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The answer to this question depends on how you define “strong,” but the hardest, or least compressible, known material as of this writing (December 2007) are aggregated diamond nanorods (ADNRs), an allotrope (variety) of carbon consisting of highly compressed and interlinked nanotubes. Aggregated diamond nanorods have a bulk modulus, or hardness measure, of 491 gigapascals (GPa), whereas conventional diamond only has a modulus of 442 GPa. Aggregated diamond nanorods can scratch both diamond and ultrahard fullerite, another allotrope of carbon which was the previous record-holder for hardness.
Aggregated diamond nanorods were first synthesized by physicists in 2005 at the University of Bayreuth in Germany. The team, led by Natalia Dubrovinskaia, used a custom-designed 5000-metric-ton (5 million kilogram) anvil press on a sample consisting of conventional fullerenes (also known as buckyballs, the element C60). By compressing these buckyballs and heating them to 2500 degrees Kelvin, it was possible to create this new allotrope of carbon. The material consists of carbon nanotubes with diameters between 5 and 20 nanometers and lengths of about a micrometer each.
The physical appearance of aggregated diamond nanorods is similar to that of a metal which scatters different colors of light, giving it a slightly rainbow-like surface. That it appears like a metal is unusual because carbon’s other allotropes (soot, graphite, diamond, etc) rarely do, except for perhaps graphite.
Aggregated diamond nanorods are also denser than diamond by a factor of 0.2 – 0.4%, making them the most dense form of carbon known. A contributing factor to the material’s hardness is thought to be the random orientation of the nanorods making it up. Because the physical structure of the nanorods are a fine mesh, like Kevlar®, the material is also shatterproof, unlike diamond.
Testing has shown that using aggregated diamond nanorods-tipped tools to machine steel gives a tool piece that wears down more slowly than diamond and allows higher precision. When it becomes economically feasible to mass-produce aggregated diamond nanorods, they may indeed replace diamonds as an industrial abrasive and material for tooltips.