THE EFFECT OF IRON OXIDE ON THE MECHANICAL AND AGEING PROPERTIES OF Y-TZP CERAMIC.
SIVANESAN SIVAKUMAR
1,a, HSIEN LOONG TEOW
2,b*, RAMESH SINGH
3,c, ALI NIAKAN
3, NOBUYUKI MASE
4,d1School of Engineering, Taylor’s University, Taylor's Lakeside Campus, No. 1 Jalan Taylor's, 47500, Subang Jaya, Selangor DE, Malaysia
2School of Engineering, INTI International College Subang, No. 3, Jalan SS15/8, 47500, Subang Jaya, Selangor DE, Malaysia
3Department of Mechanical Engineering, University of Malaya, 50603, Kuala Lumpur, Malaysia
4Green Energy Research Division, Research Institute of Green Science and Technology,Department of Engineering,Shizuoka University, Japan
a[email protected], b[email protected],
c[email protected], d[email protected]
Keywords: Y-TZP, Zirconia, Iron Oxide, Densification, Mechanical Properties, Ageing
Abstract. Small amounts of iron oxide (Fe2O3) was added to the commercially available 3 mol% Y- TZP as a sintering aid over a temperature range of 1250°C to 1500°C. Sintered samples were then evaluated to determine the bulk density, Vickers hardness, and fracture toughness. In addition, hydrothermal ageing experiments to determine the tetragonal phase stability were performed on selected sintered samples in superheated steam at 180°C / 10 bar for up to 24 hours. Based on the work carried out, it was revealed that additions of Fe2O3 particularly ≥ 0.3 wt% was indeed beneficial in aiding densification, improving the matrix stiffness and hardness when compared to undoped Y-TZP sintered at temperatures below 1350°C. Addition of Fe2O3 was found to have negligible effects on the fracture toughness of all samples with the exception of the 0.5 and 1 wt%
doped Y-TZP sintered above 1400°C. Hydrothermal ageing resistance of Y-TZP was found to be enhanced with the addition of Fe2O3 in the Y-TZP matrix.
Introduction
Zirconia is widely used in many engineering applications for example usage of zirconia to strengthen the silicon carbine ceramics [1], fuel cell’s oxygen sensor, heating elements and extrusion dies [2,3]. One of the most successful usage of Y-TZP ceramics are found to be in the biomedical field as orthopaedic femoral heads for total hip replacement due to its biocompatibility [4]. Y-TZP also suffers from a major weakness. This phenomenon is known to many researchers as ageing or low temperature degradation (LTD). The mechanism responsible for this LTD phenomenon has yet to be resolved [5,6]. In general, it is perceived by many researchers that additions of small amounts of dopants are able to promote densification and hence controlling the microstructure which influence its mechanical and LTD properties of the sintered Y-TZP body [7- 10]. In the present work, the effect of small amounts of Fe2O3 as sintering additives for commercially available 3 mol% Y-TZP on its densification and mechanical properties is reported.
Besides that, the effect of small amounts of Fe2O3 on the Y-TZPs ageing properties is also evaluated.
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Materials and methods Sample preparation
The 3 mol% Y-TZP as-received powder, supplied by Kyoritsu Japan, manufactured though co-precipitation method had a total impurity concentration of approximately 0.1 wt% with major impurities of SiO2, Fe2O3, TiO2 and Al2O3. The powders were then compacted at 0.3 MPa followed by cold isostatic pressing at 200 MPa to produce disc (20 mm diameter). These powders were then consolidated by employing presureless sintering in air using a rapid heating furnace manufactured by ModuTemp, Australia. The sintering temperature employed for the pressure sintering were ranged from 1250°C to 1500°C with 2 hours holding time before cooling to ambient temperature.
The as-sintered samples were then grinded on one face using silicon carbide papers ranged from grades 120 to 1200. Subsequently, these samples were polished with diamond paste of 6 µm followed by 1 µm to produce a clear surface for optical imaging device.
Results and Discussion
The effect of doping Y-TZP with varying amounts of Fe2O3 (0 to 1 wt%) on the variation between bulk density and sintering temperature is as shown in Fig. 1. Taking the theoretical density (T.D) of Y-TZP as 6.1 Mgm-3, it is found that higher bulk density can be achieved when sintered at low temperature at below 1400°C. Upon sintering at higher temperature particularly at 1500°C, the sample for 0.5 and 1 wt % exhibits a reduction in bulk density as it reduced to 98.52 % and 96.2 % respectively.
Figure 1: Effect of sintering temperature and Fe2O3 on the bulk density of Y-TZP sintered at various temperatures.
Based on Fig. 1, it is found that there is a significant reduction in the sintering temperature of Y-TZP doped with Fe2O3. For an instance, the sample with addition 0.3 wt% Fe2O3 is able to achieve 98.69 % T.D. when sintered at 1250°C but for the case of undoped Y-TZP, it requires sintering up to 1350°C to achieve a similar bulk density.
