Scientists Discover Material Harder Than Diamond February 12th, 2009 By Lisa Zyga in Physics / Materials
 Adiamond ring. Scientists have calculated that wurtzite boron nitrideand lonsdaleite (hexagonal diamond) both have greater indentationstrengths than diamond. Source: English Wikipedia.
(PhysOrg.com)-- Currently, diamond is regarded to be the hardest known material inthe world. But by considering large compressive pressures underindenters, scientists have calculated that a material called wurtziteboron nitride (w-BN) has a greater indentation strength than diamond.The scientists also calculated that another material, lonsdaleite (alsocalled hexagonal diamond, since it’s made of carbon and is similar todiamond), is even stronger than w-BN and 58 percent stronger thandiamond, setting a new record.
Thisanalysis marks the first case where a material exceeds diamond instrength under the same loading conditions, explain the study’sauthors, who are from Shanghai Jiao Tong University and the Universityof Nevada, Las Vegas. The study is published in a recent issue of Physical Review Letters.
“The new finding from our results is that large normal compressivepressures under indenters can transform certain materials (such as w-BNand lonsdaleite) into new superhard structures that are harder thandiamond,” coauthor Changfeng Chen from the University of Nevada, LasVegas, told PhysOrg.com. “This is a new mechanism that can be used to design new superhard materials.”
The scientists explain that the superior strength of w-BN andlonsdaleite is due to the materials’ structural reaction tocompression. Normal compressive pressures under indenters cause thematerials to undergo a structural phase transformation into strongerstructures, conserving volume by flipping their atomic bonds. Thescientists explain that w-BN and lonsdaleite have subtle differences inthe directional arrangements of their bonds compared with diamond,which is responsible for their unique structural reaction.
Under large compressive pressures, w-BN increases its strength by78 percent compared with its strength before bond-flipping. Thescientists calculated that w-BN reaches an indentation strength of 114GPa (billions of pascals), well beyond diamond’s 97 GPa under the sameindentation conditions. In the case of lonsdaleite, the samecompression mechanism also caused bond-flipping, yielding anindentation strength of 152 GPa, which is 58 percent higher than thecorresponding value of diamond.
“Lonsdaleite is even stronger than w-BN because lonsdaleite is madeof carbon atoms and w-BN consists of boron and nitrogen atoms,” Chenexplained. “The carbon-carbon bonds in lonsdaleite are stronger thanboron-nitrogen bonds in w-BN. This is also why diamond (with a cubicstructure) is stronger than cubic boron nitride (c-BN).”
Until recently, normal compressive pressures under indenters havenot been included in the calculations of ideal shear strengths ofcrystals from first principles, but latest developments have enabledresearchers to consider their effects, resulting in surprisingdiscoveries like the one shown here. Still, experimenting with w-BN andlonsdaleite will be challenging, since both materials are difficult tosynthesize in large quantities. However, another recent study has takena promising approach to producing nanocomposites of w-BN and c-BN,which may also provide a way to synthesize nanocomposites containinglonsdaleite and diamond.
In addition, by showing the underlying atomistic mechanism that canstrengthen some materials, this work may provide new approaches fordesigning superhard materials. As Chen explained, superhard materialsthat exhibit other superior properties are highly desirable forapplications in many fields of science and technology.
“High hardness is only one important characteristic of superhardmaterials,” Chen said. “Thermal stability is another key factor sincemany superhard materials need to withstand extreme high-temperatureenvironments as cutting and drilling tools and as wear, fatigue andcorrosion resistant coatings in applications ranging from micro- andnano-electronics to space technology. For all carbon-based superhardmaterials, including diamond, their carbon atoms will react with oxygenatoms at high temperatures (at around 600°C) and become unstable. Sodesigning new, thermally more stable superhard materials is crucial forhigh-temperature applications. Moreover, since most common superhardmaterials, such as diamond and cubic-BN, are semiconductors, it ishighly desirable to design superhard materials that are conductors orsuperconductors. In addition, superhard magnetic materials are keycomponents in various recording devices.”
More information: Pan, Zicheng; Sun, Hong; Zhang, Yi; andChen, Changfeng. “Harder than Diamond: Superior Indentation Strength ofWurtzite BN and Lonsdaleite.” Physical Review Letters 102, 055503 (2009).
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