Effect of Short Time Sintering on the Mechanical Properties of Undoped Zirconia Ceramics
Sivanesan Sivakumar
1,a *,Ramesh Singh
2,b,Teow Hsien Loong
3,c, Yong Leng Chuan
4,dand Jeffrey Kong Chin Leong
5,e1,4,5
Taylor’s University, No.1, Jalan Taylor’s, 47500 Subang Jaya, Selangor, Malaysia
2Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia.
3No. 3, Jalan SS15/8, 47500 Subang Jaya, Selangor, Malaysia
a[email protected], b[email protected],
c[email protected], d[email protected],
Keywords: zirconia, short time sintering, mechanical properties, undoped
Abstract. Sintering parameters are undoubtedly among the many factors that influence the mechanical properties and hydrothermal ageing resistance of tetragonal zirconia ceramics. In this research, the effect of using short holding times i.e. (1 min., 30 min. and 1 hour) as compared to the conventional 2 hours during sintering of 3 mol% Yttria Tetragonal Zirconia Polycrystals (3Y-TZP) on the mechanical properties were systematically investigated. The research revealed that holding time of 1 minute and sintered at 1400oC yielded a high relative density (above 95% of theoretical density) and high Young’s modulus (above 180 GPa) without compromising on tetragonal phase stability and mechanical properties. The study also revealed that the bulk density is an important parameter governing the matrix stiffness of 3Y-TZPs and that grain size strongly influences the transformability and consequently, the toughness of 3Y-TZPs. The toughness of the ceramic was observed to increase steeply when grains exceeded 0.52 µm, which has been identified as the critical grain size for toughening.
Introduction
Zirconia has been commonly used in the aerospace industry mainly in the coating of turbine blades [1]. Besides the aerospace industry, zirconia ceramics have also gained much interest for industrial applications since the discovery of transformation toughening phenomena [2]. For an instance, yttria stabilized zirconia exhibits high values of fracture toughness [3,4], making them a suitable candidate for a host of wide range of structural applications such as cutting tools, valve guides, extrusion dies, abrasive tools, etc. [5].
It is a general fact that increase in grain size during pressureless sintering depends on the forming method and the firing temperature [6]. Nanocrystalline ceramics often have a high degree of agglomeration, which in turn causes the development of two types of pores: inter-agglomerate pores (microns) and intra-agglomerate pores (nanometric) within the agglomerate itself [7].
Traditionally, sintering of zirconium ceramics is done through one cycle of heating, holding and cooling. Heating is done to achieve temperatures in the range of 1500oC to 1550oC until maximum density is reached, then holding for approximately 2 hours and finally cooling down to room temperature. It is during the heating stage that grain size increases continuously causing an adverse effect to the mechanical properties of sintered zirconia. A new technique developed by Chen and Wang, known as two stage sintering (TSS) might prove to be a promising approach in obtaining highly-densed nano grained ceramics. TSS has been successfully applied to ZnO, Ni – Cu – Zn ferrite [8,9], BaTiO3 [10] and Al2O3 [11-13].
The aim of this study is to show that further insights can be gained during sintering by reducing the holding time from the conventional 2 hours to shorter holding times ranging from 1 minute to 2 hour.
All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, www.ttp.net. (#70774028-23/10/16,19:48:53)
Experimental Procedure
Characterization. The bulk density of the sintered samples was measured based on Archimedes’
principle using an electronic balance retrofitted with a density determination kit (Mettler Toledo, Switzerland). The Young’s modulus by sonic resonance was determined for rectangular samples using a commercial testing instrument (GrindoSonic: MK5 “Industrial”, Belgium). The modulus of elasticity or Young’s modulus is calculated using the experimentally determined resonant frequency (ASTM E1876-97, 1998) and the values were found to be consistent regardless of the number of test performed for each samples. Fracture toughness (K1c) and Vickers hardness measurement (Future Tech., Japan) were made on polished samples using the Vickers indentation method. The indentation load was kept constant at 100 N and a loading time of 10 seconds was employed. The values of K1c were computed using the equation modified by [14].
Results and Discussion
Bulk Density. The variation of bulk density of 3Y-TZP samples for the entire range of holding times investigated at sintering temperatures of 1250oC to 1500oC is shown in Fig. 1. With the exception of the 2-hour and 1-hour holding time sintered samples, all other 3Y-TZP sintered samples exhibited similar densification trend i.e. sharp increase in density from 1250oC to 1400oC.
