The role of a tribofilm and wear debris in the tribological behaviour of nanocrystalline Ni–Co electrodeposits

The tribofilm and debris generated from wear tests acting as a third body can make important contributions to a tribo-system. The effect of tribofilm on sliding friction and wear depends strongly on the applied sliding conditions, and it is still lack of understanding due to insufficient experimental studies. In the present research, the electrodeposited nanocrystalline Ni–Co coatings with different cobalt content were prepared. The worn surfaces as well as the wear debris were characterised by surface analysis techniques after dry sliding tests against a stainless steel ball. The tribofilms containing iron from the counterparts were formed on the worn surface of the coatings (less than 60 at.% Co), which exhibited high coefficients of friction and wear rates. No tribofilm or iron transfer from the pin was found on the Co-rich coatings (more than 70 at.% Co) but a dramatic friction reduction of 50% and improved wear resistance were experienced. The wear debris contains a mixture of face-centred cubic (fcc) metallic phase and fcc oxidised phase, irrespective of the coating composition. The oxidised debris cannot form an efficient lubricative film to promote separation of the sliding surfaces.

► Nanocrystalline Ni–Co coatings with different Co contents were electrodeposited.
► Tribofilms on coatings (<60%<60% Co) were not beneficial to tribological properties.
► No tribofilm or Fe transfer from the pin was found on Co-rich coatings (>70%>70% Co).
► Improved tribological properties of Co-rich coatings were due to the hcp structure.
► Debris contained a mixture of fcc metallic and oxidic phase without hcp structure.