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Highly-Translucent, Strong and Aging-Resistant 3Y-TZP Ceramics for Dental Restoration by Grain Boundary Segregation

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Highly-Translucent, Strong and Aging-Resistant 3Y-TZP Ceramics for Dental Restoration by Grain Boundary Segregation Acta Biomaterialia 16 (2015) 215–222 Contents lists available at ScienceDirect Acta Biomaterialia journal homepage: www.elsevier.com/locate/actabiomat Highly-translucent, strong and aging-resistant 3Y-TZP ceramics for dental restoration by grain boundary segregation ⇑ Fei Zhang a, , Kim Vanmeensel a, Maria Batuk b, Joke Hadermann b, Masanao Inokoshi c, Bart Van Meerbeek c, Ignace Naert c, Jef Vleugels a a Department of Materials Engineering, KU Leuven, Kasteelpark Arenberg, 44, B-3001 Heverlee, Belgium b Electron Microscopy for Materials Science (EMAT), University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium c KU Leuven BIOMAT, Department of Oral Health Sciences, KU Leuven & Dentistry, University Hospitals Leuven, Kapucijnenvoer, 7, blok a, B-3000 Leuven, Belgium article info abstract Article history: Latest trends in dental restorative ceramics involve the development of full-contour 3Y-TZP ceramics Received 22 November 2014 which can avoid chipping of veneering porcelains. Among the challenges are the low translucency and Received in revised form 16 January 2015 the hydrothermal stability of 3Y-TZP ceramics. In this work, different trivalent oxides (Al2O3,Sc2O3, Accepted 26 January 2015 Nd O and La O ) were selected to dope 3Y-TZP ceramics. Results show that dopant segregation was a Available online 4 February 2015 2 3 2 3 key factor to design hydrothermally stable and high-translucent 3Y-TZP ceramics and the cation dopant radius could be used as a controlling parameter. A large trivalent dopant, oversized as compared to Zr4+, Keywords: exhibiting strong segregation at the ZrO grain boundary was preferred. The introduction of 0.2 mol% Ceramic structure 2 La O in conventional 0.1–0.25 wt.% Al O -doped 3Y-TZP resulted in an excellent combination of high Dental restorative material 2 3 2 3 Zirconia translucency and superior hydrothermal stability, while retaining excellent mechanical properties. Aging Ó 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Translucency 1. Introduction environment, roughening of the surface finish, esthetic degrada- tion, loss of mechanical properties and even catastrophic failure The increased esthetic demands of patients for both anterior [12–14]. Long-term stability of Y-TZP ceramics is therefore crucial and posterior teeth are driving the development of dental restor- for biomaterial applications. ative materials from traditional porcelain-fused-to-metal (PFM) Adding a small amount of alumina has proven to be effective in to all-ceramic restorations [1,2]. Yttria-stabilized tetragonal zirco- increasing the aging resistance of 3Y-TZP ceramics [15]. 0.25 wt.% nia polycrystalline (Y-TZP) ceramics, commonly containing 3 mol% alumina was normally added, but lower amounts of alumina (to yttria, have been widely accepted as a promising material for 0.1 or 0.05 wt.%) are used in new generations of high-translucent fabricating dental crowns and fixed dental prostheses (FDP) 3Y-TZP ceramics [5,16]. because of their superior biocompatibility [3], excellent mechani- The low translucency of conventional 3Y-TZP ceramics is its cal properties and high esthetic potential [4–6]. The brittleness of major drawback to mimic the optical properties of natural tooth most dental restorative ceramics limited their use only for small enamel [17,18]. An esthetic porcelain veneer is therefore needed anterior restorations [1]. 3Y-TZP ceramics are tough and strong, to cover the opaque zirconia. However, chipping or de-bonding allowing the fabrication of larger all-ceramic restorations and in of the veneering porcelain from the zirconia framework is com- the highly-loaded molar area [4,7,8], which is mainly due to the monly reported in clinics [14,19]. Although fracture of the strong transformation toughening effect of submicrometer sized tetrago- zirconia substructure is rarely reported, the contribution of strong nal zirconia [9,10]. core materials to the performance of all-ceramic restorations was However, 3Y-TZP ceramics suffer from low-temperature degra- obviously offset by the veneering fractures [20]. Therefore, high- dation (LTD), i.e. the spontaneous transformation of the tetragonal translucent Y-TZP ceramics are becoming popular in restorative to monoclinic ZrO2 phase in the presence of water (hydrothermal dentistry in order to make full-contour zirconia restorations aging) [11,12]. Hydrothermal aging can result in enhanced wear without the need for esthetic veneering porcelain [21–23]. rates with the release of small zirconia particles in the surrounding Although lowering the amount of alumina addition from 0.25 to 0.1 or 0.05 wt.