Superplasticity in Cubic Yttria-Stabilized Zirconia with Intergranular Silica A.A

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 eú ϭ AdϪpsnexpͩ ͪ (2) phase only slightly enhanced the strain rate com- RT pared to pure 8Y-CSZ. where eú 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. These cavities were aligned paral- exponent, s is the applied stress, n is the stress lel to the direction of the compressive stress. Such exponent, Q is the activation energy for the rate cavities could not be found in the 5 wt% silica con- controlling deformation mechanism, R is the gas taining samples. Samples containing 1 wt% BS constant, and T is the absolute temperature. Eq. (2) glass also demonstrated extensive cavitation during may be utilized to calculated stress exponent (n) compression creep under high stresses. Most and activation energy (Q) for creep. The stress samples containing BS failed during compression exponent (n) may be calculated from the slope of creep and grain boundary separation was evident the line obtained by plotting ln(s) vs. ln(eú). Fig. 5 from SEM examinations. is the plot of ln(s) vs. ln(eú) for 8Y-CSZ with 5 Extensive dynamic grain growth was observed wt% silica at temperature range of 1300–1500 °C. in 1 wt% silica samples during deformation. After The stress exponents calculated from the slope of ° 30 h at 1400 C, the grain size of 1 wt% silica these lines are also listed for various temperatures. samples was only 2.4 µm under static annealing conditions but 6.2 µm after 13% strain. The grain size of 5 wt% silica samples annealed under static conditions at 1400 °C increased by a factor of two in 10 h. In 5 wt% samples deformed at 1400 °C, a twofold increase in grain size was complete in 1 h. The samples containing 5 wt% silica did not grow larger than 1.4 µm in size during deformatio- n. Most samples containing 5 wt% silica were cap- able of undergoing at least 200% true strain in compression without failure. Fig. 4 shows an example of uniform deformation observed. At 1500 °C and 45 MPa, samples could easily undergo over 500% engineering strain in 1 h. Strain rate during high temperature deformation Fig. 5. High-temperature deformation of 8Y-CSZ+5 wt% sil- may be represented by: ica at 1300–1500 °C and calculated stress exponents. A.A. Sharif, M.L. Mecartney / Acta Materialia 51 (2003) 1633Ð1639 1637

The activation energy for creep is calculated viscosity phases with a lower solubility act as a from the slope of the line obtained by plotting barrier against diffusion. Grain growth enhance- ln(eú) vs. 1/T at a constant stress. The values of the ment in 8Y-CSZ occurs in the presence of silicate activation energies calculated here are listed in Fig. intergranular phases that contain low valency cat- 6. An increase in activation energy was observed ions, i.e. Li1+ and Ba2+, that are network modifiers with increasing stress with values of activation and tend to break open the network and lower vis- energy increasing from 341 kJ/mol at 10 MPa to cosity and increase diffusivity [16]. Higher valency 411 kJ/mol at 45 MPa. cations, in contrast, tend to be network formers, In order to determine if an increase in the grain which result in a closer network and consequently size during dynamic grain growth of 5 wt% silica higher viscosity, lower diffusion rates, and lower samples would result in strain hardening, a com- solubility. ° parative study was conducted at 1400 C. First While borosilicate glass has high viscosity [20] samples were deformed slowly at low stresses and low solubility (4 mol% and 1 mol% for zir- causing some strain induced grain growth, fol- conia and yttria, respectively [16]), pure silica has lowed by a jump test at higher stresses. Samples an even lower solubility of 1.4 mol% and Ͻ0.1 µ with an initial grain size of 0.5 m were also mol% for the same phases [23]. These values of directly deformed at the high stresses. Similar solubility were all measured at room temperature strain rates were observed for both samples at and it should be noted that solubility may be higher higher stresses. Despite the presence of some grain at high temperatures. Silica also has a higher glass growth, no resultant strain hardening could be transition temperature and a higher viscosity than detected. the borosilicate glass at the deformation tempera- tures [20,21]. Due to the lower solubility of zir- conia and yttria in silica and higher viscosity of 4. Discussion silica (and slower diffusion), grain growth is 4.1. Grain growth expected to be slower in silica containing ceramics than those containing borosilicate, a premise con- While low viscosity, high solubility, amorphous firmed by Fig. 2. In addition, if an intergranular intergranular phases provide an easy diffusion path liquid phase wets the grain boundaries at high tem- for ceramic components during grain growth, high perature, the presence of an intergranular second phase must lower the interfacial energy of the grain boundaries and this will also result in grain size control by reducing the driving force for grain growth. Relatively insignificant grain growth was observed in 5 wt% silica containing samples when compared to samples of pure 8Y-CSZ, however dynamic grain growth did occur. Hence, the postu- lation that dynamic grain growth does not take place in the presence of glassy phases [18] is not valid for this system. Despite some dynamic grain growth, however, strain hardening due to grain growth was not observed even in samples deformed at higher temperatures. The fact that grain size of these samples never grew beyond 1.4 µ Fig. 6. Arrhenius type plot for calculating activation energies m, may explain the lack of any significant strain for high temperature deformation of 8Y-CSZ+5 wt% silica hardening in this ceramic. under 10–45 MPa compressive stress. 1638 A.A. Sharif, M.L. Mecartney / Acta Materialia 51 (2003) 1633Ð1639

