Influence of Tantalum on phase stability and mechanical properties of WB 2
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Research Letter
Influence of Tantalum on phase stability and mechanical properties of WB2 Christoph Fuger, Vincent Moraes, and Rainer Hahn , Christian Doppler Laboratory for Application Oriented Coating Development, Institute of Materials Science and Technology, TU Wien, A-1060 Wien, Austria Hamid Bolvardi, Oerlikon Balzers, Oerlikon Surface Solutions AG, FL-9496 Balzers, Liechtenstein Peter Polcik, Plansee Composite Materials GmbH, D-86983 Lechbruck am See, Germany Helmut Riedl , and Paul Heinz Mayrhofer, Christian Doppler Laboratory for Application Oriented Coating Development, Institute of Materials Science and Technology, TU Wien, A-1060 Wien, Austria; Institute of Materials Science and Technology, TU Wien, A-1060 Wien, Austria Address all correspondence to Christoph Fuger at [email protected] (Received 30 November 2018; accepted 8 January 2019)
Abstract Based on density functional theory, we recently suggested that metastable α-WB2 is a promising candidate combining very high hardness with high toughness. These calculations further suggested that the addition of Tantalum supports the crystallization of α-structured W1−xTaxB2−z, with only minor reduction in toughness. Thus, various Ta containing WB2-based coatings have been synthesized using physical vapor deposition. With increasing Ta content, the hardness increases from ∼41 GPa (WB2) to ∼45 GPa (W0.74Ta0.26B2). In situ micromechanical cantilever bending tests exhibit fracture toughness KIC values of 3.7 to 3.0 MPa√m for increasing Ta content (single-phased up to 26 at.% Ta).
Introduction To increase the lifetime of tooling or engineering components, it is necessary to protect them from environmental impacts, chemical attacks, and adverse influences arising from hightemperature applications. One well-established way to achieve this goal is by protective coatings such as nitrides, carbides, and borides of transition metals (TM). While the production of wear- and corrosion-resistant hard materials based on nitrides[1] and carbides[2] is well-accepted, the industrial application of thin films based on borides is rather limited.[3,4] Due to the fact that transition metal borides (TMB) have highly attractive properties like chemical stability,[5] high hardness,[6] and good electrical and thermal conductivity,[7] this material class is highly interesting for application-oriented coating developments. Although, TMBs show a high stoichiometric variety,[8] especially diborides tend to have outstanding mechanical properties.[9] Representatives of the TMB2 material system that show extraordinary characteristics are, for example, TiB2 (hardness values up to 70 GPa) and MgB2 (high-temperature superconductor with a critical temperature of 39 K).[10] Latest studies on diborides show that the compounds prefer to crystallize mainly in two related hexagonal structures: AlB2 structure type (α, space group 191––P6 mmm−1) favored by early TM diborides or W2B5−x-based structure (ω, space group 194––P63 mmc−1) preferredly formed by late TMB.[11] A characteristic for the A
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