Low-friction sliding without lubrication – five working material concepts and tribological challenges
The most fundamental rule for achieving low-friction sliding is to keep the area of contact small and the shear stress low. Of course, this beneficial combination is most often realised by using hard contacting materials and adding a lubricant. However, in some cases fluid lubricants are undesirable, and in others they may temporarily or locally be missing even when intended to be present.
In the absence of a lubricant, the combined requirements of small contact area and low shear stress poses contradicting demands on the contacting materials. A small contact area requires a hard material (i.e. hard to deform plastically) while a low shear stress requires a superficial layer that is easy to deform plastically.
Fortunately, there are several possible routes for materials to get around the contradictory demands. All such possibilities share a common fundament; during tribological contact the material has to transform in such a way that the shear resistance of its outermost layer becomes reduced relative to the hardness of the underlying material.
This may be achieved in several ways and can involve all sorts of material transformations; phase transformations, deformation hardening, diffusion, smoothening, melting, crystallisation, crystalline rotation, etc. The effect on the friction from these transformations is often very dramatic.
This presentation will introduce five such working concepts:
- Surface softening by selective alloy removal and self-ordering
- Self lubrication on the atomic scale
- Deep deformation hardening with superficial softening
- Smoothening of ultra-hard coating and stabilisation of molecular surface cover
- Friction melting of thin surface layer
Illustrating examples be given for each of the concepts, collected from recent research on thin coatings, thick coatings and bulk materials. The mechanisms will be explained, and their potentials demonstrated. Last, a number of outstanding challenges for a widened practical use of these coating concepts will be presented.