How does the amorphous Si2BC3N powder form by mechanical alloying？

Silicoboron carbonitride (Si-B-C-N) ceramics are attractive for numerous applications such as thermal protection parts owing to their remarkably high microstructural stability and good high-temperature properties. Among the available methods for synthesizing metastable Si-B-C-N powders with amorphous and nanocrystalline structures, organic precursor-derived processes are widely used as they can meet the high-temperature processing demands. However, this process requires complex operations like strict water-free and oxygen-free conditions, which are expensive to achieve and maintain. Moreover, maintaining these conditions require costly and toxic raw materials that are also detrimental to human health and the environment.

Recently, mechanical alloying has been identified as a promising alternative method for synthesizing amorphous Si-B-C-N powders. Despite its simplicity and cost-effective advantages, the process is affected by various parameters which in turn affects the sinterability of the alloyed powders. Extensive research has been conducted to evaluate the influence of ball milling parameters as well as the influence of boron and carbon on the mechanical alloying process. However, effective application of mechanical alloying in synthesizing Si-B-C-N powders requires a thorough understanding of the synthesis mechanism, which has not been fully explored to date.

Motivated by the promising results of the previous studies, a team of Harbin Institute of Technology researchers including Prof. Dechang Jia, Yu Zhou, Zhihua Yangand Dr. Bin Liang (who works at Tsinghua University now) studied the structural evolution of 2Si-BN-3C mixture during mechanical alloying to understand the synthesis mechanism, optimization and control of milling products. Specifically, the changes in the chemical bonds, phases, morphologies, microstructure and elemental distribution were investigated using various techniques such as X-ray photoelectron spectroscopy, X-ray diffraction, Raman and TEM. The authors also discussed the next synthesis and development of Si-B-C-N system materials. The research is currently published in the Journal of the American Ceramic Society.

Compared to the mechanical alloying of Si-C mixture, the mechanical alloying of 2Si-BN-3C mixture provided perfect amorphous Si₂BC₃N powders in a shorter time, due to the dilution effects of the atomic concertation hindering the microdiffusion and mechanochemical reaction between Si and C. The particle sizes changed differently with the increase in the milling times. For instance, the particle sizes first decreased to sub-micrometer level in the first one hour, then slightly increased after three hours before eventually decreasing to the nanoscale level to attain equilibrium after ten hours of milling. Furthermore, h-BN and graphite reached amorphization before c-Si because of the weak van der Waal forces leading to easier cleavage along the basal planes. Most importantly, crystallite refinement–induced amorphization was identified to be the main synthesis mechanism of amorphous Si₂BC₃N powders. Crystallite refinement led to an increase in grain boundaries, internal strain and defects that accelerated nanocrystalline-to-amorphous transformation by improving the atomic diffusion pathways.

In summary, the researchers explored the synthesis mechanism of amorphous Si₂BC₃N powders by studying the structural evolution of the 2Si-BN-3C mixture during mechanical alloying. Results revealed crystallite refinement–induced amorphization as the primary synthesis mechanism of amorphous Si₂BC₃N powders. The results would also offer more insights into the synthesis ofmulti-component materials, especially Si-based brittle systems, by mechanical alloying. In particular, the authors, in a statement to Advances in Engineering, pointed out that the results would be useful in advancing future research on atomic diffusion and mixing process of Si-BN-C with different chemical compositions.