The potential of indium as a soft metal lubricant replacement for lead

Abstract

A review was carried out comparing the properties of several soft metals to identify the most suitable lubricant alternative to lead. Indium emerged as the most promising candidate.
A method for producing thin-film indium coatings via physical vapour deposition was developed. Coatings were deposited onto 52100-steel discs and bearing balls and assessed for their adhesion, purity and morphology. The friction and lifetime were then assessed in vacuum through spiral orbit tribometry (SOT) and the tribological performance compared with that of lead and MoS2.
The indium coatings displayed a microstructure consisting of evenly distributed nodules of pure indium. Coatings on bearing-steel discs exhibited both good adhesion and high chemical purity. However on bearing-steel balls, adhesion was weaker.
During SOT testing indium transferred readily from the coated ball to the uncoated SOT plates. The coatings displayed low friction (lower than for lead and comparable to that of MoS2). Whilst indium coating lifetimes exceeded those of MoS2, they were appreciably shorter than those observed with lead coatings of similar thickness.