Mechanical and Tribological Performance of Graphite/Silicon Nitride Composites: A Comparison between Pressureless and Spark Plasma Sinter Processing

Graphite particle (GP)-reinforced silicon nitride (Si₃N₄) composites were fabricated using pressureless sintering (PLS) and spark plasma sintering (SPS) in the presence of Y₂O₃-AlN-SiO₂ ternary system. Different densification behaviors of the specimens fabricated by PLS and SPS were observed. While increasing GP content drastically reduced matrix densification during PLS at 2023 K (1750 °C) with 2 hours dwell, SPS at 1923 K (1650 °C) for only 10 minute under 50 MPa pressure resulted in much dense composites even up to 3.5 wt pct GP loading. Mechanical and tribological characterizations revealed that SPS-ed composites can offer improved performance compared to pure Si₃N₄. SPS-ed 1.5 wt pct GP/Si₃N₄ composite offered the highest resistance to wear up to 20 N normal load. Wear rate (W _R) of that composite reduced by ~24 pct than that obtained for pure Si₃N₄ (W _R ≈ 1.14 × 10⁻³ mm³/N m). Furthermore, W _R of SPS-ed 2.5 wt pct GP/Si₃N₄ composite at F _N = 10 to 20 N was found to be only ~1/8th of W _R values of PLS-ed 2.5 wt pct GP/Si₃N₄ composite (^10N W _R ≈ 6.61 × 10⁻³ mm³/Nm; ^20N W _R ≈ 9.45 × 10⁻³ mm³/Nm). Present study indicated promising opportunity of SPS for fabricating improved Si₃N₄ composites through GP addition. The reinforcing particles not only rendered its self-lubrication effect to SPS consolidated composites but also significantly promoted the rate of matrix densification during SPS cycle by the virtue of its high thermal and electrical conducting nature.