Effects of CeO
2Addition on Slip - Cast Yttria Tetragonal Zirconia Polycrystals Toughened Alumina (ZTA)
Sivakumar Sivanesan
1,a*, Teow Hsien Loong
1,b, Satesh Namasivayam
1,cand Mohammad Hosseini Fouladi
1,d1Taylor’s University Lakeside Campus, No. 1 Jalan Taylor’s, 47500, Subang Jaya, Selangor Darul Ehsan, Malaysia
a[email protected], b[email protected],
c[email protected], d[email protected]
Keywords: Y-TZP, CeO2, Microstructural, Mechanical, ZTA
Abstract. Addition of CeO2 into ZTA and its effects on microstructure and mechanical properties were investigated. CeO2 was detected with significant amounts only above 10 wt%. Viscosity was measured for slurry preparartions and characterization of mechanical properties of ZTA. Additions of CeO2 of more than 10 wt% surpassed the solubility limit and formed Ce2Zr3O10. Ce2Zr3O10
increased the tetragonality factor, prevented excessive grain growth through a pinning effect, which is attributed to the segregation of Ce2Zr3O10 to the grain boundaries and showed a peak in fracture toughness with a value of 9.3 MPam1/2 with 10 wt% additions of CeO2. Further additions of CeO2
reduced ZTA’s mechanical strength. Maximum value of Hv was 17700 MPa with 10 wt% CeO2. Porosities have been attributed as the underlying reason as to why theoretical density were always higher than measured densities.
Introduction
The wide usage of Yttria Tetragonal Zirconia Polycrystals (Y-TZP) may be due to its superior fracture toughness (6-15 MPam1/2), high fkexural strength (1000 – 1500 MPa) and commendable wear resistance [1-9]. Alumina is characterized with relatively low fracture toughness (< 5MPam1/2) and flexural strength (< 600 MPa) [10,11]. Zirconia has been introduced into alumina as a sintering aid in a wide variety of research [12-16]. Introduction of zirconia into alumina is capable of forming a solid solution which in turn, enhances density through lattice defect formations [17]. The author [17] attributed stress induced transformation toughening effect and other crack deflection toughening mechanisms as the main underlying reason governing the enhanced mechanical properties. However, fine grained ceramics are hard to manufacture due to the high chances or occurences of inhomogeneous densification. Inhomogeneous densification leads to intermittent stresses causing pore like cracks and reltively large voids [18]. Lange [19] reported that fabrication defects were the main reason for the low fracture toughness of hot-pressed composites. Due to the inherent fabrication defects, many researchers have resorted to the use of sintering additives such as Cr2O3, NiO, TiO2 and MgO to achieve small grain sizes and promote higher densities at low sintering temperatures [20]. The inclusion of sintering aids increases critical grain size of ZrO2
which in turn reduces the polymorphic transformation temperatures. However a nominal amount of stabilizer must be added for effective transformation, above which, the tetragonal grains become non-transformable [21,22]. This study attempts to understand further the effect of CeO2 addition (1- 20 wt%) on the microstructure and mechanical properties of ZTA-CeO2 ceramics.
Experimental Procedure
Starting Materials. 3 mol% yttria and Al2O3 (Kyoritsu Co. Ltd., Japan) supplied the commercially available powder used in this study. Material properties of the as-received powder is shown in Table 1.
Key Engineering Materials Submitted: 2019-03-31
ISSN: 1662-9795, Vol. 814, pp 340-346 Revised: 2019-04-26
doi:10.4028/www.scientific.net/KEM.814.340 Accepted: 2019-04-28
© 2019 Trans Tech Publications Ltd, Switzerland Online: 2019-07-29
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 Ltd, www.scientific.net. (#506985559-26/07/19,08:07:29)
Table 1: Characteristics of the Starting Powder 3 mol% Yttria and Al2O3 by Kyoritsu Co. Ltd.
