“It is about three to four times harder than most steels,” said Professor Emilia Morosan, the lead scientist on a new study. “It's four times harder than pure titanium, which is what's currently being used in most dental implants and replacement joints.
“This compound is not difficult to make, and it's not a new material,” she added.
In fact, the atomic structure of the material, with its atoms tightly packed in a ‘cubic’ crystalline structure that's often associated with hardness, was previously known. It's not even clear that Prof 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.
“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,” Prof Morosan explained. “When we tried to grind up titanium-gold, we couldn't. I even bought a diamond (coated) mortar and pestle, and we still couldn't grind it up."
Prof 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 an almost pure crystalline form of the beta version of the alloy. At lower temperatures, the atoms tend to arrange in another cubic structure - the alpha form of titanium-3-gold. The alpha structure is about as hard as regular titanium. It appears that labs that had previously measured the hardness of titanium-3-gold had measured samples that largely consisted of the alpha arrangement of atoms.
For biomedical implants, for example, two key measures are biocompatibility and wear resistance. Because titanium and gold by themselves are among the most biocompatible metals and are often used in medical implants, the team believed titanium-3-gold would be comparable. Tests by colleagues at the University of Texas MD Anderson Cancer Centre in Houston determined that the new alloy was even more biocompatible and wear-resistant than pure titanium.
Prof Morosan said her group is planning to conduct follow-up tests to further investigate the crystal structure of beta titanium-3-gold and to see if chemical dopants might improve its hardness further.