Mechanical properties of ultrananocrystalline diamond prepared in a nitrogen-rich plasma: A theoretical study

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Jeffrey T. Paci,¹ Ted Belytschko,² and George C. Schatz¹
¹Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208-3113, USA
²Department of Mechanical Engineering, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208-3111, USA

Received 27 April 2006; revised 5 September 2006; published 13 November 2006

Weexamine the mechanical properties of ultrananocrystalline diamond (UNCD) produced byplasma-enhanced chemical vapor deposition, with a focus on thin filmscreated with high levels of nitrogen in the plasma. Amodel with several of the attributes of the corresponding experimentalUNCD is developed and its properties explored. Simulations are performedusing semiempirical quantum mechanics and density functional theory. Our resultspredict a Young's modulus of 0.69 TPa, failure strain of 0.13,and a tensile fracture stress of 61 GPa which are 66%,100%, and 61%, respectively, of those predicted for UNCD producedin the absence of nitrogen. As in the case ofUNCD produced without nitrogen in the plasma deposition, the fracturestress ( sigma _f=61 GPa) is very large compared to that observed experimentally;these indicate that the experimental specimens contain large defects andsome estimates are made of the size of these defectsusing the Griffith formula with the surface energy computed here.The effect of nitrogen on the mechanical properties of atom-wideUNCD grain boundaries is also investigated. Throughout, the accuracy ofthe various simulation methods is compared and evaluated.

URL:	http://link.aps.org/doi/10.1103/PhysRevB.74.184112
DOI:	10.1103/PhysRevB.74.184112
PACS:	62.20.Mk; 62.25.+g; 81.05.Uw 62.20.Mk Fatigue, brittleness, fracture, and cracks 62.25.+g Mechanical properties of nanoscale materials 81.05.Uw Carbon, diamond, graphite: fabrication, treatment, testing and analysis YEAR: 2006
KEYWORDS:	diamond, thin films, plasma CVD, quantum theory, density functional theory, Young's modulus, fracture, surface energy, grain boundaries

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