Friction reduction by water-soluble ammonium thiometallates

Lubricant bases for metalworking applications make extensive use of water-soluble additives to reduce friction and wear. In order to do such task, these additives must form a lubricating film that separates the contact surfaces thus imparting good surface finishes to the worked parts. This paper presents a study on the tribological performance of aqueous solutions of ammonium thiomolybdate and ammonium thiotungstate. Tests were carried out on a pin-on-disc tribometer for a steel–aluminum contact while keeping load, entrainment speed, sliding distance, temperature, and concentration of the additive constant to study the lubrication effect of these two salts. Chemical analysis of the wear track indicates the presence of an in-contact-formed solid film enriched with MoS₂ and WS that reduces friction markedly.