On the use of ceramic PVD coatings to replace metallic coatings in electrical contacts

Abstract

In electrical contacts the combination of high normal pressures and soft contact materials results in large contact areas and low contact resistance, but also in massive plastic deformation and wear. This paper examines some of the potential advantages and disadvantages of replacing one of the soft metallic coatings in electrical power connectors with a hard ceramic coating. Three ceramic PVD coatings (Ti/TiN, Nb/NbN and CrN) all 2–3 μm thick, deposited on copper, were laboratory tested under fretting conditions against a silver coating and compared with the corresponding traditional contact between two silver surfaces. The model contact consisted of two crossed cylinders (Ø 10 mm) continuously powered with 10 a d.c. Five combinations of normal force (20–60 N) and vibration stroke (15–30 μm), all resulting in gross weld regime fretting for silver mated to silver [1,2], were tested. The fretting contacts were primarily studied with respect to contact resistance behaviour and silver surface damage. Despite the relatively high resistivity of the ceramic materials the contact resistance levels were quite low. This is primarily due to the extensive contact area growth during fretting out also due to the fretting aiding in removing thin oxide or corrosion films from the ceramic coating surface. Furthermore, the silver deformation was less severe in the silver-to-ceramic contact than in the corresponding silver-to-silver contact. For comparison, the influence of thin coatings of relatively high resistivity on the electrical potential and the temperature distributions was investigated using a single-spot contact finite element model.