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The effects of CeO2 addition on the physical and microstructural properties of ZTA-TiO2 ceramics composite
Accepted Manuscript The effects of CeO2 addition on the physical and microstructural properties of ZTATiO2 ceramics composite Ahmad Zahirani Ahmad Azhar, Siti Hajar Mohamad Shawal, Hanisah Manshor, Afifah Mohd Ali, Nik Akmar Rejab, Ezzat Chan Abdullah, Zainal Arifin Ahmad PII:
S0925-8388(18)33413-3
DOI:
10.1016/j.jallcom.2018.09.173
Reference:
JALCOM 47592
To appear in:
Journal of Alloys and Compounds
Received Date: 11 July 2018 Revised Date:
13 September 2018
Accepted Date: 15 September 2018
Please cite this article as: A.Z.A. Azhar, S.H. Mohamad Shawal, H. Manshor, A.M. Ali, N.A. Rejab, E.C. Abdullah, Z.A. Ahmad, The effects of CeO2 addition on the physical and microstructural properties of ZTA-TiO2 ceramics composite, Journal of Alloys and Compounds (2018), doi: https://doi.org/10.1016/ j.jallcom.2018.09.173. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Ahmad Zahirani Ahmad Azhar a, Siti Hajar Mohamad Shawal a, Hanisah Manshor b,*, Afifah Mohd Ali a, Nik Akmar Rejab c, Ezzat Chan Abdullah d and Zainal Arifin Ahmad c
Department of Materials and Manufacturing, Faculty of Engineering, International Islamic University Malaysia, P.O. Box 10, 50728 Kuala Lumpur, Malaysia.
b
Department of Science in Engineering, Faculty of Engineering, International Islamic University Malaysia, P.O. Box 10, 50728 Kuala Lumpur, Malaysia.
c
Structural Materials Niche Area, School of Materials and Mineral Resources Engineering, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia.
d
Malaysia-Japan International Institute of Technology (MJIIT), Universiti Teknologi Malaysia Kuala Lumpur, Jalan Semarak, 54100 Kuala Lumpur, Malaysia.
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The Effects of CeO2 Addition on the Physical and Microstructural Properties of ZTA-TiO2 ceramics composite
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ABSTRACT: The effect of CeO2 addition ranging from 0 wt. % to 7 wt. % on phase, microstructural evolution, physical and mechanical properties of ZTA-3 wt. % TiO2 ceramic composite were investigated. The samples were prepared by solid-state mixing
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and sintered at 1600°C for 1hr under pressureless condition. Samples were then characterized by XRD, SEM, densitometer and Vickers indentation method. Based on
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XRD analysis, m-ZrO2 began to diminish at 1 wt.% CeO2 while secondary phases, i.e. Ce0.7Zr0.3O2and Zr0.4Ti0.6O2 initiated at 3 wt.% CeO2 addition. SEM images showed finer grain sizes was produced upon increasing amount of CeO2 up to 5 wt.%, corresponding to higher average grain intercept (AGI) values.From the results obtained, the optimum
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amount of CeO2 addition was at 5 wt. % which yielded the highest bulk density (4.41 g/cm3), firing shrinkage (21.94%), hardness (1580.10HV) and fracture toughness (9.77 MPa.m1/2 ). This is contributed by the grain refinement and the highest amount of
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secondary phases formed, especially Zr0.4Ti0.6O2 . However, with an excessive addition of CeO2, i.e more than 5 wt.%, grain sizes enlarged and the amount of secondary phases
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reduced, which degraded the mechanical properties of ZTA-3 wt. % TiO2.
KEYWORDS: ZTA-TiO2 ceramic composite, CeO2, fracture toughness, Vickers hardness,
microstructure
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1. INTRODUCTION Among the ceramics that can be found in the shell of the earth, alumina (Al2O3) is considered one of the most abundant. Established as the most frequently used ceramic
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material, it comes with several exceptional characteristics. This includes chemical and thermal constancy, resiliency, high melting point, as well as favourable mechanical and insulation properties. Due to these advantages, Al2O3 is very much in demand for a wide range of engineering applications [1–6]. On the downside, however, Al2O3 is saddled with
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a low fracture toughness level of approximately 4 – 5 MPa.m1/2.. A considerable number
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of studies have been conducted on the alumina-zirconia ceramics composite system to overcome the low fracture toughness. These studies were focused on enhancing the system’s mechanical features particularly its strengthening mechanisms [7–9]. Zirconia is utilized as a sintering aid in the Al2O3 matrix. Its applicability for this role is
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attributed to its capacity to make possible the formation of a solid solution, and to bring about the densification process through the attendance of the lattice defect [7]. It has been verified that the fracture toughness of ZrO2 ranges from 6 to 10 MPa.m1/2. In comparison
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to Al2O3 ceramics, this level of fracture toughness is generally twice as high [10]. Wang & Stevens, (1989) [7] and Asharaf et al., (2014) [10] are of the opinion that the presence of
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ZrO2 grains in the Al2O3 matrix facilitates the materialization of a stress-induced toughening mechanism. This is brought about by the transformation of the metastable tetragonal ZrO2 into a thermodynamically stable monoclinic state as the crack tip area undergoes stress. This transformation comes with an escalation in volume expansion which serves to hinder crack propagation. As such, the fracture toughness of Al2O3 is enhanced.
