Design and processing of NbC-reinforced self-lubricating iron matrix composites via in-situ synthesis and vacuum impregnation

Abstract

Self-lubricating metal matrix composites (SLMMCs) are materials that offer a combination of high wear resistance and low friction coefficients in severe tribological conditions, resulting from their combination of ceramic reinforcements and solid lubricants. In this work, a combination of in-situ precipitation and vacuum impregnation was employed to fabricate an iron matrix composite reinforced with niobium carbide (NbC), resulting in a novel self-lubricating metal matrix composite. This unique approach enables the use of composites as self-lubricating materials by enhancing interfacial adhesion between the matrix and the reinforcements, thereby preventing the detachment of reinforcement particles—a common cause of third-body wear. Moreover, the surface surrounding the pores is composed almost entirely of NbC particles, acting as an ideal reservoir for graphite by preventing the sealing of these pores. The composite was produced in situ from the reaction between graphite and Fe₂Nb intermetallic powders. A theoretical design using CALPHAD simulations, followed by an experimental analysis to optimize the graphite content and sintering temperature to achieve a microstructure consisting of fine micrometric and submicrometric carbides dispersed in a ferritic matrix. The amount of porosity was tailored for vacuum impregnation with graphite in order to induce solid lubrication. Tribological characterization was performed in a ball-on-flat configuration under dry sliding conditions. The material exhibited wear rates of around 1.23 × 10⁻⁶ mm²/N.m against a harder counter body, while also inducing friction coefficients in the 0.08 – 0.1 range, primarily attributed to the effective utilization of porosity as lubricant reservoirs and the reinforcement of the material due to NbC precipitation. This innovative approach enables the creation of high-performance self-lubricating materials and has the potential to be extended to several other combinations of metals, ceramics, reinforcements, and solid lubricants.