PLASTICITY OF NANOCRYSTALLINE ZIRCONIA CERAMICS AND COMPOSITES

The deformation strain rate of nanocrystalline Y-TZP shows an increase by a factor 4 if the grain size decreases from 200 to 100 nm. Real su perplastic deformation (strain rate > 10(-4) s(-1)) is observed in the se materials at relative low temperature (1100-1200 degrees C). Grain- boundary analysis indicates (partial) removal of an ultra-thin (1 nm), yttrium-rich grain boundary layer after deformation. Uniaxial pressur e-assisted sintering techniques (=sinter-forging) provide the opportun ity of large shear strains during densification. Sinter-forging experi ments on zirconia-toughened alumina (15 wt% ZrO2/85 wt% Al2O3) resulte d in a dense composite within 15 min at 1400 degrees C and 40 MPa, wit h effective shear strains up to 100%. Sinter-forging of Y-TZP and ZTA gives an increase in strength, reliability and fracture toughness. The se improvements are caused by the large shear strains that result from the removal of processing flaws. Also, the number of microcraks at th e grain boundaries and the interatomic spacing between the grains are reduced by the forging techniques, resulting in a strengthening of the grain boundaries if compared with pressureless sintering. K-1C values of 10 MPa root m are obtained for Y-TZP, while no classical stress-in duced phase transformation toughening is observed. Sinter-forged ZTA s amples showed a better wear resistance than free sintered ones. (C) 19 97 Elsevier Science Limited and Techna S.r.l.