In-situ micro-asperity investigation of real contact area formation during sliding with the effects of roughness and normal load considered

Real contact area is a challenging phenomenon in tribology, where despite established theoretical models, detailed experimental studies are still lacking. This work is focused on the evolution of the real contact area of multi-asperity steel contacts during sliding, by using sub-micron characterisation of entire multi-asperity contact area and simultaneous measurement of the friction force. Steel-sapphire contact was monitored by using an in-situ optical system in real time during 5 mm of unidirectional sliding in surface roughness range Ra = 0.1–1.0 µm and normal loads up to the nominal contact pressure equal to steel yield strength (Y). The results showed that the initial real contact area caused by static loading increased during sliding. At low nominal contact pressure of 0.3Y, the increase in real contact area was the largest: for Ra = 0.1 µm it went from 9.5% of the nominal area under static conditions to 15.0% after sliding, a 56% relative increase, and for Ra = 1.0 µm it went from 5.0% to 11.5% after sliding, a 106% increase. The increase in real contact area was smaller at higher nominal contact pressures of 0.6Y and 1.0Y. At 1.0Y for Ra = 0.1 µm the real contact area went from 18.0% to 21.8% after sliding and for Ra = 1.0 µm it went from 9.4% to 12.9% after sliding. The coefficient of friction increased with roughness at the lowest nominal contact pressure 0.3Y (for Ra = 0.1 µm it was 0.24 and for Ra = 1.0 µm it was 0.29) but became independent of roughness at higher contact pressures (on average 0.18).