4.2. High temperature deformation increasing grain size (m = 1/n, hence, m decreases with increasing grain size). In addition to controlling grain size, the presence Values of activation energy for creep of Y-CSZ of an amorphous glassy phase may enhance grain with 5 wt% silica calculated from Fig. 6 are in the boundary sliding (GSB) and rotation. An increase range of 341–411 kJ/mol. Thes values are similar in the strain rate during high-temperature defor- to the activation energies for lattice diffusion of mation of silica containing samples compared to Zr4+ and Y3+ ions, 391 and 423 kJ/mol, respect- pure samples is in part due to an increase in the ively [27], and the activation energy for the grain ease of GBS, with an increase in the amount of growth of pure 8Y-CSZ that is in the range of 300– silica resulting in an improved strain rate. The 400 kJ/mol [11,16]. In fine-grained ceramics, it has decrease in the stress exponent by addition of silica been proposed that superplasticity occurs with shown in Fig. 3 confirms an increased resistance GBS accommodated by a diffusional mechanism to flow localization in the presence of silica glass to inhibit cavitation [26]. One model for the rate compared to BS glass and pure samples. controlling mechanism would be an accommodat- The inverse of stress exponent (n) is called strain ing diffusional mechanism with an activation rate sensitivity exponent (m) and is a measure of energy in the range of 340–410 kJ/mol. ability of the material to undergo elongation with- These values for activation energy of defor- out neck formation. Superplasticity is generally mation for Y-CSZ with 5 wt% silica are lower than associated with m Ͼ 0.4 [18,25]. The value of m those values reported for Y-TZP, Y-CSZ, and varies from zero to unity. A small value of m is related composites, whose values range from 500– indicative of easy neck formation during tensile 700 kJ/mol. Kajihara et al. [23], however, found deformation, hence, lack of superplasticity. In con- that there was a transition temperature of 1380 °C trast, values of m close to 1 indicates resistance to above which there was a decrease in the activation flow localization and resistance to neck formation energy for superplastic deformation of 2.5Y-TZP during tensile deformation, hence, the material is containing 5 wt% silica. 2.5Y-TZP samples with- able to undergo large tensile deformations. Since out added silica had activation energies in the neck formation is the onset of failure during tensile range of 530 kJ/mol throughout the temperature deformation, for superplastic deformation it is range. However, for 2.5Y-TZP with 5 wt% silica desirable to have an m value close to unity. The added, at temperatures below 1380 °C an acti- values of m calculated from the inverse of n in Fig. vation energy of 635 kJ/mol was obtained while 5 range from 0.77 at 1300 °C to 0.59 at 1500 °C. an activation energy of 182 kJ/mol was found at The decrease in the value of m with increasing high temperatures. This change in the value of acti- temperature observed here may be due to a change vation energy was postulated to be the result of a in viscosity of the grain boundary phase and grain change in viscosity of the supercooled liquid silica size of the ceramic with increasing temperature. at 1380 °C. The present investigations could not Without a secondary phase, m remains constant or substantiate a change in deformation mechanism of increases with increasing temperature since there silica containing Y-CSZ at the temperature range is an increase in resistance to flow localization with of 1300–1500 °C; more data would be required to increasing temperature [16]. With viscous inter- substantiate such transition. granular phases, as the viscosity of the grain The activation energy for superplastic defor- boundary phase is decreased at higher tempera- mation increased with increasing stress as shown tures, the cohesion strength between the grains is in Fig. 6. At higher stresses, the rate of plastic decreased resulting in easy cavitation. Similarly, as deformation is higher than those of lower stresses. the grain size is increased, there is a tendency to However, since the rate controlling mechanism is exhibit flow localization and that results in smaller diffusion-dependent, viz. time dependent, higher values of m. Noting the grain sizes in Fig. 2 for strain rates do not allow for diffusion to catch up. compositions listed in Fig. 3, it is evident that the This results in an enhanced cavitation and degra- magnitude of the stress exponent n increases with dation of superplastic deformation. A.A. Sharif, M.L. Mecartney / Acta Materialia 51 (2003) 1633Ð1639 1639

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