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ACCEPTED MANUSCRIPT Additionally, Al2O3 is considerably affected by the incorporation of TiO2 [11] and CeO2 [12]. According to Dhar et al., (2015) [13], TiO2 incites the sintering and grain development of Al2O3. This serves to boost the bulk density and hardness of ZTA. An addition of a mere 3 wt. % of TiO2 is all it takes to realize optimal hardness [14]. As for
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CeO2, its inclusion swelled the bulk density and lowered the proportion of porosity to register an improvement in the mechanical properties of Al2O3. The results attained by Dhar et al., (2015) revealed that the grain dimensions of ZrO2 and Al2O3 grew in tandem
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with the amount of CeO2 added to Al2O3 [15]. Kumar et al., (2004) [16] forwarded that CeO2 performs the role of a stabilizer to facilitate the full transition of zirconia into its
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tetragonal phase (t-phase). This involves the conversion of zirconia into tetragonal zirconia polycrystals (TZP). Deville et al., (2003) [17] opined that similarities in the Zr4+ valence permits the substitution of the Y3+ ions by the Ce4+ ions. While ceria stabilized zirconia toughened alumina (CSZ-TA) has been applied for biomedical procedures, Y3+
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remains the stabilizer of choice for almost all other applications [18–22]. Although many works have been made into improving the fracture toughness of zirconiatoughened alumina (ZTA), several possibilities remain unexplored. Among them is the
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inclusion of CeO2 in the ZTA system with a TiO2 additive. Accordingly, this study takes
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on the task of examining the viability of using CeO2 to improve the mechanical properties of the ZTA-TiO2 ceramic composite.
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2. METHODS 2.1 Materials and Fabrications Alumina (99.7%, 0.7µm), yttria stabilized zirconia (5.4 mol%, Goodfellow, 94.5%, 0.1 –
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2.0µm), Titania (>99.0%, 0.13µm) and cerium (IV) oxide-gadolinium doped (Sigma Aldrich, 90.0%with 10 mol% gadolinium doped, <0.5µm) were the preliminary powders employed for this work. While TiO2 was set at 3.0 wt. %, the composition of CeO2 was
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fluctuated between 0.0 wt. % and 7.0 wt. % during testing. The composition of ZTA for Al2O3 and YSZ was fixed at a ratio of 4:1 of the residual mass.
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ZTA-TiO2 samples with varying measures of CeO2 were readied for testing. A wet-mixing procedure was used to merge the powders with an adequate quantity of acetone. An oven set at 100oC was then employed to dry the powders for a period of 10 hours. All the powder mixtures were put through a 75µm strainer and subsequently, a uniaxial hydraulic
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hand press set at 200MPa was used to compress the powder mixtures into a 12mm diameter mould. To conclude the process, the green pellets were subjected to pressureless
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sintering at 1600°C for a period of 1hour with a heating and cooling rate of 5 °C/minute. The bulk density of the samples was gauged by way of a Densimeter GK-300, the degree
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of shrinkage was determined through alterations in the size of the samples. Phase analysis was conducted with the use of an X-Ray diffractometer D8 Advance Bruker with CuK radiation functioning at 40 kV, 40 mA with the 2θ ranges 10˚ - 90˚. Scanning was carried out at the rate of 0.034˚/s. A scanning electron microscope (InTouchScope JSM-IT100) was engaged to scrutinize the microstructure of the samples. Subsequent to the coating of the samples with palladium, the standard ASTM E112 – 13 linear intercept procedure was done on the SEM images to determine the average grain intercept (AGI) values. The 4
ACCEPTED MANUSCRIPT Vickers hardness test was performed on samples which were caused to endure a load of 30 kgf for a period of 15 seconds. The resulting surface cracks were measured, and the equation formula was used to compute the fracture toughness values of the samples [23].
(1)
య/మ
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ܭூ = 0.00824
KIc represents the fracture toughness (MPa.m1/2), P the pressure (MPa), c the sum of the
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half length of Vickers diagonal indentation, a (µm), and the length of the radial crack size,
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l (µm).
3. RESULTS AND DISCUSSION
While Figure 1 displays the XRD analyses for each sample with varying amounts of CeO2,
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Table 1 displays the percentage of every phase found in the ZTA-TiO2-CeO2 ceramic composite which was directly obtained from the rietvield refinement of XRD data. As can be observed, the major diffraction peaks are Al2O3 and t-ZrO2. A minor diffraction peak of
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Baddeleyite (monoclinic ZrO2) shows up at 0 wt. % and 0.5 wt. % addition of CeO2. A further addition of CeO2 culminates in the disappearance of this peak. This outcome is
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substantiated by the result obtained by the investigation conducted by Rejab et al. (2013) [15]. As in our investigation, the addition of CeO2 exceeding 0.5 wt. % led to the total disappearance of M-ZrO2 from the composition. For sample from 0 wt. % CeO2 to 1.0 wt. % CeO2, the peak of CeO2 are not visible. For XRD analysis, peaks for any materials that are below 1.0 wt. % will not be detected since the peaks are masked with background radiation. For sample with more than 1 wt. % CeO2, addition of CeO2 has incorporated 5
ACCEPTED MANUSCRIPT onto ZrO2 forming Ce0.7Zr0.3O2 which can be seen in Figure 1 (designated as a purple star). The addition of 3, 5 and 7 wt. % of CeO2 resulted in the appearance of fresh diffraction peaks. These peaks were recognized as Zr0.4Ti0.6O2 and Ce0.7Zr0.3O2. Ce0.7Zr0.3O2 was the
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result of an excess addition of CeO2. This peak came with a cubic fluorite structure as well as an outstanding thermal stability [24–26]. Then again, this finding contradicted to the work of Rejab et al. (2013) [15]. Their work found the formation of merely a sole
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secondary phase (Ce2Zr3O10) at 10 wt. % and 15 wt. % CeO2 addition. This contradiction
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can be traced to the fact that unlike the study conducted by Rejab et al. (2013), this work included TiO2 as an additive into the ceramic composite. Also, the inclusion of 10 to 20 wt. % CeO2 into the ZTA system while excluding both TiO2 and Y2O3 will lead to the
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emergence of the secondary phase CeAl11O18 [27].
