Si containing transition metal diborides (TM-Si-B2±z) – Advanced ternary compounds for high temperature oxidative environments

Abstract

In the exploration of novel MAB phases owning unusual and unique properties, hexagonal transition metal diborides often constitute the basis for developments of these atomically layered arrangements – obtaining in the simplest form at least 2-dimensional boron hexagon sheets. With this contribution, we want to introduce ternary Si containing transition metal diborides (TM = Cr, Hf) as promising compounds for extreme conditions. In contrast to classical 2D solids but also well-known SiC alloyed bulk ceramics, we directly synthesized TM-Si-B2±z solid solutions by physical vapor deposition (PVD). Based on the limited kinetics during growth all TM-Si-B2±z coatings investigated were stabilized in the AlB2-type structure. Above well-defined Si contents Cr-Si-B2±z and Hf-Si-B2±z exhibit strongly retarded oxidation kinetics with kp values being below 10-11 kg2m-4s-1 at 1100 °C. Long term oxidation tests at 1200 °C confirmed the protective ability, i.e. the initially 2.60 µm thin Hf0.20Si0.23B0.57 exhibits an oxide scale thickness of only 1.5 µm after 60 h. Along with a broad set of high-resolution characterization techniques describing the phase stability (HR-TEM, APT, SIMS) as well as the fracture behaviour by micromechanical testing, the study suggests these ternary diborides as alternatives for mechanically and oxidative harsh environments.