Titanium is the leading material for artificial knee and hip joints because it's strong, wear-resistant and nontoxic, but an unexpected discovery by Rice University physicists shows that the gold standard for artificial joints can be improved with the addition of some actual gold.
"It is about 3-4 times harder than most steels," said Emilia Morosan, the lead scientist on a new study in Science Advances that describes the properties of a 3-to-1 mixture of titanium and gold with a specific atomic structure that imparts hardness. "It's four times harder than pure titanium, which is what's currently being used in most dental implants and replacement joints."
Morosan, a physicist who specializes in the design and synthesis of compounds with exotic electronic and magnetic properties, said the new study is "a first for me in a number of ways. This compound is not difficult to make, and it's not a new material."
In fact, the atomic structure of the material—its atoms are tightly packed in a "cubic" crystalline structure that's often associated with hardness—was previously known. It's not even clear that Morosan and former graduate student Eteri Svanidze, the study's lead co-author, were the first to make a pure sample of the ultrahard "beta" form of the compound. But due to a couple of lucky breaks, they and their co-authors are the first to document the material's remarkable properties.
"This began from my core research," said Morosan, professor of physics and astronomy, of chemistry and of materials science and nanoengineering at Rice. "We published a study not long ago on titanium-gold, a 1-to-1 ratio compound that was a magnetic material made from nonmagnetic elements. One of the things that we do when we make a new compound is try to grind it into powder for X-ray purposes. This helps with identifying the composition, the purity, the crystal structure and other structural properties.
"When we tried to grind up titanium-gold, we couldn't," she recalled. "I even bought a diamond (coated) mortar and pestle, and we still couldn't grind it up."
Morosan and Svanidze decided to do follow-up tests to determine exactly how hard the compound was, and while they were at it, they also decided to measure the hardness of the other compositions of titanium and gold that they had used as comparisons in the original study.
One of the extra compounds was a mixture of three parts titanium and one part gold that had been prepared at high temperature.
What the team didn't know at the time was that making titanium-3-gold at relatively high temperature produces...