Acta Materialia 51 (2003) 1633–1639 www.actamat-journals.com Superplasticity in cubic yttria-stabilized zirconia with intergranular silica A.A. Sharif a,∗, M.L. Mecartney b a California State University, Los Angeles, Department of Mechanical Engineering, 5151 State University Drive, Los Angeles, CA 90032-8153, USA b University of California, Irvine, Department of Chemical Engineering & Materials Science, Irvine, CA 92697-2575, USA Received 1 November 2002; received in revised form 22 November 2002; accepted 27 November 2002 Abstract The effect of amorphous silicate additions on grain growth and high-temperature deformation of 8 mol% cubic yttria stabilized zirconia was investigated. Fine-grained (0.5 µm) samples were produced by addition of 5 wt% colloidal silica. Dynamic grain growth was limited by the presence of this inert intergranular amorphous phase with low solubility for zirconia and yttria. Strain rates as high as 5 × 10Ϫ3 sϪ1 at 1500 °C were observed under compression, similar to those observed in superplastic tetragonal yttria stabilized zirconia. Over 180% true strain (505% engineering strain) could be obtained within 1 h at 1500 °C. The stress exponent for deformation was calculated to vary from 1.3–1.7 at temperatures of 1300–1500 °C, respectively. Activation energies for superplastic deformation in the range of 340–410 kJ/mol were obtained for applied stresses of 10–45 MPa. 2003 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Ceramics; High temperature deformation; Grain growth 1. Introduction ible only a decade ago. High strain rate defor- mation of ceramics at high temperatures is rapidly Superplastic deformation, with uniform and approaching speeds used during metal forging. The extensive plasticity at high temperatures, may pro- most prevalent example of high temperature super- vide a convenient and cost effective means of near plasticity is yttria stabilized tetragonal zirconia net shape forming in ceramics. Recent advances in polycrystals (Y-TZP) [1–4]. Y-TZP ceramics have the science and technology of superplastic defor- fine grain sizes (Ͻ1 µm) and abnormally sluggish mation of ceramics has moved the field close to grain growth rates during high temperature defor- the production of intricate parts which was imposs- mation; they are therefore perfect candidates for superplastic deformation. Yttria stabilized cubic zirconia (Y-CSZ) is for- ∗ Corresponding author. Tel.: +1-323-3434478; fax: +1- med at higher concentrations of yttria (Y2O3)in 323-3435004. solid solution with zirconia (ZrO2) than Y-TZP [5]. E-mail address: [email protected] (A.A. Sharif). Y-CSZ is widely used as solid electrolytes [6–8] 1359-6454/03/$30.00 2003 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. doi:10.1016/S1359-6454(02)00564-5 1634 A.A. Sharif, M.L. Mecartney / Acta Materialia 51 (2003) 1633–1639 and thermal barrier coatings [9,10]. Typically, Y- improved superplastic behavior for Y-TZP by CSZ ceramics have large grain sizes (Ͼ10 µm) and addition of silicate glassy intergranular phases due high grain growth rates [11,12]. The inherently to the ease of GBS in the presence of a liquid phase large grain sizes and extensive static and dynamic [4,20–23]. This research investigates the use of grain growth during high temperature deformation amorphous silicate intergranular phases to refine limit any possibility of superplasticity in pure Y- the grain size and promote GBS in Y-CSZ. CSZ. Microstructural design of superplastic ceramics requires an ultrafine grain size that is stable against coarsening during fabrication and deformation 2. Experimental [2,3,13,14]. Grain size refinement is one route to promoting superplasticity in metals and ceramics [2,15]. In order to achieve the high strain rates Commercially available 8Y-CSZ powder required for superplastic deformation of Y-CSZ (Tosoh, Japan) was used to prepare samples with ceramics, it is necessary to limit intrinsic grain no additives (hereafter called “pure samples”) and growth during sintering and prevent dynamic grain samples with 1 wt% (2.7 vol%) and 5 wt% (12.5 growth during high temperature deformation while vol%) colloidal silica (Nissan Chemicals, NY) and promoting grain boundary sliding (GBS). 