Does combination of a lubricant anti-wear additive and a hard coating double the surface protection?

For a contact interface under boundary lubrication, wear protection relies on both the lubricant physicochemical characteristics and the surface mechanical properties. Anti-wear additives—traditionally zinc dialkyldithiophosphate (ZDDP) and tricresyl phosphate (TCP) and, more recently, oil-soluble ionic liquids—are important components of many lubricants. They are designed to protect the contact surface via forming a tribofilm in situ via thermomechanically triggered chemical reactions. Concurrently, various hard coatings have been developed with promise for reducing friction and wear. Common ceramic coatings include titanium nitrides (TiN, TiAlN, CrN, etc.), carbides (WC-Co, VC, Cr₃C₂, etc.), and borides (TiB₂, AlMgB₁₄, etc.). Diamond-like-carbon (DLC) coatings are among the most successful carbon-based thin films. While both lubricant anti-wear additives and hard coatings are being used in many industrial applications, a fundamental question remains: would using them together provide double surface protection? The short answer is: not necessarily and sometimes an incompatible additive-coating pair could even accelerate the wear loss. This talk uses a few case studies, synergistic or antagonistic, to reveal the mechanisms behind the compatibilities between the lubricant chemistry and coating composition. Tribochemical reactions at the contact interface could be either beneficial to form a self-healing tribofilm or detrimental leading to excessive tribocorrosion. Results provide critical insights for future lubricant and coating development and implementation.