Dynamics of Neutral and Charged Nanodiamonds in Aqueous Media Confined between Gold Surfaces under Normal and Shear Loading

Abstract

The dynamics of cubo-octahedral nanodiamonds (NDs) with three different surface treatments and confined in aqueous environments between gold surfaces under shear and normal loading conditions have been characterized via molecular dynamics (MD) simulations. The treatments consisted of carboxyl (-COO^-) or amino (-NH₃ ⁺) groups attached to the NDs, producing either negatively or positively charged NDs, respectively, and hydrogen-terminated surfaces producing neutral NDs. Simulations were performed in the presence and absence of induced image charges to explore the impact of electrostatic interactions on friction and surface deformation. Significant deformation of the gold surfaces was observed for negatively charged NDs placed between gold surfaces under external loads that were sufficient to displace water from the contact. Rolling and relatively high friction levels were also observed for the negatively charged NDs under the same conditions. In contrast, the neutral and positively charged NDs exhibited sliding behavior with only minor deformation of the gold surfaces. The results suggest that the size of the surface functional group plays a major role in determining whether NDs slide or roll on solid contacts. Higher friction levels were also observed in conjunction with induced image charges in the gold contacts. The results demonstrate how surface functionalization and surface-induced charges can work in combination to profoundly influence tribological performance.

Figure 1

Illustration of the ND simulation system. The red and blue spheres represent carbon and hydrogen atoms, respectively, while the brown spheres represent the solvated counter ions, the white rods represent water molecules, and the green and yellow spheres represent the surface functional groups. The gold surfaces are represented by gold and black spheres, gold for deformable layers and black for rigid layers.

Figure 2

Illustration of the initial contact states of NDs between sliding gold surfaces viewed from the Y-direction under different loads. The sliding velocity is 10 m/s. Gold and black colored atoms on the surface represent the deformable layers and rigid layers, respectively. The red and blue spheres represent carbon and hydrogen atoms, respectively, the brown spheres represent the solvated counter ions, the white rods represent water molecules, and the green and yellow spheres represent the surface functional groups.

Figure 3

Morphology of lower slabs after 2000 ps, color-coded in grayscale according to the position on the z-axis. V = 10 m/s, P = 666.7 MPa.

Figure 4

Relative sliding speed (upper section) and the cumulative rolling angle (bottom section) of NDs from their initial state versus simulation time. The blue lines represent the results of simulations that take account of the surface-induced charges. The red dashed lines represent the results of simulations without involving the surface-induced charges. (a)−(c) Neutral NDs at loads of 333.3, 666.7, and 1000.0 MPa, respectively. (d)−(f) Positively charged NDs at loads of 333.3, 666.7, and 1000.0 MPa, respectively. (g)−(i) Negatively charged NDs at loads of 333.3, 666.7, and 1000.0 MPa, respectively.

Figure 5

Friction force versus simulation time under various applied loads in the presence and absence of surface-induced charges. The average friction forces are plotted on the right vertical axis. The blue dashed lines represent results under load 333.3 MPa. The red lines represent results under load 666.7 MPa. The yellow dot dash lines represent results under load 1000.0 MPa. (a) Neutral NDs with induced charges (left) and without induced charges (right). (b) Positively charged NDs with induced charges (left) and without induced charges (right). (c) Negatively charged NDs with induced charges (left) and without induced charges (right).