Fig. 3 shows the FT-IR analyses of the RA, SRA, and PRA. The peak at 1705 cm–1 corresponded to the carbonyl stretching vibration in the RA. The new absorption peaks at 3237, 1602, and 1740 cm–1 are assigned to the characteristic absorptions of the —NH and carbonyl in the amide group, and the peak for —OH was not observed. The characteristic absorption peaks of S=O and C—S were observed at 1342, 1158, and 763 cm–1. When the RA reacted with methyl alcohol and PDCP, new absorption peaks at 1213 cm–1 and 1014–930 cm–1 were assigned to the characteristic absorption peaks of the P=O and P—O—C. A peak for —OH was not observed.
The 1H NMR analysis results for the RA, SRA, and PRA are shown in Fig. 4. The 1H NMR (400 MHz, CDCl3), for RA, δ= 1.98 (—OH), δ= 3.60 (—CH—OH), δ= 5.23–5.55 (—CH=CH—). For the SPA, δ= 4.74 (—NH), δ= 7.25–8.00 (benzene-H), δ= 4.13 (—CH2—O—). For the PRA, δ= 0.85 (—CH3), δ= 1.06–1.66 (—CH2—CH2—), δ= 2.78 (—CH2—COOH), δ= 5.36 (—CH=CH—). δ= 3.43–4.25 (—CH2CH(OH)CH2OH), δ= 2.01 (—OH). δ= 7.00–7.30 (benzene-H), δ= 3.30–3.70 (—CH2—O—).
Fig. 5 shows the COF and WSD of water at different SRA and PRA concentrations under a load of 200 N for 30 min. It can be observed that the addition of SRA and PRA improved the anti-wear and anti-friction properties of pure water at all concentrations.
The COF of the SRA/water solutions increased with SRA concentration (from 0.5 wt% to 2.0 wt%). As for the PRA in water, the COF gradually decreased with increased concentration. The PRA produced significantly lower COF values as compared with the SRA. The trend of the WSD is similar to that of the COF, indicating that the SRA and PRA improved the anti-friction and anti-wear performance of pure water. This was ascribed to the long alkyl chains and active sulfur and phosphorus form a boundary lubrication film that serves as a physical lubrication film and tribochemical reaction film on the stainless steel surface. In addition, The COF of the SRA/water solutions increased with the SRA concentration. This be ascribed to the metal sulfide film is relatively brittle and the sulfur element is more corrosive than the phosphorus element under high temperature and pressure (Ding et al., 2018; He et al., 2018; Wu et al., 2019c).
The adsorption and reaction activities of the additive molecules and extreme pressure elements are closely related to the extreme pressure properties. Fig. 6 shows the relationship between the SRA and PRA concentration (in water) and PB values. The PB value of water is 95 N, indicating that water has difficulty with physical adsorption to the film on the stainless steel surface, and thus has difficulty lubricating the surface. The addition of the SRA and PRA improved the PB values. The PB values increased with increased SRA and PRA concentrations. At 2.0 wt% SRA and PRA, the PB values for the SRA were 842 N and 8.7 times that of pure water, while the values for the PRA were 725 N and 7.6 times that of pure water. The results contribute to the synergistic effect of long fatty chains, active sulfur and phosphorus elements in the SRA and PRA, which indicates that the SRA and PRA can produce physical adsorption and chemical reactions film on the stainless steel surface. However, at the same concentration, the PB value of the SRA is much higher than that of the PRA. This can be ascribed to greater reactivity of sulfur and adsorption with friction metal surfaces under high loads. In addition, metal sulfide films have good heat resistance (Bai et al., 2018; Bhaumik et al., 2018; Yu et al., 2019).
To investigate the lubricating mechanisms of the SRA and PRA, we analyzed worn steel surfaces using SEM and XPS. The SEM morphologies for stainless steel surfaces lubricated by various concentrations of the SRA and PRA are shown in Fig. 7 and Fig. 8. As the SRA concentration increased, wear scars became widen and deepen, and it showed adhesive wear and abrasive wear. It indicated that the lubricating mechanisms of the SRA was mainly adhesive and abrasive wear. However, as the PRA concentration increased, wear scars became shallower and smoother, and the adhesive and abrasive wear weaken. This meaned that pure water with the PRA could form the boundary film on the stainless steel surface to reduce the friction. Meanwhile, at the same concentrations, abrasive scars smaller with the PRA additive than with the SRA, indicating better anti-friction and anti-wear performance with the PRA (Xia et al., 2018; Kerni et al., 2019; Wu et al., 2019a).
Figs. 9 shows the XPS survey spectra of the PRA and SRA samples in which the characteristic C, O, P, S, and Fe elements are detected. Data for the analysis are shown in Table 1. The binding energy of C1s at 284.8 and 288.8 eV are assigned to C—H, C—C, C=O, and C—O. It indicates that the PRA and SRA samples formed a physical adsorption film composed of carbon, esters, and so on. The XPS peaks of O1s appear at the binding energies of 529.4 eV, which might be ascribed to FeO and Fe2O3. The peaks at 530.3 and 532.6 eV are ascribed to Fe(OH)3 and Fe2(PO4)3. The binding energy of P2p at 133.2 eV might be ascribed to PO43–. The S2p peaks at 161.7 and 168.9 eV were attributed to ferrous sulfide and ferrous sulfate. The Fe2p peaks at approximately 710.0 and 723.4 eV combined with the O1s peaks at 529.4, 530.3 and 532.6 eV, indicating the existence of iron oxide and iron phosphate (Fe2O3, FeO, FeSO4, and Fe2(PO4)3) on the stainless steel surface (Gao et al., 2017; Wu et al., 2017; Zhang et al., 2019).
Element Binding energy (eV) Assignment C1s 284.8, 288.8 C—H, C—C, C=O, C—O O1s 529.4, 530.3, 532.6 Carbon oxide, iron oxide compounds P2p 133.2 PO43– S2p 161.7, 168.9 Ferrous sulfide Fe2p 710.0, 723.4 Iron oxide
Table 1. The XPS results of worn surfaces lubricated with 2.0 wt% PRA and SRA.
Given the SEM and XPS results, we found that the PRA and SRA formed effective lubricant films of iron sulfide, iron oxide, and iron phosphate compounds and thus, improved the lubrication properties of water solutions.
Based on the above analysis, the PRA and SRA solutions primarily offered boundary lubrication. The possible lubrication mecha-nisms of the PRA and SRA were summarized as the follows and in Fig. 10 and Fig. 11. As for the PRA, fatty chain had good adsorption, and its lubrication mechanism was that the long carbon chain molecules of fatty acid could be more effectively adsorbed on the metal surface to form a physically adsorbed lubricating film, and exerted a certain anti-friction and anti-wear effect. As the pressure in-creased, the physical adsorption film ruptured, and the friction chemical reaction lubrication film appeared, which was the coverage of the organic phosphorus and inorganic nitrogen film formed on the surface of the friction pair, and the long-chain fatty acid chain acted to reduce the direct contact between the steel ball and the oxygen in the water, thereby preventing the frictional wear and oxidation of the steel ball.
The possible lubrication mechanisms of the SRA was similar to that of the PRA. First, the polar groups of the SRA were physically adsorbed onto the stainless steel surface to form a physical and chemical adsorption layer, and this adsorption was due to the synergistic action of the long aliphatic chain and the highly active sulfur. Second, the highly active sulfur elements reacted with the friction pair to rapidly form a more stable chemical reaction film (iron sulfide, iron oxide etc.) and reduced direct contact with the friction pair surfaces.
Thus we found how the PRA and SRA improved the extreme pressure and friction properties of water solutions.