Pushing the grain size limit of pressureless-sintered alumina nanocrystalline ceramics by non-oxidizing atmospheres

Nanocrystalline ceramics are of interest due to their refined grains and expected ductility, yet their pressureless sintering is challenging considering the large coarsening driving force and the coupled densification-coarsening kinetics. Of special interest is the pressureless sintering of alumina nanocrystalline ceramics, with fundamental significance to ceramic processing science and technology. Previously reported efforts offered dense alumina nanocrystalline ceramics with average grain size G _avg down to 34 nm and with a coarsening ratio β (defined as the ratio of average grain size after and before sintering) of 7.2 from nanoparticles to dense alumina ceramics, by synergistically optimized α-Al ₂O ₃ nanoparticles and pressureless two-step sintering in air. Here we further pushed G _avg down to 29 nm and β down to 5.8, by changing air to non-oxidizing atmosphere (e.g., N ₂, Ar, and vacuum). We identified pronounced difference in sintering kinetics in air versus in N ₂ , Ar, or vacuum, which is unexpected for wide-bandgap, redox-insensitive oxide materials of alumina. With smaller initial nanoparticles (e.g., <3.5 nm) and better optimized sintering conditions, it may be possible to pressureless-sinter dense alumina ceramics with G _avg below 20 nm.