3.1 Friction-reducing behaviors
Mean coefficient of friction (COF) and worn surface roughness of each tests were summarized in Table 2. Variation of COF with test duration were also displayed in Fig. 2. It is observed that COF of base oil PAO4 experienced a drastic fluctuation during test and give the highest mean COF of 0.349. The introduction of anti-wear additives greatly alleviated severe friction conditions on the metal interfaces. Among which, AW 316, a phosphate amine anti-wear additive achieved the lowest mean COF of 0.082, that is almost 76% reduction of COF compared with base oil. Besides, COF of ZDDP was 0.093 which indicates that ZDDP works very well on the steel-copper friction pair. 353, an ashless anti-wear additive with typical DDP (dialkyldithiophosphate) group also showed a relatively low COF of 0.118. All of these three additives share a common characteristic that they all chemically active additives and have a relatively high total acid number (TAN). TAN of of AW 316, ZDDP and 353 were 260 mgKOH/g, 140 mgKOH/g and 136 mgKOH/g, respectively. High acid value indicates these additives need less active energy to initial tribochemical reaction, on other words, they are more prone to take part into the tribochemical reactions during sliding. By contrast, mean COF of TCP and [P8888][DEHP] were 0.170 and 0.199, respectively, which are much higher than their counterparts. TAN of TCP and [P8888][DEHP] are around 0.1 mgKOH/g which are much lower than above active additives means that they are chemically inert. Overall, the friction-reducing order of the tested anti-wear additives are AW 316 > ZDDP > 353 > TCP > [P8888][DEHP]. What’s more, worn surface roughness of copper plate also display a similar order which is consistent with friction reducing performances, demonstrating COF is closely related with the surface roughness.
Table 2
Mean coefficient of friction and worn surface roughness of copper flat
Item number | Composition | Mean Coefficient of friction | Worn Surface roughness |
a | PAO4 base oil | 0.349 | 0.155 |
b | PAO4 + 0.8% ZDDP | 0.093 | 0.049 |
c | PAO4 + 0.8% AW316 | 0.082 | 0.042 |
d | PAO4 + 0.8% [P8888][DEHP] | 0.199 | 0.124 |
e | PAO4 + 0.8% 353 | 0.118 | 0.048 |
f | PAO4 + 0.8% TCP | 0.170 | 0.161 |
3.2 Anti-wear performances
Figure 3 showed the wear volume of copper plate after tribological tests of different lubricants. PAO4 yield the highest wear volume of 72.9 × 10− 3 mm3. The addition of anti-wear additives greatly enhances the wear protection performance of base oil. It’s worth to mention that AW 316 produce the lowest wear volume of 6.15 × 10− 3 mm3 which is more than an order of magnitude smaller than base oil. Other anti-wear additives also reduce the wear volume to a great level. The wear protection properties of the selected additive follow the order of AW 316 > ZDDP > 353 > [P8888][DEHP] > TCP. The general trend of anti-wear behaviors is similar to their friction reducing performances.
3.2 Worn surfaces analysis
Worn surfaces of copper plates were ultrasonically cleaned with petroleum ether after tribological tests. Both steel ball and copper plate were examined with SEM. Figure 4 showed the top view images and 3D morphologies of worn surfaces. As observed, PAO4 (a1, a2, a3) showed the largest and deepest wear scar among these lubricants. Deep and large grooves and large bumps were clearly detected on the wear scar of PAO4 indicating server tribology and both abrasive wear and adhesive wear occurred on the rubbing surfaces. For PAO4 with 0.8% ZDDP, the wear scar is much narrow and shallow and clear separate patches can be seen on the surfaces which is similar to the tribofilm (pad-like structure) formed on steel-steel interfaces. AW 316 exhibited best tribological properties among these additives, worn surfaces of AW 316 is much smoother than pure PAO4 and PAO4 with ZDDP. Although the wear scar of AW 316 is a litter wider than ZDDP, the depth of wear scar is much shallow than ZDDP. The wider and smoother wear scar usually means a larger contact area and explained the lowest COF of PAO with AW 316.
