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

Nico A. Gaida, Norimasa Nishiyama, Atsunobu Masuno, Astrid Holzheid, Hiroaki Ohfuji, Ulrich Schürmann, Christin Szillus, Eleonora Kulik, Jozef Bednarcik, Oliver Beermann, Christopher Giehl, Lorenz Kienle

Research output: Contribution to journalArticlepeer-review


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.

Original languageEnglish
Pages (from-to)323-332
Number of pages10
JournalJournal of the American Ceramic Society
Issue number1
Publication statusPublished - 2017 Jan 1
Externally publishedYes


  • alumina
  • hardness
  • nanocomposites
  • silica

ASJC Scopus subject areas

  • Ceramics and Composites
  • Materials Chemistry

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