Densification and polymorphic transition of multiphase Y2O3 nanoparticles during spark plasma sintering

Multiphase (MP) monoclinic and cubic Y2O3 nanoparticles, 40 nm in diameter, were densified by spark plasma sintering for 5–15 min and100 MPa at 1000 °C, 1100 °C, and 1500 °C. Densification started with pressure increase at room temperature. Densification stagnated during heating compared to the high shrinkage rate in cubic single-phase reference nanopowder. The limited densification of the MP nanopowder originated from the vermicular structure (skeleton) formed during the heating. Interface controlled monoclinic to cubic polymorphic transformation above 980 °C led to the formation of large spherical cubic grains within the vermicular matrix. This resulted in the loss of the nanocrystalline character and low final density.

► Multiphase monoclinic and cubic Y2O3 nanoparticles were densified by spark plasma sintering.
► Densification stagnated compared to the high shrinkage rate in cubic single-phase nanopowder.
► Vermicular structure formed during heating limited densification of the multiphase nanopowder.
► Large cubic grains formed by polymorphic transformation led to the loss of the nano-crystallinity.