Effects of Normal Load, Sliding Speed, and Surface Roughness on Tribological Properties of Niobium under Dry and Wet Conditions

The effects of normal load, sliding speed, and surface roughness on the friction and wear of high-purity niobium (Nb) during sliding without and with an introduction of water were systematically investigated. Increasing the normal load or sliding speed decreased the friction of the Nb under the both dry and wet conditions because the increased wear of the Nb decreased the interfacial shear strength between the steel ball and Nb by promoting the surface roughening and the production of wear debris. However, the Nb tested at the lowest sliding speed under the lowest normal load with water exhibited the lowest friction and wear due to the formation of oxide layer on the wear track. The friction and wear of the Nb tested under the dry condition decreased with increased surface roughness because the higher interfacial shear strength between the steel ball and smoother Nb resulted in the earlier breakdown of the native oxide layer and direct contact between the steel ball and Nb. However, increasing the surface roughness of the Nb increased its friction and wear under wet conditions, probably due to the easier breakdown of the oxide layer that formed on the rougher surface during sliding. The tribological results clearly showed that the introduction of water during sliding had a significant influence on the tribological properties of the Nb.