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ACCEPTED MANUSCRIPT As illustrated in Table 1, the mineral srilankite was perceived before the addition of 0-1.0 wt. % CeO2. According to Willgallis et al. (1983) [28], srilankite (ZrTi2O6) is among the compositions present in the zirconium titanate (Zr,Ti)O2 disordered solid solution. The zirconium titanate solid solution is recognized as the intermediate compound of the ZrO2–
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TiO2 system [29,30] that takes form when the measure of TiO2 in the ZrO2 system is in between 42 and 67 mol% [31]. This is attributed to the fact that with an escalation in the amount of CeO2, the proportion of ZrO2 becomes lesser than that of TiO2. The absence of
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srilankite beginning from the addition of 3 wt. % CeO2 signifies that the content of TiO2 surpassed 67 mol%. The formation of an ordered solid solution, namely Zr0.4Ti0.6O2, then
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materialized. The development of the secondary phase is due to the addition of material surpassing the solubility limit. This is in agreement with the work of Gao et al. (2010) [32].
Figure 2 shows the microstructure of ZTA-TiO2 with different composition of CeO2.
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Al2O3 and YSZ are represented by dark and white grains respectively. These grains are evenly spread out and no existence of significant agglomeration. The existence of CeO2 and TiO2 grains is untraceable as only a negligible amount of these compounds was used
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for this investigation. The AGI values with varying levels of CeO2 are exhibited in Figure
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3. As can be observed, the higher the AGI value, the smaller is the size of the grain. According to Figure 3, the grain development of Al2O3 is influenced by the addition of CeO2. The grain development of Al2O3 was hindered by the introduction of CeO2 into the ZTA-TiO2 ceramic composite. And at 5 wt. % CeO2, the microstructure was enhanced when the AGI values achieved their highest value with the least possible grain growth. Another contributory factor to this situation is the decrease in unit cell volume as the ratio of Ti to Zr increased. This is attributed to the lesser ionic radius of Ti4+ in comparison to 7
ACCEPTED MANUSCRIPT Zr4+ [33] as soon as Zr is replaced by Ti. Any additional inclusion of CeO2 is deemed a surplus which will no longer impede the grain development of Al2O3. Figure 4 illustrates the firing shrinkage of the ZTA-TiO2 ceramic composite at varying compositions of CeO2. The graph displays an ascendant trend of firing shrinkage from 0
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wt. % CeO2 to 5 wt. % CeO2 (21.19% to 21.94%). Then again, an inclusion of CeO2 above 5 wt. % led to a dip in the firing shrinkage value. While the rise in firing shrinkage value is contributed by the grain refinement of the respective phases, the decline in shrinkage is
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attributed to the rather refractory inclination of the rare earth oxides [34].
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The compressed arrangement of small grains serves to decrease the dimension of the pores. The study conducted by Mitra et al. (2002) [34] indicated that an elevated sintering temperature or a raised CeO2 content will lead to a decrease in the size of the pores. Apparently, the decrease in porosity of the sample contributes towards an elevation in the
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firing shrinkage.
The study conducted by Mangalaraja et al. [35] yielded a completely contrasting outcome to that attained through this investigation. Their efforts saw a progressive dip in the firing
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linkage percentage as the CeO2 inclusion was increased from 0 to 5 vol. %. This could be due to their exclusion of TiO2 from the composition. The absence of TiO2 would have
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affected the grain development performance in such a manner that the level of porosity escalated. In the opinion of Manshor et al. (2015) [11], the introduction of 3wt. % of TiO2 into ZTA improves the grain refinement of Al2O3 to bring about a decrease in the level of porosity. This is in line with the assumption that the inclusion of CeO2 up to 5 wt. % serves to raise the value of the AGI. This is illustrated in Figure 3.
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ACCEPTED MANUSCRIPT The bulk density data concurs with this outcome. As shown in Figure 5, the density of sintered ceramics exhibited a rising movement from 0 wt. % of CeO2 (4.14 g/cm3) until it arrived at its peak of 5 wt. % of CeO2 (4.41g/cm3). A rise in the degree of densification, and a decrease in the level of porosity contributed towards a boost in bulk density. Rejab
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et al., (2013) [12] recorded similar findings in their work. They reported that the bulk density of the sintered samples recorded a linearly rise from 4.13 g/cm3 to 4.40 g/cm3. This rise in bulk density of the sintered samples can be attributed to the enhanced AGI values
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which reduced grain sizes. This is in line with the findings of Rejab et al., (2013) [15]. On the other hand, the addition of 7 wt. % of CeO2 brought about a substantial decline in bulk
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density of the sintered ceramics composite. This decline is attributed to the surplus quantity of CeO2 that is no longer an effective hindrance to the grain development of Al2O3. The veracity of this finding is backed by the reduced AGI value. A reduced AGI value signifies irregular grain sizes, and irregular grain sizes impede densification. Rejab
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et al., (2013) [12] revealed that the decline in density with an addition of CeO2 in excess of 5 wt. % is attributed to the development of Ce2Zr3O10. The hardness level of ZTA-TiO2 ceramic composite climbed steadily from 1512.64 HV to
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1580.10 HV at 0 wt. % and 5.0 wt. % CeO2 respectively. However a slight drop in the
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hardness level to 1569.51 HV was discerned at 7.0 wt% CeO2. The enhancement in the hardness level of the ceramic composite can be put down to the smaller grain sizes with elevated AGI values. This is made clear by the existence of numerous grain boundaries and diverse angles/orientations that impede the movement of dislocations during indentation. This circumstance raises the capacity of the ceramic composite to defy deformation. The decline in the ceramic composite’s Vickers hardness value when the CeO2 content went beyond 5 wt. % is attributed to the decreased quantity of Al2O3 9
ACCEPTED MANUSCRIPT content. This is portrayed in Table 1. An earlier effort by Rejab et al., (2013) [12] uncovered that the ZTA ceramic composite’s hardness value is enhanced with an increase in the CeO2 content. This is because the increased CeO2 content improves densification while significantly decreasing the porosity of the ceramic composite. On the other hand,
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Naga et al., (2017) [27] put forward that the ZTA ceramics composite’s hardness is adversely affected by the addition of CeO2. They opined that this negative outcome is linked to the development of cerium hexaaluminate (CeAl11O18) and cerium zirconate
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(Ce2Zr3O10).
