CERAMICS BASED ON ZIRCONIA WITH A LOW SINTERING TEMPERATURE

Powder Metallurgy Progress, Vol.14 (2014), No 3 148 CERAMICS BASED ON ZIRCONIA WITH A LOW SINTERING TEMPERATURE V. V. Smirnov, S. V Smirnov, A. I. Krylov, O. S. Antonova, M. A. Goldberg, L. I. Shvorneva, D. D. Titov, Ľ. Medvecký, A. Baikin, S. M. Barinov Abstract Ceramics based on zirconia with a low sintering temperature were studied. The additives were injected into the powders after mechanical activation in a planetary mill. The additive was based on sodium silicate in an amount of 5 wt.%. Mechanical activation decreases the sintering temperature of samples by 100 150 C, and the injection of additive allowed reaching the nonporous state at temperatures below 1350 C. The main phase of sintered samples is tetragonal zirconia with crystal size from 100 to 400 nm. The flexural strength was about 550 MPa, fracture toughness 8.1 MPa m 0.5. Keywords: biomaterials, zirconia, mechanical properties INTRODUCTION Ceramic products based on zirconia have been used as critical constructional parts in machine and aerospace industry due to high strength and fracture toughness. Also zirconia ceramics are widely used in medicine as medical instruments and implants, because of its high chemical inertness and durability [1]. To expand the range of applications, increase reliability and useful lifetime of zirconia ceramics, it is necessary to improve the mechanical properties. It is possible to increase the strength by reducing the grain size in tetragonal (t- ZrO 2 ) zirconia ceramics sintered to a dense state. To obtain fine-grained (crystal size under 1 μm) dense ceramics, two main conditions have to be fulfilled: usage of nanosized powders and minimization of the recrystallization process rate during sintering. Despite extensive studies of zirconia materials, ceramics with melt additives are almost unknown. Application of melt additives may lead to significant decrease of sintering temperature and microcrystalline or nanocrystalline material formation. The purpose of this work is a development of synthesis methods for zirconia nanosized powders and applying additives which induce liquid phase sintering and produce fine-grained dense materials with a low sintering temperature. EXPERIMENTAL Nanosized powders of ZrO 2 were prepared by two methods. The method 1 is thermal decomposition of ZrONO 3 nh 2 O by reaction (1). The method 2 is chemical precipitation from ZrOCl 2 8H 2 O aqueous solution by reaction (2) with ammonia aqueous solution as precipitator. To produce t-zro 2 phase there was an addition of YCl 3 6H 2 O to the starting reagents in amounts such as to obtain 2 and 3 mol.% Y 2 O 3. To decrease powder aggregation, polyacrylamide in ammonia aqueous solution was added (method 2). Powders Valeriy V. Smirnov, Sergey V. Smirnov, A.I. Krylov, Olga S. Antonova, Margarita A. Goldberg, L. I. Shvorneva, D. D. Titov, Alexander Baikin, Sergey M. Barinov, Baikov Institute of Metallurgy and Materials Science, Russian Academy of Sciences, Moscow, Russia Ľubomír Medvecký, Institute of Materials Research, Slovak Academy of Sciences, Košice, Slovak Republic