Japan
Chemical Composition Al2O3 Y-TZP
Al2O3 wt% 99.80 <0.01
ZrO2 wt% - 94.70
SiO2 wt% 0.02 <0.01
Fe2O3 wt% 0.01 <0.01
Na2O wt% 0.06 -
MgO wt% 0.05 -
TiO2 wt% - <0.01
Y2O3 wt% - 5.20
Mean particle diameter (µm) 0.3 0.3-0.4
Specific Surface Area (m2/g) - 9-12
Specific Gravity 3.96 5.73
Slurry Preparation. Viscosity was used to determine the most effective conditions for the preparation of slurry. Polyacrylate (D-3019, Germany) was used as the dispersant whilst deionized water served as the dispersant. Viscosity measurements were taken at three time intervals ranging from 15 s to 1 minute using a viscometer (Brookfield, UK). Initial composition of the powder was constituted of 80 wt% alumina and 20 wt% Y-TZP. Samples with varying CeO2 additions ranging from 1 wt% to 20 wt% and were labeled 1Ce-ZTA, 5Ce-ZTA, 10Ce-ZTA, 15Ce-ZTA and 20Ce- ZTA. Slips were then ball milled for 240 minutes using Y-TZP as the milling media. Post filtration, slips were stirred magnetically in a vacuum desiccator for 2 h to eradicate trapped air bubbles. After soaking for 1 day, green rectangular samples (~ 5 × 15 × 50 mm) were removed and dried in air for 1 day prior to drying in high temperature environment of 110oC. Samples were then sintered at 1550oC in air for 120 minutes at a heating rate of 5oC/min. Post sintered samples were then ground and polished.
Characterization and Evaluation. Bulk densities were measured using the water immersion technique. X-ray diffraction of polished samples was carried out at room temperature using Cu-Kα as the radiation source to determine phase compositions. Monoclinic (m) and tetragonal (t) content were obtained using Toraya et al. [23]. Hv was obtained using an indentation load of 10 N for 15 seconds. Grain size was determined using Mendelson’s line method [24].
Results and Discussion
XRD traces with varying CeO2 additions revealed YTZP (Zr0.935Y0.065)O1.968 and Alumina that corresponded well with ICDD files No. 01-078-1808 (Zr,Y)O2 and No.00-010-0173 (Al2O3). The presence of (Zr0.935Y0.065)O1.968 refers to metastable (t) grains which are prone to undergo transformation toughening. The effect of metastable (t) grains zirconia polycyrstals in increasing the toughness and strength was deliberated to a great extent by [25-28]. These authors explained that the deformation associated with (t) to (m) transformation of Y-TZP is responsible for the reduction in the stress that is often seen to occur at the tip of a propagating crack. Traces of CeO2 were not seen in traces from 1 wt% to 5 wt%. CeO2 traces were evident only when doped with CeO2 content of more than 10 wt%. At 10 wt% and above, Ce2Zr3O10 peaks corresponding to ICDD No.00-026-
Key Engineering Materials Vol. 814 341
0359 were evident in all traces. These results comply well with the findings from [29,30]. The variation of tetragonaility factor (c/a) with CeO2 addition is shown in Fig. 1.
Figure 1: Effect of CeO2 addition on the tetragonality factor of Ce-ZTA
Results from Fig. 1 show that there is no significance difference between the tetragonality factor without additions of CeO2 and with 1 wt% addition of CeO2. From 1 wt% to 5 wt% additions of CeO2, the tetragonality factor reduced from 1.44 to 1.43 which could be attributed to the significant increase in lattice dimensions due to the varying ionic radii of Ce4+ (0.96Å ) as compared to Y3+
(Y3+: 0.92Å). The enhancement in the tetragonality factor above additions of 5wt% of CeO2 was attributed to the formation of Ce2Zr3O10 when the solubility limit was surpassed.
Grain size and texture are pivotal factors leading to the enhancement in densities of ceramics. The variation of average grain size with varying CeO2 compositions is shown in Figure 2. The average grain size of ZTA increased from 1.9 µm to 2.4 µm with an increase in CeO2 additions from 0 wt%
to 20 wt%. The increase in grain sizes with a concomitant increase in additions of CeO2 could be attributed to the fact that grain shapes changed due to surpassed solubility limits of CeO2 when added to ZTA as shown in Fig. 3. These results do conform to findings from [29]. The author further explained that abnormal growth such as platelet growth occurred among ZrO2 grains with the maximum average grain size being 2.32 µm. More significant grain size increase up to additions of 10 wt% i.e. from 1.9 µm to 2.3 µm can be observed when compared to increase in average grain sizes from 10 wt% to 20 wt%. It may be postulated that above 10 wt%, the solubility limit of CeO2
into ZTA composites have been exceeded leaving a relative abundance of the new phase Ce2Zr3O10
to exist at grain boundaries.
