3.1 Leaf-surface wax chemical components
The chemical compositions of Leaf-surface wax were analyzed using GC-MS, The result showed in Table 1. The major components of SOL are hydrocarbons, the major components of EJT are hydrocarbons and alcohols, and for FPM the major components are esters and hydrocarbons. The other components listed in table are a bit less.
Table 1
The chemical components and contents of wax samples
| SOL | EJT | FPM |
Hydrocarbons | 87.59% | 36.53% | 17.12% |
Alcohols | 0.56% | 29.25% | - |
Esters | 3.39% | - | 25.17% |
Ketones | 0.31% | 13.92% | 1.66% |
Ethers | 0.69% | 0.51% | 4.26% |
Carboxylic acids | 0.48% | - | - |
Others | 6.98% | 19.79% | 51.79% |
Because alcohol, acid and ester are good antifriction and anti-wear additives in lubricating oils, and the content of alcohol and ester in the SOL, EJT, FPM waxes is different, what is the difference of their lubricating performance? |
3.2 Friction reduction and anti-wear properties
Figure 2 shows the COF and WSW of steel-aluminum contact at 20 N. It can be seen that when the mass fraction of EJT and FPM is 1.6%, the COFs of them are the lowest ones, and the COF is the lowest one when the mass fraction of SOL is 1.2%. The situations of WSW for EJT and FPM are similar: the 1.6% additives have the lowest WSW, while the WSW is the lowest one when the mass fraction of SOL is 2.0%.
Figure 3 shows the COF and WSW of steel-aluminum contact at different loads when the additives are 1.2% SOL, 1.6% EJT and 1.6% FPM, respectively. It can be seen that the COF of all the lubricants increase with the loads increase, and the WSW of all the lubricants increase with the loads increase.
The results show that the lubrication performance of alcohol is better than that of wax containing ester and other components for steel-aluminium friction pairs.
Figure 4 shows the COF and WSW of steel-steel contact at 50 N. It can be seen that when the mass fraction of EJT and FPM is 1.6%, the COFs of them are the lowest ones. The effect of additives for reducing WSW is not very obvious.
Figure 5 shows the COF and WSW of steel-steel contact at different loads when the additives are 2.0% SOL, 1.6% EJT and 1.6% FPM, respectively. It can be seen that the COF and WSW of all the lubricants increase with the loads increase.
The results show that the lubrication properties of alcohol and ester are similar for steel and steel friction pairs.
3.3 Wear scar morphology
Figure 6 suggests the wear scar surface morphologies on the aluminum discs at 40 N. It can be seen that the surface lubricated by SE has very serious damage, both abrasive wear and adhesive wear. In terms of the other four surfaces, SE lubricating surface is the roughest one and EJT lubricating surface is the smoothest one.
Figure 7 displays the wear scar surface morphologies of the steel discs. As steel has a higher hardness than aluminum, the surfaces are smoother. The surface lubricated by SE is the roughest and the other four surfaces are not much different, therein EJT lubricating surface is smoothest.
3.4 Wear scar composition analysis
To investigate the chemical component on the surface of wear scar, the TOF-SIMS analysis was employed to analyze the ions on the surface of aluminum disc. For ease of analysis processing, the data were normalized by Al. The highest ion intensity of Al + was chosen as the normalized intensity of positive ion mass spectrum while the highest ion intensity of AlO- as the normalized intensity of negative ion mass spectrum. As shown in Fig. 1 and Fig. 2, in terms of the measured positive ions, there are many kinds of hydrocarbon ions (such as C2H5+) and common positive ions such as H+. With respect to the measured negative ions, there are some carboxylic acid ions (such as C5H9O2-), several alcohol ions and some other common negative ions such as OH-.
Table 2 shows the data that ion normalized intensity area data for the wear scar surface. As can be seen, there are most and least positive ion content on the surface lubricated by EJT and FPM, respectively. The short carbon chain (C1 ~ 3) content is less and long carbon chain (C4 ~ 9) content is more on the surface lubricated by SOL. In terms of total negative ion content, there is FPM > SOL > EJT. In terms of the short carbon chain (C < 7) positive ion intensities, in general, EJT > SOL > FPM.
Table.2 The ion normalized intensity area on the wear scar suface
| SOL | EJT | FPM |
Positive ion intensity |
CH3+ | 0.10464 | 0.11582 | 0.09796 |
Al+ | 1 | 1 | 1 |
C2H3+ | 0.66228 | 0.76411 | 0.61342 |
C2H5+ | 0.68668 | 0.76617 | 0.59548 |
C3H3+ | 0.36714 | 0.39667 | 0.31499 |
C3H5+ | 0.94943 | 1.01830 | 0.80190 |
C3H7+ | 0.87671 | 0.94258 | 0.70426 |
C4H9+ | 0.32020 | 0.32395 | 0.23890 |
C5H8+ | 0.03131 | 0.03101 | 0.02516 |
C6H9+ | 0.05882 | 0.06071 | 0.05071 |
C7H10+ | 0.00352 | 0.00345 | 0.00316 |
C8H12+ | 0.00131 | 0.00130 | 0.00119 |
C9H12+ | 0.00139 | 0.00136 | 0.00116 |
Negative ion intensity |
CH3− | 1.53690 | 1.29460 | 1.83630 |
O2− | 0.98106 | 0.87963 | 0.82999 |
C2HO− | 7.63280 | 7.11460 | 8.89660 |
AlO− | 1 | 1 | 1 |
C2H2O2− | 2.88580 | 2.58670 | 3.45730 |
C3H3O2− | 6.53470 | 5.81060 | 7.08570 |
C5H9O2− | 1.22700 | 0.70320 | 1.17790 |
C7H13O2− | 0.72031 | 0.42460 | 0.67404 |
3.5 Discussion
It could be inferred from above results that the improvement of the friction surface for two kinds of friction pairs lubricated by waxes are better than those of the pure SE. The leaf-surface waxes lubricating surfaces are smoother because the wax lubricants form a lubricating film and reduce the contact between friction pairs [15–16].
It can be seen from the above analysis that in terms of the optimal wax additive concentration at 40N load of steel-aluminum friction pair, there are less hydrocarbons and more polar negative ions on the worn surface lubricated by FPM, so that FPM lubricants have lower COF and WSW in this condition. It can be inferred that the polar negative ions have better friction reduction and anti-wear abilities than hydrocarbon ions because of their polarity which can makes the lubricating film firmer.