For decades, materials scientists have faced a persistent challenge with advanced ceramics: making them stronger often makes them more prone to fracture. This hardness-toughness trade-off is a major obstacle for materials like transition metal carbides, which are essential for high-temperature equipment, cutting tools, and wear-resistant components. While strengthening techniques like spinodal decomposition—a process that separates a material into distinct nanoscale phases—have shown promise, their full potential has been capped by the ceramics’ inherently high “stacking fault energy” (SFE), which restricts the formation and movement of dislocations that are crucial for improving toughness.

Now, a team of researchers led by Shandong University and Zhengzhou University has introduced an innovative and effective solution. Guided by first-principles calculations, they demonstrated that systematically incorporating nitrogen into a (Ti, Zr)C carbide ceramic could significantly lower its SFE. This unlocked a previously suppressed toughening mechanism, enabling a synergistic leap in both hardness and toughness.

“For a long time, making ceramics tougher often meant making them less hard, and vice versa. Our work shows a pathway to get the best of both worlds,” says Weibin Zhang, the corresponding author of the study and a professor at Shandong University. “By manipulating the material at a fundamental level—reducing the stacking fault energy—we unlocked a powerful toughening mechanism that was previously dormant. The result is a ceramic that is not only harder but also much more resistant to cracking.”

The study revealed a fascinating microstructural evolution. The nitrogen-induced low SFE, when combined with a controlled high-temperature aging treatment, first promoted the formation of numerous stacking faults. As the treatment continued, these faults transformed into active sources for dislocations, generating a dense dislocation network throughout the material—a feature rarely seen in conventional carbide ceramics.

“The key was realizing that high stacking fault energy was the bottleneck. Our calculations pointed to nitrogen as the perfect element to solve this problem,” Zhang explains. “This allowed the spinodal decomposition process to not only create strengthening interfaces but also to generate a high density of dislocations. These dislocations dissipate energy from propagating cracks, which is the fundamental source of the enhanced toughness.”

The results were striking. The team found an optimal composition—a (Ti, Zr)C ceramic with 25% nitrogen content—that, after aging, exhibited a hardness increase of about 40% and a fracture toughness increase of nearly 50% compared to the initial material. However, the researchers also noted that excessive nitrogen content or over-aging could be detrimental, causing the microstructure to coarsen and diminishing the performance gains. This highlights the importance of precise control over both composition and processing.

Looking ahead, the team plans to apply this new design principle to other advanced ceramic systems. “Our ultimate goal is to establish a new design paradigm for ultra-high-performance ceramics,” adds Zhang. “The next step is to explore whether this ‘stacking fault energy engineering’ approach can be applied to other high-entropy or complex carbide systems. We believe this strategy can create materials for even more demanding applications, such as next-generation aerospace engines and advanced machining tools.”