An explanation of the high temperature creep of yttria tetragonal zirconia nanocrystals
Introduction
Over the past several years, research on ultra-fine ceramics has been motivated by the desire to improve sinterability and ductility. It is widely admitted that superplasticity can be obtained at lower temperatures or at high strain rates by decreasing grain size, so net-shaping can be available at lower temperatures. On the other hand, superplastic joining can also be improved by decreasing grain size [1]. In the last decades, the superplastic behaviour of fine-grained yttria tetragonal zirconia polycrystals (YTZP) in the submicrometric range has been extensively studied since the first publication by Wakai et al. [2]. A revision can be found in [3]. Grain boundary (gb) sliding has been considered to be the principal microscopic mechanism present during high temperature deformation of this material; however, the accommodation mechanism is still the object of discussion [4]. This is the reason why a deep insight into the deformation mechanisms and their potential applications in YTZP nanoceramics has become an attractive and dynamic research goal. It is only quite recently that fully-dense materials have been available [5], [6], so previous works on this topic are very limited. A first study on the main features of YTZP high temperature deformation was reported by Gutiérrez-Mora et al [7] and Lorenzo-Martı́n et al. [8].
In this work, results on the high temperature mechanical behaviour of YTZP nanoceramics are presented, and they are interpreted in terms of recent models reported in literature for the high temperature plasticity features of nanoceramics.
Section snippets
Creep tests
Two different sets of 1.7 mol% Y2O3-stabilized tetragonal ZrO2 polycrystals with different contents of glassy phase were used. One set was composed of YTZP samples with impurity contents lower than 0.1 wt% (named as “pure” YTZP) from now on; the impurity content in the second set was higher and could form an amorphous phase (hereby named “impure YTZP”). Chemical analysis of the impure samples revealed the glassy phase composition of 80 wt% SiO2, 15 wt% Al2O3, and 5 wt% SrO. Experimental
Results
Creep rates can be deduced from the strain (ε) versus time (t) plots recorded during compression tests, with ε=ln(l/l0) where l is the instantaneous length and lo the initial length. Strain rate () versus strain (typical creep curves) have been plotted for each set of samples.
Data were fitted to the generally-admitted constitutive high temperature creep equation in ceramics [10]:being G the shear modulus of YTZP, b the Burgers vector of dislocations in YTZP, σ the
Physical implications of the mechanical and microstructural evidences
The mechanical results and particularly the values of the stress exponents were approximately equal to 2, and the fact that no microstructural changes were observed after deformation leads to the grain boundary sliding as a primary deformation mechanism. This statement have already been verified elsewhere for submicrometric YTZP samples at high stresses and quite recently for nanometric ones [3], [8]. Despite the similarities between the submicrometric and the nanometric YTZP samples, some
Conclusions
The high temperature mechanical properties of YTZP nanoceramics with a mean grain size of 50 nm have been studied. Compression tests at constant load were performed in a range of temperatures between 1150 and 1200 °C. The deformation mechanism of YTZ nanocrystals under these compression tests is grain boundary sliding. The main features of the plastic behaviour fit into recent models proposed in the literature, in which yttria segregation at the grain boundaries is an essential issue. The
Acknowledgements
This work has been made possible by the financial support of the Spanish Ministerio de Ciencia y Tecnologı́a through the project MAT2000-0622. R. C. acknowledges the Israel Ministry of Science for the financial support.
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