% can increase the translucency, the hydrothermal stability is simultaneously lowered [16,24]. Moreover, hydrother- ⇑ Corresponding author. Tel.: +32 16 37 77 38. E-mail address: [email protected] (F. Zhang). mal aging of full-contour Y-TZP restorations could be more crucial, http://dx.doi.org/10.1016/j.actbio.2015.01.037 1742-7061/Ó 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. 216 F. Zhang et al. / Acta Biomaterialia 16 (2015) 215–222 since they are in direct contact with the oral fluid. On the other Transmission electron microscopy (TEM) analysis was per- hand, another strategy of increasing the yttria content and intro- formed to examine the distribution of dopant cations (La3+,Nd3+, ducing cubic phase zirconia was also used to improve the translu- Al3+ or Sc3+), Y3+and Zr4+ around the grain boundaries. Electron cency of Y-TZP ceramics [5,16,25]. Aging-resistant Y-TZP ceramics transparent samples were prepared by ion-milling with an Ion can be obtained, but this strategy sacrifices their excellent flexural Slicer (EM-09100IS, Jeol, Japan). High angle annular dark field scan- strength and fracture toughness due to the partial loss of the ning transmission electron microscopy (HAADF-STEM) images and transformation toughening effect of tetragonal zirconia [5,26]. energy-dispersive spectroscopy (STEM-EDS) elemental maps were Therefore, the aim of this work was to design hydrothermally obtained on a FEI Titan 60-300 ‘‘cubed’’ transmission electron stable and high-translucent 3Y-TZP ceramics while retaining their microscope operated at 200 kV. 5–7 grain boundaries in each inherent excellent mechanical properties. ceramic were analyzed. Quantitative elemental mappings were The macroscopic properties of polycrystalline materials depend acquired to calculate the element concentration profile across on the grain size and the atomic structure and chemistry of the the grain boundary using ESPRIT 1.9 software. grain boundaries. As a birefringent material, the optical properties of Y-TZPs are strongly influenced by the structure of the grain 2.3. Assessment of aging kinetics boundaries [27]. The chemistry of the zirconia grain boundary was reported to be critical for the aging behavior of Y-TZP ceramics In vitro accelerated hydrothermal experiments were used to age [28–30]. Furthermore, the phase composition [31] and grain size the ceramics. Double-sided mirror polished specimens were [32] of Y-TZP ceramics are controlled by the grain boundary struc- autoclaved at 134 °C and 0.2 MPa in water vapor. The amount of ture, which in turn influences the translucency [5,33], aging behav- tetragonal to monoclinic phase transformation was determined ior [13] and mechanical properties [34]. Therefore, in this work, we by X-ray diffraction (XRD, 3003-TT, Seifert, Ahrensburg, Germany) explored the strategy of controlling the composition of the zirconia using Cu Ka radiation at 40 kV and 40 mA. XRD patterns were grain boundaries at the nanometer level using the atomic radius of recorded in the range of 27–33° (2h)byh–2h mode with a scan an additional dopant oxide as a grain boundary engineering tool. speed of 2 s/step and a scan size of 0.02° (the X-ray penetration depth was calculated to be 7.5 lm (Cu Ka with 98% absorption)). 2. Materials and methods The monoclinic phase content (Vm) was calculated according to the formula of Garvie et al. [36] and Toraya et al. [37]: 2.1. Material preparation À111 111 1:311 ðIm þ Im Þ V m ¼ À111 111 101 ð1Þ Pure tetragonal zirconia was doped with trivalent oxides having 1:311 ðIm þ Im ÞþIt a different cation radius Al3+ (53.5 pm) < Zr4+ (84.0 pm) Sc3+ (87.0 pm) < Y3+ (101.9 pm) < Nd3+ (110.9 pm) < La3+ (116.0 pm) with I, the intensity of monoclinic (À111 and 111) and tetragonal [35]. Formulations were made with each of the trivalent oxides (101) phase peaks indicated by the subscripts m and t. The monoclinic phase content was plotted as a function of aging (Nd2O3 (Chempur, purity of 99.9%), La2O3 (Chempur, purity of time to indicate the aging kinetics. For each curve, at least 3 spec- 99.99%), Sc2O3 (abcr GmbH & Co. KG, purity of 99.9%), Al2O3 imens (6 exposed surfaces) were tested and the average result was (TM-DAR, purity of 99.99%)) by mixing with tetragonal ZrO2 nano- powder (grade TZ-3Y, Tosoh, Japan) on a multidirectional mixer reported. Note that, at 0 h of aging, XRD patterns were recorded in (Turbula type T2C, Basel, Switzerland) for 24 h in ethanol using the range of 20–90° (2h) to identify the phase composition of the 5 mm Y-TZP milling balls. The mol% was indicated in the nomen- non-aged specimen. Furthermore, in order to compare the aging kinetics of different clature expect for the Al2O3 dopant which was indicated in wt.% ceramics, the aging kinetics were rationalized by fitting the trans- (for example 3Y-0.1La means 3 mol% Y2O3 and 0.1 mol% La2O3, formation curves with the Mehl–Avrami–Johnson (JMA) equation, 3Y-0.25Al means 3 mol% Y2O3 and 0.25 wt.% Al2O3). The mixed suspension was further processed by bead milling as follows [11]: Ò (DISPERMAT SL, Germany) for 3 h at 5000 rpm using 1 mm ZrO2 V m n beads (grade TZ-3Y, Tosoh, Japan).
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