Subsequently the densities of all these samples were found to fluctuate in the range of 5.629 Mgm-3 to 5.995 Mgm-3 up to a temperature of 1500oC. The 2-hour sintered 3Y-TZP samples showed an increased in density when the sintering temperature was increased from 1250oC (5.812 Mgm-3) to 1450oC (5.978 Mgm-3). However, after 1450oC, the density was found to reduce from 5.978Mgm-3 to 5.629 Mgm-3. This reduction could be attributed to the cubic (c) phase formation in the 3Y-TZP samples sintered at 1500oC for 2 hours.
Figure 1: Effect of sintering temperature on the bulk density as a function of holding time As shown in Fig. 1, the minimum density was obtained when 3Y-TZP samples were sintered at 1350oC with a holding time of 1 minute was 5.612 Mgm-3, corresponding to a relative density of
~92% of the theoretical density. On the other hand, the highest density obtained was 5.995 Mgm-3 corresponding to a relative density of ~98% of the theoretical density when 3Y-TZP samples were sintered at 1450oC with a 2 hours holding time. It is also evident from Fig. 1 that generally, when the sintering temperature is increased, density increases regardless of the holding time. Increasing density with increasing temperature and holding time has been observed by [15]. These authors observed that relative density approached a plateau of >90% of theoretical density after 1 hour of sintering 3Y-TZP samples in the range of 1250oC to 1400oC. In this work, the same trend can be observed. It was found that regardless of the sintering temperature, all samples attained a minimum density of 85% of theoretical density at 1hour holding time. As such it may be concluded that this
4 4,2 4,4 4,6 4,8 5 5,2 5,4 5,6 5,8 6
1250 1300 1350 1400 1450 1500
Bulk Density (Mgm-3)
Sintering Temperature (ºC)
1 min.
30 min.
1 h 2 h
results correlates well with the work of [11,15]. Representative SEM micrographs from the thermally etched surfaces of the sintered 3Y-TZP samples for 1 minute holding time are presented in Fig. 3. Many studies have shown that open dispersed pores can retard grain boundary migration via the pinning effect [11,16,17]. This is evident from the SEM micrographs as seen in Fig. 2(a).
The open pores that existed within this 3Y-TZP sample retarded the grain boundary migration and thus, increased densities without significant grain growth. However, as sintering proceeded with time, the microstructure evolved as shown in Fig. 2(c) and Fig. 2(d). It was discovered that open pores diffused and formed smaller closed pores as shown in Fig. 2(b) and Fig. 2 (c). As a result, the collapse of open pores to form smaller closed pores diminished the pinning effect and encouraged grain growth (Fig. 2(d)). At this stage, grain growth was more significant than densification. For instance, densities varied mildly from 5.734 Mgm-3(94%) to 5.917 Mgm-3 (97%) while the grain size increased more aggressively from 0.28 µm to 0.82 µm.
(a) (b)
(c) (d)
Figure 2: SEM micrographs from the thermally etched samples, which were sintered at 1 minute:
(a) 1250ºC, (b) 1350ºC, (c) 1400ºC and (d) 1500ºC. Pores are clearly visible in Fig. 2(a) and grains are the granular structure as seen in Fig. 2(c).
Young’s Modulus. The effect of varying holding time and sintering temperature on the Young’s modulus (E), of 3Y-TZPs sintered in the range of 1250oC to 1500oC is shown in Fig. 3. Generally, the Young’s modulus of all the Y-TZP samples was found to increase with increasing temperature and holding time up to a temperature of 1350oC. Above 1350oC, the values of Young’s modulus for all the samples begin to show very little fluctuation. The Young’s modulus for these samples fluctuated between a maximum of 208.5 GPa for 3Y-TZP sintered at 1350oC with a holding time of 2 hours to a minimum of 199.6 GPa for the 3Y-YZP sample sintered at 1350oC with a holding time of 1 minute. It can be generally observed that at lower temperatures (< 1300oC), reduction in holding time of 3Y-TZP samples sintered within the range of 1250oC to 1300oC resulted in a reduction in Young’s modulus. The lowest Young’s modulus of 110 GPa was observed for the 3Y- TZP sample sintered at 1250oC with a holding time of 1 minute. The highest value of Young’s modulus observed for 3Y-TZPs sintered below 1300oC was 197.3 GPa which was obtained from the 3Y-TZP sample sintered at 1300oC with a holding time of 2 hours. Generally a Young’s modulus of 200 GPa and above can be obtained for samples sintered with a temperature of 1350oC regardless of the holding time employed. Thus it may be observed that at high temperatures (1350oC to 1500oC), holding time does not affect the Young’s modulus of 3Y-TZP samples. This results correlates well with the work of [17]. The authors discovered that by reducing the sintering holding time from 2 hours to 1 hour no significant change in the Young’s modulus was obtained.