The fracture toughness of the ceramics composite (Figure 7) was heightened with the increase in CeO2 content from 0 wt. % (6.13 MPa.m1/2) to 5 wt. % CeO2 (9.77 MPa.m1/2). At 0 wt. % and 0.5 wt. % CeO2, the reduced fracture toughness values can be put down to the attendance of m-ZrO2 phases in the ceramics composite. This was verified by the XRD
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pattern (Figure 1). The inclusion of CeO2 exceeding 0.5 wt. % led to a reduction of the mZrO2 phase and an enhancement in the fracture toughness of the ceramics composite. The
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escalation in fracture toughness upon the addition of 3 wt. % and 5 wt. % of CeO2 is linked to the emergence of the Zr0.4Ti0.6O2 phase. However, at 7 wt. % CeO2, the lessening
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amount of the Zr0.4Ti0.6O2 phase brought about a dip in the fracture toughness level. This dip in the fracture toughness level upon the addition of 7 wt.% CeO2 can also be attributed to the attendance of a lesser quantity of ZrO2. In such a circumstance, the ZrO2 toughening mechanism is considerably weakened [2]. It is also found out that the amount of Ce2Zr3O10 for 7 wt.% CeO2 sample is the highest, which is 2.2%. This result is in accordance to previous works by Duran et al. (1990) [36] and Rejab et al. (2013) [15] where the presence of Ce2Zr3O10 will reduce the mechanical properties of ceramic. 10
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4. CONCLUSION Testing was conducted on the phase evolution, the microstructure, and the physical as well as mechanical characteristics of the CeO2 doped ZTA-TiO2 ceramic composite. While the
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departure of the monoclinic phase of ZrO2 commenced at 1 wt. % CeO2, the emergence of the secondary phases began at 3 wt. % CeO2. At 5 wt. % CeO2, the levels of density, shrinkage, hardness and fracture toughness rose to arrive at their respective peak values of 4.41g/cm3, 21.94%, 1580.10HV and 9.77MPa.m1/2. The introduction of CeO2 in excess of
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5 wt. % led to a drop in these levels. This is most likely attributed to the development of
ACKNOWLEDGEMENT
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big grain sizes and the decrease in the quantity of secondary phases.
The authors are grateful to the International Islamic University Malaysia for its financial
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[29] L.W. Coughanour, R.S. Roth, V.A. DeProsse, Phase equilibrium relations in the systems lime-titania and zirconia-titania, J. Res. Natl. Bur. Stand. 52 (1954).
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[30] A.E. McHALE, R.S. Roth, Low‐temperature phase relationships in the system ZrO2‐TiO2, J. Am. Ceram. Soc. 69 (1986) 827–832.
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[31] U. Troitzsch, D.J. Ellis, High-PT study of solid solutions in the system ZrO2-TiO2: The stability of srilankite, Eur. J. Mineral. 16 (2004) 577–584.
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[32] L. Gao, M. Zhou, Y. Zheng, H. Gu, H. Chen, L. Guo, Effect of zinc oxide on yttria doped ceria, J. Power Sources. 195 (2010) 3130–3134.
[33] U. Troitzsch, A.G. Christy, D.J. Ellis, The crystal structure of disordered (Zr, Ti) O 2 solid solution including srilankite: evolution towards tetragonal ZrO2 with increasing Zr, Phys. Chem. Miner. 32 (2005) 504–514. [34] N.K. Mitra, S. Das, S. Maitra, U. Sengupta, A. Basumajumdar, Effect of CeO2 on 15
ACCEPTED MANUSCRIPT the sintering behaviour of zirconia–alumina composite, Ceram. Int. 28 (2002) 827– 833. [35] R.. Mangalaraja, B.. Chandrasekhar, P. Manohar, Effect of ceria on the physical, mechanical and thermal properties of yttria stabilized zirconia toughened alumina,
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Mater. Sci. Eng. A. 343 (2003) 71–75. doi:10.1016/S0921-5093(02)00368-4.
[36] P. Duran, M. Gonzalez, C. Moure, J.R. Jurado, C. Pascual, C. (1990). A new
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materials science. 25(12) (1990) 5001-5006.
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tentative phase equilibrium diagram for the ZrO2-CeO2 system in air. Journal of
List of Figures 16
ACCEPTED MANUSCRIPT Figure 1: X-ray diffraction analyses for all samples with different compositions of CeO2. Figure 2: Microstructure of ZTA-TiO2 ceramics composite with different composition of CeO2 at 1000 magnification (a) 0 wt.% CeO2, (b) 0.5 wt.% CeO2, (c) 1.0 wt.% CeO2, (d) 3.0 wt.% CeO2, (e) 5.0 wt.% CeO2, (f) 7.0 wt.% CeO2. Figure 3: Average Grain Intercept of ZTA-TiO2 at different composition of CeO2.
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Figure 4: Firing Shrinkage of ZTA-TiO2 ceramics composite with different composition of CeO2. Figure 5: Bulk Density of ZTA-TiO2-CeO2 ceramics composite.
