The controlled crystallisation of high strength leucite glass-ceramics

Abstract

Leucite glass-ceramics are widely used in Dentistry, and show favourable aesthetics. Undesirable brittle failure however, is associated with uneven leucite crystal distribution, and the thermal mismatch between the glass matrix and larger leucite crystals. This thesis focuses on glass design and leucite microstructural control in order to produce high strength, aesthetic, reliable and processable leucite glass-ceramics. Glasses were designed using Appen Factors to control the thermal expansion coefficient, density and refractive index. Zirconia and niobium oxide were also introduced in to the glasses to investigate their nucleating effects on leucite crystallisation. Glasses were characterised using Dilatometry, Light Microscopy, Differential Scanning Calorimetry and High Temperature X-ray Diffraction. A series of two-step crystallisation heat treatments on all glasses were carried out to control the leucite crystal size, morphology and volume fraction. The biaxial flexural strength (BFS) of the experimental and commercial control IPS Empress Esthetic glass-ceramic was measured using the ball on ring test. Appen Factors were proven as a useful tool for glass design. Phase crystallisation and the crystallisation mechanisms in the leucite glass-ceramic system were dependant on the glass composition. Leucite crystallisation without the presence of titanium dioxide was possible. Leucite glass-ceramics with evenly distributed fine tetragonal leucite crystals (Mean (SD) 0.15 (0.09) μm2) were achieved by ball milling of the parent glass followed by optimised two-step heat treatments. The glass-ceramic showed minimal matrix microcracking and a high BFS value of 252.4 (38.7) MPa and m value of 8.7, which is statistically higher than the IPS Empress Esthetics glass-ceramic with a BFS of 165.5 (30.6) MPa and m value of 6.3. Leucite crystal size, morphology and distribution depended on the parent glass composition and the thermal heat treatment. Highly crystallised leucite microstructures with orientated fibres, spheres and rosette shaped domains were synthesised based on different glass compositions, and showed a high BFS of 212.2 (28.2) MPa and m value of 8.5. High strength, reliable and translucent leucite glass-ceramics were successfully heat extruded to produce dental restorations.