A diffusion approach for plasma synthesis of superhard tantalum borides
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FOCUS ISSUE
THE SCIENCE AND TECHNOLOGY OF VAPOR PHASE PROCESSING AND MODIFICATION OF SURFACES
A diffusion approach for plasma synthesis of superhard tantalum borides Aaditya Rau1, Kallol Chakabarty2, William Gullion3, Paul A. Baker2, Ilias Bikmukhametov4, Richard L. Martens5, Gregory B. Thompson4, Shane A. Catledge2,a) 1
Department of Mechanical Engineering, Whiting School of Engineering, The Johns Hopkins University, Baltimore, Maryland 21218-2682, USA Department of Physics, Center for Nanomaterials and Biointegration (CNMB), The University of Alabama at Birmingham, Birmingham, Alabama 35394-1170, USA 3 Department of Physics, Brigham Young University—Idaho, Rexburg, Idaho 83460, USA 4 Department of Metallurgical & Materials Engineering, The University of Alabama, Tuscaloosa, Alabama 35487, USA 5 Alabama Analytical Research Center, Tuscaloosa, Alabama 35487, USA a) Address all correspondence to this author. e-mail: [email protected] 2
Received: 31 July 2019; accepted: 8 November 2019
Microwave plasma chemical vapor deposition (MPCVD) was used to diffuse boron into tantalum using plasma initiated from a feedgas mixture containing hydrogen and diborane. The role of substrate temperature and substrate bias in influencing surface chemical structure and hardness was investigated. X-ray diffraction shows that increased temperature results in increased TaB2 formation (relative to TaB) along with increased strain in the tantalum body-centered cubic lattice. Once the strained tantalum becomes locally supersaturated with boron, TaB and TaB2 precipitate. Additional boron remains in a solid solution within the tantalum. The combination of precipitation and solid solution hardening along with boron-induced lattice strain may help explain the 40 GPa average hardness measured by nanoindentation. Application of negative substrate bias did not further increase the hardness, possibly due to etching from increased ion bombardment. These results show that MPCVD is a viable method for synthesis of superhard borides based on plasma-assisted diffusion.
Introduction Metal borides have long been a subject of interest due to their desirable properties [1], such as increased hardness, wear resistance, and chemical stability [2]. Of these compounds, borides of tantalum have come into the focus due to their excellence in these properties [3]. Specifically, these properties include high hardness, high mechanical strength and wear resistance, high melting point, chemical stability, and high electrical and thermal conductivity. As a result, tantalum borides are a good candidate in high-temperature mechanical applications, in application as hard refractory materials and as conducting components with good wear and corrosion resistance [4]. A wide variety of techniques have been used to synthesize tantalum borides, including traditional powder or pack boriding [5], high temperature/pressure compression in diamond anvil cells [6, 7], and chemical vapor deposition (CVD) using tantalum-based precursors [8]. However, these methods are not without dra
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