Figure 6: Vickers hardness of ZTA-TiO2 ceramics composite with different
List of Table
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Figure 7: Fracture toughness of ZTA-TiO2-CeO2 ceramics composite.
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Table 1: Percentages of phases present in ZTA-TiO2-CeO2 ceramics composite
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ACCEPTED MANUSCRIPT Table 1: Percentages of phases present in ZTA-TiO2-CeO2 ceramics composite CeO2 (wt %) 0
Al2O3 (wt %) 81.1
t-ZrO2 (wt %) 12.2
m-ZrO2 (wt %) 4.4
Srilankite (wt %) 2.3
0.5
81.7
16.8
1.2
0.3
0
0
1.0
81.4
13.9
0
4.7
0
0
3.0
79.4
14.8
0
1.7
0.7
3.4
5.0
80.0
12.8
0
1.3
0.7
5.2
7.0
80.0
11.7
0
1.9
2.2
4.2
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Zr0.4Ti0.6O2 (wt %) 0
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Ce0.7Zr0.3O2 (wt %) 0
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Research Highlights 1. Effects of CeO2 addition on the ZTA-TiO2 systems were investigated. 2. Small additions of CeO2 lead to improved mechanical properties.
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3. Effects of excessive amount of CeO2 addition have been discussed.
S0925-8388(18)33413-3
DOI:
10.1016/j.jallcom.2018.09.173
Reference:
JALCOM 47592
To appear in:
Journal of Alloys and Compounds
Received Date: 11 July 2018 Revised Date:
13 September 2018
Accepted Date: 15 September 2018
Please cite this article as: A.Z.A. Azhar, S.H. Mohamad Shawal, H. Manshor, A.M. Ali, N.A. Rejab, E.C. Abdullah, Z.A. Ahmad, The effects of CeO2 addition on the physical and microstructural properties of ZTA-TiO2 ceramics composite, Journal of Alloys and Compounds (2018), doi: https://doi.org/10.1016/ j.jallcom.2018.09.173. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Ahmad Zahirani Ahmad Azhar a, Siti Hajar Mohamad Shawal a, Hanisah Manshor b,*, Afifah Mohd Ali a, Nik Akmar Rejab c, Ezzat Chan Abdullah d and Zainal Arifin Ahmad c
Department of Materials and Manufacturing, Faculty of Engineering, International Islamic University Malaysia, P.O. Box 10, 50728 Kuala Lumpur, Malaysia.
b
Department of Science in Engineering, Faculty of Engineering, International Islamic University Malaysia, P.O. Box 10, 50728 Kuala Lumpur, Malaysia.
c
Structural Materials Niche Area, School of Materials and Mineral Resources Engineering, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia.
d
Malaysia-Japan International Institute of Technology (MJIIT), Universiti Teknologi Malaysia Kuala Lumpur, Jalan Semarak, 54100 Kuala Lumpur, Malaysia.
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a
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The Effects of CeO2 Addition on the Physical and Microstructural Properties of ZTA-TiO2 ceramics composite
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ABSTRACT: The effect of CeO2 addition ranging from 0 wt. % to 7 wt. % on phase, microstructural evolution, physical and mechanical properties of ZTA-3 wt. % TiO2 ceramic composite were investigated. The samples were prepared by solid-state mixing
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and sintered at 1600°C for 1hr under pressureless condition. Samples were then characterized by XRD, SEM, densitometer and Vickers indentation method. Based on
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XRD analysis, m-ZrO2 began to diminish at 1 wt.% CeO2 while secondary phases, i.e. Ce0.7Zr0.3O2and Zr0.4Ti0.6O2 initiated at 3 wt.% CeO2 addition. SEM images showed finer grain sizes was produced upon increasing amount of CeO2 up to 5 wt.%, corresponding to higher average grain intercept (AGI) values.From the results obtained, the optimum
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amount of CeO2 addition was at 5 wt. % which yielded the highest bulk density (4.41 g/cm3), firing shrinkage (21.94%), hardness (1580.10HV) and fracture toughness (9.77 MPa.m1/2 ). This is contributed by the grain refinement and the highest amount of
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secondary phases formed, especially Zr0.4Ti0.6O2 . However, with an excessive addition of CeO2, i.e more than 5 wt.%, grain sizes enlarged and the amount of secondary phases
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reduced, which degraded the mechanical properties of ZTA-3 wt. % TiO2.
KEYWORDS: ZTA-TiO2 ceramic composite, CeO2, fracture toughness, Vickers hardness,
microstructure
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1. INTRODUCTION Among the ceramics that can be found in the shell of the earth, alumina (Al2O3) is considered one of the most abundant. Established as the most frequently used ceramic
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material, it comes with several exceptional characteristics. This includes chemical and thermal constancy, resiliency, high melting point, as well as favourable mechanical and insulation properties. Due to these advantages, Al2O3 is very much in demand for a wide range of engineering applications [1–6]. On the downside, however, Al2O3 is saddled with
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a low fracture toughness level of approximately 4 – 5 MPa.m1/2.. A considerable number
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of studies have been conducted on the alumina-zirconia ceramics composite system to overcome the low fracture toughness. These studies were focused on enhancing the system’s mechanical features particularly its strengthening mechanisms [7–9]. Zirconia is utilized as a sintering aid in the Al2O3 matrix. Its applicability for this role is
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attributed to its capacity to make possible the formation of a solid solution, and to bring about the densification process through the attendance of the lattice defect [7]. It has been verified that the fracture toughness of ZrO2 ranges from 6 to 10 MPa.m1/2. In comparison
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to Al2O3 ceramics, this level of fracture toughness is generally twice as high [10]. Wang & Stevens, (1989) [7] and Asharaf et al., (2014) [10] are of the opinion that the presence of
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ZrO2 grains in the Al2O3 matrix facilitates the materialization of a stress-induced toughening mechanism. This is brought about by the transformation of the metastable tetragonal ZrO2 into a thermodynamically stable monoclinic state as the crack tip area undergoes stress. This transformation comes with an escalation in volume expansion which serves to hinder crack propagation. As such, the fracture toughness of Al2O3 is enhanced.
