Affordable spark plasma sintering of superhard Al-doped ZrB2 nanoceramics from ZrH2, B, and Al mechanically activated by shaker milling

Abstract

A route, based on the short-time shaker milling of a 16ZrH2+29B+1Al powder mixture followed by its low-temperature reactive spark plasma sintering (SPS), is presented for the affordable fabrication of novel superhard ZrB2 ceramics. Firstly, it is shown that just 1 h of shaker milling intimately mixes and significantly refines the ZrH2, B, and Al particles, mechanically activating the powder mixture. Secondly, it is shown that the shaker-milled 16ZrH2+29B+1Al powder mixture is very sinterable and can be optimally densified by reactive SPS at only 1350 °C, one of the lowest sintering temperatures ever reported for ZrB2, resulting in unusual fully-dense superhard (27 GPa) Al-doped ZrB2 nanoceramics. This exciting super-hardness is due to both Hall-Petch hardening (or grain-boundary hardening, because the ZrB2 grains are nanometric) and solid-solution hardening (because the ZrB2 nanograins are doped with Al). SPS below 1350 °C leads to much softer, non-dense ZrB2 nanomaterials (ZrB2–Zr3Al2 nanocomposites for SPS at 1100 °C and 1150 °C but Al-doped ZrB2 nanoceramics for SPS at 1200°C-1300 °C), and SPS above 1350 °C to increasingly softer, fully-dense ZrB2 materials (Al-doped ZrB2 ceramics for SPS at 1400 °C but ZrB2–Zr3Al2 composites for SPS at 1450 °C and 1500 °C) with bimodal microstructures increasingly dominated by micrometre-sized grains. It is also shown that neither shaker milling nor reactive SPS alone is sufficiently effective, but their synergic combination is. Finally, it is postulated that the attributes and fabrication affordability of these novel Al-doped ZrB2 nanoceramics make them potentially very appealing for structural applications.