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

Abstract

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.