Sintering behaviour and properties of graphene oxide-doped Y-TZP ceramics
Introduction
Yttria-tetragonal zirconia polycrystals (Y-TZP) are known for its excellent strength, high fracture toughness and good wear resistance [1], [2], [3], thus making it one of the most sought after material for dentistry, orthopaedics and host of engineering applications such as oxygen sensors and electrolytes in fuel cells [4], [5]. These excellent mechanical properties of Y-TZP is associated with its ability to undergo transformation toughening as reported initially by Garvie et al. [6]. In this mechanism, the stress produced by a propagating crack will be absorbed by the metastable tetragonal (t) grains thus causing them to transform into monoclinic (m) symmetry [7], [8]. This martensitic diffusionless (t) to (m) phase transformation occurs at room temperature and is accompanied by a 3–5% volume expansion resulting in formation of compressive stress around the crack tip, hence preventing further crack propagation [9], [10].
One of the major drawbacks of Y-TZP ceramic is its predisposition to hydrothermal ageing, also known as low-temperature degradation (LTD), leading to a spontaneous (t) to (m) phase transformation and a concomitant deterioration of mechanical properties [11], [12]. The LTD occurs when the Y-TZP ceramic is exposed in water at temperatures of 65–300 °C or in hot aqueous solution resulting in the undesirable (t) to (m) phase transformation [13], [14], [15]. The LTD or ageing was first discovered by Kobayashi et al. [16] who reported that the Y-TZP exhibited a spontaneous monoclinic phase transformation when exposed to humid environment, progressing from the free surface to the interior of the ceramic, accompanied by macro- and micro-cracks and eventually leading to catastrophic failure of the ceramic. Since the discovery of LTD, numerous research were conducted to study the factors governing ageing and to suppress the devastating effect of LTD [17], [18], [19], [20]. Although the underlying LTD mechanism is still under debate but it has been widely accepted that the ingression of the hydroxyl ions from the test medium into the zirconia lattice structure resulted in localised stress being generated that in turn destabilises the tetragonal grains [21], [22].
To date an economical solution has yet to be found to completely eradicate the LTD, nevertheless there have been reports on the beneficial effect of sintering additives or dopants such as CuO, Nb2O5, MnO2 and Al2O3 [1], [23], [24], [25] in retarding the rate of phase transformation. It has been suggested that the ageing-induced (t) to (m) transformation starts at the grain boundary regions and progresses towards the grain centre. As such, the addition of sintering additives could help in modifying grain boundary regions, promote densification at low sintering temperature and enhance the mechanical properties of Y-TZP [26], [27], [28]. Ramesh et al. [29] studied the effect of adding small amount of MnO2 on the densification and mechanical properties of Y-TZP. They found that samples containing ≥0.3 wt% MnO2 exhibited 97.5% relative density and Young's modulus above 200 GPa as compared to 91.8% and 175 GPa for the undoped ceramic when sintered at 1250 °C. Wu et al. [23] studied the effect of Al2O3 on the sintering behaviour of Y-TZP and the authors found that Al2O3 had a significant effect in supressing LTD and that the phase transformation rate increases with the increase of sintering temperature. Ramesh et al. [30] studied the LTD behaviour of undoped and CuO-doped Y-TZP in superheated steam. They observed that after exposure for 200 h, the 0.05 wt% CuO-doped zirconia showed some resistance to LTD and exhibited about 37% monoclinic content as compared to 85% in the undoped Y-TZP. The authors concluded that the dissolution of yttria near the grain boundary regions due to hydroxyl reaction led to the phase transformation.
In this present study, the effect of doping small amounts of graphene oxide (GO) on the densification and mechanical properties of 3 mol% Y-TZP ceramic was investigated. Furthermore the LTD resistance of GO-doped zirconia was evaluated in an autoclave containing superheated steam at 180 °C/10 bar vapour pressure.
Section snippets
Sample preparation
A commercially available 3 mol% Y-TZP powder was obtained from Tosoh Corporation, Japan. The starting zirconia powder had a total impurity concentration up to 0.1 wt% with Al2O3, SiO2, Fe2O3 and Na2O as major impurities. Varying amount of high purity graphene oxide (0.05, 0.1, 0.2, 0.5 and 1 wt%) obtained from Sigma-Aldrich, USA was doped with the Y-TZP powder through attrition milling (Union Process, USA) at 550 rpm for 30 min with zirconia balls (5 mm diameter) used as the grinding medium and
Results and discussion
The XRD analysis conducted on the as-received Y-TZP powder indicated the presence of ~17% monoclinic (m) phase and ~83% tetragonal (t) phase content. Similar phase profile was found in all the GO-doped Y-TZP powders as depicted in Fig. 1, thus indicating that the dopant had negligible effect on the phase content of the starting zirconia powder. After sintering, all the sintered samples exhibited a fully tetragonal phase regardless of the amount of dopant addition and sintering temperature.
The
Conclusions
In this present work the effect of addition of small amounts of graphene oxide as sintering additives on the densification and properties of Y-TZP were evaluated and contrasted with that of undoped Y-TZP. It was found that the addition of GO did not disrupt the tetragonal phase stability in Y-TZP throughout the sintering regime. The dopant was beneficial in enhancing the densification and improved the mechanical properties pf Y-TZP when sintered at low sintering temperature, particularly at 1200
Acknowledgements
The authors thank University of Malaya for providing the funding for this research under the FRGS Grant no. of FP056-2015A and PPP Grant no. PG055-2015A.
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2022, Journal of the European Ceramic SocietyCitation Excerpt :The monoclinic content in the composites incorporating e-GNP, FLG or rGO nanostructures after ageing for 24 h is considerably lower than the measured one in monolithic zirconia. This result indicates that rGO is potentially very effective also in restricting the ageing-induced t-m transformation, in contrast with the accelerated LTD trend found when GO is added to air-sintered zirconia [72]. Furthermore, the 1FLG1250 and 2.5e-GNP1300 composites are very attractive since they exhibit similar LTD restriction than the 2.5rGO one with a lower FLG amount or with e-GNP obtained from cost-effective GNP.