Hard Stuff: Cooked diamonds don’t dent

Popping diamonds into a high-pressure oven for a few minutes can render the famously hard minerals even harder, researchers have found. In particular, pressure-cooking a recently developed type of synthetic diamond has yielded the hardest single-crystal diamond material ever tested, claims Russell J. Hemley of the Carnegie Institution of Washington (D.C.). Single-crystal diamond has a consistent geometric order of atoms throughout, making it desirable for uses ranging from jewelry to electronics.

The new material is so hard that tools used to gauge hardness left no mark on several of the crystals, Hemley and other researchers say. In fact, the researchers broke equipment worth about $10,000 in their attempts at measurement.

Because the treated diamonds are also highly tough, or fracture-resistant, they may prove superior for many uses, Hemley and other researchers say. They suggest the material may serve as anvils for high-pressure research, coatings for cutting tools and biomedical implants, and wafers for electronics that must operate under extreme conditions.

Hemley and his colleagues at the Carnegie Institution, Los Alamos (N.M.) National Laboratory, and Phoenix Crystal Corp. in Ann Arbor, Mich., present their findings in the March Physica Status Solidi (a).

Some materials researchers are skeptical about the hardness measurements that Hemley’s team reports. Michael Popov of the Max-Planck Institute for Chemistry in Mainz, Germany, and the Russian Academy of Sciences in Moscow says that the team should have used a different, also standard, technique that some other recent hardness investigations have employed.

In the new data’s defense, Hemley notes that he and his colleagues used their measurement method on a variety of types of natural and synthetic diamonds of known hardness and obtained the expected values.

To create the diamonds that were so hard as to be unmeasurable, the group started with a technique that Hemley and another team, including some of the same Carnegie scientists, had previously developed (SN: 9/14/02, p. 165: Better-Built Diamonds: Fast growth, purity may multiply uses). In a process known as chemical-vapor deposition (CVD), the researchers rapidly deposited carbon atoms onto an ordinary diamond. They then cut the new diamond free from the substrate.

However, extraordinary hardness required another step: heating to 2,000°C under pressure like that found 150 kilometers or so beneath Earth’s surface. The new study indicates that natural diamonds of one of the two types tested also show big jumps in hardness when subjected to the heat-and-squeeze treatment.

The treatment is a type of annealing, a widely used process for modifying metals, metal alloys, and other materials. For diamonds, annealing had previously been used only to change the mineral’s color.

Tetsuo Irifune of Ehime University in Matsuyama, Japan, calls the work a “significant advance in synthesizing hard, single-crystal diamond.” He says he shares Popov’s doubts about the accuracy of the new hardness measurements. Nevertheless, “it seems clear that the new CVD diamond is harder than normal natural and conventional synthetic diamonds,” Irifune says.

“The [hardening] mechanism is not well understood and should be further explored,” he adds.

Hemley says that his team has studies under way with that goal in mind.