Development of SiO2 - Li2O - ZrO2 dental glass-ceramic systems with extended lifetimes: crystallization mechanisms and resistance to subcritical crack growth

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Glass-ceramics based on Lithium silicates today constitute the most commercially successful restorative material for dental applications. Compared to typical feldspar-based dental ceramics, lithium disilicate (Li2Si2O5) crystals benefit from a preferred crystal growth along the crystallographic c-axis. Elongated crystal phases enable the tailoring of anisotropic microstructural architectures in congruence with those employed by natural design, such as in dental enamel. The deliberate orientation of microstructural features appears to be a cornerstone in biology´s strategy to achieve optimal mechanical performance, an approach applicable to inorganic materials as well. Unfortunately, the currently existent lithium disilicate systems are particularly susceptible to stress-assisted subcritical crack growth in the glass phase. This phenomenon is accelerated by the development of thermal residual stresses between the glass and crystal phases. Especially affected are glass-ceramics in the system SiO2 – Li2O – ZrO2. The incremental addition of ZrO2 to the base glass alters the typical nucleation and crystallization kinetics known for lithium disilicates, involving the heterogeneous nucleation of Li2SiO3 and the later transformation to Li2Si2O5 at higher temperatures. This results in a residual glass phase enriched with ZrO2. The underlying mechanisms involved in this process, the relationship to the SiO2/Li2O ratio, and their ultimate imbrications in shaping the glass network structure, are still poorly understood. As part of an interdisciplinary project, the effects of compositional variations in the SiO2 – Li2O – ZrO2 glass system will be thoroughly investigated. The kinetics of nucleation and crystallization constitute the central focus of our interest, including a detailed characterization of the glass network structure and properties of the residual glass with respect to slow crack growth resistance and influence of thermal residual stresses. The implications on the local and global mechanical behavior will be probed systematically with focus on the optimization of microstructural design and phase compatibility, aiming on the development of dental ZrO2-containing lithium disilicates for extended clinical lifetimes.