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ACCEPTED MANUSCRIPT Additionally, Al2O3 is considerably affected by the incorporation of TiO2 [11] and CeO2 [12]. According to Dhar et al., (2015) [13], TiO2 incites the sintering and grain development of Al2O3. This serves to boost the bulk density and hardness of ZTA. An addition of a mere 3 wt. % of TiO2 is all it takes to realize optimal hardness [14]. As for
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CeO2, its inclusion swelled the bulk density and lowered the proportion of porosity to register an improvement in the mechanical properties of Al2O3. The results attained by Dhar et al., (2015) revealed that the grain dimensions of ZrO2 and Al2O3 grew in tandem
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with the amount of CeO2 added to Al2O3 [15]. Kumar et al., (2004) [16] forwarded that CeO2 performs the role of a stabilizer to facilitate the full transition of zirconia into its
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tetragonal phase (t-phase). This involves the conversion of zirconia into tetragonal zirconia polycrystals (TZP). Deville et al., (2003) [17] opined that similarities in the Zr4+ valence permits the substitution of the Y3+ ions by the Ce4+ ions. While ceria stabilized zirconia toughened alumina (CSZ-TA) has been applied for biomedical procedures, Y3+
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remains the stabilizer of choice for almost all other applications [18–22]. Although many works have been made into improving the fracture toughness of zirconiatoughened alumina (ZTA), several possibilities remain unexplored. Among them is the
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inclusion of CeO2 in the ZTA system with a TiO2 additive. Accordingly, this study takes
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on the task of examining the viability of using CeO2 to improve the mechanical properties of the ZTA-TiO2 ceramic composite.
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2. METHODS 2.1 Materials and Fabrications Alumina (99.7%, 0.7µm), yttria stabilized zirconia (5.4 mol%, Goodfellow, 94.5%, 0.1 –
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2.0µm), Titania (>99.0%, 0.13µm) and cerium (IV) oxide-gadolinium doped (Sigma Aldrich, 90.0%with 10 mol% gadolinium doped, <0.5µm) were the preliminary powders employed for this work. While TiO2 was set at 3.0 wt. %, the composition of CeO2 was
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fluctuated between 0.0 wt. % and 7.0 wt. % during testing. The composition of ZTA for Al2O3 and YSZ was fixed at a ratio of 4:1 of the residual mass.
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ZTA-TiO2 samples with varying measures of CeO2 were readied for testing. A wet-mixing procedure was used to merge the powders with an adequate quantity of acetone. An oven set at 100oC was then employed to dry the powders for a period of 10 hours. All the powder mixtures were put through a 75µm strainer and subsequently, a uniaxial hydraulic
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hand press set at 200MPa was used to compress the powder mixtures into a 12mm diameter mould. To conclude the process, the green pellets were subjected to pressureless
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sintering at 1600°C for a period of 1hour with a heating and cooling rate of 5 °C/minute. The bulk density of the samples was gauged by way of a Densimeter GK-300, the degree
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of shrinkage was determined through alterations in the size of the samples. Phase analysis was conducted with the use of an X-Ray diffractometer D8 Advance Bruker with CuK radiation functioning at 40 kV, 40 mA with the 2θ ranges 10˚ - 90˚. Scanning was carried out at the rate of 0.034˚/s. A scanning electron microscope (InTouchScope JSM-IT100) was engaged to scrutinize the microstructure of the samples. Subsequent to the coating of the samples with palladium, the standard ASTM E112 – 13 linear intercept procedure was done on the SEM images to determine the average grain intercept (AGI) values. The 4
ACCEPTED MANUSCRIPT Vickers hardness test was performed on samples which were caused to endure a load of 30 kgf for a period of 15 seconds. The resulting surface cracks were measured, and the equation formula was used to compute the fracture toughness values of the samples [23].
(1)
య/మ
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ܭூ = 0.00824
KIc represents the fracture toughness (MPa.m1/2), P the pressure (MPa), c the sum of the
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half length of Vickers diagonal indentation, a (µm), and the length of the radial crack size,
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3. RESULTS AND DISCUSSION
While Figure 1 displays the XRD analyses for each sample with varying amounts of CeO2,
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Table 1 displays the percentage of every phase found in the ZTA-TiO2-CeO2 ceramic composite which was directly obtained from the rietvield refinement of XRD data. As can be observed, the major diffraction peaks are Al2O3 and t-ZrO2. A minor diffraction peak of
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Baddeleyite (monoclinic ZrO2) shows up at 0 wt. % and 0.5 wt. % addition of CeO2. A further addition of CeO2 culminates in the disappearance of this peak. This outcome is
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substantiated by the result obtained by the investigation conducted by Rejab et al. (2013) [15]. As in our investigation, the addition of CeO2 exceeding 0.5 wt. % led to the total disappearance of M-ZrO2 from the composition. For sample from 0 wt. % CeO2 to 1.0 wt. % CeO2, the peak of CeO2 are not visible. For XRD analysis, peaks for any materials that are below 1.0 wt. % will not be detected since the peaks are masked with background radiation. For sample with more than 1 wt. % CeO2, addition of CeO2 has incorporated 5
ACCEPTED MANUSCRIPT onto ZrO2 forming Ce0.7Zr0.3O2 which can be seen in Figure 1 (designated as a purple star). The addition of 3, 5 and 7 wt. % of CeO2 resulted in the appearance of fresh diffraction peaks. These peaks were recognized as Zr0.4Ti0.6O2 and Ce0.7Zr0.3O2. Ce0.7Zr0.3O2 was the
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result of an excess addition of CeO2. This peak came with a cubic fluorite structure as well as an outstanding thermal stability [24–26]. Then again, this finding contradicted to the work of Rejab et al. (2013) [15]. Their work found the formation of merely a sole
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secondary phase (Ce2Zr3O10) at 10 wt. % and 15 wt. % CeO2 addition. This contradiction
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can be traced to the fact that unlike the study conducted by Rejab et al. (2013), this work included TiO2 as an additive into the ceramic composite. Also, the inclusion of 10 to 20 wt. % CeO2 into the ZTA system while excluding both TiO2 and Y2O3 will lead to the
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emergence of the secondary phase CeAl11O18 [27].
