Synthesis of Al₂O₃/SiO₂ nano‐nano composite ceramics under high pressure and its inverse Hall–Petch behavior

Abstract

We report the synthesis of alumina/stishovite nano‐nano composite ceramics through a pressure‐induced dissociation in Al2SiO5 at a pressure of 15.6 GPa and temperatures of 1300°C‐1900°C. Stishovite is a high‐pressure polymorph of silica and the hardest known oxide at ambient conditions. The grain size of the composites increases with synthesis temperature from ~15 to ~750 nm. The composite is harder than alumina and the hardness increases with reducing grain size down to ~80 nm following a Hall–Petch relation. The maximum hardness with grain size of 81 nm is 23 ± 1 GPa. A softening with reducing grain size was observed below this grain size down to ~15 nm, which is known as inverse Hall–Petch behavior. The grain size dependence of the hardness might be explained by a composite model with a softer grain‐boundary phase.

Transparent polycrystalline nanoceramics consisting of triclinic Al2SiO5 kyanite and Al2O3 corundum

“…13 Three samples were fabricated at 10 GPa and 1200, 1400, and 1600°C, encapsulated within inert platinum. The second stage anvils were tungsten carbide cubes with a truncated edge length of 11 mm.…”

Microstructural effects on hardness and optical transparency of birefringent aluminosilicate nanoceramics

“…The latter was fabricated by using deformable punch spark plasma sintering (DP‐SPS) . In contrast, hardening and softening according to the Hall‐Petch and inverse Hall‐Petch relation were observed for spinel prepared by high‐pressure spark plasma sintering and high‐pressure high‐temperature synthesis, nanocrystalline magnesium oxide (MgO), and alumina/silica composite ceramics . These ceramics indicate that the inverse Hall‐Petch effect, where the Vickers hardness starts to decrease with decreasing grain size, mainly occurs below a range of average grain size between 30 and 100 nm.…”

“…These ceramics indicate that the inverse Hall‐Petch effect, where the Vickers hardness starts to decrease with decreasing grain size, mainly occurs below a range of average grain size between 30 and 100 nm. Based on the nanocrystalline MgO and alumina/silica composite ceramics, a model was developed describing the grain size dependence of the hardness of a statistically homogenous dense polycrystalline ceramics . This so‐called “percolative composite model” considers the crystals to be harder than the noncrystalline grain boundaries and was applied to the kyanite‐based ceramics of this study.…”

“…These anvils compressed a pressure medium consisting of a 5% Cr 2 O 3 doped octahedral MgO with edge length of 18 mm (and 14 mm for the 6‐rams LVP) with cylindrical sample and resistive heater (LaCrO 3 ) inside. Further details of the high‐ PT syntheses are described elsewhere . The sample with highest transparency was reproducible synthesized in three experiments at 10 GPa and 1200°C.…”

“…Light scattering models, based on the Rayleigh‐Gans‐Debye scattering theory, predict that polycrystalline materials consisting of randomly orientated crystals, which are optically anisotropic (birefringent), can be transparent if the grain sizes are decreased to the sub‐micrometer scale and the porosity is close to zero. Several studies demonstrate that the material hardness can simultaneously be enhanced by reducing the grain size following the Hall‐Petch relation or by fabricating composite ceramics, in which the second phase is harder than the matrix …”