Elastic constants, Vickers hardness, and fracture toughness of fluorrichterite-based glass-ceramics.

OBJECTIVES: To determine the elastic constants, Vickers hardness, and indentation fracture toughness of fluorrichterite glass-ceramics in the system SiO2-MgO-CaO-Na2O-K2O-F. METHODS: Five glass compositions were prepared with increasing sodium content. The compositions were melted at 1400 degrees C for 2 h and cast into 60 x 12 mm rods. Discs (1.5 x 12 mm) were cut from the glass rods, nucleated in the temperature range 600-650 degrees C for 1 h and crystallized at 900 degrees C for 0.5 h. The density of the glass-ceramics was measured by Archimedes' method. The elastic constants were determined by standard ultrasonic velocity technique. The fracture toughness was evaluated by the indentation technique. The indentation crack patterns and microstructure were investigated by scanning electron microscopy. The crystalline phases were identified by X-ray diffraction. A commercially available lithium disilicate glass-ceramic served as control. RESULTS: The fluorrichterite glass-ceramics containing 3.8 and 5.6 wt% sodium had the lowest mean fracture toughness. The glass ceramic containing 1.9 wt% sodium and the control material had the highest mean fracture toughness. The values obtained for these two materials (2.26 +/- 0.15 MPa m0.5 and 2.29 +/- 0.31 MPa m0.5, respectively), were not significantly different (p = 0.997). There was no linear relationship between the amount of sodium present in the glass-ceramic composition and the mean fracture toughness. The fluorrichterite glass ceramics exhibited a dual microstructure consisting of both fluorrichterite and mica crystals. SIGNIFICANCE: Higher fracture toughness appeared to be associated with a higher density of fluorrichterite crystals.