Enhancing Silicon Carbide Densification: Spark Plasma Sintering with Si‑SiC Nanocomposite Additives from Thermal DC Plasma

Abstract

Si‑coated SiC (Si‑SiC) nanocomposite particles were synthesized via non‑transferred arc thermal DC plasma from solid‑state SiC powder. These particles were employed as sintering additives in spark plasma sintering (SPS) of micron‑sized SiC. The optimal 10 wt % Si‑SiC loading, combined with 0.2 wt % activated carbon, yielded a relative density of 97.1 % and a Vickers hardness of 31.4 GPa at 1800 °C for a 1‑min hold. This work demonstrates a low‑temperature, high‑density SiC route that leverages nano‑size effects and exothermic silicon‑carbon reaction bonding.

Background

Silicon carbide ceramics are prized for their exceptional high‑temperature hardness, wear resistance, low thermal expansion, and corrosion tolerance, making them indispensable in turbine blades, diesel engines, aerospace, and nuclear reactors. However, the covalent Si–C bond and sluggish self‑diffusion hinder densification without additives. Traditional routes—solid‑state sintered SiC (SSS‑SiC) and liquid‑phase sintered SiC (LPS‑SiC)—require temperatures above 1850 °C and often introduce secondary phases that reduce high‑temperature performance. Reaction‑bonded SiC (RB‑SiC) offers lower sintering temperatures but suffers from poor density. Thus, reducing sintering temperature while maintaining mechanical integrity is a key research challenge.

Nanoparticulate SiC has emerged as a promising avenue; its high surface area accelerates sintering and can enhance final properties. We therefore developed a method to coat SiC with a thin Si layer via non‑transferred thermal DC plasma, creating a Si‑SiC composite nanoparticle that serves as an effective, minimal‑additive densifier.

Experimental

Micron‑Sized SiC Synthesis

Si (average 25 µm, 99.9 %) and activated carbon (32 µm) were mixed in a 1:1.5 mol ratio and ball‑milled for 15 h. The mixture was calcined at 1300 °C for 2 h in Ar (1 L min⁻¹). The resulting β‑SiC powder was ground for further use.

Plasma‑Generated Si‑SiC Nanoparticles

Using a non‑transferred arc thermal plasma reactor, the micron‑SiC powder was fed through a 2‑mm pipe at 70 V and 1 g min⁻¹. The system operated at 200 Torr, 30 L min⁻¹ Ar, 3 L min⁻¹ H₂, and 300 A (45 V). Nanoparticles were collected from the reactor walls; the yield was 80–85 %.

Spark Plasma Sintering

Mixtures of micron‑SiC and Si‑SiC nanoparticles (5–15 wt %) were compacted in a 20 mm graphite die under 80 MPa. SPS was performed in vacuum (10⁻² Torr) with a heating rate of 600 °C min⁻¹, targeting 1600–1800 °C. Holding times ranged from 0 to 1 min at 1800 °C. Some samples received 0.1–0.2 wt % activated carbon to promote reaction bonding. Density was measured by Archimedes; Vickers hardness by a 10 kgf indenter.

Characterization

XRD (Cu‑Kα) assessed phase composition; SEM (JSM‑5900) and TEM (JEM‑2010) examined morphology. Shrinkage displacement was recorded during sintering.

Results and Discussion

XRD confirmed β‑SiC in both micron‑ and nano‑samples; the latter also contained trace Si and SiO₂ from partial decomposition and oxidation. SEM showed 2–5 µm micron particles and 20–70 nm Si‑SiC nanoparticles (surface area 69 m² g⁻¹).

Increasing sintering temperature improved relative density and hardness; at 1800 °C and 1 min hold, the 90:10 (SiC:Si‑SiC) mixture reached 88.2 % density and 21.2 GPa hardness. Adding 0.2 wt % activated carbon further increased these values to 97.1 % density and 31.4 GPa hardness, indicating effective reaction bonding between free silicon in the nanoparticles and carbon.

Micrographs revealed grain growth from 2–4 µm at 1800 °C, with densification accelerating under the exothermic silicon–carbon reaction. Shrinkage data corroborated enhanced densification, especially in samples containing activated carbon.

Mechanistically, the Si‑SiC nanoparticles act in two ways: their high surface area promotes rapid diffusion (nano‑size effect), and the liberated silicon reacts exothermically with added carbon, locally raising temperature and accelerating sintering (reaction‑bonding effect).

Conclusions

We demonstrated that Si‑coated SiC nanoparticles, produced by thermal DC plasma, serve as an efficient, low‑additive densifier for SiC ceramics. A 10 wt % loading combined with 0.2 wt % activated carbon yields a 97.1 % relative density and 31.4 GPa Vickers hardness at 1800 °C, surpassing conventional methods. The dual contribution of nano‑size diffusion enhancement and silicon–carbon reaction bonding offers a viable route to high‑performance SiC at reduced temperatures.