Influence of Heating Rate on Reinforcement Degradation during Spark Plasma Sintering of Solid Lubricant-Reinforced Copper Composites

Copper composites reinforced with solid lubricants are promising potential solutions for electric sliding mobility systems, where the material employed should possess a blend of good electrical conductivity, sufficient hardness, and reduced frictional losses. However, the thermal stability of such reinforcement during consolidation remains a significant challenge, as longer exposure times at higher sintering temperatures to obtain better relative density come with a trade-off with reinforcement functional property degradation, which eventually affects the composite's performance. The present work aims at studying the effect of the heating rate of spark plasma sintering (SPS) on the microstructure and properties of copper composites reinforced by various layered materials. Copper composites containing equal vol.% of reinforcement were fabricated through a powder metallurgy route followed by SPS. Three different layered reinforcements, hexagonal boron nitride (hBN), molybdenum disulfide (MoS₂), and graphite, were incorporated into the copper matrix. Sintering was performed at a constant temperature with heating rates of 50, 100, and 200 °C min^-1. The consolidated composites were characterized in terms of microstructure, electrical conductivity, Vickers hardness, and nanoindentation responses. The results revealed a strong reinforcement-dependent response to SPS heating rate. Higher heating rates effectively suppressed reinforcement degradation due to reduced thermal exposure, but due to the limited grain growth, electrical conductivity was reduced. Conversely, lower heating rates promoted grain coarsening and enhanced electrical conductivity but increased reinforcement degradation and reduced hardness. Nanoindentation analyses highlighted the influence of reinforcement stability on the mechanical characteristics of the matrix-reinforcement interfaces. The performance of self-lubricating copper composites is strongly affected by their processing history. Since microstructural evolution, reinforcement stability, and interfacial characteristics are established during consolidation, careful selection of sintering parameters is essential for achieving balance between the electrical, mechanical, and tribological properties.