High-gravity activated SHS of large bulk Al₂O₃/ZrO₂ (Y₂O₃) nanocrystalline composites

Abstract

Large bulk Al₂O₃/ZrO₂ (Y₂O₃) composites were in-situ prepared by SHS under varied high-gravity from ZrO₂ + Y₂O₃powder blends with an added thermit mixture. Investigated was the effect of high gravity on the microstructure, crystal growth, and properties of synthesized materials. The XRD data suggest that high gravity did not bring about any change in the phase constitution of the composite ceramics and that the ceramic matrix was composed of α-Al₂O₃, t-ZrO₂, and m-ZrO₂. SEM and EDS data show that, with increasing level of high gravity, the morphology of the ceramic microstructures transformed from the cellular eutectics to the rodshaped colonies, and the volume fraction and aspect ratio of the rod-shaped colonies increased while the rodshaped colonies were refined. Above 200 g, the microstructures of composite ceramics developed as the randomlyorientated rod-shaped colonies with a symmetrical triangular dispersion of tetragonal ZrO₂ fibers of 300 nm in the average diameter. Relative density, hardness, flexural strength and fracture toughness simultaneously reached the highest values of 98.6%, 18.6 GPa, 1248 MPa, and 15.6 MPa m^1/2 as the maximum high-gravity level of 250 g was achieved. An increase in the relative density and hardness of the ceramics with increasing gravity level was attributed to the acceleration of gas escape from SHS melts and the elimination of shrinkage cavity in the ceramics under the action of high-gravity field. The increase in fracture toughness results from the enhancement of the coupled toughening mechanisms while the increase in flexural strength comes from the refinement of the microstructures, decrease in critical defect size, and achievement of high fracture toughness.