Influence of Surface Design on the Solid Lubricity of Carbon Nanotubes-Coated Steel Surfaces

Abstract

Topographically designed surfaces are able to store solid lubricants, preventing their removal out of the tribological contact and thus significantly prolonging the lubrication lifetime of a surface. The present study provides a systematic evaluation of the influence of surface structure design on the solid lubrication effect of multi-walled carbon nanotubes (MWCNT) coated steel surfaces. For this purpose, direct laser writing using a femtosecond pulsed laser system is deployed to create surface structures, which are subsequently coated with MWCNT by electrophoretic deposition. The structural depth or aspect ratio of the structures and thus the lubricant storage volume of the solid lubricant is varied. The frictional behavior of the surfaces is recorded using a ball-on-disk tribometer and the surfaces are thoroughly characterized by complementary characterization techniques. Efficient lubrication is achieved for all MWCNT-coated surfaces. However, and in contrast to what would be expected, it is shown that deeper structures with larger lubricant storage volume do not lead to an extended lubrication lifetime and behave almost equally to the coated unstructured surfaces. This can be attributed, among other things, to differences in the final surface roughness of the structures and the slope steepness of the structures, which prevent efficient lubricant supply into the contact.