1 wt% (2.7 vol%) borosilicate (BS) glass Previous investigations on Y-CSZ have found (Specialty Glass, FL) of composition 83.3 mol% that amorphous silicate intergranular phases could SiO2, 1.5 mol% Al2O3, 11.2 mol% B2O3, 3.6 mol% be used to limit grain growth at high temperatures Na2O3, and 0.4 mol% K2O. Right circular cylinders if these intergranular phases had a low solubility of nearly full density were made from these pow- for zirconia and yttria [16,17]. In addition, it has ders as described elsewhere [16]. For grain growth been postulated that dynamic grain growth occur- studies, samples were polished to a 0.05 µm finish, ring during deformation may not occur in ceramics ultrasonically cleaned in acetone and methanol, with glassy phases [18]. Hence, a glassy phase may and placed in a rapid-heating box furnace for be utilized to limit concurrent grain growth in 8Y- annealing times of 3, 10, 25, 50, 75, and 100 h in CSZ while enhancing GBS in the presence of a air at 1400 °C. A heating rate of 100 °C per minute viscous phase during high-temperature deformatio- was used. The samples were cooled to room tem- n. perature inside the furnace. The reported grain size The addition of appropriate intergranular phases values are the average intercept length multiplied can modify grain growth by influencing not only by 1.74 [24]. the grain boundary mobility but also the grain Samples for creep experiments were prepared boundary interfacial energy. A simple model for with a diameter of 2.5 mm and a height of 4.5 mm. grain growth can be given by [19]: High temperature deformation of these samples at the temperature range of 1300–1500 °Cwas dnϪdn ϭ 2Mg⍀t (1) o accomplished in a compression creep furnace where d is the instantaneous grain size at time t, (ATS, Inc. PA) between two SiC rods under quasi- = do is the initial grain size at time t 0, n is the constant stress conditions. Assuming a uniform grain growth exponent, M is the mobility, g is the diameter throughout the samples, increase in the grain boundary energy, and ⍀ is the atomic vol- cross sectional area was calculated for increments ume. It can be seen that the grain growth rate can of decrease in sample height. A chart was prepared be limited by reducing the mobility (M) and the correlating the extensometer reading to stress. This grain boundary energy (g). In addition, an inter- chart was used during the experiment to keep the granular second phase may also enhance superplas- stress constant by increasing the load as the sample ticity by increasing the resistance to cavity diameter increased during creep testing. For all nucleation and enhancing grain-boundary sliding experiments, the creep furnace reached the testing and rotation. In general, studies have found an temperatures in 3 h. A.A. Sharif, M.L. Mecartney / Acta Materialia 51 (2003) 1633–1639 1635 3. Results Fig. 1 shows the comparison of the initial micro- structures of (a) pure 8Y-CSZ, (b) 8Y-CSZ with 1 wt% BS glass, (c) 8Y-CSZ with 1 wt% silica, and (d) 8Y-CSZ with 5 wt% silica. The initial grain size of pure 8Y-CSZ was around 3 µm. The grain size of 1 wt% BS samples was 3.2 µm whereas a submicron grain size of 0.8 µm was obtained for 1 wt% silica samples and a grain size of 0.5 µm was obtained for 5 wt% silica samples. In all cases, the intergranular glass phase was not dispersed uni- Fig. 2. Comparison of the grain growth of pure 8Y-CSZ to those containing 1 wt% borosilicate glass, 1 wt% silica, and 5 formly along the grain boundaries but appeared to ° agglomerate at multiple grain junctions. wt% silica at 1400 C. Annealing the samples at 1400 °C for 100 h pro- duced pure 8Y-CSZ samples with a grain size of 12 µm. With the same heat treatment the grain size wt% silica samples is compared to those of pure of 1 wt% BS glass increased to 8 µm, the grain 8Y-CSZ and 1 wt% borosilicate glass containing size of 1 wt% silica samples increased to 3 µm, samples in Fig. 3. Strain rates for the 5 wt% silica but the grain size of 5 wt% silica samples was only samples were about an order of magnitude greater 1.9 µm (Fig. 2). A higher amount of pure silica than those obtained in 1 wt% silica samples and was more effective in limiting grain growth. two orders of magnitude greater than those in pure High temperature deformation of 1 wt% and 5 8Y-CSZ. The presence of 1 wt% of the borosilicate Fig. 1. Comparison of the initial microstructures (a) pure 8Y-CSZ, (b) 8Y-CSZ with 1 wt% borosilicate glass, (c) 8Y-CSZ with 1 wt% colloidal silica, and (d) 8Y-CSZ with 5 wt% colloidal silica. 1636 A.A. Sharif, M.L. Mecartney / Acta Materialia 51 (2003) 1633–1639 Fig. 3. Comparison of the steady state strain rates of various Fig. 4. Samples of 8Y-CSZ+5 wt% silica before deformation ° samples at 1400 C. and after deformation at 1450 °C. ϪQ e˙ ϭ AdϪpsnexpͩ ͪ (2) phase only slightly enhanced the strain rate com- RT pared to pure 8Y-CSZ. where e˙ denotes steady state strain rate, A is a con- Cavities were observed in the 1 wt% silica con- stant, d is the grain size, p is the inverse grain-size taining samples.