Effect of heat treatment on fracture toughness K(IC) and microstructure of a fluorcanasite-based glass-ceramic

Abstract

Purpose: The purpose of this study was to test the hypothesis that the increase in fracture toughness of a fluorcanasite-based glass-ceramic is a linear function of crystal volume fraction.

Materials and methods: A total of 60 specimen bars (20 x 5 x 2 mm(3)) were cut from parent glass blocks, polished, annealed, randomly divided into six groups, nucleated at 680 degrees C/4 hr, and crystallized at the following temperatures and times: (1) 850 degrees C/0.5 hr, (2) 850 degrees C/1 hr, (3) 850 degrees C/3 hr, (4) 750 degrees C/6 hr, (5) 800 degrees C/6 hr, or (6) 850 degrees C/6 hr. Indentation flaws were produced by a microhardness indenter at the center of one surface, and the prepared specimens were subjected to three-point flexure loading with the severely flawed surface under tension at a crosshead speed of 0.5 mm/min. Flexural strength and fracture toughness (K(IC)) were calculated based on the indentation-strength technique. Crystal volume fraction (V(c)) was determined by quantitative stereology of scanning electron images of each group of ceramic specimens. Statistical analysis was performed using ANOVA and Duncan's multiple comparison test (alpha= 0.05).

Results: The mean K(IC) and V(c) values ranged from 2.7 to 3.9 MPa m(1/2) and 37% to 71% within the crystallization temperature range of 750 to 850 degrees C. Five statistical subsets of groups 1, 2/4, 3, 5, and 6 were determined as a function of crystallization temperature and holding time (Duncan's multiple comparison analysis; alpha= 0.05). The lowest and highest K(IC) and V(c) values were associated with Groups 1 (850 degrees C/0.5 hr) and 6 (850 degrees C/6 hr), respectively. Fracture toughness increased linearly as a function of crystal volume fraction (correlation coefficient R(2)= 0.67). The fracture toughness increased by 45% when the crystal volume fraction increased by 92%.

Conclusions: Mean K(IC) values increased as a linear function of crystal volume fraction in a fluorcanasite-based glass-ceramic within the crystallization temperature range of 750 to 850 degrees C and at isothermal crystallization time range of 0.5 to 6 hours. The control of crystallization temperature and isothermal holding time should be optimized to generate tougher, more reliable ceramic prostheses in the shortest period of time.