Role of SiC and Si3N4 reinforcing particles in the tribological performance of graphite-based composites

Abstract

Graphite is used extensively for electrodes, in metal processing and for furnace linings but its lifetime in such applications is limited by poor wear resistance. Here SiC or Si3N4 particles are formed in situ within the porous microstructure of bulk graphite by Si infiltration followed by heat treatment under Ar or N2 respectively. Their tribological performances are evaluated at the macro- and microscale. Unlubricated pin-on-disc (1.8 N load) tests were conducted to compare the coefficient of friction and wear rates for the composites with bulk graphite, while (1 mN load) nanoscratch tests were used to measure the same properties for the individual constituents. The high hardness and Young’s modulus of the Si3N4 and SiC reinforcements compared to the graphite matrix increases the wear resistance of the composites relative to the unreinforced graphite (33% and 65% of the wear rate, respectively) which show abrasive wear, as cracking at the macroscale. Microstructurally, abrasive wear takes place by ploughing in the matrix and through fracture in the particles. The Si3N4 reinforcement shows similar hardness but considerably greater capacity to accommodate plastic-elastic deformation without fracture and thus a stronger bonding than the SiC reinforcement. As a result, Si3N4 is less brittle than the SiC reinforcement and thus is more effectively retained within the graphite matrix leading to better wear resistance.