# Concurrent optimization of fracture toughness, thermal conductivity, and tribological behavior in Cf/Si3N4 composites via phase driven selection

## TL;DR

Researchers made carbon fiber-reinforced silicon nitride composites with high toughness, thermal conductivity, and friction resistance by controlling the initial silicon nitride phase.

## Contribution

The study introduces a microstructural design strategy using initial Si3N4 phase control to optimize multiple functional properties in Cf/Si3N4 composites.

## Key findings

- Composites from α-Si3N4 achieved 96.53% density, 10.87 MPa m0.5 fracture toughness, and 66 W/m K thermal conductivity.
- The α-Si3N4 composite showed stable friction behavior with a COF of ≈ 0.46 due to self-reinforced β-Si3N4 microstructure.
- β- and γ-Si3N4 composites had lower performance due to increased porosity and secondary phase formation.

## Abstract

Carbon fiber–reinforced silicon nitride (Cf/Si3N4) composites were fabricated by spark plasma sintering (SPS) using α-, β-, and γ-Si3N4 powders to clarify the influence of the initial Si3N4 phase on microstructural evolution and functional properties. The results show that the starting phase significantly affects densification behavior, phase transformation, and mechanical and tribological performance. The composite derived from α-Si3N4 achieved the highest relative density (96.53%) and exhibited an optimal balance of fracture toughness (10.87 MPa m0.5), thermal conductivity (66 W/m K), and stable friction behavior (COF ≈ 0.46). This performance is attributed to the in-situ formation of a self-reinforced β-Si3N4 microstructure during the α → β phase transformation, which promotes crack deflection, crack bridging, and effective load transfer in synergy with carbon fibers. In contrast, β- and γ-Si3N4–based composites showed lower densification or excessive hardness associated with increased porosity and secondary phase formation. These findings demonstrate that controlling the initial Si3N4 phase provides an effective microstructural design strategy for developing high-performance Cf/Si3N4 composites for thermostructural applications such as aerospace brake discs.

The online version contains supplementary material available at 10.1038/s41598-026-44244-7.