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ACCEPTED MANUSCRIPT As illustrated in Table 1, the mineral srilankite was perceived before the addition of 0-1.0 wt. % CeO2. According to Willgallis et al. (1983) [28], srilankite (ZrTi2O6) is among the compositions present in the zirconium titanate (Zr,Ti)O2 disordered solid solution. The zirconium titanate solid solution is recognized as the intermediate compound of the ZrO2–
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TiO2 system [29,30] that takes form when the measure of TiO2 in the ZrO2 system is in between 42 and 67 mol% [31]. This is attributed to the fact that with an escalation in the amount of CeO2, the proportion of ZrO2 becomes lesser than that of TiO2. The absence of
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srilankite beginning from the addition of 3 wt. % CeO2 signifies that the content of TiO2 surpassed 67 mol%. The formation of an ordered solid solution, namely Zr0.4Ti0.6O2, then
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materialized. The development of the secondary phase is due to the addition of material surpassing the solubility limit. This is in agreement with the work of Gao et al. (2010) [32].
Figure 2 shows the microstructure of ZTA-TiO2 with different composition of CeO2.
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Al2O3 and YSZ are represented by dark and white grains respectively. These grains are evenly spread out and no existence of significant agglomeration. The existence of CeO2 and TiO2 grains is untraceable as only a negligible amount of these compounds was used
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for this investigation. The AGI values with varying levels of CeO2 are exhibited in Figure
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3. As can be observed, the higher the AGI value, the smaller is the size of the grain. According to Figure 3, the grain development of Al2O3 is influenced by the addition of CeO2. The grain development of Al2O3 was hindered by the introduction of CeO2 into the ZTA-TiO2 ceramic composite. And at 5 wt. % CeO2, the microstructure was enhanced when the AGI values achieved their highest value with the least possible grain growth. Another contributory factor to this situation is the decrease in unit cell volume as the ratio of Ti to Zr increased. This is attributed to the lesser ionic radius of Ti4+ in comparison to 7
ACCEPTED MANUSCRIPT Zr4+ [33] as soon as Zr is replaced by Ti. Any additional inclusion of CeO2 is deemed a surplus which will no longer impede the grain development of Al2O3. Figure 4 illustrates the firing shrinkage of the ZTA-TiO2 ceramic composite at varying compositions of CeO2. The graph displays an ascendant trend of firing shrinkage from 0
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wt. % CeO2 to 5 wt. % CeO2 (21.19% to 21.94%). Then again, an inclusion of CeO2 above 5 wt. % led to a dip in the firing shrinkage value. While the rise in firing shrinkage value is contributed by the grain refinement of the respective phases, the decline in shrinkage is
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attributed to the rather refractory inclination of the rare earth oxides [34].
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The compressed arrangement of small grains serves to decrease the dimension of the pores. The study conducted by Mitra et al. (2002) [34] indicated that an elevated sintering temperature or a raised CeO2 content will lead to a decrease in the size of the pores. Apparently, the decrease in porosity of the sample contributes towards an elevation in the
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firing shrinkage.
The study conducted by Mangalaraja et al. [35] yielded a completely contrasting outcome to that attained through this investigation. Their efforts saw a progressive dip in the firing
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linkage percentage as the CeO2 inclusion was increased from 0 to 5 vol. %. This could be due to their exclusion of TiO2 from the composition. The absence of TiO2 would have
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affected the grain development performance in such a manner that the level of porosity escalated. In the opinion of Manshor et al. (2015) [11], the introduction of 3wt. % of TiO2 into ZTA improves the grain refinement of Al2O3 to bring about a decrease in the level of porosity. This is in line with the assumption that the inclusion of CeO2 up to 5 wt. % serves to raise the value of the AGI. This is illustrated in Figure 3.
