Traction and nonequilibrium phase behavior of confined sheared liquids at high pressure

Nonequilibrium molecular dynamics simulations of confined model liquids under pressure and sheared by the relative sliding of the boundary walls have been carried out. The relationship between the time-dependent traction coefficient, $\mu \left(t\right)$, and the state of internal structure of the film is followed from commencement of shear for various control parameters, such as applied load, global shear rate, and solid-liquid atom interaction parameters. Phase diagrams, velocity and temperature profiles, and traction coefficient diagrams are analyzed for pure Lennard-Jones (LJ) liquids and a binary LJ mixture. A single component LJ liquid is found to form semicrystalline arrangements with high-traction coefficients, and stick-slip behavior is observed for high pressures and low-shear velocities, which is shown to involve periodic deformation and stress release of the wall atoms and slip in the solid-liquid boundary region. A binary mixture, which discourages crystallization, gives a more classical tribological response with the larger atoms preferentially adsorbing commensurate with the wall. The results obtained are analyzed in the context of tribology: the binary mixture behaves like a typical lubricant, whereas the monatomic system behaves like a traction fluid. It is discussed how this type of simulation can give insights on the tribological behavior of realistic systems.

Figure 1

Schematic representation of the system.

Figure 2

Phase diagram for (a) the monatomic and (b) the binary mixture systems. For the monatomic system (a), two phase diagrams are shown, one for $c=1$ (top) and $c=0.1$ (bottom). The main phases are labeled on the figure (CL stands for central localization and AM for asymmetric melting), and the solid black lines indicate the boundary between two phases. The shaded areas (blue online) highlight broad transition regions between coexisting phases.

Figure 3

(a) Atom positions for the liquid-like phase, for a 100 MPa, 100 m/s, $c=1$ system. The different colors indicate the displacement $\Delta x=x\left(t_{\mathrm{final}}\right)-x\left(t_0\right)$ of the atoms at the end of the simulation, with respect to their original positions, with the distance unit being $\sigma$. (b) Corresponding velocity profile. (c) Density profile for 100 MPa, 100 m/s, c $=$ 0.1, and c $=$ 1.

Figure 4

System formed by solid LJ walls and a confined binary mixture. The large atoms are green and the smaller ones are blue online. Alignment of these molecules in distinct layers at the boundary can be seen. The right-hand frame shows velocity profiles (at a wall speed of 20 m/s) for the system at pressures of 100 MPa and 4 GPa.

Figure 5

(a) Velocity profile of the system with a solid confined phase ($P=4$ GPa, $v=2$ m/s, $c=1$). Closed symbols are used for the velocity of the wall atoms, open symbols for the velocity of the confined system. (b) Traction coefficient $\mu$ showing a series of “peaks” indicating the stick-slip behavior of the system. In the highlighted peak, the (blue online) solid line shows the “stick” behavior, the (red online) dashed line shows slip. (c) Velocity profile for the system during the stick-slip cycle highlighted in panel (b). The open symbols represent the confined system and the closed symbols are for the walls. The diagrams show the system configuration as the traction increases and decreases.

Figure 6

(a) Plug-slip configuration, after ca. $\sim 7$ ps of MD from the start of the production phase of the simulation, for $P=1$ GPa, $v=100$ m/s, and $c=1$. The light coloring (light green online) of the central section of the system in the left frame shows that it has not moved with respect to its original position. (b) Close-up of the boundary layer. The confined atoms close to the ordered wall boundary (yellow/orange online) show more disorder and higher movement from their original position than those further away from the wall (light green online). (c) Traction coefficient for the whole duration of the simulation.

Figure 7

Instantaneous configuration and velocity profiles for a system exhibiting (a) central localization (left diagram, for $c=0.5$ and $P=500$ Mpa) and (b) asymmetric melting (right diagram, $c=0.25$ and $P=500$ Mpa). The melted areas are shown in a lighter colour (green online) at the centre of the CL system and at the bottom boundary of the AM system.

Figure 8

Time evolution of (a) Wall separation $h$ for a PS, CL, and AM system and (b) traction coefficient $\mu$ for a CL, and AM system at 500 Mpa, for the first 6 ns of a simulation.

Figure 9

Number density evolution as a function of time from the start of the simulation in the confined section of the system, for (a) a PS-to-AM transition and (b) a PS-to-CL transition.

Figure 10

Time-averaged traction coefficients for $c=1$ and $c=0.1$ and different pressures in the monatomic LJ system. The pressure and wall speeds used are indicated on the phase diagram in the insets.

Figure 11

Traction coefficient for all pressures and velocities for a binary mixture.

Figure 12

Temperature profiles for the central part of the system and a section of the walls for $c=1$ and $c=0.1$ for all pressures at 100 m/s. The boundary between walls and confined system is indicated by vertical dashed lines, with the bottom wall being on the left and the top wall on the right. The temperature on the vertical axis is in reduced units. On the horizontal axis, $l$ is the height of the system considered in the computation of the temperature and $\Delta l$ is the length of each “slice” in which the system has been divided.

Figure 13

Temperature profile for all pressures at 100 m/s for the binary mixture. The temperature is shown on the vertical axis and the $z$ coordinate on the horizontal axis. LJ units have been used. The calculation has been performed for the confined liquid only. The symbols represent the simulated data, whereas the solid line represents the theoretical fit of Eq. (2). The thermal conductivities found by fitting the simulation data to Eq. (2) are: ${\lambda }_{500\mathrm{MPa}}=37.38$, ${\lambda }_{1\mathrm{GPa}}=42.74$, ${\lambda }_{2\mathrm{GPa}}=58.37$, ${\lambda }_{4\mathrm{GPa}}=79.63$, in reduced units.

Figure 14

(a) Viscosity as a function of the wetting parameter $c$, for simulations at $P=100$ MPa for a monatomic LJ confined liquid. (b) Shear stress/shear rate diagram for $P=100$ MPa, 1 GPa, 4 GPa, and all velocities, for a confined binary mixture. (c) Close up of the shear/shear rate graph $b$ for $P=100$ for a binary mixture.