Nucleation efficacy and flexural strength of novel leucite glass-ceramics
Introduction
Glass-ceramics are widely used in biomedical and industrial applications and their efficient, low-cost synthesis are important parameters to consider when scaling new materials for dental applications. The production of mainstream glass-ceramics is via a high temperature melt/quench synthesis of glasses and their subsequent heat treatments, to control the crystal phase and the residual glass. Careful control of these processes is important as they can influence the chemical, thermal, mechanical, biological and optical properties of the final glass-ceramic [1], which has implications for their clinical performance and the patient acceptance of these materials. In particular, leucite glass-ceramics are used in dentistry for the construction of all-ceramic restorations including crowns, inlays and veneers and for fusing to metal substrates to produce porcelain fused to metal bridges, implants and crowns [2].
Glass-ceramics are synthesized by the controlled nucleation and crystallization of the produced glass [3], so that a high-volume fraction of fine crystals can be produced which are thermally compatible with the residual glass phase. They are produced via a surface or bulk crystallization process, with the former being the most prevalent [4]. The literature indicates leucite glass-ceramics can be crystallized via a surface crystallization mechanism [5]. There is however also experimental evidence to suggest that these processes can happen sequentially according to specific glass powder sizes in leucite glass-ceramics [6].
The current authors previously utilized surface crystallization mechanisms to reduce crystal size and increase crystal area fraction (24.9–29.3%), creating fine (<1 μm2) and nano-sized (<0.1 μm2) leucite glass-ceramics [7], [8] with high flexural strengths (253.8–255.0 MPa) and producing low enamel wear [9]. These formulations however contained very low quantities of titanium dioxide (0.3 mol%), which is a copious nucleating agent used successfully in many glass systems to effect bulk crystallization. This is due to its ability to induce phase separation by its displacement from the glass network, in combination with a divalent cation, and effecting a change in its coordination state. The resultant structural changes to the local glass network and medium range reorganization encourages nucleation [10]. A number of precursor titanate phases have also been associated with the efficacious crystal growth of other glass-ceramic systems [11], [12].
The aim of the study is therefore to synthesize a novel aluminosilicate glass designed using Appen factors with increased TiO2 content [13], to induce any potential titanate phase formation found in other glass-ceramic systems in order to optimize the nucleation process. Optimization of the nucleation mechanism in surface crystallized leucite glass-ceramics may allow more efficacious glass-ceramic manufacture and improvements in the microstructure and mechanical reliability.
Section snippets
Glass synthesis
An alumino-silicate glass was commercially synthesized (Cera Dynamics Ltd, Stoke-on-Trent, UK) by heating reagents in a high temperature custom made furnace (James Kent, UK) at 10 °C/min to 1550 °C (5 h hold). The glass was of the following X-ray fluorescence (based on BSEN ISO 12677:2011) composition (mol%); SiO2 (69.4%), Al2O3 (10.5%), K2O (12.0%), CaO (1.8%), TiO2 (1.3%), Na2O (2.4%), Li2O (1.9%), B2O3 (0.7%). The glass frit was quenched in water and ball milled using a two-stage industrial
Differential thermal analysis results
The results of the DSC nucleation experiment are shown in Fig. 1, exhibiting a maximum at ≈602 °C. An experiment was also carried out to assess the hold time (0.5, 1, 2 and 3 h) at the nucleation temperature and 1 h was found to be the most efficacious.
Biaxial flexural strength results
The results of the BFS tests are listed in Table 1. The mean BFS and characteristic strength values for the experimental leucite glass-ceramics (groups 1, 2, 3 and 4) were not significantly different (p > 0.05, Table 1). There was no significant
Discussion
In the current work the starting glass, nucleated glass powders and frit specimen (592 °C, 1 h hold) were largely amorphous (Fig. 6a and b), with signs of phase separation (Fig. 2c and e). EDX was however inconclusive due to decomposition of these areas during analysis. A nucleation and growth process producing phase separated domains, random in size and low in connectivity could be responsible, including later stage coarsening [19], [20]. The TiO2 (1.3 mol%) content in the glass causing the
Acknowledgements
The Authors gratefully acknowledge funding and support for this project from Dr. Brian Schottlander (Davis Schottlander Davis Ltd.). Dr. R. Bailey (School of Engineering and Materials Science, QMUL) is acknowledged for help with the SEM. A. Boebenroth (Fraunhofer IMWS) is acknowledged for TEM sample preparation. Dr. Xu CaO (Cera Dynamics Ltd) for glass synthesis and supplying X-ray fluorescence data. Dr. H. Toms NMR facility manager (School of Biological and Chemical Sciences, QMUL) is kindly
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