Materials Science Forum Vol. 694 (2011) pp 304-308 Online available since 2011/Jul/27 at www.scientific.net © (2011) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/MSF.694.304
Pressureless Sintering of Electro-Conductive Zirconia Composites Mahdi Amiriyan1,a, Ramesh Singh2,b, Iis Sopyan3,c, Meenaloshini Satgunam1,d, Ranna Tolouei1,e, Teng Wan Dung4,f 1
Ceramics Technology Laboratory, University Tenaga Nasional, Kajang, Selangor, Malaysia. 2
Centre of Advanced Manufacturing & Materials Processing (AMMP), Department of Engineering Design and Manufacture, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia 3
Department of Manufacturing and Materials Engineering, Faculty of Engineering, International Islamic University Malaysia (IIUM), Kuala Lumpur, 50728 Malaysia 4
Ceramics Technology Group, SIRIM Berhad, 40911 Shah Alam, Selangor, Malaysia a
d
[email protected],
[email protected],
[email protected],
[email protected],
[email protected],
[email protected]
Keywords: Pressureless sintering, Densification, Yttria-stabilized tetragonal zirconia, zirconium diboride, electrical conductivity.
Abstract. In the present work, 3 mol% Yttria-stabilized tetragonal zirconia (Y-TZP) composite containing 25 wt.% of zirconium diboride (ZrB2) was prepared via pressureless sintering method in an inert atmosphere over the temperature range of 1350-1550°C for one hour. The effect of zirconium diboride content in the zirconia matrix, as well as the sintering temperature on densification, phase stability and electrical properties of sintered samples have been studied. The results revealed that there was a significant increased in electrical conductivity of sintered samples when 25 wt.% of ZrB2 is incorporated into Y-TZP matrix. Introduction Yttria Stabilized Tetragonal Zirconia (Y-TZP) offers many advantages over the other advanced ceramics mainly due to its unique self-healing properties known as transformation toughening [1]. Y-TZP doped with 3 mol% yttrium oxide has exhibited excellent mechanical properties such as high fracture toughness due to the transformation toughening phenomenon [2,3]. Because of this unique property, Y-TZP has been extensively used in many engineering applications such as to produce engine parts, valves, cutting tools and moulds, sandblasting nozzles and biomedical implants [4,5]. In the last two decades, researchers in ceramics fields focused on reduction of manufacturing cost to produce ceramics parts. Machining of advanced ceramics such as Y-TZP ceramics normally accounts for the major high cost in producing the parts. For instance, grinding and cutting by using diamond tooling are common machining methods for ceramics, but is often very expensive and time-consuming. In addition, fabrication of a very complicated ceramic item using diamond grinding route is almost impossible. Development of electro-conductive ceramics (at room temperature) would be advantageous as components could be fabricated to near-net shape with wire electro discharge machining (WEDM) [6]. WEDM can be successfully applied to ceramics provided the electrical resistivity of the material is lower than 100-300 Ω.cm [6]. Various secondary electro-conductive phases such as WC[7], TiC[8], TiCN[8], ZrB2[9] and TiB2[10] have been used to prepare electro-conductive zirconia. One of the most interesting combinations is Y-TZP with zirconium diboride (ZrB2). Zirconium diboride is a non-oxide ceramic with special properties such as high electrical conductivity at room temperature and high Vickers hardness [9].