0.25 µm 0.25 µm
0.25 µm 0.25 µm
Figure 3:Variation in the Young’s modulus of 3Y-TZPs sintered in the range of 1250oC to 1500oC A striking observation that can be made from Fig. 3 is that, as sintering proceeded from 1450oC (207.6 GPa) to 1500oC (204.73 GPa), a reduction in the Young’s modulus for samples sintered with 2-hour holding time is evident. This can be explained through the cubic (c) phase formation resulting in a reduction in bulk density for Y-TZP samples sintered at 1500oC for 2 hours.
Vickers Hardness. The effect of sintering temperature and holding time on the Vickers hardness of 3Y-TZP is shown in Fig. 4. It can be noted from Fig. 4 that the variation of Vickers hardness for 3Y-TZP samples sintered in the range of 1250oC to 1500oC with various holding times follows the same trend as the variation of the bulk density with sintering temperature. The results obtained in the present work confirmed that longer holding times are required for the attainability of 3Y-TZP ceramics with higher hardness. The hardness of 3Y-TZP samples with a 1 minute holding time was its lowest when sintering was performed at 1250oC and soon increased rapidly to 13.1 GPa when sintered at 1400oC before decreasing to 10.2 GPa when sintered at 1500oC.
Figure 4: Effect of holding times on the Vickers hardness of 3Y-TZP samples sintered from 1200oC to 1500oC
The hardness trend of the 30 minutes, 1 hour and 2 hours holding time samples shared the same trend with an increase of 110.8%, 97.3% and 84% respectively when sintered from 1250oC to 1400oC. However, when sintering proceeded to higher temperatures, the hardness values for the 30 minutes, 1 hour and 2 hours samples reduced to 12.6 GPa, 12.5 GPa and 12.2 GPa, respectively.
Generally regardless of the holding time, the hardness of all 3Y-TZP samples started to decrease when a sintering temperature of more than 1400oC was employed. This trend of increasing hardness until a maximum is attained at a certain sintering temperature followed by a continuous decline thereafter with further sintering as seen in the present work is in agreement with the works of [18].
The decrease in hardness of the 3Y-TZP samples sintered above 1400oC could be mainly due to two factors. The first being the decrease in the densities of the sample and the second being the increase in grain size. The increase in grain size seems to be the dominating factor in this work since the densities of all samples was higher than 97% of the theoretical density at temperatures above
80 100 120 140 160 180 200 220
1250 1300 1350 1400 1450 1500
Young's Modulus (GPa)
Sintering Temperature (ºC)
1 min.
30 min.
1 h 2 h
0 2 4 6 8 10 12 14
1250 1300 1350 1400 1450 1500
Vickers Hardness (GPa)
Sintering Temperature (ºC)
1 min.
30 min.
1 h 2 h
1400oC. In the case of the 2 hours sintered samples, the hardness reduced after sintering at 1450oC.
This is because hardness is strongly dependent on bulk density, which decreased considerably after sintering at 1450oC due to the presence of cubic phase.
Fracture Toughness. The effect of sintering temperature and holding times on the fracture toughness (KIc) of 3Y-TZP samples sintered between 1250oC to 1500oC with holding times varying from 1 minute to 2 hours is shown in Fig. 5. In general, sintering up to a temperature of 1350oC had negligible effect on the fracture toughness of 3Y-TZPs. The fracture toughness of these samples was found to vary between 4.7 MPam1/2 to 5.1 MPam1/2. The fact that the KIc values did not fluctuate significantly, shows that sintering below 1350oC had a negligible effect on the tetragonal (t) phase stability of 3Y-TZP. Above 1400oC, samples sintered within a holding time of 1 hour, showed a similar trend up to a maximum sintering temperature of 1500oC. However, above 1400oC, sintering of 3Y-TZP with a holding time of 1 and 2 hours, respectively showed an increase in fracture toughness. This observation was more pronounced in the 3Y-TZP samples sintered above 1400oC with a holding time of 2 hours. The increase in fracture toughness of this sample was from 5.6 MPam1/2 to 7.7 MPa1/2 at 1450oC and 1500oC, respectively. The sharp increase in fracture toughness (5.6 MPam1/2 to 7.7 MPam1/2) with decreasing tetragonal (t) phase content (91.9% to 86.9%) was in agreement with the work of [19].
Figure 5: Fracture toughness of sintered samples
Conclusion
In general, the mechanical properties of zirconia ceramics that enable its wide usage in the industry are bulk densities of > 95%, Young’s Modulus > 180 GPa, Vicker’s Hardness > 10 GPa and Fracture Toughness in the range of 4 – 7 MPam1/2 [20]. The values exhibited by 3Y-TZP sintered at 1400oC for 1 minute are 97%, 200.2 GPa, 13.1 GPa and 5.12 MPam1/2, respectively. Thus for the first time, it is shown in this work that a conventional sintering route with a holding time of 1 minute does produce dense Y-TZP ceramic having minimum properties for structural applications.
