Surface yttria content and hydrothermal ageing behaviour of two step sintered 3Y-TZP ceramics

: In this work the effect of surface yttria concentration on hydrothermal ageing of two-step sintered (TSS) 3 mol. % yttria stabilised tetragonal zirconia polycrystalline (3Y-TZP) ceramics was investigated. Green compacts were prepared by bench pressing and sintered using a two-step sintering with first step sintering temperature of 1400°C for one minute and second step sintering temperature of 1200°C at different holding times of 2, 10 and 20 hours. Surface morphology and surface yttria content of the sintered samples were investigated by using Xray florescence spectroscopy (XRF) and field emission scanning electron microscopy (FESEM), respectively. It was found that the TSS effectively suppressed the grain growth. Increasing the holding time increased densification. Also, an increment in the surface yttria content with increasing holding time was observed. The ageing studies were conducted in an autoclave containing super-heated steam at 180°C and 10 bar pressure for 100 hours. Ageing resistance increased with increasing second step sintering time. The enhanced hydrothermal degradation of lower holding time samples can be attributed to the deficiency of surface yttria. Effect of ageing on surface yttria concentration and surface morphology were also investigated by using XRF and FESEM, respectively. The surface yttria concentration decreased with ageing which confirms leach out of surface yttria during hydrothermal ageing.


INTRODUCTION
Zirconium composites are derived from natural minerals, mainly zircon (ZrSiO 4 ) and baddeleyite (ZrO 2 ). Zirconia was first found from Zircon from Sri Lankan beach sand (Kreidl, 1942). Zirconium oxide is stabilised in their tetragonal phase by 3 mol.% yttrium oxide. 3 mol.% yttria-stabilised tetragonal zirconia polycrystalline (3Y-TZP) ceramic is one of the most widely used ceramic materials in various applications. It is used as grinding media, cutting tools, spares for chemical pumps, roller guides, and tweezers (Jue et al., 1991;Liang et al., 2009). Yttria stabilised tetragonal zirconia polycrystals (Y-TZP) are increasingly used as biomaterial particularly in dental and bone restorations in recent years due to its superior mechanical properties, chemical and environmental inertness, biocompatibility and aesthetic nature (Piconi & Maccauro, 1999;Chevalier, 2006;Flinn et al., 2012). The martensitic nature of tetragonal to monoclinic phase transformation is one of the extreme technological importance in zirconia ceramics (Subbarao et al., 1974). Transformation toughening in zirconia is a landmark innovation in obtaining high strength and toughness ceramic as first reported by Garvie et al. (1975). The retention of tetragonal phase is an essential prerequisite

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Journal of the National Science Foundation of Sri Lanka 49 (2) for toughening, since in the presence of a propagating crack the tetragonal particles are induced to transform to the monoclinic phase with an accompanying volume expansion of 3-5%. Consequently, an advancing crack is subjected to a compressive stress, its progress is either halted or retarded, and the material is toughened. This is quite similar to the toughening mechanism in some steels and as such zirconia has been labelled 'ceramic steel' (Garvie et al., 1975). However, the mechanical properties deteriorate rapidly in the presence of moisture at relatively low temperature (65-500°C), a phenomenon known as low temperature degradation (LTD) or hydrothermal ageing (Lawson, 1995). During hydrothermal ageing, the metastable tetragonal zirconia surface grains transform to monoclinic phase accompanied with volume expansion. Volume expansion in the compact ceramic leads to microcracks (Kobayashi et al., 1981), grain pullout and finally surface roughening (Lawson, 1995;Bartolomé et al., 2004;Hernández-Montoya et al., 2012), which eventually leads to property degradation and failure in the ceramic sample.
Grain size of the 3Y-TZP was controlled by tailoring the sintering condition (Mazaheri et al., 2008;Sutharsini et al., 2018Sutharsini et al., , 2017Sutharsini et al., , 2014bVignarajah et al., 2019). Grain size increased with increasing sintering temperature (Ramesh & Gill, 2001;Ramesh et al., 2013Ramesh et al., , 2016Ramesh et al., , 2018Sutharsini et al., 2014a;b, 2017Ting et al., 2016;Ubenthiran et al., 2018;Dhuban et al., 2019;Ng et al., 2019;Vignarajah et al., 2019). However, high temperature sintering is important for densification which directly influences the mechanical properties. Two step sintering is a promising approach to achieve high densities with controlled grain growth (Mazaheri et al., 2008;Sutharsini et al., 2014bSutharsini et al., , 2017Sutharsini et al., , 2018Vignarajah et al., 2019). In the two-step sintering technique, the samples are sintered at higher temperature for a short time to achieve critical grain size and then it is sintered at lower temperature for a long time to reach maximum densification. As reported earlier, the TSS samples showed excellent hydrothermal ageing resistance and high densification . It was also observed that ageing resistance increased with increasing holding time but without any significant changes in the grain size. In this work, different ageing behaviours of the two-step sintered 3Y-TZP ceramic with different holding times were investigated but with the same grain size. The surface yttria content of the samples was investigated and the results were analysed.