Viscous flow mechanism might be responsible for the rapid densification at lower temperature as this increases the contact area of the particles in a compact solid which in turn enhances densification as it promotes mass diffusivity of the zirconia matrix. These findings are in agreement in the works of Zhang et al. [11]. The Vickers hardness of the Y-TZP doped with Fe2O3
is as shown in Fig. 2. The hardness for Y-TZP doped with 0.3 wt% Fe2O3 was found to be one of
91 92 93 94 95 96 97 98 99 100
1200 1250 1300 1350 1400 1450 1500 1550
Relative Bulk Density (%)
Sintering Temperature (°C) 0 wt% Fe 0.05 wt% Fe 0.1 wt% Fe 0.3 wt% Fe 0.5 wt% Fe 1 wt% Fe
the highest when sintered at low temperature i.e. 1250°C as it achieved hardness of 13.4 GPa. As the sintering temperature increases, it reaches a maximum hardness value of 13.7 GPa when sintered at 1350°C. However, increasing the sintering temperature above 1350°C was undesirable as it revealed that the hardness value decreasing as sintering temperature increases. As the sintering temperature increases from 1400°C to 1500°C, the hardness value decreases from 13.6 GPa to 12.6 GPa.
Figure 2: Effect of Fe2O3 addition on the Vickers hardness of Y-TZPs at various sintering temperatures.
The effect of Fe2O3 addition on the fracture toughness of Y-TZP sintered at various temperatures is as shown in Fig. 3. It is found that addition of Fe2O3 up to 0.3 wt% do not have any effect on the fracture toughness as the values of fracture toughness maintained between a range of 4.54 to 5.03 MPam1/2. As it is established that the transformation toughening mechanism are influenced by the phase stability of the (t) grains, the values obtained for fracture toughness can be used as an indicator to indicates the phase stability of (t) grains in the zirconia matrix [12]. In general, if the (t) grains are in metastable state, it would result in high value of fracture toughness.
Based on the present work, there is no indication of enhancement in fracture toughness for samples doped with up to 0.3 wt% Fe2O3. On the other hand, for the Y-TZP doped with 1 wt% Fe2O3, when sintering temperature was increased from 1400°C to 1450°C, the fracture toughness increased from 5.02 to 5.72 MPam1/2 and following an increase of sintering temperature to 1500°C, the fracture toughness increased drastically to 7.34 MPam1/2. The result obtained is in agreement with the XRD phase analysis whereby presence of monoclinic (m) phase was detected in this sample about ~4.5
%.
Based on these results computed, it can be suggested that there might be a mechanism responsible for the abrupt increase in fracture toughness. This mechanism was triggered causing yttria segregation in the Y-TZP matrix. This is in agreement with the data obtained from the XRD phase analysis as there were small amount of monoclinic phase was detected from the samples for 1 wt% Fe2O3 when sintering temperature of 1500°C.
9.0 10.0 11.0 12.0 13.0 14.0 15.0
1200 1250 1300 1350 1400 1450 1500 1550
Vickers Hardness (GPa)
Sintering Temperature (°C) 0 wt% Fe 0.05 wt% Fe 0.1 wt% Fe 0.3 wt% Fe 0.5 wt% Fe 1 wt% Fe
Figure 3: The effect of various amounts Fe2O3 addition on the fracture toughness of Y-TZPs at various sintering temperatures.
On the other hand, the remaining tetragonal grains would remain in metastable state and also having optimum amount of yttria required would readily undergoes transformation toughening when stress is induced caused by propagating crack hence resulting in enhancement of fracture toughness as shown in Fig. 3.
Figure 4: The effect of hydrothermal ageing on the monoclinic phase content development in Y- TZPs sintered at 1350°C.
The effect of Fe2O3 addition on the hydrothermal ageing of Y-TZP’s tetragonal phase stability has also been studied. The samples sintered at 1350°C were selected and exposed to superheated steam for a duration of up to 24 hours. The effect of hydrothermal ageing on the monoclinic phase development in Y-TZPs sintered at 1350°C is as shown in Fig. 4. Based on the observation in Fig. 4, the hydrothermal ageing resistance properties were significantly improved for the samples of Y-TZPs doped with Fe2O3 as compared to the undoped Y-TZP. The 0.3 wt% Fe2O3- doped samples only begins to undergo phase transformation slowly when exposed to more than 6 hours and attained about 7% content after 24 hours of exposure. The observations obtained shows that addition of small amount Fe2O3 is beneficial in enhancing Y-TZP’s ageing properties. Further works are needed to understand the role of Fe2O3 on supressing the (t) to (m) phase transformation.
However, it is believed that Fe2O3-rich compounds formed would have prevented the hydroxyl reactions with Yttria close to the grain boundary regions [13].
4 4.5 5 5.5 6 6.5 7 7.5 8
1200 1250 1300 1350 1400 1450 1500 1550 Fracture Toughness (MPam1/2)
Sintering Temperature (°C) 0 wt% Fe
0.05 wt% Fe 0.1 wt% Fe 0.3 wt% Fe 0.5 wt% Fe 1 wt% Fe
0 10 20 30 40 50 60 70 80 90 100
0 5 10 15 20
Monoclinic Content (%)
Ageing Time (h)
0 wt% Fe 0.05 wt% Fe 0.1 wt% Fe 0.3 wt% Fe
Conclusions
The beneficial effect of Fe2O3 addition in enhancing mechanical properties and retarding the hydrothermal degradation of Y-TZPs ceramics has also been presented in this work. In the present work, it is revealed that addition 0.3 wt% Fe2O3 is the most effective in enhancing mechanical properties since only 7% of monoclinic phase was detected after 24 hours as compared to while samples doped with 0.5 wt% and 1 wt% exhibited more than 20% monoclinic phase within the same duration. This shows the efficacy of 0.3 wt% Fe2O3 in retarding ageing induced phase transformation.
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