Effect of Glass Powder Size on Leucite Glass-Ceramic Crystallisation

Methods: Glasses were designed using Appen Factors, synthesised and attritor milled sequentially for up to 8 hours. Glasses were characterised using Particle Size Analysis, Dilatometry, Differential Scanning Calorimetry (DSC), High Temperature X-ray Diffraction (HTXRD) and Inductively Coupled Plasma - Mass Spectrometry (ICP-MS). Two-step heat treatments were carried out to investigate the leucite glass-ceramic crystallisation. Disc specimens (14mm diameter x 2mm thickness) were fabricated by heat pressing or sintering. Experimental and control (Empress Esthetic (EA)) specimens were tested using the Biaxial Flexural Strength (BFS, ball on ring) test at a crosshead speed of 1mm/min. The microstructures were viewed using Secondary Electron Microscopy (SEM).

Results: Attritor milling reduced glass powder size (D₅₀) from 11.37 µm (starting glass, SG) to 0.45 µm (8 hour milled, 8HM). Tetragonal leucite was detected in all crystalline glass-ceramics. Glass milling and heat treatment produced a reduction in leucite crystal size, Mean (SD) SG: 0.99 (0.59) µm² to 8HM: 0.16 (0.10) µm². The EA crystal size was 1.7 (2.0) µm². The leucite crystal number increased from 558 (SG) to 3378 (8HM). The glass-ceramic thermal expansion coefficient (100-400ºC) was: 19.0 x10^-6/K (SG) and 17.2 x10^-6/K (8HM). The mean biaxial flexural strength MPa (SD) of the test groups was SG: 153.2 (21.7), 4HM (4 hour milled) 252.4 (38.7), 8HM: 219.5 (54.1) and EA: 165.5 (30.6). The glass-ceramic 4HM and 8HM had a higher BFS and characteristic strength than the other reported test groups (P < 0.05), however there was no significant difference between BFS values for SG and EA (P > 0.05).

Conclusion: A relationship between the glass powder size and the leucite crystal size and number was established. High strength fine grained leucite glass-ceramics were produced.