Temperature-driven self-lubrication in TiB2±x coatings via boron oxide formation

• Super-stoichiometric TiB 2.9 outperforms TiB 1.5 in friction reduction with increasing temperature. • TiB 2.9 friction drops by 55% at elevated temperatures compared to room temperature. • Boron oxide formation governs the low friction and self-lubrication of TiB 2±x at high temperatures. Transition metal borides represent an auspicious family of materials for functional coatings due to their refractory nature, including high melting points, extreme hardness, and outstanding resistance to wear. Additionally, they act as model systems to study in-operando self-lubrication through the formation of friction-reducing boron oxide layers. The prevailing temperature and composition strongly influence the boron oxide formation. In this study TiB 2±x thin films have been used to explore the fundamentals of this, so far, not fully understood temperature driven tribo-reaction. Tribological tests indicate that sputter deposited super-stoichiometric TiB 2.9 exhibits a significantly lower friction coefficient (∼0.3) compared to sub-stoichiometric TiB 1.5 (∼0.4) when tested at 500 °C. This reduction is attributed to the higher oxidation rate of super-stoichiometric TiB 2.9 further promoted through the presence of B-rich tissue phases at the column boundaries. Moreover, although the coatings showed slightly higher wear rates at 500 °C than at room temperature, the overall wear remained low – most notably for the oxidation-prone super-stoichiometric TiB 2.9 , which also displayed the strongest friction-reducing effect. In summary, these results highlight the potential of boron oxide for solid self-lubrication.