Water Lubrication of Stainless Steel using Reduced Graphene Oxide Coating

Abstract

Lubrication of mechanical systems using water instead of conventional oil lubricants is extremely attractive from the view of resource conservation and environmental protection. However, insufficient film thickness of water due to low viscosity and chemical reaction of water with metallic materials have been a great obstacle in utilization of water as an effective lubricant. Herein, the friction between a 440 C stainless steel (SS) ball and a 440 C stainless steel (SS) plate in water lubrication could be reduced by as much as 6-times by coating the ball with reduced graphene oxide (rGO). The friction coefficient with rGO coated ball in water lubrication was comparable to the value obtained with the uncoated ball in oil lubrication. Moreover, the wear rate of the SS plate slid against the rGO coated ball in water lubrication was 3-times lower than that of the SS plate slid against the uncoated ball in oil lubrication. These results clearly demonstrated that water can be effectively utilized as a lubricant instead of oil to lower the friction and wear of SS components by coating one side with rGO. Implementation of this technology in mechanical systems is expected to aid in significant reduction of environmental pollution caused by the extensive use of oil lubricants.

Figure 1

(a) TEM (up-left) and 2-D AFM (up-right) images of the graphene oxide sheet. Graph (bottom) is the 2-D surface profile of position marked with yellow line presented in the 2-D AFM image. 1L and 2L in the graph indicate one and two layers of graphene oxide, respectively. Inset scale bar is 200 nm. (b) Schematic of the electrodynamic spraying system composed of injection controller, power supply and 2-axis moving stage. Rectangular dotted line indicate the location of the samples during the ESP. The schematic was drawn by H.-J. Kim using conventional 3D modeling software. (c) Photograph of multiple samples place on the 2-axis moving stage after the ESP deposition with 100 mL injection volume of GO. Inset scale bar is 1 cm.

Figure 2

(a) XRD pattern of GO (black) and rGO (red). The marked shift of (002) peak from GO (9.2°) to rGO (24.5°) was obtained with reduction process. (b) ATR-FTIR spectra of GO (black) and rGO (red). Blue lines indicate the location of C-O and –OH peaks at the wavelengths of 1030 cm⁻¹ and 3428 cm⁻¹, respectively. (c) XPS depth profiles of the rGO coated SS ball after hydrazine treatment. The pink dotted line indicates the depth at which full penetration of the coating has been reached. This point was determined by noting the beginning of Fe element detection that was associated with the SS ball.

Figure 3

(a) Schematic of the reciprocating type of a tribotester with fluid injector attached to provide continuous supply of water or oil lubricant to the sliding system. Schematic indicated with blue dotted line shows the magnified image of the test region. The schematic was drawn by H.-J. Kim using conventional 3D modeling software. (b) Friction coefficient under different lubricant conditions with respect to the number of sliding cycles. Inset red graph shows the friction coefficient of rGO-water lubrication condition for the first 3000 cycles. (c) Steady-state friction coefficients with respect to different experimental conditions. Each bar is represented by an average value with standard deviation for 3 tests. (d) Friction coefficient of rGO-water lubrication condition with respect to different normal loads. (e) Friction coefficient of oil lubrication condition with respect to different normal loads. Each data point is represented by an average value with standard deviation of 3 tests.

Figure 4

Optical images of wear track (up) and surface profiles (down) of positions marked with red dotted line presented in the optical images after the sliding tests under 50 mN normal load for (a) rGO-water (b) oil and (c) water lubricant conditions. Inset scale bar is 200 μm. (d) Wear rate of the steel specimen under various lubrication conditions. Each bar is represented by an average value with standard deviation of 3 tests. (e) Wear rate of the steel specimen under water and oil lubrication conditions with respect to different normal loads. Each data point is represented by an average value with standard deviation of 3 tests.

Figure 5. XPS analysis of rGO coated SS ball after the sliding test under water lubrication condition.

Optical microscope image (left) and corresponding XPS spectrum of C 1 s (right) of the position marked with a red dot in the optical image of (a,b) pristine rGO coated SS ball. (c,d) rGO coated SS ball after 50000 cycles of sliding test. (e,f) rGO coated SS ball after 100000 cycles of sliding test. Inset graph in optical images represent 2-D surface profile of the position marked with a magenta dotted line for (a) pristine rGO coated SS ball (a) rGO coated SS ball after 50000 sliding cycles and (e) rGO coated SS ball after 100000 sliding cycles.