METHODOLOGY
Green samples were prepared by using 3 mol.% yttriastabilised tetragonal zirconia polycrystalline powder (Kyoristu, Japan) and the pressing method reported elsewhere (Ubenthiran et al., 2013;Sutharsini et al., 2014a;b, 2017). Two step sintering technique was used to sinter the samples. Green samples were initially heated to the first step sintering temperature of 1400 °C and held at that temperature for 1 min, cooled down to 1200 °C and held at the same temperature for different holding times of 2 h, 10 h and 20 h. Thereafter, the samples were allowed to cool down to room temperature. The heating and cooling rate was fixed at 10 °C/min. Assintered samples were aged in a stainless-steel autoclave containing superheated water steam at the temperature 180°C and pressure 10 bar up to 100 h.
Crystal structure of the samples was investigated by using X-ray diffraction (XRD) (PANalytical Empyrean, Netherlands). Phase transformation was estimated by using the method proposed by Toraya et al. (1984). Surface yttria content of the as-sintered and aged 3Y-TZP ceramics was measured using a micro-XRF analyser. The XRF measurements were conducted at 20 kV and 700 μA with a spot size of 30 μm on as-sintered and aged samples by using Orbis micro-XRF analyzer. Hundred points were randomly selected on the surface to obtain average surface yttria concentration and distribution. Microstructural evaluation was conducted by using FESEM (Auriga Zeiss Ultra-60). The samples were polished and thermally etched before FESEM investigation for clear identification of grains. Average grain sizes of the samples were calculated by line intercept method (ASTM, 2013).

Results
Microstructural properties of the as-sintered 3Y-TZP samples were studied using FESEM. Figure 1 shows the FESEM images of as-sintered samples. The micrographs revealed uniform grain size and distribution regardless of sintering holding time as shown in Figure 1  2 hours, 10 hours and 20 hours were measured as 0.29 ± 0.16 μm, 0.29 ± 0.021 μm 0.28 ± 0.024 μm, respectively.
There were no significant changes observed in the samples regardless of the holding time.
shows the FESEM images of as-sintered samples. The micrographs revealed uniform grain size and distribution regardless of sintering holding time as shown in Figure 1 (a)-(c). Grain size of the samples as-sintered at holding times of 2 hours, 10 hours and 20 hours were measured as 0.29 ± 0.16 μm, 0.29 ± 0.021 μm 0.28 ± 0.024 μm, respectively. There were no significant changes observed in the samples regardless of the holding time. The surface yttria content of the as-sintered samples was also measured. The effect of holding time on surface concentration is shown in Figure 2. As the holding time increased, surface yttria concentration increased and a narrow distribution is observed for long holding times. The average surface yttria content of the samples sintered with holding times of 2 hours, 10 hours, and 20 hours were measured as 2.48 ± 0.19, 2.62 ± 0.15, and 2.74 ± 0.09 mol %, respectively. Hence, it was evident that the surface yttria content increases with increasing holding time. Moreover, the yttria distribution curve got narrower with increased holding time. The surface yttria content of the as-sintered samples was also measured. Effect of holding time on surface concentration is shown in Figure 2. As the holding time increased, surface yttria concentration increased and a narrow distribution is observed for long holding times. The average surface yttria content of the samples sintered with holding times of 2 hours, 10 hours, and 20 hours were measured as 2.48 ± 0.19, 2.62 ± 0.15, and 2.74 ± 0.09 mol %, respectively. Hence, it was evident that the surface yttria content increases with increasing holding time. Moreover, the yttria distribution curve got narrower with increased holding time.

Discussion
The experimental data clearly shows that the two-step sintering suppress the grain growth of 3Y-TZP ceramics. There were no significant changes observed in the grain size even if the holding time was extended up to 20 hours. As a result, it is possible that the sintering profile (1400 °C as first step temperature for 1 min and 1200°C as second step temperature with different holding times of 2 hours, 10 hours and 20 hours) used in this work lies within the kinetic window of 3Y-TZP . However, an increment in relative density was observed in a previous study . As