Element composition on the copper worn surfaces were also examined using EDS and listed in Table 3. It can be seen in the table that besides oxygen and copper, a substantial amount of Fe (4.46 %) were found on the worn surface lubricate with pure PAO4 proves that abrasive wear occurred during sliding. Element mappings on steel ball (see supporting information S1) also confirm the transfer of Cu and Fe. Notably, 31.95 % Zn were detected on the worn surfaces lubricated by PAO4 with ZDDP demonstrates the formation of zinc rich boundary lubrication films. All of the signature elements of AW 316 were found on the worn surfaces. Distinguished from steel-steel contacts where little or no N were observed for amine phosphate anti-wear additives, worn surface of copper exists abundant nitrogen. Due to the high reactivity between sulfur and copper, 353 showed the highest sulfur content among these additives indicating sulfur plays a dominate role on the formation of boundary film. For TCP and [P8888][DEHP], only few phosphorus (around 1 %) were detected on the worn surfaces suggests that limited or inadequate tribochemical reaction took place on the interfaces which explains their poor tribological performances.
Table 3
Elements composition on worn surfaces of copper plates obtained from EDS analysis
Elements (wt.%) | a | b | c | d | e | f |
O | 7.45 | 10.26 | 8.73 | 6.00 | 6.96 | 6.18 |
Fe | 4.46 | 1.38 | 4.88 | 2.30 | 3.09 | 1.24 |
Cu | 88.08 | 54.02 | 81.24 | 90.72 | 83.36 | 91.45 |
P | - | 2.04 | 2.62 | 0.99 | 1.57 | 1.12 |
S | - | 0.36 | - | - | 5.02 | - |
N | - | - | 2.52 | - | - | - |
Zn | - | 31.95 | - | - | - | - |
Total | 100.00% | 100.00% | 100.00% | 100.00% | 100.00% | 100.00% |
3.4 FIB-TEM analysis
In order to further understand the tribological mechanism of lubricants, FIB-TEM technique was applied to analysis the tribofilm generated with the lubrication of PAO4 + AW 316. It is identified that a white tribofilm with a thickness of 10–15 nm is formed the worn surface as shown in Fig. 5. Underneath the tribofilm is a subsurface plastic deformation zone with a thickness of about 250 nm. It is observed that the tribofilm is even and flat which is consistence with the SEM results (Fig. 4 c2). It was this tribofilm protected the rubbing pair by separating the direct contact of the friction pairs. Figure 5f depicts the phosphorus content along the surface depth. It is clearly shown that the phosphorus content decrease with the depth of surface, when the depth is higher than about 12 nm the phosphorus become almost 0, which further demonstrates the film thickness of the boundary lubrication film.
3.5 XPS analysis
To get a further insight of chemical composition of tribofilm on copper disc, XPS spectra of key elements of PAO + ZDDP and PAO + AW316 were analyzed. The Cu2p peaks at 932 eV and 951.8 eV can be ascribed to CuS for PAO + ZDDP while peak at 932.6 can be assigned to Cu2O for PAO + AW316 [14–15]. P2p of PAO + ZDDP exhibit a binding energy at 134.4 eV, indicating that the phosphorus in the worn surface exists in the pentavalent oxidation state and possibly apparently as P-O bond species. A typical peak near 132.8 eV in the high-resolution spectra of P2p of PAO + AW316 (Fig. 6d) can be assigned to P-C bond [16–18]. It is seen that the binding energy of S2p is 160.89 eV for PAO + ZDDP which corresponds to S-C compounds [19, 20]. Figure 6h shows N1s of PAO + AW316 give a strong peak appears at 399.8 eV and 398.5eV, which is most possibly identified as Cu-N [21, 22]. The O1s peak of PAO4 + AW316 (Fig. 6d) at approximately 531.6 eV can be assigned to C–O bonding. Above XPS results suggest that tribo-chemical reactions occurred between anti-wear additives and copper during the tests. These processes generate metal oxides, phosphate and sulfur or nitrogen oxides or amide. These boundary films can effectively protect against the direct contact of rubbing surface and reduce the friction coefficient of the tribology system and resulting a better tribological properties.