As the hardest substance known, diamond is ideal for cutting rock and other tough stuff. But diamond is costly, and it degrades when machining steel and other ferrous metals because of reactions that create softer iron carbides. For cutting steel, the first choice is cubic boron nitride, which is almost as hard, resisting 40 to 50 gigapascals (GPa) of pressure compared with diamond’s 70 to 100 GPa. But manufacturing the substance requires high temperatures (1,500 degrees Celsius) and extreme pressures (5 GPa), which make it expensive.
A low-pressure method may provide a less expensive route to ultrahard materials. “We wanted to design new hard materials, rather than finding them using trial-and-error methods,” says physical chemist Sarah Tolbert of the University of California, Los Angeles. The hardness of diamond and boron nitride arises because of short, covalent bonds that keep the constituent atoms strongly stuck together and rigid in all directions. certain “ultraincompressible” metals are rigid in two dimensions: they resist squeezing—the electrons inside them strongly repel one another—but they are relatively soft because their atoms are arranged in layers that can slide past one another. Tolbert and her colleagues found a way to harden these metals by introducing another element that covalently bonds to these layers, preventing them from slipping without disrupting incompressibility.