One of the Strongest, Lightest Metals Ever Made Is Less Dense Than Water

Here's a block of metal that is so light, its density so low, that it floats on water. It can also withstand intense pressures and appears to be ideal to use to manufacture boats.

There are, of course, several types of metals that float on water—but few of them are useful as an actual building material.

Earlier this year, we visited the laboratory of Nikhil Gupta, a researcher at NYU Polytechnic University who is designing some of the strongest materials ever devised, in conjunction with researchers at Deep Springs Technology. Gupta works with a class of material called "syntactic foam," which are composite materials that are filled with hollow particles.

A piece of one of Gupta's metals looks and feels solid. But at the edges of any given piece of material, you can see holes because the spherical particles don't always line up with the edge (it's kind of like how you can see holes in swiss cheese). These hollow particles, of course, are why the metal is so light.

When we visited Gupta's lab, he was excited about a new class of magnesium-alloy syntactic foam that always floats on water. He said it was one of the strongest metals for its weight ever developed. At the time, he didn't want to speak too much about it, because he was waiting for a paper announcing the discovery to be printed by a scientific journal.

The paper has now been accepted, and Gupta's material seems impressive indeed. It's the first metallic syntactic foam that has a density lower than 1 gram per centimeter^3, which is the same density as water. Anything lighter than that will float, as this one does.

"Such [materials] can open up buoyancy-related applications in marine vessel structures," Gupta wrote in a paper published in the International Journal of Impact Engineering.

The foam is 44 percent stronger than similar, aluminum-based foams, and each individual sphere within the foam can withstand pressure of more than 25,000 pounds per square inch before breaking, which is roughly 100 times the pressure exerted by water coming out of a firehose.

Gupta said that the material will likely be used in military boats and amphibious transports, and that it can also help make cars lighter. He suspects that it'll be ready for mass production and use in military prototypes within three years.

Surprisingly, these materials aren't all that expensive, according to Gupta—the raw materials are common, and there are many factories that make syntactic foams. The difficulty is in the basic science of creating them and evaluating their properties. Gupta's lab has all sorts of machines to bend, twist, compress, pull, and otherwise stress-test a material.

"If there's a way to break something, we can do it here," he told me.