Figure 2: Average grain size with varying amounts of CeO2
Ce2Zr3O10 could have promoted a pinning effect thus retarding further siginificant grain growth for samples with more than 10 wt% CeO2. Energy dispersive X-ray analysis (EDX) indicates that CeO2
was found both in the interior of the grain and also at the grain boundaries. The interior distribution 342 Advanced Materials and Engineering Materials VIII
of CeO2 into ZTA was well within the solubility limit and excess CeO2 was displaced to the grain boundaries.
Figure 3(a) (left): Microstructure of 15Ce-ZTA with darker grains representing the corundum and lighter grains representing Y-TZP. Figure 4 (right): EDX analysis of 15CE-ZTA
The results for the fracture toughness of ZTA composites with varying additions of CeO2 is shown in Fig. 5.
Figure 5: Fracture toughness of ZTA with varying additions of CeO2
It can be seen that fracture toughness increased slightly from 5.9 MPam1/2 to 6.2 MPam1/2 when CeO2 additions was increased up to 5 wt%. A significant increase in fracture toughness was observed in samples with 10 wt% CeO2. Fracture toughness increased from 6.2 MPam1/2 to 9.3 MPam1/2. This significant increase could be attributed to the average grain size that increased from 2.3 µm in 5Ce-ZTA to 2.4 µm in 10 Ce-ZTA. The observation of fracture toughness increasing with increased grain size has been supported by [29, 31-33]. Rejab et al. [29] have also further explained that the increase in fracture toughness can be attributed to the presence of CeO2 in ZTA as this would represent a good solid solubility between Ce4+ and Y3+ in the (Zr,Y,Ce)O2 phase that tends to stabilize the transformation of tetragonal (t) to monoclinic (m) inside ZTA. Further addition of CeO2 was not beneficial in the enhancement of fracture toughness of ZTA. This could be attributed to the formation of Ce2Zr3O10 which tends to reduce mechanical properties of the overall ceramic [30]. The current study does correlate well with the findings of [30].
Fig. 6 shows the variation of Vicker’s hardness with varying amounts of CeO2 addition into ZTA ceramics.
(a) (b)
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Figure 6: Variation of Vicker’s Hardness with varying amounts of CeO2 in ZTA
It may be realised that addition of CeO2 does increase the hardness up to 10 wt%. Hardness increased from 15600 MPa without any CeO2 addition and increased to a miximum of 17700 MPa with an addition of 10 wt% CeO2. Further additions of CeO2 was found to reduce the hardness of ZTA samples. This reduction could be attributed to the formation of Ce2Zr3O10. These results do concur with the findings from [29]. Rejab et al further explained that hardness is greatly influeneced by density and increase in hardness was caused by higher densificaions. The author corelated hardness to density by stating that ceramics have no plastic deformation attributes and thus would not have the ability to absorb any energy transfered to these materials once a crack is intiaited. This would allow the crack to have a higher propoensity to propagate until fracture occurs.
Fig. 7 shows the variation of measured and theoretical densities with varying amounts of CeO2
addition into ZTA ceramics. It can be seen that the theoretical density is always higher than the measured densidty regardless of the amount of CeO2 added. This could be attributed to the formation of porosities whiah was most probably propmoted during sintering. Porosity reduces the strength of ceramics by providing ceramics with lesser effective surface area to withstand any applied load. Consequetly higher densities would result in enhanced mechanical strength and reduced porosities, however, higher porosities would be detrimental to the strength of a ceramic.
Figure 7: Variation of measured and theoretical densities with varying amounts of CeO2 additions Conclusion
The beneficial effects of CeO2 in enhancing the mechanical properties of ZTA has been revealed.
Formation of new phases should be anticipated with increasing amount of CeO2 additions due to the solubility limit. The new phases formed would not be beneficial in enhancing the strength of the ceramic. As such, careful consideration should be taken into account for optimum mechanical 344 Advanced Materials and Engineering Materials VIII
strength when additions of CeO2 is incorporated into ZTA in order to inhibit formation of new phases that may prove to be detrimental to the strength of ZTA.
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