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Thus, the aim of the present work is to evaluate the effects of incorporating 25 wt.% ZrB2 on phase stability, density, stiffness and electrical conductivity (electrical resistivity) of 3mol% Y-TZP matrix. Experimental Procedures The 3 mol% yttria-stabilized zirconia starting powder used in this work was manufactured by Kyoritsu Ltd., Japan under the code name of KZ-3YF according to the hydrolysis method and spray dried method to obtained free-flowing ready-to press powder. The zirconium diboride used in this work was obtained from a commercial available ZrB2 powder (99% purity, Wako, Japan) and amount of that used was 25 wt%. The 3Y-TZP and ZrB2 were mixed in 150 mls. ethanol via ultrasonification followed by ball milling for 1 hour using zirconia balls as milling media. After the mixing, the slurry was dried, crushed and sieved into powder form. The obtained composite powder and undoped Y-TZP powder were uniaxial pressed at 2.5-3.0 MPa into circular discs (20 mm diameter and 3 mm thickness) and rectangular bars (4 × 13 × 32 mm) and subsequently were cold isostatic pressed at 200 MPa (Riken Seiki, Japan). Compacted green samples were sintered at temperature ranging from 1350-1550°C in a tube furnace with the standard heating and cooling rate of 5 °C/min and soaking time of 1 hour in argon flow. Phase analysis by X-ray diffraction (XRD) (Shimadzu, Japan) of the sintered samples was carried out under ambient conditions using Cu Kα as the radiation source operating at 35 kV in step mode with a 0.02° 2θ step and a count time of 0.5 s per step over the 2θ range 10–80° which covers the monoclinic (m-ZrO2), tetragonal (t-ZrO2) and zirconium diboride related peaks. The bulk densities were obtained by water immersion technique (Mettler Toledo, Switzerland). The elastic modulus or Young's modulus was calculated using the experimentally determined resonant frequency [11] and the values were found be consistent regardless of the number of test performed for each sample. Results and Discussion X-Ray Diffraction XRD patterns obtained from sintered monolithic Y-TZP and Y-TZP/25 wt. % ZrB2 composite at the three different temperatures are presented in Fig. 1. From XRD plots it can be seen that predominate crystalline phases was zirconium diboride (ZrB2) and/or zirconia (t-ZrO2 and m-ZrO2). For the monolithic zirconia, the detectable monoclinic phase was found to increase with increasing sintering temperature as shown in Fig. 1. In fact, 7% of m-ZrO2 phase was observed in the sample sintered at 1350°C while a larger amount of 28% was recorded when the sintering temperature increased to 1550°C. This results show that the sintering temperature plays a role in determining the extent of tetragonal to monoclinic phase transformation in the monolithic zirconia. In the case of YTZP/ZrB2 composites, there is no clear trend in the amount of m-ZrO2 formation with varying sintering temperature. Densification, Young’s Modulus and Electrical Resistivity The bulk density, relative density and Young's modulus variations with sintering temperature for undoped Y-TZP and Y-TZP/ZrB2 composites are summarized in Table 1. Although the undoped YTZP was densified to near theoretical density at temperatures below 1500°C in a normal atmosphere [12-14], in the current research it seems that argon gas has a significant effect on bulk density during sintering and hinders the densification of Y-TZP monoliths. Table 1 shows that the monolithic Y-TZP samples sintered at 1550°C in an inert atmosphere exhibited almost 98% theoretical density. The theoretical density of Y-TZP and ZrB2 were taken as 6.10 and 6.09 Mg m−3 respectively. While the samples sintered at 1350°C and 1450°C have only 91% theoretical density. In the case of Y-TZP/ZrB2 composites, an increase in bulk density has been observed as a result of increasing sintering temperature.
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It is well documented that there are two major cause for the low densification of ZrB2 containing composites i.e. the large difference in initial powder particle sizes (nanometer for Y-TZP and micrometer for ZrB2) and the lower self-diffusion coefficient of ZrB2 particles [15,16]. Nonetheless, it has been reported that full densification of Y-TZP/ZrB2 composites are possible by using hot isostatic pressed (HIP) [17, 18].
(i)
(ii)
(iii) Fig.1. X-Ray diffraction patterns of (a) monolithic Y-TZP and (b) Y-TZP/25 wt.% ZrB2 composites, sintered at (i) 1350°C, (ii) 1450°C and (iii) 1550°C. (Keys: m = monoclinic; t = tetragonal; B = ZrB2) The variation of Young's Modulus (E) of sintered samples with increasing sintering temperature revealed the beneficial effect of ZrB2 in enhancing the matrix stiffness of Y-TZP/ZrB2 composites. The E value of the undoped Y-TZP was approximately 210 GPa when sintered at 1550°C as compared to >245 GPa for Y-TZP containing 25 wt.% ZrB2 as shown in Table 1. However, as the sintering temperature was increased, the E value of the undoped ceramic started to rise slightly and reached a maximum of 210 GPa at 1550°C. In general, the Young's modulus of all the Y-TZPs studied correlated well with the sintered bulk density, i.e. the E value rose with increasing density as shown in Table 1. Chatterjee et al.[19] have reported that the effective composition range for the conductive zirconia composites is from 30 to 50 wt.% ZrB2. However, the 25 wt.% ZrB2 containing composite in the current work exhibited low electrical conductivity regardless of sintering temperature. Although the monolithic Y-TZP had high electrical resistivity at room temperature, incorporation of a secondary electro-conductive zirconium diboride phase decreased the electrical resistivity to approximately 25 Ω.cm which is reasonably enough for WEDM applications [6].
