Extended glaze firing improves flexural strength of a glass ceramic.

OBJECTIVE

To investigate the effect of firing protocols on flexural strength, surface roughness, and crystalline structure of a leucite-based glass ceramic.

METHODS

Discs produced by automated machining were distributed into five groups (n=30) according to the applied firing protocols, conducted above (790°C) or below (575°C) the ceramic transition temperature (Tg) (625±20°C): C - control, no heat treatment; G790 - glaze firing (790°C) for 1.5 min (manufacturer-recommended); G790-SC - G790 modified by slow cooling; EGF790-SC - extended G790 for 15 min, with slow cooling; and EF575-SC - extended firing below Tg at 575°C for 15 min, with slow cooling. Discs were subjected to biaxial flexural tests and results were assessed using Weibull analysis. Surface roughness was measured before and after treatments. One specimen from each group was used for X-ray diffraction (XRD).

RESULTS

Highest values of characteristic strength (σ0) were obtained for EGF790-SC (211.7MPa). Regimens EF575-SC, G790-SC, and G790 produced σ0 values (167.9, 157.7, and 153.7MPa, respectively) lower than the control (C) (187.7MPa). The Weibull modulus (m) was statistically similar between groups. All treatments reduced the mean roughness (Ra) of the specimens. Extended cycles (EGF790-SC, EF575-SC) decreased the mean amplitude (Rz). XRD revealed no crystalline phase transformation and slight changes in size of leucite crystallites.

SIGNIFICANCE

Increased values of fracture strength and decreased surface roughness for a leucite-reinforced glass ceramic are achieved by extended glaze firing after machining. Crystalline structure is not modified. Glaze cycles, manufacturer-recommended or modified by slow cooling, and firing below Tg, significantly reduce fracture strength.