8(a). The maximum peak of the spectrum was at 284.80 eV, corresponding to the main graphite peak. The two peaks 284.55 eV and 284.10 eV were mainly caused by benzene ring, and there were peak at 285.71 eV and 288.27 eV indicating that it contained (CH2)n and carbon-nitrogen single bonds[28, 29]. Figure 8(b) was the N 1s region curve fitted. The two peaks 399.42 eV and 399.72 eV indicated the presence of triazole[30]. Combined with the composition of additive PN, it could be speculated that the friction surface contained derivatives of mannich base containing benzotriazole. Figure 8(c) and (d) were the fitted curves of O 1s and Fe 2p, respectively. The 531.01 eV and 534.03 eV were the peaks of Oxygen hydrogen bond and oxygen phosphorus bond, and the peak of 529.5 eV indicated that there were FeO or Fe2O3[31]. The peak of Fe 2p meant that there were divalent Fe and trivalent Fe. Therefore, it could be concluded that the worn surface contained FeO, Fe2O3, FeOOH[32] and a small amount of FePO4[33].
As it was shown in Fig. 9, additive PN contained triazole ring, and it had good affinity with metal and could quickly adhere to the metal surface during the friction process. Then PN was decomposed into phosphoric acid group and derivative of Mannich base containing benzotriazole by tribochemical reaction. The nitrogen atoms in triazole ring had lone pair electrons, which would make them form a dense chemical protective film on the metal surface, preventing the metal ions into oil and weakening its catalytic oxidation effect on oil. Therefore, PN additives could exert excellent anti-friction properties at high temperatures (Fig. 6). In addition, XPS analysis showed that there were FeO, Fe2O3, FeOOH and a small amount of FePO4 on the worn surface. These Fe compound could form a transfer film layer to reduce friction and wear.