Materials Science and Engineering A 394 (2005) 353–359 Superplasticity of zirconia–alumina–spinel nanoceramic composite by spark plasma sintering of plasma sprayed powders Xinzhang Zhou a , Dustin M. Hulbert a , Joshua D. Kuntz a , Rajendra K. Sadangi b , Vijay Shukla b , Bernard H. Kear b , Amiya K. Mukherjee a,∗ a Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616, USA b Center for Nanomaterials Research, Department of Ceramic and Materials Engineering, Rutgers University, Piscataway, NJ 08854, USA Received 23 August 2004; accepted 18 November 2004 Abstract Zirconia 3 mol% yttria–alumina–alumina magnesia spinel nanoceramic composite was synthesized by spark plasma sintering of plasma sprayed particles. For compacts sintered from high energy ball milled powders, superplasticity was observed at temperatures between 1300 and 1450 ◦ C and at strain rates between 10 -4 and 10 -2 s -1 , while for those without high energy ball milling, deformation at the same temperature and strain rate range did not show superplastic behavior. Also, the apparent activation energy (945 kJ/mol) of the high energy ball milled batch was much higher than that of the same composite processed from nanopowder mixtures (621 kJ/mol). The flow stresses were also higher at the same temperatures and strain rates. The difference may be related to the unique low angle grain boundaries in the grains that nucleated and grew from the metastable phase inside the plasma sprayed agglomerate at elevated temperatures. Such boundaries were not intrinsically easy to slide. © 2004 Elsevier B.V. All rights reserved. Keywords: Nanoceramics; Superplasticity; Plasma spray; Spark plasma sintering (SPS); Low angle grain boundary; High energy ball milling (HEBM) 1. Introduction Superplasticity was first widely studied in metals and al- loys. The constitutive relationship for superplastic deforma- tion usually takes the form of the Mukherjee–Bird–Dorn equation [1]: ˙ ε = A D 0 Gb kT  b d  p σ G  n e -Q/RT (1) in which G is the elastic shear modulus, b the Burger’s vec- tor, k the Boltzmann’s constant, T the absolute temperature, d the grain size, p the grain-size dependence coefficient, n the stress exponent, Q the activation energy, D 0 the diffu- sion coefficient and R the gas constant. The inverse of n is termed the strain rate sensitivity m. Grain-boundary sliding is generally the predominant mode of deformation during ∗ Corresponding author. Tel.: +1 530 752 1776; fax: +1 530 752 9554. E-mail address: akmukherjee@ucdavis.edu (A.K. Mukherjee). the superplastic flow. Plastic deformation by grain-boundary sliding is generally characterized by n = 2 (or m = 0.5) and an apparent activation energy that is typically either equal to that for lattice diffusion or for grain-boundary diffusion. From Eq. (1), it is clear that at a constant temperature and stress, high strain rate is more easily realized in specimens with smaller grains. With the development of ceramic pro- cessing, the particle sizes are now made increasingly smaller into the nanometer range and so is the probability of realizing increasingly finer grain sizes in dense compacts. Superplas- ticity in ceramics has also been studied since the first ob- servation of fine-structure superplasticity in yttria-stabilized tetragonal zirconia (YTZP) by Wakai et al. [2]. A number of fine-grained polycrystalline ceramics have also demonstrated superplasticity, such as YTZP [3], magnesia-doped alumina [4], and alumina reinforced YTZP [5]. Unfortunately, the su- perplastic temperatures were typically above 1450 ◦ C and the strain rates were relatively low (10 -4 s -1 or lower). Re- cently, Kim et al. [6] realized a high strain rate of 0.1 s -1 0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2004.11.037