China claims super ceramics for nuclear reactors, hypersonic jets

Scientists at Harbin University in China have devised a novel two-step process that uses in-situ reactive spark plasma sintering (SPS) with ZrC, TiSi2, and B4C as raw materials to produce ultra-high-temperature ceramics (UHTCs).

These new-age materials are critical for applications such as next-generation nuclear energy, hypersonic flight, and advanced propulsion systems operating in extreme environments. 

Zirconium carbide (ZrC) is a leading material for building UHTCs due to its exceptionally high melting point and solid-state stability. However, large-scale application of the material remains a distant dream due to its poor sinterability and intrinsic brittleness, raising doubts about its long-term structural stability. 

Sinterability is the ability of a powder to form a solid mass without reaching the full liquefaction stage. A lot of factors are at play in this process, but for ZrC, sinterability is hindered by the need for extremely high processing temperatures. Different approaches applied in the past have helped address some of ZrC’s limitations, but they have often improved some facets at the expense of others. 

Novel two-step process

Researchers Boxin Wei and Yujin Wang stepped up to address these shortcomings by switching to a two-step process.

“The core challenge we aimed to address was how to simultaneously enhance both densification behavior and fracture resistance in ZrC ceramics,” explained Boxin Wei, associate professor at Harbin University of Science and Technology, in a press release. 

“Our approach was to leverage a carefully designed sequence of in-situ reactions that would not only promote low-temperature densification but also create a hierarchical microstructure with reinforcing phases operating at different length scales,” added Wei. 

In the first step, carried out at 2912 Fahrenheit (1600 degrees Celsius), TiSi2 preferentially reacts with B4C to form TiB2 and SiC. The sintering schedule consists of three minutes at this temperature, then raising it to 3272 Fahrenheit (1800 degrees Celsius) to separate the reaction-dominated and diffusion-dominated processes. 

The silicon atom released in the first step then reacts with the ZrC matrix to produce ZrSi2 and secondary SiC. The liquid-phase sintering and interdiffusion of Zr and Ti, lead to the formation of (Zr,Ti)C and (Ti,Zr)B2 solid solutions.

Refined microstructures

“The lower-temperature hold prioritizes completion of the in-situ reactions, generating a high density of fine TiB2 and SiC nuclei while intentionally limiting matrix grain growth,” explained Wang in the press release.

“With these pinning phases already dispersed throughout the microstructure, the subsequent high-temperature sintering achieves full density while the nanoscale particles effectively suppress grain coarsening from the outset.” 

By adding 30 mol% TiSi2 and 15 mol% B4C, the researchers successfully obtained a refined sub-microstructure with a grain size below 500 nm. With these grain sizes, the material developed by the researchers demonstrated a flexural strength of 824 ± 46 MPa and a fracture toughness of 7.5 ± 0.5 MPa·m1/2, well above those previously reported for ZrC-based materials. 

High-resolution transmission electron microscopy also showed that the orientation of secondary SiC within the (Zr,Ti)C matrix reduces lattice mismatch and improves stress transfer. The researchers also observed a lower rate peak at higher temperatures during its two-step process, confirming that the major reactions were completed in the first step. 

“This work demonstrates that careful control of reaction sequence and thermal history can fundamentally alter the microstructure-property relationships in carbide ceramics,” concluded Wei.

The research findings were published in the Journal of Advanced Ceramics.