References
[1] Daniel Soares de A., Carlos A.A.C., Cosme R.M.S., Thermal Barrier Coating Electron Beam Deposition Physical Vapour Deposition of Zirconia codoped with Yttria and Niobia, J. of Aer.
Tech. * Mgmt., 2, (2010) 195-202.
[2] Garvie R.C., Hannick R.H.J. & Pascoe R.T. , Ceramic Steel? Nature, 258, (1975) 5537 .
[3] Tsukuma K., Kubota Y. &Tsukidate T., “Thermal and Mechanical Properties of Y2O3-Stabilized Tetragonal Zirconia Polycrystals,” In Advances in Ceramics, Vol. 12, Claussen N., Ruhle M. &
Heuer A.H. (eds.) (The Am. Ceram, Soc., Columbus, Ohio, 1984) pp. 382-390.
4 4,5 5 5,5 6 6,5 7 7,5 8
1250 1300 1350 1400 1450 1500
Fracture Toughness (MPam1/2)
Sintering Temperature (ºC) 1 min.
30 min.
1 h 2 h
[4] Masaki T., Mechanical Properties of Y-PSZ after Ageing at Low Temperature. Int. High Tech.
Ceram., 2, (1986) 85-98.
[5] Ramesh S. & Gill C. (2001).Environmental Degradation of CuO-doped Y-TZP Ceramics.
Ceram. Inter., 27, 705-711.
[6] Basu B.,Toughness Tailoring of Yttria – Stabalized tetragonal Zirconia Ceramics.Int. Mater.
Rev., 50, (2005b) 239-256.
[7] Bowen P. & Carry C. ,From Powdered to Sintered Pieces: Forming, Transformations and Sintering of Nanostructured Ceramic Oxides. Pow. Tech.,128, (2002) 248 – 255.
[8] Wang H., Aboushelib M.N. &Feilzer A.J.Strength Influencing Variables on CAD / CAM Zirconia Frameworks. Dent. Mater., 24, (2008) 633-638.
[9] Dobson P., “Unleashing The Potential of Nanotechnology,” In Materials World (The Institute of Materials, UK., April 2004) pp. 25-28.
[10] Mazaheri M., Simchi A. &Fard F.G., Densification and Grain Growth of nanocrystalline 3Y- TZP During Two Step Sintering. J. Euro. Ceram., Soc., 28, (2008) 2933-2939.
[11] Chen I. & Wang X.,Sintering Dense Nanocrystalline Ceramics Without Final Stage Grain Growth.Nature, 404, (2000) 168-171.
[12] Bravo L.A., Morikawa Y., Kawahara M. & Mayo M.J., Fracture Toughness of NAnocrystalline Tetragonal Zirconia with Low Yttria Content. Acta Mater. 50, (2002) 4555 – 4562.
[13] Kanellopoulos P. & Gill C. ,Hydrothermal Ageing of Yttria-Stabilised Zirconia, Sintered at 1300oC-1325oC: The Effect of Copper Oxide Doping and Sintering Time Variations. J. Mater. Sci., 37, (2002) 5075-5082.
[14] Bernard –G.G. &GuizardC.,Spark Plasma Sintering of a Commercially Available Granulated Zirconia Powder –II. Microstructure After Sintering and Ionic Conductivity. Acta Mater., 55, (2008) 3493.
[15] Li W. &Gao L., “Rapid Sintering of NanocrystallineZrO2(3Y) by Spark Plasma Sintering,” J.
Eur. Ceram. Soc., 20, (2000) 2441-2445.
[16] Kim H.T., Han Y.H., Mechanical Behaviour of Zirconia / Alumina Composites. Ceram Inter., 30, 1107-1113 (2004).
[17] Hjerppe J., Vallitu P.K., Froberg K. &LassilaL.V.J. Effect of Sintering Time on Biaxial Strength of Zirconium Dioxide. Dent. Mater., 25, (2009) 166-171.
[18] Hodgson S.N.B., Cawley J. &Clubley M., The Role of Al2O3 Impurities on the Microstructure and Properties of Y-TZP. J. Mater. Process. Tech., 92-93, (1999a) 85-90.
[19] Lange F.F., Dunlop G.L. & Davis B.I., Degradation During Ageing of Transformation Toughened ZrO2-Y2O3 Materials at 250oC. J. Am. Ceram. Soc.,69,(1986) 237-240.
[20] Introduction to Zirconia, Magnesium electron Ltd., UK No. (113, 1986).
