| dc.description.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. | en_US |
