Effects of the Electric Double Layer Characteristic and Electroosmotic Regulation on the Tribological Performance of Water-Based Cutting Fluids

Abstract

The electroosmosis effect is a complement to the theory of the traditional capillary penetration of cutting fluid. In this study, based on the electric double layer (EDL) characteristics at friction material/solution interfaces, the influences of additives and their concentrations on capillary electroosmosis were investigated, and a water-based cutting-fluid formulation with consideration to the electroosmosis effect was developed. The lubrication performance levels of cutting fluids were investigated by a four-ball tribometer. The results show that the EDL is compressed with increasing ionic concentration, which suppresses the electroosmotic flow (EOF). The specific adsorption of OH^- ions or the dissociation of surface groups is promoted as pH rises, increasing the absolute zeta potential and EOF. The polyethylene glycol (PEG) additive adsorbed to the friction material surface can keep the shear plane away from the solid surface, reducing the absolute zeta potential and EOF. The electroosmotic performance of cutting fluid can be improved by compounding additives with different electroosmotic performance functions. Furthermore, electroosmotic regulators can adjust the zeta potential by the electrostatic adsorption mechanism, affecting the penetration performance of cutting fluid in the capillary zone at the friction interface. The improvement in the tribological performance of cutting fluid developed with consideration given to the electroosmosis effect is attributed to the enhancement of the penetration ability of the cutting fluid and the formation of more abundant amounts of lubricating film at the interface.

Keywords: electrical double layer; electroosmosis; tribological performance; water-based cutting fluid; zeta potential.

Figure 1

Photograph of capillary EOF velocity measurement device of cutting fluid.

Figure 2

Schematic diagram of (a) the tangential streaming potential test system, (b) the measurement cell, and (c) the parallel-plate microchannel.

Figure 3

EOF velocities of (a) lubricity additive, (b) EP additive, and (c) pH buffer aqueous solutions.

Figure 4

Streaming potentials of (a) PEG400, (b) MOA-3P, and (c) TEA aqueous solutions on AISI 52100 steel and alumina ceramic surfaces.

Figure 5

Zeta potentials of (a) PEG400, (b) MOA-3P, and (c) TEA aqueous solutions on AISI 52100 steel and alumina ceramic surfaces.

Figure 6

(a) Streaming potentials, (b) zeta potentials, and (c) EOF velocities of compound cutting fluids with different concentrations of MOA-3P.

Figure 7

(a) Streaming potentials, (b) zeta potentials, and (c) EOF velocities of WCF cutting fluids with different CHAPS and CTAB concentrations.

Figure 8

The COF curves for steel–steel friction interface under the lubrication of WCF cutting fluid with different concentrations of (a) CHAPS and (b) CTAB. The average (c) COF and (d) WSD for steel–steel friction interface under the lubrication of WCF cutting fluid containing different concentrations of electroosmotic regulators.

Figure 9

SEM images of worn surfaces and EDS spectra of the dashed areas under the lubrication of (a,a’) WCF containing 0.8 mmol/L CTAB, (b,b’) WCF base fluid, and (c,c’) WCF containing 0.8 mmol/L CHAPS.

Figure 10

Schematic diagram of (a) the surface charge of AISI 52100 steel and alumina ceramic, (b) the EDL and its potential distribution, (c) the effects of additives on the zeta potential of the EDL.

Figure 11

Schematic diagram of (a) capillary electroosmosis of cutting fluid at friction interface and (b) the regulation mechanism of electroosmotic regulators CHAPS and CTAB.