Self-ordering of the Lubricant Film

• Introduction

• Some examples

  • Hard lubricant, N_l=1

  • Hard lubricant, N_l=3

  • Curved substrate

• Dependence on V_ll

• Conclusion   

Introduction

As was shown above, the use of solid lubricants may be a very promising way, especially in micro-devices, where conventional lubricants may often be ineffective or may even work like a glue. In an ideal case of contact of two rigid crystalline incommensurate surfaces the static frictional force is zero (moreover, the same is true even for commensurate surfaces if they are not perfectly aligned). Analogously, if the lubricant film has a crystalline structure and is confined between two flat substrates with the ideal structure, the friction coefficient in such perfect-sliding system may be as low as  μ ~ 10^–3 to 10^–2  or even lower. The critical velocity of the transition from stick-slip to smooth sliding is also quite small,  v_c~10^–2c. Unfortunately, such an ideal system can hardly be reproduced experimentally: even specially prepared surfaces are not perfectly smooth on a mesoscopic scale, and a lubricant has typically numerous structural defects. As a result, the static frictional force  f_s  is large enough, and the solid lubricant will melt at the onset of sliding. Then, at stick, the film solidifies back, but, again, either in an amorphous state or in a state with many defects, because the cooling of the confined film is very rapid due to very good thermal contact with the substrates. According to simulation, in such a system one finds μ > 0.1 and v_c~ 0.1c, i.e., the tribological characteristics are even more worse than those of liquid lubricants. In what follows we will explore whether the system itself may approach the desired perfect-sliding regime for a suitable choice of the solid lubricant. Indeed, the energy pumped into the system due to external driving has to be converted into heat. Therefore, the effective lubricant temperature T^* must increase during sliding. It is this increase of the temperature that leads to melting of the lubricant in the melting-freezing mechanism of stick-slip. However, if T^* will remain lower than the melting temperature T_m, the lubricant film could remain solid and, at the same time, its structure may become more ordered due to annealing of structural defects, especially if T^* will be close to T_m.  Because T_m is determined by the interactions within the lubricant, one may play with the parameters of interaction looking for a situation when T_m  is close to (but higher than) T^*. Below we show that, indeed, a suitable choice of the parameters may lead to the desired self-ordering of the lubricant and consequently to the low values of the frictional forces.

Some examples

Hard lubricant, N_l=1

Sliding-induced ordering of the  N_l=1  system for  v_spring= 0.1

	Configuration before self-ordering
	Configuration after self-ordering

Hard lubricant, N_l=3

Sliding-induced ordering of the  N_l=3  system for  v_spring= 0.3  &  movie (avi 1.5 Mb)

Curved substrate

Sliding-induced ordering of the  N_l=3  system with  V_ll= 0.5 (so that  V_ll/V_sl= 1.5) and the curved top substrate for  v_spring= 0.1

Configurations:
before self-ordering (commensurate)	after self-ordering (incommensurate)
movie (mpeg 7.2 Mb)	movie (mpeg 9 Mb)
full movie (mpeg 15 Mb)

Dependence on V_ll (interaction in the lubricant)