Hollow nanoparticles of WS2 as potential solid-state lubricants

Abstract

Solid lubricants fill a special niche in reducing wear in situations where the use of liquid lubricants is either impractical or inadequate, such as in vacuum, space technology or automotive transport. Metal dichalcogenides MX2 (where M is, for instance, Mo or W and X is S or Se) are widely used as solid lubricants. These materials are characterized by a layered structure with weak (van der Waals) inter-layer forces that allow easy, low-strength shearing. Within the past few years, hollow nanoparticles (HNs) of MX2 with structures similar to those of nested carbon fullerenes and nanotubes have been synthesized. Here we show that these materials can act as effective solid lubricants: HN-WS2 outperforms the solid lubricants 2H-MoS2 and 2H- WS2 in every respect (friction, wear and lifetime of the lubricant) under varied test conditions. We attribute the outstanding performance of HN-WS2 to its chemical inertness and the hollow cage structure, which imparts elasticity and allows the particles to roll rather than to slide.