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et al., (2013) [12] recorded similar findings in their work. They reported that the bulk density of the sintered samples recorded a linearly rise from 4.13 g/cm3 to 4.40 g/cm3. This rise in bulk density of the sintered samples can be attributed to the enhanced AGI values
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which reduced grain sizes. This is in line with the findings of Rejab et al., (2013) [15]. On the other hand, the addition of 7 wt. % of CeO2 brought about a substantial decline in bulk
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density of the sintered ceramics composite. This decline is attributed to the surplus quantity of CeO2 that is no longer an effective hindrance to the grain development of Al2O3. The veracity of this finding is backed by the reduced AGI value. A reduced AGI value signifies irregular grain sizes, and irregular grain sizes impede densification. Rejab
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et al., (2013) [12] revealed that the decline in density with an addition of CeO2 in excess of 5 wt. % is attributed to the development of Ce2Zr3O10. The hardness level of ZTA-TiO2 ceramic composite climbed steadily from 1512.64 HV to
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1580.10 HV at 0 wt. % and 5.0 wt. % CeO2 respectively. However a slight drop in the
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hardness level to 1569.51 HV was discerned at 7.0 wt% CeO2. The enhancement in the hardness level of the ceramic composite can be put down to the smaller grain sizes with elevated AGI values. This is made clear by the existence of numerous grain boundaries and diverse angles/orientations that impede the movement of dislocations during indentation. This circumstance raises the capacity of the ceramic composite to defy deformation. The decline in the ceramic composite’s Vickers hardness value when the CeO2 content went beyond 5 wt. % is attributed to the decreased quantity of Al2O3 9
ACCEPTED MANUSCRIPT content. This is portrayed in Table 1. An earlier effort by Rejab et al., (2013) [12] uncovered that the ZTA ceramic composite’s hardness value is enhanced with an increase in the CeO2 content. This is because the increased CeO2 content improves densification while significantly decreasing the porosity of the ceramic composite. On the other hand,
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Naga et al., (2017) [27] put forward that the ZTA ceramics composite’s hardness is adversely affected by the addition of CeO2. They opined that this negative outcome is linked to the development of cerium hexaaluminate (CeAl11O18) and cerium zirconate
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(Ce2Zr3O10).
The fracture toughness of the ceramics composite (Figure 7) was heightened with the increase in CeO2 content from 0 wt. % (6.13 MPa.m1/2) to 5 wt. % CeO2 (9.77 MPa.m1/2). At 0 wt. % and 0.5 wt. % CeO2, the reduced fracture toughness values can be put down to the attendance of m-ZrO2 phases in the ceramics composite. This was verified by the XRD
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pattern (Figure 1). The inclusion of CeO2 exceeding 0.5 wt. % led to a reduction of the mZrO2 phase and an enhancement in the fracture toughness of the ceramics composite. The
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escalation in fracture toughness upon the addition of 3 wt. % and 5 wt. % of CeO2 is linked to the emergence of the Zr0.4Ti0.6O2 phase. However, at 7 wt. % CeO2, the lessening
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amount of the Zr0.4Ti0.6O2 phase brought about a dip in the fracture toughness level. This dip in the fracture toughness level upon the addition of 7 wt.% CeO2 can also be attributed to the attendance of a lesser quantity of ZrO2. In such a circumstance, the ZrO2 toughening mechanism is considerably weakened [2]. It is also found out that the amount of Ce2Zr3O10 for 7 wt.% CeO2 sample is the highest, which is 2.2%. This result is in accordance to previous works by Duran et al. (1990) [36] and Rejab et al. (2013) [15] where the presence of Ce2Zr3O10 will reduce the mechanical properties of ceramic. 10
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4. CONCLUSION Testing was conducted on the phase evolution, the microstructure, and the physical as well as mechanical characteristics of the CeO2 doped ZTA-TiO2 ceramic composite. While the
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departure of the monoclinic phase of ZrO2 commenced at 1 wt. % CeO2, the emergence of the secondary phases began at 3 wt. % CeO2. At 5 wt. % CeO2, the levels of density, shrinkage, hardness and fracture toughness rose to arrive at their respective peak values of 4.41g/cm3, 21.94%, 1580.10HV and 9.77MPa.m1/2. The introduction of CeO2 in excess of
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5 wt. % led to a drop in these levels. This is most likely attributed to the development of
ACKNOWLEDGEMENT
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big grain sizes and the decrease in the quantity of secondary phases.
The authors are grateful to the International Islamic University Malaysia for its financial
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materials science. 25(12) (1990) 5001-5006.
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tentative phase equilibrium diagram for the ZrO2-CeO2 system in air. Journal of
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ACCEPTED MANUSCRIPT Figure 1: X-ray diffraction analyses for all samples with different compositions of CeO2. Figure 2: Microstructure of ZTA-TiO2 ceramics composite with different composition of CeO2 at 1000 magnification (a) 0 wt.% CeO2, (b) 0.5 wt.% CeO2, (c) 1.0 wt.% CeO2, (d) 3.0 wt.% CeO2, (e) 5.0 wt.% CeO2, (f) 7.0 wt.% CeO2. Figure 3: Average Grain Intercept of ZTA-TiO2 at different composition of CeO2.
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Figure 4: Firing Shrinkage of ZTA-TiO2 ceramics composite with different composition of CeO2. Figure 5: Bulk Density of ZTA-TiO2-CeO2 ceramics composite.
Figure 6: Vickers hardness of ZTA-TiO2 ceramics composite with different
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Figure 7: Fracture toughness of ZTA-TiO2-CeO2 ceramics composite.
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Table 1: Percentages of phases present in ZTA-TiO2-CeO2 ceramics composite
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ACCEPTED MANUSCRIPT Table 1: Percentages of phases present in ZTA-TiO2-CeO2 ceramics composite CeO2 (wt %) 0
Al2O3 (wt %) 81.1
t-ZrO2 (wt %) 12.2
m-ZrO2 (wt %) 4.4
Srilankite (wt %) 2.3
0.5
81.7
16.8
1.2
0.3
0
0
1.0
81.4
13.9
0
4.7
0
0
3.0
79.4
14.8
0
1.7
0.7
3.4
5.0
80.0
12.8
0
1.3
0.7
5.2
7.0
80.0
11.7
0
1.9
2.2
4.2
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Zr0.4Ti0.6O2 (wt %) 0
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Ce0.7Zr0.3O2 (wt %) 0
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Research Highlights 1. Effects of CeO2 addition on the ZTA-TiO2 systems were investigated. 2. Small additions of CeO2 lead to improved mechanical properties.
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3. Effects of excessive amount of CeO2 addition have been discussed.