Fabrication, Kinetics and Crystallography of Lithium Disilicate Glass-Ceramics

Abstract

This research project mainly focuses on three aspects of investigations on lithium disilicate glass-ceramics, namely, fabrication, kinetics, and crystallography. One of the objectives is to fabricate a high-strength glass-ceramic in complex lithium disilicate glass systems for dental restorative applications by means of compositional design of parent glass and heat treatment optimization. The second objective is to understand the nucleation mechanism and crystallization kinetics of lithium disilicate glasses by in situ and real-time observations of phase transition using X-ray diffraction (XRD) technique with the state-of-the-art synchrotron radiation. In the meantime, it is of interest to systematically study the evolution of crystallographic structure of the involved crystalline phases (mainly lithium metasilicate and/or disilicate), which is refined by Rietveld method from the as-received synchrotron XRD data, and discuss the relationship of phase transition and crystallographic evolution. Firstly, a lithium disilicate glass-ceramic, based on a well-documented SiO2–Li2O–Al2O3– P2O5–ZrO2 glass system, was developed. This glass ceramic has a three-point flexural strength of 307 MPa and Vickers hardness of 7.9–8.2 GPa. The glass-ceramic demonstrated a zigzag crack path, suggesting the fracture mechanism of crack deflection. The formation of cubic zirconia (c-ZrO2) with a spherical shape was identified (Chapter 4). Built on the experimental findings of Chapter 4, other new glass compositions (glasses B, C, E, and F) were designed, with MgO being a common additive. Glass-ceramics from glass B (Al2O3-free system) showed an optimized flexural strength of 439±93 MPa and Vickers indentation fracture (VIF) toughness of 0.93–1.29 MPa∙m1/2. Very limited amount of lithium metasilicate (LS) was detected by synchrotron radiation during glass crystallization. The investigation indicates that the crystallization temperature has a more profound effect than holding time on the phase transformation, morphology and crystallite size of these glass-ceramics (Chapter 5). In addition, a high strength lithium disilicate glass-ceramics with 562±107 MPa were fabricated in a novel glass composition (glass C) with both MgO and Al2O3 additives and crystallised at 505°C/60min + 605°C/60min + 810˚C/120min. Such a three-stage annealing profile was optimized step by step. Remarkably in this glass-ceramic, the netting and interlocking cluster morphology of lithium disilicate (LS2) crystals can be controllably achieved, which is highly correlated with the superior mechanical properties of final products (Chapter 6). Its formation mechanism is still unclear and worthy to be investigated in future. According to the type of crystallization sequences of LS and LS2, the developed glass-ceramics can be categorized into the following three types. Type I is the simultaneous nucleation of LS and LS2 phases, followed by the transformation of LS to LS2. Type II is that LS nucleates first and then transforms to LS2 at a higher temperature. Type III is that LS2 directly forms in the glasses without the formation of LS. Practically, glasses with types I and II reaction sequences are preferred because LS phase has very attractive machinability that enables the utilization of CAD/CAM technology for the fabrication of dental products with complex shapes. Secondly, we investigated the role of P2O5, the nucleation and crystallization kinetics in a complex lithium disilicate glass (glass Bo) by in situ XRD measurements and Rietveld refinements. As evidenced by high-resolution synchrotron powder diffraction data, the nucleation of LS and LS2 is triggered by the steep compositional gradients around the disordered Li3PO4 (LP) structural units in the glass matrix. The isothermal nucleation and crystallization kinetics were investigated as well. Accordingly, the Avrami exponent n and activation energy of silicate phases were estimated by a modified JMAK equation and Arrhenius assumption (Chapter 7). Owing to its lower activation energy, the LS phase was evidenced to nucleate ahead of LS2 at the very beginning of glass nucleation. Surprisingly, the phase transformation of glass upon heating and that of glass melt upon cooling were different; that is, no LS2 was detected during fast cooling of the glass melt. It indicates that the driving force of the cooling process is not enough for activating the nucleation of LS2 phase. Apart from the phases LP, LS, and β-cristobalite, two hexagonal phases with very close lattice constants to β-quartz were observed and they showed near zero expansion behaviour (Chapter 8). The non-isothermal crystallization kinetics of lithium disilicate glasses (glasses B and Bo) was depicted by the evolution of phase fraction and crystallite size as a function of annealing temperature (Chapter 5 and Chapter 7). It is found that the growth of LS2 in glasses of type I is probably constrained by LS at the nucleation stage. Thirdly, from the investigations into the crystallography of the above-discussed three types of glass-ceramics, a common phenomenon of structural response of Li2Si2O5 along its c axis to other silicon-related phases during glass crystallization was observed unprecedentedly, which was evidenced from the relationship between the crystallographic evolution and phase transition in glasses (AG1, C, and E/F). In addition, the crystallographic evolution of LS2 phase in glass Bo was investigated upon heating and cooling, respectively. It is confirmed in glasses Bo and C that the lattice parameter c of LS2 shows a typical “V”-shape trend in lithium disilicate glasses of type I. In other words, it drops non-linearly when LS is present in the glass-ceramic then rises up linearly after LS disappears. Moreover, upon heating the c axis of LS demonstrates a non-linear shrinking trend; while upon cooling, it has a linear expanding trend against the increasing temperature (i.e. positive correlation). Thus, a close correlation between the structures of LS2 and LS phases during phase transformation is suggested.