Powder Metallurgy Progress, Vol.14 (2014), No 3 149 produced by both methods were heated at 400 C for 1 to 100 hours. As a result, the powders became ultrafine and there was no auxiliary reaction product. Powders produced by method 1 were densely aggregated zirconia powders. The powders produced by method 2 formed larger aggregates with lower density (Fig.1). ZrONO 3 nh 2 O ZrO 2 + NO 2 + nh 2 O (1) ZrOCl 2 8H 2 O + O 2 ZrO 2 +НCl + 8H 2 O (2) Pre-study of sintering shows that method 2 powders sintered at smaller temperatures than powders from method 1. Therefore the more active method 2 powders were used for sintering. Fig.1. Powders microstructure obtained by method 1 (а), method 2 (b). 80000x magnification. After calcination the powders were mechanically activated (MA) in a planetary mill in cylindrical teflon jars using zirconia balls for 60 minutes. The cylindrical samples, after mechanical activation, samples were pressed at 100 MPa, resulting in porosity of about 60%. Porosity of the samples was determined in accordance with the procedure described in Ref. [2]. For characterization of MA influence on sintering, continuous shrinkage of ceramic samples, with size of 4 х 4 х 25 mm, was studied at a heating rate of 15 C/min on the Dilatometer DIL 402 C (Netzsch, Germany). Dilatometric results were used for calculation of the sintering activation energy by equilibration (3) Q = R tgα (3), where R universal gas constant, tgα slope ratio in the coordinate frame ln((l/lо)/dt) 1/T according to Ref. [3], L sample size shrinkage at a given temperature, L 0 - starting sample size. Sintering additive based on sodium silicate, which melted at 1050-1100 C [4], was injected in the amount of 5 wt.% to zirconia powders. Zirconia powders were pressed at 100 MPa to sample size 4 x 4 x 40 mm. and sintered in air at temperatures from 1100 to 1350 C. Phase composition was determined by Xray diffraction (XRD) analysis (Shimadzu XRD6000 diffractometer, CuKб, radiation) using JCPDS Powder Diffraction File data. Microstructural analysis was conducted by scanning electron microscopy (TeScan Vega II SBU) and transmission electron microscopy (JeolJEM-2100). The specific surface area (S BET ) of the powders was determined by low temperature nitrogen BET measurements (Micromeritics TriStar analyzer). Mechanical tests were made on an Instron 5581 tensile machine. Flexural strength σ was determined in three-point bending on samples 40x4x4 mm using the formula σ = 3R L / 2B W 2. For fracture toughness (method SENB, lateral

Powder Metallurgy Progress, Vol.14 (2014), No 3 150 incision thickness of 200 µm [5]), K 1C = 3P L a 1/2 Y/2 B W 2, where P - breaking load, mm; L - distance between brackets, mm; a - the length of the incision, mm; Y - calibration function, which depends on the ratio a/w [6]; B - width of the sample, mm; W - the height of the sample, mm. RESULTS AND DISCUSSION Studies of the powders after thermal ageing at 400 C (Fig.2) showed that increase of aging time resulted in gradual crystallization of the amorphous material into a crystalline t - ZrO 2 (100% main peak - 2 Θ = 30.269 [7]. This was expressed in the formation of a peak in the angle of ~ 30 (2Θ, deg) for 100 hours of ageing instead of a halo, which was observed for the heat-treated powder after smaller ageing times. Fig.2. XRD patterns of the powders thermally aged at 400 C for 1, 2, 20, 50 and 100 hours. 180 Surface area, m 2 /g 160 140 120 100 80 60 40 20 0 Time, h 1 5 20 50 100 Fig.3. Effect of ageing time on the specific surface area of the sample.

Powder Metallurgy Progress, Vol.14 (2014), No 3 151 After ageing, dispersion of the powders increased from 41 to 161 m 2 /g (Fig.3). This effect could be explained by the crystallization of amorphous particles with the formation of smaller crystals, and this is confirmed by XRD analysis (Fig.2). Assuming that the particles have a spherical shape with a smooth surface and similar sizes, the surface area can be related to the average particle size using the following geometrically derived equation: d = 6000 / (S BET с), where d is the average particle size (nm), с the density (g/cm 3 ) and S BET the specific surface area (m 2 /g) [8]. a) b) Fig.4. Shrinkage dependence on temperature, without MA (a), with MA (b).

Powder Metallurgy Progress, Vol.14 (2014), No 3 152 The calculations showed that the thermal ageing time reduced the size of particles from 25 nm for1 hour to 6 nm for 100 hours. Thus, as a result of thermal ageing, nanodispersed powders were formed with a high specific surface area - 160 m 2 /g and a particle size of about 6 nm. According to dilatometric studies MA powders showed a significant influence on the sintering processes. Samples after MA have faster shrinkage. After MA powders have an intensive shrinkage in the range 900-1300 C, but without MA powders have intense shrinkage in a wider range 800-1500 C (Fig.4). Activation energy of sintering was calculated for powders with the MA operation. In accord with the obtained graphs of shrinkage, portions with one type of sintering mechanism were chosen. This was expressed in the linear dependence in coordinates ln ((L/Lo)/dt) - 1/T (Fig.5) for samples with and without MA, which were selected in the temperature range 950-1200 C. Calculations showed that the powders without MA have Q = 108 kj/mol. These data match with the Q for t-zro 2 ceramic materials, sintered from nanoparticles. The activation energy of sintering of these powders is only 100-150 kj/mol, significantly lower than, for example, powders with a particle size of about 1-3 microns [3]. In the case of using MA, Q is reduced to an even lower value of 49 kj/mol. This value is more than 2 times lower compared to materials without MA. Thus MA has a significant impact on the increase of powder sintering activity. Although the main compaction of material occurs up to 1250 C (Fig.4), a dense sintered body can be achieved only at higher temperatures - 1500 C. In order to intensify the sintering, an additive was injected to the powders. Fig.5. Shrinkage rate dependence on temperature. This allowed to significantly reduce the sintering temperature down to 1150 C (Fig.6).

