A review and a fundamental theory of silicon nitride tribochemistry

► We propose a general model for silicon nitride lubrication. ► We propose a general mechanism for silicon nitride tribochemical reactions. ► Tribochemical products act as lubricants and protective film in silicon nitride. The present work aims at providing a better understanding of the tribology of silicon nitride, an engineering ceramic with many applications in the automotive and aerospace industry. The tribological properties of silicon nitride depend on tribochemical reactions, which modify both surface composition and morphology. In air or humid conditions, hydroxylated silicon oxide is the main product of tribochemical wear, which flattens surface decreasing stresses on asperities and reducing wear rate and friction level. Severe wear is dominated by fracture wear modes. Boundary lubrication with n-alcohols is also characterized by tribochemical wear, which leads to the formation of silicon alkoxides and polysiloxanes. The working model is based on the tribo-emission process of low-energy electrons during friction with generation of positively charged silicon sites and nitrogen free radicals.