Anisotropic mechanical properties of ultra-incompressible, hard osmium diboride
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S.H. Tolbert Department of Chemistry and Biochemistry and California NanoSystems Institute, University of California–Los Angeles, Los Angeles, California 90095
R.B. Kanerb) Department of Chemistry and Biochemistry, Department of Materials Science and California NanoSystems Institute, University of California, Los Angeles—Los Angeles, California 90095 (Received 10 December 2007; accepted 14 March 2008)
Borides of high electron density metals such as Os show promise as hard materials. Arc-melting elemental osmium and boron under an argon atmosphere produced osmium diboride (OsB2). Both a Vickers diamond microindenter and a Berkovich nanoindenter were used to measure hardness. Vickers microindentation indicates that the hardness of OsB2 increases significantly with decreasing applied load. The average hardness reaches approximately 37 GPa as the applied load is lowered to 0.245 N. The hardness is found to be highly dependent on the crystallographic orientation. For the {010} grains, along the 〈100〉 direction, the average hardness is significantly higher than that in the orthogonal 〈001〉 direction. Cracks associated with pop-in events in the nanoindentation load–displacement curves are observed in the {010} grains. The measured Young’s modulus of OsB2 is 410 ± 35 GPa, which is comparable to that obtained from first-principles calculations.
I. INTRODUCTION
A prerequisite for designing a super-hard material is a high bulk modulus so that the material does not undergo any significant volume decrease under uniform applied pressure.1 Recent experiments show that osmium is the most incompressible metal with a reported bulk modulus ranging from 395 to 462 GPa,2–4 which is comparable to that of diamond (443 GPa), the most incompressible substance known. However, the hardness values of osmium (4 GPa) and diamond (70–100 GPa) differ significantly because while diamond possesses strong, directional covalent bonding, osmium exhibits weaker, nondirectional, metallic bonding. To increase the hardness of transition metals without significantly decreasing their bulk modulus, small nonmetallic elements such as boron, carbon, and nitrogen can be incorporated into the structure to form short, strong covalent bonds.5 For osmium, this means adding boron since osmium carbide is either
Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/JMR.2008.0221 J. Mater. Res., Vol. 23, No. 6, Jun 2008
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metastable under ambient conditions or has a very narrow temperature-stability range6 and osmium nitride has only been synthesized under extreme conditions (greater than 50 GPa and 2000 K).7 In the Os–B system, there are three known osmium borides: OsB1.1, Os2B3, and OsB2. Among these, OsB2 has the highest boron to osmium ratio, so it is the most likely to be a hard material since OsB2 should contain the greatest number of Os–B bonds. The experimentally measured bulk modulus for OsB2 ranges from 365 to 395 GPa depending on the va
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