Carbon-based solid lubricants: An overview

Solid lubrication is an excellent option to reduce friction and wear in dry sliding conditions, as well as a secondary source of protection in fluid-lubricated systems. Considering the most widely used solid lubricants, carbon-based lubricants are among the most versatile for tribological use. The reasons for this include their availability and their innate ability to adopt various nanostructures. The crystalline ordering of carbon has been identified as the main factor governing its tribological behavior. This work presents and discusses the most significant findings from an ongoing research program aimed at developing carbon-based solid lubricants. In this sense, several carbon-based materials with diverse levels of nanostructural order have been studied: three-dimensional (3D) crystalline graphite (GR); novel carbide-derived two-dimensional (2D) turbostratic carbon (CDC) produced from Fe–SiC (GSF) and B₄C–Cr₃C₂ (GBC) solid-state reactions; NH₃ plasma-functionalized multilayer graphene (MLG); in situ fluorinated diamond-like carbon (DLC) (isfDLC); vertically aligned carbon nanotube (CNT) films. Initially, we present a brief description of the processing route for obtaining these materials, followed by their microstructural characterization and a synthesis of the most relevant aspects of tribological evaluation. Dry sliding tests in reciprocating motion were employed with different tribo-pair geometries, specimens and counter-body materials, varied surface topography, and diverse routes to add solid lubricants to the contact (vacuum impregnation of sintered steels, drop-casting, self-lubricating composites and vertically aligned films). The results provide a holistic view of the nature of the tribolayers formed by these materials. Finally, a new micro-Raman (µRaman) analysis technique for quantifying the point and line defects of the carbon present in tribolayers is used to correlate the lubrication and degradation mechanism of carbonaceous solid lubricants with their initial nanostructure and testing conditions. This technique provides new insights into the nature of tribolayers produced by carbon-based solid lubricants.