Superplasticity of CuO-doped ZrO2(Y2O3)

Materials Science and Engineering, A 168 (1993) 45-47

45

Superplasticity of CuO-doped ZrO2(Y203) W. Hendrix, S. Kuypers, J. Vangrunderbeek, J. Luyten and W. Vandermeulen Materials Division, V.I.T. 0., Vlaamse Instelling voor Technologisch Onderzoek, B-2400 Mol (Belgium)

Abstract Low temperature superplastic behaviour (1100 °C) was demonstrated in creep compression tests of three different CuOdoped yttria stabilized tetragonal zirconia polycrystalline (YTZP) materials, synthesized using commercially available powders. The microstructures obtained were analysed by different characterization techniques such as ceramography, impedance spectroscopy, transmission electron microscopy and X-ray diffraction. Although some observations point to the formation of a thin liquid phase which could explain the low temperature superplasticity of these CuO-doped YTZP materials, such a phase could not be detected.

1. Introduction The recent discovery of superplasticity in ceramics by Wakai et al. [1, 2] opens the way to net-shape superplastic forming of these materials. The practical application of such a process requires a high deformation rate at a relatively low temperature. The aim of the present study was the development of a material which would satisfy these requirements. Previous experiments [3] have shown that ZrOz-based materials are the most suitable for modification for low temperature superplasticity. Their grain boundaries have good cohesive strength and grain growth at the superplastic deformation temperature is rather limited. In analogy with the work of Chen et al. [4] in a previous investigation, yttria stabilized tetragonal zirconia polycrystalline (YTZP) powders doped with 0.3 mol% CuO were sintered and tested for superplastic behaviour. Figure 1 shows that, as a result of the addition of CuO, the compression creep rate under a load of 40 MPa in the temperature range 1100-1200°C is increased by two orders of magnitude [5]. In the present contribution we attempt to clarify the mechanism of this low temperature superplasticity.

diffraction (XRD) and ceramography. Table 1 summarizes the properties of the ceramic materials used in this study. The grain size was estimated from SEM observations of fracture surfaces.

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TABLE 1. Characteristics of the ceramics Composition

Density (% TD)

Grain size (#m)

Crystal structure (XRD)

3Y-TZP, Lonza,

96.3

0.3

Tetragonal

3Y-TZP, Tosoh, + 0.3 mol% CuO

97.7

0.3

Tetragonal (f.c.c., Mon)

2.5Y-TZP, Tioxide, + ().3 mol% CuO

94.9

0.3

Tetragonal (f.c.c., Mon)

2. Experimental procedure Fine ZrO2 (Y203) powders with 2.5-3 mol% Y203 were supplied by the companies Tioxide, Lonza and Tosoh. These base powders were mixed with 0.3 mol% CuO, uniaxially cold pressed and sintered at 1250 °C. After sintering, the ceramics were characterized using scanning and transmission electron microscopies (SEM, TEM), impedance spectroscopy (IS), X-ray

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© 1993 - Elsevier Sequoia. All rights reserved

46

I Superplasticity of CuO doped YTZP

W. Hendrix et aL

The superplastic behaviour was tested by compression creep tests under constant load on pellets (7 mm in diameter and 7 or 13.7 mm in height) at temperatures ranging from 1000°C to 1400°C. The activation energy of deformation was determined from the strain rate between 3% and 5% strain. The sintering behaviour at 1250 °C of materials with up to 1 mol% CuO was also studied in order to improve the superplastic behaviour. However, no characterization tests have as yet been performed.

3. Results and discussion Figure 2 shows the creep rate v s . stress for different materials and test temperatures. It can be seen that the effect of different starting powders on the creep rate is generally within one order of magnitude. The effect of temperature on the creep rate can be described by an activation energy. At a stress of 40 MPa, values of 480, 500 and 490 kJ mol -t are obtained for the three starting materials (Lonza, Tosoh and Tioxide). These values are of the same order as those found by Hwang and Chen [6] for their 2 Y T Z P + CuO ceramic (445 kJ mol-~). These authors proposed Coble creep and grain-boundary-phase enhanced diffusional creep, and liquid-enhanced interface creep, as the dominant deformation mechanisms below and above the eutectic temperature (1130 °C). From the present creep test results, however, no conclusions on the existence of two mechanisms can be drawn since the activation energy appears to be the same below and above the eutectic temperature. Until now, our T E M observations have not revealed a thin layer at the grain boundaries. This is contradictory to some observations reported in the literature [6]. However, IS did indicate grain boundary modifications due to the addition of CuO. The grain boundary

ion conductivity measured in CuO-containing specimens was strongly increased [7], whereas the lattice ion conductivity of the grain interior remained unchanged (Fig. 3). The sintering experiments performed on two powders with different CuO contents show that for normal heating rates, the final density decreases for CuO contents above 0.5 tool% (Fig. 4). Specimens sintered with a higher heating rate did not show such a remarkable drop in density. The lack of densification after relatively slow heating has been explained by the loss of a liquid phase during heating. At high heating rates the sintering temperature is supposed to be reached with less loss, and increased densification can then be expected. The CuO content of the present material was only 0.3 mol%, so if a liquid phase were formed it would only be present in very small amounts. It may also be the case that only CuO-rich

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W. Hendrix et al.

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Superplasticity of CuO doped YTZP

grain boundaries are formed, which would explain the IS and T E M results,

4. Conclusions (1) Although some optimization of the synthesis is needed, the low temperature superplastic behaviour of 3 Y T Z P ceramics doped with 0.3 tool% C u O can be demonstrated. (2) T h e r e are only indirect indications that a liquid film is formed around the grains at about 1130°C, enhancing the diffusion and enabling low temperature superplastic deformation.

47

are also indebted to J. Cooymans and G. Verreyt for technical assistance.

References 1 F. Wakai, S. Sakaguchi and Y. Matsuno, Adv. Ceram. Mater., 3 (1986) 259-263. 2 F. Wakai, Br. Ceram. Trans. J., 88 (1989) 205-208. 3 J. Luyten, W. Hendrix, J. Sleurs and W. Vandermeulen, Proc. Mechanics of Creep in Brittle Materials, Leicester, September 2-4, 1991. 4 I.-W. Chen and L. A. Xue, J. Am. Ceram. Soc., 73 (9)(1990)

2585-2609. 5 J. Luyten, W. Hendrix, J. Sleurs, W. Vandermeulen and A. Stalios, Proc. Ist Pacific Rim Int. Conf. on Advanced Materials and Processes, Hangzhou, June 23-27, 1992.

Acknowledgments T h e authors wish to thank the group headed by P. Diels for careful characterization of the materials. T h e y

6 C.-M. J. Hwang and I.-W. Chen, J. Am. Ceram. Soc., 73 (6) (1990) 1626-1632. 7 J. Vangrunderbeek, J. Luyten, S. Kuypers, W. Hendrix and F. De Schutter, Proc. 2nd Int. 3)'rap. on Electrochemical Impedance Spectroscopy, Santa Barbara, (¼, July 12-17, 1992.