Objectives: This study examined how the addition of zirconia to glass compositions affected (1) the subsequent transformation of glasses to glass-ceramics upon heat treatment and (2) their mechanical properties with a view to the materials' potential use as machinable glass-ceramics for use in CAD/CAM dentistry.
Methods: Three novel glasses were produced based upon a parent ternary composition 90 % mol barium fluormica, 6 % mol cordierite and 4 % mol calcium phosphate. To this original composition zirconia was added incrementally (5, 10 and 15 % mol ZrO2).
Glasses were heat treated to produce glass-ceramics. SEM, TEM and X-ray Diffraction were performed to look at the microstructure and the nature of crystallisation in the glass-ceramics produced. Biaxial flexural strengths and micro-hardness were determined.
Results: SEM indicated that all samples had a random distribution of interlocking discs or plate like mica crystals embedded within a glassy matrix. X-ray Diffraction showed a dominant barium fluormica phase and a small amount of monoclinic zirconia. TEM micrographs indicated that zirconia was only partially dissolved in the glass melt (£1 atomic % of the total zirconia). The surface dissolution of zirconia caused the particles to have a rounded appearance. The remaining zirconia crystals acted as nucleating agent for mica crystallisation.
Addition of 5, 10 and 15 % mol zirconia increased the biaxial flexural strength to 137±4, 168±11 and 170±22 MPa respectively in comparison with that of the parent glass ceramic (100±15 MPa). Hardness values of the glass ceramics with 5, 10 and 15 % mol zirconia were 171.97, 145.7 and 134.3kg/m2 HV respectively.
Conclusion: Biaxial flexural strength of the glass ceramics increased significantly with addition of zirconia. Zirconia acted as a nucleating agent for the subsequent crystallisation of mica. This work is supported by The Ministry of Heath, Kingdom of Saudi Arabia.