Controlling friction is one of the top priorities for many tribologists. The friction in bearings has to be reduced to increase the energy efficiency of numerous devices, while friction in transmission systems has to be increased for effective power transmission.
Recently, a joined group of researchers from various universities in China addressed the frictional properties of metallic glasses. These metallic alloys posses a number of interesting properties, such as an isotropy, high elasticity, high hardness, superior corrosion resistance, low internal friction. One of the promising applications of metallic glasses is the development of highly hydrophobic surfaces (using micro-patterning) of long lifetime.
The long term performance of the metallic glass based hydrophobic surface will be determined by the frictional and consequently wear behavior, which are in turn controlled by the surface pattern. The question is then, what type of pattern would be most suitable for the friction minimization and lifetime maximization?
In case of lubricated contacts, friction can be reduced by the surface patterning. Optimally designed patterns improve load carrying capacity of the lubricant film. It is also suggested that the textured areas may serve as oil pockets, but also help to release the wear particles from the contact by trapping in the patterns.
In case of dry friction, the surface patterning, for example honeycomb structures, can also be used to tune friction. When the surface pattern is imposed on the surface, the real contact area is altered and it is well known that in micro-scale, the friction force is proportional to the real contact area and shear strength :
The real contact area is proportional to the apparent contact area, and hence the decrease of the pitch of the honeycomb reduces the real contact area. Therefore, the friction is reduced. However, as it was discussed by the researchers, the decrease of friction is only seen down to a ceratin value and increase in friction is seen aftewards. This is ascribed to the effect of ploughing which start to dominate when the real contact area is so small (local contact pressure is high) that the pattern start to penetrate to the surface. The friction force of a spherical asperity is proportional to the penetration depth, which in turn scales with the asperity load as . On the other hand, the , where is the normal load. Taking this into account, one can get:
Therefore, the total friction coefficient is composed of two components and can be written as follows:
The corresponding friction coefficient is shown in Fig. 1. A clear optimum can be seen which is a result of a competition of two friction effects.
Further details can be found in the original article: Ning Li, Erjiang Xu, Ze Liu, Xinyun Wang and Lin Liu, Tuning apparent friction coefficient by controlled patterning bulk metallic glasses surfaces, doi:10.1038/srep39388.