Powder Metallurgy Progress, Vol.14 (2014), No 3 153 a) b) c) Fig.6. Microstructure of the sintered samples at 1150 C (a) 1250 C (b), 1350 C. (c). Sintered samples were characterized by the absence of pores with a medium particle size of about 100 nm. The particles were coated with an amorphous glassy phase formed by melting of the additive. As the temperature increased, observed was the rapid growth of crystals, up to 200-250 and 300-400 nm at 1250 C and 1330 C, respectively (Fig.6 b, c). Despite the fine-crystal structure, the samples had a low flexural strength of 260 MPa for 1150 C and 230 MPa for 1350 C sintering temperatures. Low mechanical properties can be explained by the content of the low strength monoclinic (m-zro 2 ) modification, the amount of which grows with increasing temperature. For example, at 1150 C m - ZrO2 amount is about 50% and at 1400 C it increases up to 70% of the crystalline phases. In order to stabilize (reducing the m-zro 2 content), the amount of Y 2 O 3 was increased from 2 to 3 mol.%. As a result, the sintered materials which contained 100% t- ZrO 2 were obtained (Fig.7).

Powder Metallurgy Progress, Vol.14 (2014), No 3 154 Fig.7. XRD pattern of the sintered samples containing 3 mol.% Y 2 O 3. The flexural strength increased to 550 MPa and fracture toughness (K 1 c) reached the value 8.1 MPa m 1/2. Material showed nonporous homogeneous microstructure with an average crystal size of about 200 nm (Fig.8). Fig.8. The microstructure of the sintered samples containing 3 mol. % Y 2 O 3. CONCLUSIONS Powders with a high S BET 160 m 2 /g were obtained as a result of development of the thermal ageing technology. Application of MA to zirconia materials promotes intensification of the sintering process. By using a sintering additive based on sodium silicate, new nanoparticulate ceramic material based on ZrO 2 with a crystal size of 100 nm and sintered to a dense state at 1150 C was obtained.

Powder Metallurgy Progress, Vol.14 (2014), No 3 155 Acknowledgements This work is supported by the Russian Foundation for Basic Research, Grant No. 14-08-00575а. REFERENCES [1] Barry, CC., Grant, NM.: Ceramic Materials: Science and Engineering. Springer, 2007. 16 p. [2] Lukin, ES., Andrianov, NT. In: Technical analysis and control of ceramic manufacturing. Moscow : Stroyizdat, 1986, p. 272 [3] Bakunov, VS., Belyakov, AV., Lukin, ES., Shayahmetov, US. In: Oxide ceramics and refractories. Sintering and creep. Ed. V.S. Bakunov. Moscow : RCTU, 2007, p. 584 [4] Kim, SS., Sanders, TH.: J. Am. Ceram. Soc., vol. 74, 1991, no. 8, p. 1833 [5] Barinov, SM., Shevchenko, VY. In: Strength of technical ceramic. Moscow : Nauka, 1996, p. 159 [6] Srowley, JE. In: Fracture. Vol. 4. Moscow : Mashinostroenie, 1977, p. 47 [7] Máleka, J., Beneša, L., Mitsuhashi, T.: J. Powder Diffraction, vol. 12, 1997, no. 2, 1997, p. 96 [8] Curran, DJ., Fleming, TJ., Towler, MR., Hampshire, S.: J Mater Sci: Mater Med, vol. 21, 2010, p. 1109