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Abstract
The dry friction of surfaces with nanoscale roughness and the possibility of using micropatterning to tailor friction by manipulating contact area is investigated. Square wave patterns produced on samples from silicon wafers (and their unstructured equivalent) were slid against unstructured silicon counter surfaces. The width of the square wave features was adjusted to vary the apparent feature contact area. The existence of nanoscale roughness was sufficient to ensure Amontons’ first law (F = μP) on both structured & unstructured samples. Somewhat counterintuitively, friction was independent of the apparent feature contact area making it difficult to tailor friction via the feature contact area. This occurred because, even though the apparent feature contact area was adjusted, the surface roughness and nominal flatness at the contact interface was preserved ensuring that the real contact area and thereby the friction, were likewise preserved. This is an interesting special case, but not universally applicable: friction can indeed be adjusted by structuring provided the intervention leads to a change in real contact area (or interlocking)– and this depends on the specific surface geometry and topography.
Highlights
Friction is the mechanism responsible for transmitting tangential force across an interface when materials are in contact
We contend that contact area is the key factor governing the influence of surface structure on friction and this is what we investigate in the present paper
A square wave pattern was used for the structured silicon surfaces and the contact arrangement involved these samples in contact with a flat unstructured silicon sample
Summary
Friction is the mechanism responsible for transmitting tangential force across an interface when materials are in contact. Another study indicating the somewhat inconclusive nature of the dry friction results is by Pettersson and Jacobson [39] These authors had a smooth steel ball in reciprocating sliding contact with a square wave structured flat silicon surface having either a TiN or DLC coating. The structures pro duced by laser surface texturing have a high degree of irregularity making it impossible to guarantee a specific real contact area and sec ond, very many of the friction studies have involved the ball-on-flat test arrangement which is problematic for a few reasons: the ball-on-flat has its own contact area versus load relation producing a contact area pro portional to P2/3, only a small number of the nano or micro features will make contact and each will make contact to a different extent – all of which makes it difficult to control the real contact area. It is interesting to determine what kind of friction law is obeyed for both the structured and unstructured instances of these highly idealised surfaces
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