Effects of interfacial contact states on tribological behaviour in micro-sheet forming

• Parallel micro-grooves surface was chosen to reduce the friction in microforming.
• Increase-ratio was firstly selected in analysing size effects on friction.
• The friction coefficient decreased with the decrease of the plat width.
• Evolution of real contact area was adopted to reveal effects of contact pressure.

With the miniaturization of parts, obvious size effects of friction occur in microforming processes when liquid lubricants are applied. To improve the situation, a kind of micro-structured surface composed of parallel grooves was chosen to reduce the friction in micro-strip drawing. The load-bearing strips between the grooves are called “plats”. To evaluate the degree of size effects on friction, the increase-ratio of friction coefficient was firstly selected in analysing the effect of the contact states on the tribological behaviour. It was found that the degree of size effects on friction decreased with increase of contact pressure, surface roughness and sliding speed, and also with decrease of the width of the plats of the micro-structured surface. The mechanism of the effects of contact pressure and of the micro-structured surface were mainly studied by developing the calculation equations of the friction coefficient considering the evolution of the real contact area under different contact pressures and the plat width as the feature dimension of the micro-structured surface. For the micro-structured surface, the friction coefficient for microforming was inversely proportional to the number of plats in the contact area. The results of theoretical analysis results are in good agreement with experimental results. It also can be concluded that the micro-textured surface is an important means of reducing the friction in micro-sheet forming.