Effect of Ni–Bi Alloying on the Microstructure and Self-Lubricating Properties of CoCrNi-Based Coatings Across a Wide Temperature Range

Abstract

This study investigates the use of bismuth (Bi) as a lubricating additive in laser cladding Co-based composite coatings, addressing the issue of Bi segregation and agglomeration through Ni–Bi alloying. The microstructure, mechanical properties, and tribological properties of composite coatings with varying Ni–Bi contents are systematically evaluated at temperatures between 30 and 800 °C. Analysis reveals that Ni and Bi elemental powders are successfully alloyed to form BiNi and Bi₃Ni intermetallic compounds following vacuum sintering. Incorporation of Ni–Bi alloying powder significantly enhances the friction coefficient and wear rates of the composite coating across the temperature range. Adhesive wear and abrasive wear are identified as primary wear mechanisms. Notably, the formation of BiNi, multiple oxides, and Bi₁₆CrO₂₇ compounds on the surface of the 85:15 (at%) Bi:Ni composite coating at 600 °C created a self-lubricating friction layer, synergistically reducing friction. Consequently, compared to Co-based alloy coatings without Ni–Bi alloying, the composite coating exhibited a three-fold reduction in friction coefficient and a two-order-of-magnitude improvement in wear rate, demonstrating exceptional tribological properties.