Fluorcanasite glass-ceramics for dental applications
Fluorcanasite, a chain silicate glass ceramic, displaying a combination of high flexural strength and high fracture toughness in comparison with currently available resin-bonded ceramics, is being developed as a material that should be easy to fabricate into a dental restoration. Previous work has focussed on producing a chemically durable formulation of fluorcanasite. Unfortunately, in an attempt to achieve low solubility, the mechanical properties of this material have been compromised. The aim of this study was to assess the influence of compositional changes on the crystallisation and fracture toughness of the fluorcanasite crystalline phases. This study has focused on compositional variations to the tluorcanasite composition of 6OSi~-8Na20-7K20-15CaO-l OCaF2. Fluorite additions were attempted to ascertain the extent to which the 'known' nucleating agent influences the crystalline phase development. Zirconia and silica additions were made to the formulation to improve the mechanical properties while maintaining the chemical solubility of the material. It was found that fluorcanasite glasses of these formulations crystallize to give a combination of canasite and frankamenite phases and at higher zirconia additions, a potassium zirconium silicate phase called wadeite. The attempted reformulations resulted in a composition (6.11Na20-S.35K20-11.46CaO- 11.46CaF2-64.82Si02-0.80Zr < h) which upon a standard two-stage heat treatment schedule crystallised to give a glass ceramic with substantially improved mechanical properties. A greater than three-fold increase was achieved in the fracture toughness in comparison to the base composition and is comparable to current commercial dental materials indicated for use as posterior restorations. This formulation has resulted in a solubility which is within the solubility limits for use as a core material (class 2, ISO 6872:1995 (E» and the glass ceramic has been shown to have adequate machinability for development using the CAD CAM process.