Titanium is the leading material for artificial knee and hip¹ joints² because it's strong, wear-resistant and nontoxic, but an unexpected discovery by Rice University physicists³ 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 implants⁵ and replacement⁶ joints."

 

Morosan, a physicist⁴ 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 previously⁷ 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 remarkable⁸ 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 structural⁹ properties.

 

"When we tried to grind up titanium-gold, we couldn't," she recalled. "I even bought a diamond (coated) mortar¹⁰ and pestle¹¹, and we still couldn't grind it up."

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