For hardness and stiffness, it's long been thought that nothing beats diamond. But Roderic Lakes and Don Stone, from the University of Wisconsin MRSEC and their colleagues have made a material that is almost ten times stiffer, by embedding small particles of barium titanate in a matrix of tin.
Barium titanate can adopt different crystal structures. In the composite, it is trapped in a high-temperature form at temperatures below the usual transition point. This gives the inclusions the strange property of negative stiffness, meaning that they bend in the opposite direction to an applied force. Warming the composite changes the balance between the mechanical properties of the inclusions and those of the matrix, which responds more conventionally to an applied force. At a particular temperature, these tendencies cancel each other almost exactly, and the material then scarcely deforms at all. This extreme stiffness is remarkable, given that neither of the materials involved are especially stiff on their own. The composite isn't, however, expected to be particularly hard or strong.