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Journal of the National Science Foundation of Sri Lanka 49 (2) reported earlier, TSS freeze the grain growth at the end of first step sintering and densification was achieved during the second step sintering. The dominant mechanism for frozen grain growth is suppression of grain boundary migration before the second stage of sintering caused by immobilizing the triple-point junctions, due to rapid cooling after first step sintering and densification achieved by grain boundary diffusion . Then it was found that TSS enhanced the hydrothermal ageing resistance which is consistent with previous work Vignarajah et al., 2019). The interesting observation here is that two step sintering revealed different ageing behaviours for different samples with the same grain size distribution. In addition, regardless of the same grain size, the tetragonal to monoclinic phase transformation decreased with increasing sintering holding time Vignarajah et al., 2019). To identify the mechanism behind the unique ageing behaviour, the surface yttria concentration of the samples was studied. A correlation between the surface yttria content and ageing behaviour of TSS samples with different holding times was found. Sample with the highest surface yttria content showed no phase transformation even after 100 hours of ageing. Also the monoclinic content after 100 hours of ageing increased with reducing surface yttria content. It suggests that uniform and high surface yttria distribution might enhanced hydrothermal ageing resistance of the sample sintered with holding time of 20 hours.
In order to study the effect of ageing on surface yttria concentration, surface yttria concentration of the assintered and 100 hour-aged sample were measured. Then, the 100 hour-aged sample was re-sintered and surface yttria content was measured. Re-sintering was done using single step sintering at 1400 °C for 2 hours. Figure 4 shows the average surface yttria concentration of the as-sintered, 100 hour-aged and re-sintered samples. It is clearly seen that the ageing reduces the surface yttria content. Decrease in yttria concentration with hydrothermal ageing is reported by some researchers (Thompson & Rawlings, 1992;Kimel & Adair, 2002;Ho & Wei, 2004;Papanagiotou et al., 2006). The reduction in surface yttria content can be explained in two ways: 1) grain pull-out and 2) yttria leach-out during ageing. Firstly, ageing induced grain pull-out expose the interior grains potentially with low yttria content. Thompson and Rawlings (1992) proposed that tetragonal to monoclinic phase transformation is a consequence of the leaching of the yttrium from the zirconia particles thereby reducing their stability. Papanagiotou et al. (2006) and Sutharsini et al. (2017) also supported that loss of yttria, which could potentially reduce the tetragonal phase stability. However, Ho and Wei (2004) observed the different trends between yttrium dissolution and monoclinic phase transformation. Hence, they proposed that yttria dissolution is not the controlling factor leading to the monoclinic phase transformation. On the other hand, Lange et al. (1986) reported the formation of α-Y(OH) 3 were observed for the sample sintered for 20 hours and hence Figure 1

Discussion
The experimental data clearly shows that the two-step sintering suppress the grain growth of 3Y-TZP ceramics. There were no significant changes observed in the grain size even if the holding time was extended up to 20 hours. As a result, it is possible that the sintering profile (1400°C as first step temperature for 1 min and 1200°C as second step temperature with different holding times of 2 hours, 10 hours and 20 hours) used in this work lies within the kinetic window of 3Y-TZP . However, an increment in relative density was observed in a previous study . As reported earlier, TSS freeze the grain growth at the end of first step sintering and densification was achieved during the second step sintering. The dominant mechanism for frozen grain growth is suppression of grain boundary migration before the second stage of sintering caused by immobilizing the triple-point junctions, due to rapid cooling after first step sintering and densification achieved by grain boundary diffusion . Then it was found that TSS enhanced the hydrothermal ageing resistance which is consistent with previous work Vignarajah et al., 2019). The interesting observation here is that two step sintering revealed different ageing behaviours for different samples with the same grain size distribution. In crystallite on the aged sample. They also postulate that there could be a possible Yttrium diffusion during ageing. However, Yoshimura et al. (1989) disproved the yttria diffusion theory due to the very slow yttria diffusion at low temperatures.
Therefore, it is suspected that the reduction in surface yttria concentration confirmed yttria leach out from the surface grain during hydrothermal ageing. It is therefore proposed that the mechanism of hydrothermal ageing relies on dissolution of yttria from 3Y-TZP ceramics. Removal of surface yttria concentration reduces the tetragonal stability of 3Y-TZP ceramics. As a result, tetragonal zirconia transformed to monoclinic zirconia with hydrothermal ageing.
can be concluded that keeping the surface yttria content nearly 3 % in the as-sintered articles would enhance the ageing resistance of 3Y-TZP ceramics.

Conflict of interest
Authors declare no conflict of interest. Therefore, it is suspected that the reduction in surface yttria concentration confirmed yttria leach out from the surface grain during hydrothermal ageing. It is therefore proposed that the mechanism of hydrothermal ageing relies on dissolution of yttria from 3Y-TZP ceramics. Removal of surface yttria concentration reduces the tetragonal stability of 3Y-TZP ceramics. As a result, tetragonal zirconia transformed to monoclinic zirconia with hydrothermal ageing.

Conclusions
It was shown that TSS can be used as a tool to achieve highly dense 3Y-TZP ceramic with controlled grain growth and excellent hydrothermal ageing resistance. Surface yttria content of the two-step sintered 3Y-TZP is not exactly 3 % and it increases with increasing second step holding time. Hence, it was postulated that the surface yttria content is an important factor in determining the ageing resistance of two-step sintered 3Y-TZP ceramic with the same grain size. Surface yttria concentration decreased after 100 hours of ageing, which is a direct evidence for yttria leach out during ageing. It can be concluded that keeping the surface yttria content nearly 3 % in the as-sintered articles would enhance the ageing resistance of 3Y-TZP ceramics.

CONCLUSIONS
It was shown that TSS can be used as a tool to achieve highly dense 3Y-TZP ceramic with controlled grain growth and excellent hydrothermal ageing resistance. Surface yttria content of the two-step sintered 3Y-TZP is not exactly 3 % and it increases with increasing second step holding time. Hence, it was postulated that the surface yttria content is an important factor in determining the ageing resistance of two-step sintered 3Y-TZP ceramic with the same grain size. Surface yttria concentration decreased after 100 hours of ageing, which is a direct evidence for yttria leach out during ageing. It