Real contact area is one of the very important parameters that control mechanical, tribological, thermal and other phenomena at contacting surfaces. While there have been many studies reported on modeling real contact area for deformable and work-hardening materials, like steels, there still do not exist models that would be generally accepted and applicable in engineering practice. Moreover, some fundamental mechanisms of how the real contact area develops and what are the expected and realistic values are the key missing information. These are the aspects discussed in this work.

We ground our results based on the experimental evaluation of real contact are using optical imaging with a very high precision, namely at 700 nm of lateral and 20 nm of vertical resolution. In a recent study of a range of material properties of metallic steel surfaces we have shown that very small real contact area (10-25 %) carry the load even under loads up to the yield strength on a nominal contact area. This indicates very high load carrying capacity of these asperities and real contact surfaces. The question arises, which mechanisms may be responsible for this. We analysed the elastic and plastic deformation contributions at the multi-asperity real-sample surfaces, at various loads up to a nominal yield strength. The results elucidate why the asperities were able to carry the load that resulted in as much as a 4–10-times-higher contact pressure than the (initial) bulk yield strength. It is proposed that for the rough surfaces this is due to the work-hardening, while for the smooth surfaces the dominant mechanisms are the work-hardening combined with the hydrostatic bulk stresses.