The answer to this question depends on how you define "strong", but at the time of writing (December 2007), the known material that is difficult or incompressible is aggregated diamond nanorods (ADNR), which is composed of a variety of carbon Allotropes (carbon) compress and interconnect nanotubes. The aggregated diamond nanorods have a total modulus or hardness measurement of 491 gigapascals (GPa), while conventional diamond has only 442 GPa. The aggregated diamond nanorods will scratch the diamond and the superhard fullerite, which is another allotrope of carbon and used to be a record holder of hardness.
The aggregated diamond nanorods were first synthesized by physicists at Bayreuth University in Germany in 2005. The research team, led by Natalia Dubrovinskaia, used a custom-designed 5,000 metric tons on samples composed of conventional fullerenes (also known as buckyballs, element C60) ( 5 million kg) anvil press. By compressing these buckyballs and heating them to 2500 Kelvin, this new carbon allotrope can be created. The material consists of carbon nanotubes with a diameter between 5 and 20 nanometers and a length of approximately one micrometer, respectively.
The physical appearance of the aggregated diamond nanorods is similar to the appearance of metals that scatter different colors of light, making its surface slightly rainbow-like. It looks unusual like metal, because apart from graphite, other allotropes of carbon (soot, graphite, diamond, etc.) rarely do so.
The density of the aggregated diamond nanorods is also 0.2-0.4% higher than that of diamond, making it a known high-density carbon. It is believed that one factor affecting the hardness of the material is the random orientation of the nanorods that constitute it. Because the physical structure of nanorods is a fine mesh like Kevlar®, unlike diamond, this material is also shatterproof.
Tests have shown that the use of aggregate tools embedded with diamond nanorods to machine steel can cause tool parts to wear more slowly than diamond and have higher accuracy. When it is economically feasible to mass-produce aggregated diamond nanorods, they can indeed replace diamond as an industrial abrasive and tool tip material.