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Table 1. Bulk density, relative density and Young’s modulus of the undoped Y-TZP and Y-TZP/25 wt.% ZrB2 samples at different sintering temperatures. Undoped YTZP Sintering Bulk Temperature Density [ºC] [gm-3]
Y-TZP/25 wt.% ZrB2
Relative Density [% ρth]
Elastic Modulus [E, GPa]
Bulk Density [gm-3]
Relative Density [% ρth]
Elastic Modulus [E, GPa]
1350
5.58
91.5
184
5.47
89.7
173
1450
5.58
91.5
188
5.81
95.3
220
1550
5.94
97.5
210
5.97
97.8
246
Conclusions In the present research Y-TZP/25 wt. % ZrB2 composites were sintered at 1350-1550°C for 1 hour under argon flow. The sintered bodies exhibited relative density of more than 97% of theoretical value. XRD analysis of sintered composites indicated that tetragonal ZrO2 structure can be retained by sintering the samples at 1550°C for 1 hour. In addition, incorporation of secondary electro-conductive zirconium diboride phase has a beneficial effect on increasing zirconia matrix stiffness. The Y-TZP/ZrB2 composites sintered at three different temperatures exhibited low electrical resistivity, suitable for machining by wire electrical discharge machining (WEDM). Acknowledgment The authors would like to thank the Ministry of Science, Technology and Innovation of Malaysia (MOSTI) for providing the financial support under the Science Fund Project no. 03-02-03-SF075. References [1] R.C. Garvie, R.H.J. Hannick and R.T. Pascoe: Nature Vol. 258 (1975) pp. 703-704. [2] R. Stevens: Zirconia and Zirconia Ceramics (Magnesium Electron Ltd., UK., 1986) pp. 2843. [3] L. Gakovic, U.S. Patent 7,214,046 (2007). [4] C. Piconi, W. Burger, H.G. Richter, A. Cittadini, G. Maccauro, V. Covacci, N. Bruzzese, G.A. Ricci and E. Marmo: Biomaterials Vol. 19 (1998) pp. 1489-1494. [5] S. Deville, J. Chevalier and L. Gremillard: Biomaterials Vol. 27 (2006) pp. 2186-2192. [6] W. Konig, D. F. Dauw, G. Levy and U. Panten: Annals of the CIRP Vol. 37 (1988) pp. 623663. [7] G. Anne, S. Put, K. Vanmeensel, D. Jiang, J. Vleugels and O. Van Der Biest: J. Eur. Ceram. Soc. Vol. 25 (2005) pp. 55-63. [8] J. Vleugels and O. Van Der Biest: J. Am. Ceram. Soc. Vol. 82 (1999) pp. 2717-2720. [9] B. Basu, J. Vleugels and O. Van Der Biest: J. Alloys Comp. Vol. 334 (2002) pp. 200-204. [10] B. Basu, J. Vleugels and O. Van Der Biest: Key Eng. Mater. (2002) pp. 1177-1180. [11] ASTM E1876-97 (1998): STANDARD Test Method for Dynamic Young's Modulus, Shear Modulus and Poisson's Ratio by Impulse Excitation of Vibration, Annual Book of ASTM Standards. [12] S. Ramesh, S. Meenaloshini, C.Y. Tan, W.J. Kelvin Chew and W.D. Teng: Ceram. Inter. Vol. 34 (2008) pp. 1603-1608. [13] N. Gupta, P. Mullick and B. Basu: J. Alloys Comp. Vol. 379 (2004) pp. 228-232. [14] S. Ramesh and C. Gill: Ceram. Inter. Vol. 27 (2001) pp. 705-711.
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[15] A. Mukhopadhyay, B. Basu, S. D. Bakshi and S. K. Mishra: Inter. J. Refractory Metals & Hard Materials Vol. 25 (2007) pp. 179-188. [16] A.L. Chamberlain, W.G. Fahrenholtz and G.E. Hilmas: J. Am. Ceram. Soc. Vol. 89 (2006) pp. 450-456. [17] S.D. Bakshi, B. Basu, S.K. Mishra: Composites Part A: Applied Science and Manufacturing Vol. 37 (2006) pp. 2128-35. [18] F. Meschke, N. Claussan, G. De Port and J. Rodel: J. Eur. Ceram. Soc. Vol. 17 (1997) pp. 843-850. [19] D. K. Chatterjee, G. S. Jarrold and S. K. Ghosh, U.S. Patent 5,827,470 (1998)
Frontier of Nanoscience and Technology doi:10.4028/www.scientific.net/MSF.694 Pressureless Sintering of Electro-Conductive Zirconia Composites doi:10.4028/www.scientific.net/MSF.694.304
