INTRODUCTION: Nanolubricants containing environmentally friendly nanoparticles have shown considerable enhancements in tribological properties under anti-wear (AW) and extreme-pressure (EP) conditions1–4 due to several reported tribological mechanisms such as surface mending, nanoparticle exfoliation, formation of a protective tribofilm, or a third-body effect5,6. For example, Li et al.1 prepared mineral base nanolubricants containing “green” calcium borate (CaBN) nanoparticles which demonstrated to have good EP, AW, and friction reducing characteristics due to the formation of a tribofilm. By a similar mechanism calcium carbonate (CaCO3) nanoparticles dispersed in a poly-alpha-olefin (PAO) base oil were able to improve the load-carrying capacity, as well as reduce wear and coefficient of friction7. Most recently, halloysite clay nanotubes (HNTs), aluminosilicate clay nanoparticles with hydrophilic characteristics, have been studied as lubricant additives2,3 due to being environmentally friendly, inexpensive, and biocompatible8–10. Formulated polymeric lubricants for metal forming applications showed improvements on the load-carrying capacity of 72% due to exfoliation of the nanoparticles at the high pressures of the test and the formation a tribofilm. In this study HNTs were dispersed in distilled water, taking advantage of their hydrophilic nature, to determine their tribological properties under EP without the presence of other additives. Water soluble semi-synthetic nanolubricants with HNTs were also characterized and compared to the distilled water nanolubricants.
METHODS: Nanofluids of distilled water and a general-purpose semi-synthetic metal working fluid with a 93% water content were prepared with HNTs at concentrations of 0.01, 0.05, and 0.10 wt.% by ultrasonication for 5min. Nanolubricants were characterized with a four-ball test using balls of an AISI 52100 steel, diameter of 12.7 mm, and surface roughness average (Ra) of 0.25 µm, according the ITeE-PIB method for testing lubricants under conditions of scuffing11. Here, a load increasing from 0 to 7200 N is applied on the upper ball that rotates at 500 rpm against three fixed balls over the course of 18 s. Scuffing load occurs when a sudden increase in the friction torque is observed during the test. Seizure load (Poz) takes place at a corresponding friction torque of 10 N.m. Wear scar diameters of the three lower balls are measured after each test with an optical microscope; then, the limiting pressure of seizure (poz), when the lubricant film disappears, is calculated by the following formula: poz=0.52(Poz)/WSD2.
RESULTS: Figure 1 shows the tribological results obtained by the extreme pressure test. Figure 1A and Figure 1B show the friction torque over time obtained for the distilled water and semi-synthetic nanofluids at linearly increasing loads. In Figure 1A HNTs delay scuffing initiation with increasing nanoparticle concentration. For distilled water scuffing takes place at 905 N, whereas at 0.01, 0.05, and 0.10wt.% it occurs at 1500 N, 3100 N, and 3200 N, respectively. For the semi-synthetic nanofluids (Figure 1B) the highest scuffing load of 5100 N is found at 0.01 wt.% compared to 3600 N for the unfilled semi-synthetic fluid. Higher nanoparticle concentration resulted in a decrease in the load required for scuffing. Figure 1C shows the limiting pressure of seizure of the distilled-water and the semi-synthetic lubricant at varying nanoparticle content. It can be observed that for the distilled water nanolubricants lower values are obtained compared to those found for the semi-synthetic lubricants.