Ductile ceramics set to become reality

A US research project lead by UCLA is developing methods to produce room temperature ductile ceramics. The study suggests they could be developed based on transition-metal carbides and possibly transition-metal nitrides, which have similar structures.

While the heat resistive properties of ceramic make it advantageous for a number of applications, the materials brittleness make practical application difficult. Ductile ceramics have become an increasingly sought-after class of material as they would be exceptionally hard, capable of withstanding extremely high temperatures and be less prone to corrosion compared to metals. Critically, however, they would have the ability to become dented or deformed without fracturing.

The breakthrough came when researchers were examining compounds called transition metal carbides. Here, the atoms are held together by three types of chemical bonds — ionic, covalent and metallic. The combination of the three bonds, researchers believe, is what makes these materials tough.

To validate that hypothesis, the team performed compression tests on single crystals of two transition-metal carbides, zirconium carbide and tantalum carbide, inside a transmission electron microscope. The compressive stress in the material exceeded 20GPa at room temperature.

It was here they observed that the crystals deformed — that is, they changed shape without cracking — at room temperature. This showed researchers that the crystals' size and orientation play an important role in the materials' mechanical behaviour.

"Ultra–high-temperature ceramics are highly desirable in aerospace and other industries where durable structural components are required to maintain their strength and stability at elevated temperatures," said Suneel Kodambaka, associate professor of materials science and engineering at UCLA. "Transition-metal carbides are attractive for these applications because they are very hard and do not melt until they reach very high temperatures.

"Our studies show how the crystals slip, an atomic-scale process that controls malleability. These results could help engineer the microstructures of ceramic components to tailor their mechanical properties."

Ductile ceramics could replace high-strength metallic alloys in many applications including aircraft engines and turbines as well as structural applications like nuclear power plants, and even car engines – to allow greater thermal efficiency.

The researchers also suggested that the ceramics hold promise for use in micro- and nano-electromechanical systems, and the radiation resistance of ceramic means it could also be applied to the space sector for a variety of applications including ultra thin foils for use as solar sails to propel spacecraft.

Additional research is being planned to further understand the atomic-scale mechanisms that affect plasticity in the materials.

Justin Cunningham

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