Coupled defect chemistry and redox dynamics in a bismuth gahnite system for self-adaptive tribological interfaces

Abstract

The development of self-adaptive, wear-resistant surfaces is critical for advancing next-generation tribological materials. In this study, a robust spinel zinc aluminate matrix is engineered with bismuth to incorporate lubricious phases that activate dynamically under sliding conditions. The strain and local structural distortion are brought about by introducing larger bismuth ions into the gahnite matrix. This doping also promotes oxygen vacancy formation, enabling a redox-active environment during wear. Under atmospheric oxygen and frictional heating, bismuth undergoes oxidative transformation at the wear track to form a lubricious bismuth oxide layer. At the same time, defect-rich, high-stress regions promote the reduction of bismuth through vacancy-mediated electron transfer, establishing a reversible redox loop. This repeated cycle enhances surface reactivity, enabling bismuth migration and the formation of surface oxides. Tribological testing using electroless NiP matrices on steel surfaces confirms bismuth redox cycling and tribofilm behavior, which critically influences the wear response. Raman spectroscopy and OSP show dynamic surface transformations and phase evolution. These findings demonstrate a synergistic interplay between redox-active dopants, structural modulation, and oxygen vacancy dynamics, offering a pathway to design chemically adaptive, high-performance tribological coatings.