Q-carbon harder than diamond
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Research Letter
Q-carbon harder than diamond Jagdish Narayan, Siddharth Gupta, and Anagh Bhaumik, Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695-7907, USA Ritesh Sachan, Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695-7907, USA; Materials Science Division, Army Research Office, Research Triangle Park, NC 27709, USA Filippo Cellini, Advanced Science Research Center, CUNY, New York, NY 10031, USA; CUNY Graduate Center, Ph.D. Program in Physics, New York, NY 10031, USA Elisa Riedo, Advanced Science Research Center, CUNY, New York, NY 10031, USA; CUNY Graduate Center, Ph.D. Program in Physics, New York, NY 10031, USA; Department of Physics, CUNY-City College of New York, New York, NY 10031, USA Address all correspondence to Jagdish Narayan at [email protected] (Received 24 January 2018; accepted 27 February 2018)
Abstract A new phase of carbon named Q-carbon is found to be over 40% harder than diamond. This phase is formed by nanosecond laser melting of amorphous carbon and rapid quenching from the super-undercooled state. Closely packed atoms in molten metallic carbon are quenched into Q-carbon with 80–85% sp3 and the rest sp2. The number density of atoms in Q-carbon can vary from 40% to 60% higher than diamond cubic lattice, as the tetrahedra packing efficiency increases from 70% to 80%. Using this semiempirical approach, the corresponding increase in Qcarbon hardness is estimated to vary from 48% to 70% compared to diamond.
Introduction The hardness of solid-state materials has played a critical role in advancement and sustenance of human civilization. This property has assumed an even more significant role in modern cutting tools needed for applications ranging from high-speed machining to deep-sea drilling and protective coatings. Diamond is the stiffest and the hardest material known to humankind. The stiffness is determined by the bulk modulus (B = 440 GPa) and Young’s modulus (E = 1015 GPa), whereas the hardness (H ) is directly proportional to the shear modulus (μ = 506 GPa) of the diamond as H = 0.151*μ. These extraordinary properties of diamond are derived primarily from its short and strong covalent bonding. The C60 fullerites under ultrahigh pressures with intramolecular distances approaching C–C bonds have been predicted to be stiffer than diamond.[1] However, the empirical modeling[1] multiplied incorrectly in-plane stress with out-ofplane strain to arrive at pressure, relating to B. They also assumed unusually large strain to arrive at the surface tension and bulk modulus of C60, and ignored Poisson’s effect in arriving at bulk modulus harder than diamond. This is tantamount to assuming Poisson’s ratio of 0.5, which is ∼500% higher than the accepted values for diamond. Furthermore, the C–C bonds in these fullerites need to be considerably shorter than C–C bond of diamond (0.154 nm sp3 bonds) for showing over 100% increase in stiffness.[2] Taking 0.143 nm for sp2 bond length, the increase in bulk modulu
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