Tribological performance and thermal conductivity of graphite-based composites infiltrated by SiC and Si3N4

  • Miguel Hernández

Student thesis: Phd

Abstract

Industrial applications of graphite demand an increase in its wear resistance without incurring losses in its advantageous properties, such as its high electrical and thermal conductivity, low coefficient of friction, its stability at high temperatures and low price. This study aimed to develop graphite-based composites infiltrated by SiC and/or Si3N4 particles with higher wear resistance and thermal conductivity. Graphite was infiltrated by a silicon slurry under vacuum, then carbonised and nitrided at 1400 ◦C to form SiC and Si3N4, respectively, in the graphite porosity. Enhancement of the wear resistance of the infiltrated graphite was investigated through examining its mechanical and tribological performances at the macroscale, using indentation and pin-on-disc tests, and at the microscale, using nanoindentation and nanoscratches. The effect of the volume fraction of SiC and Si3N4 particles on the thermal conductivity of the graphite composites was measured by the laser-flash technique. The SiC reinforcement contained nuclei grains and whisker-like structures, and the Si3N4 reinforcement included blade-like grains, fine grains and needle-like morphologies. The greater hardness and Young’s modulus of SiC (9.7 GPa and 53.3 GPa, respectively) and Si3N4 (10.9 GPa and 139.5 GPa, respectively) inclusions in comparison with the graphite matrix (0.34 GPa and 7.7 GPa, respectively), increased the wear resistance of the composites (the wear rate of g/SiC and g/Si3N4 composites was 66 and 33 % relative of the graphite) without affecting the graphite’s lubricant properties. The Si3N4 reinforcement was more effective in protecting the graphite matrix than the SiC reinforcement since accommodated more deformation without fracture, as supported by the H3/Er2 ratios, and possessed better bonding with the carbon. The thermal conductivity of graphite increased from 41 W/m·K to 53 and 64 W/m·K adding 12 and 19 vol.% of SiC and Si3N4, respectively. The effect of reinforcement enhancing the thermal conductivity of graphite was simulated by a model based on the heat conduction pathways in the graphite microstructure. The new graphite-based composites, with their higher wear resistance and thermal conductivity, produced infiltrating SiC and Si3N4, will be able to replace graphite in several applications, extending service life and enhancing performance.
Date of Award31 Aug 2021
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorPhilip Withers (Supervisor) & Ping Xiao (Supervisor)

Keywords

  • SiC
  • Si3N4
  • Thermal Composite
  • Ceramic processing
  • Tribology
  • Graphite-based composites

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