Paper SE+NS+TF-MoM6
Ab initio Guided Development of Ternary Borides: A Case Study of Ti-B-N, Ti-Zr-B, Ti-W-B, Ta-W-B, and V-W-B Systems

Monday, October 22, 2018, 10:00 am, Room 202C

Transition-metal borides are a special class of ultra-high temperature ceramics. Among these, refractory borides such as TiB₂, ZrB₂, VB₂, TaB₂, and WB₂ are attractive candidates for many applications – ranging from high temperature electrodes, cutting tools, and molten metal containment to microelectronic buffer layers – because of their thermomechanical and chemical properties, their high melting temperatures up to ~3500 ºC, and excellent high temperature strengths. However, these diborides have a comparably low fracture toughness of K_IC ~1 MPa√m (here, basically obtained by in-situ micromechanical cantilever bending tests).

How diboride materials can be designed – implementing quantum chemistry guided materials design concepts – to allow for a combination of high strength, ductility, and thermal stability, is the focus of this talk. We will use recent developments of diborides – where we applied alloying and architecture concepts (e.g., composition and/or phase modulated layers) – to explore such materials-science-based guidelines for improved properties. Especially the phase stability (with respect to chemistry and temperature) of diborides is an extremely interesting task. For example, only WB₂ (among all binary diborides, except for TcB₂) provides a G/B ratio below 0.5 (~0.34) and a positive Cauchy pressure C₁₃–C₄₄ (~73 GPa), which are typical indications for dominating non-directional bonds and thus a more ductile behavior. But WB₂provides these properties only in its metastable α-structure (AlB₂-prototype) and not for its thermodynamically stable ω-structure (WB₂-prototype). With the help of ternary diborides, such as (Ti,W)B₂ or even (Ta,W)B₂, the α-structure can be stabilized (even up to ~1200 °C). Even more important is a selective sensitivity of the α- and the ω-structure for the formation of vacancies. Especially, when using physical vapor deposition (PVD) techniques at moderate temperatures (here ~400 °C) the content of vacancies (and point defects in general) is rather high. Such defects are less penalized in the α- than in the ω-structure, allowing for growing even single-phased α-WB₂ by PVD, exhibiting hardnesses H of ~40 GPa combined with high fracture toughness of K_IC ~3 MPa√m.

With the help of superlattices, nanocolumnar and nanocomposite structures, we show that also with architectural concepts, strength (H ~45 GPa) and ductility (K_IC ~3.5 MPa√m) can be improved simultaneously.