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Introduction

Clinical studies involving leucite-reinforced glass ceramic restorations report survival rates between 76 and 97%, with bulk fracture being one of the main reasons for failure of these restorations, totaling 3–11% of failures . Cohesive chipping of the ceramic is mentioned in the literature, but this has not been considered as a failure, since this type of defect does not prevent functioning of the restorations .

Glass ceramics are widely used in indirect restorations that require aesthetic appeal, because they mimic the dental tissues and provide a mechanical strength that is greater than that of feldspathic ceramics. The improvements in the mechanical properties of these materials are a result of the higher content of crystals, such as leucite , and the development of prefabricated ceramic blocks for CAD-CAM (Computer Aided Design, Computer Aided Machining) systems, produced using a standardized process that results in a more homogeneous material . However, even with these fracture toughness advances, the ceramic components maintain their brittle behavior, accumulating residual stresses as a result of the machining process, and are thus susceptible to different failures . On the other hand, it is believed that machining might introduce thin compressive stresses to the surface of the ceramic with a positive effect on its strength . Thus, an understanding of these effects is essential for predicting possible failure mechanisms and assessing the reliability of restorations made by CAD-CAM systems.

Different heat treatments have been employed for glass ceramics in an attempt to control residual stresses , while potentially reducing some of the damage accumulated during the various steps of fabricating the restorations. These treatments are conducted at temperatures near the glass transition temperature ( T _g ), where the material viscosity is reduced, and structural rearrangements promote relaxation of internal stresses in the ceramic . However, cycles that reach temperatures close to the material softening point , in order to promote healing of possible defects, have also been reported. A relevant aspect in firing treatments for glass ceramics is the way in which the cooling stage is conducted, since it is the rate (fast or slow) at which this process occurs that will determine how much tension will be released or accumulated . Below T _g , the glass behaves as a solid and all structural contraction reverts to residual stress locked inside the material . Higher cooling rates usually result in thermal contraction and solidification being non-uniform because of temperature gradients, leading to the development of stresses , and possible weakening of the material.

Materials consisting of different phases, such as leucite-reinforced glass ceramics (with two phases, glassy and crystalline), should be treated with extreme caution because these phases behave differently during heat treatment . This situation requires thermal schemes that meet the requirements for structural balance between phases. Some studies show that, depending on the regimen adopted, alterations in the crystals, and even in the constituent phases of the materials, may occur .

There are studies showing that routine laboratory thermal cycles, such as glazing, may be associated with mechanical and structural changes, which could reduce the fracture strength of ceramic materials . Thus, these treatment schemes must be taken into account when trying to accurately predict the long-term performance of restorations. However, there is inconsistency in the literature regarding different thermal protocols applied to ceramic materials. Existing data on the influence of these treatments on structural and mechanical characteristics of the material are inconclusive.

The present study aimed to investigate the effect of different firing protocols on the fracture strength of a leucite-reinforced glass ceramic. The first hypothesis tested was that cycles that adopt heating baths at temperatures above the glass transition temperature, followed by slow cooling, would produce higher values of fracture strength. In addition, we sought to evaluate the effect of these treatments on surface roughness and crystalline structure of the ceramic (crystallite size and phase changes), testing a second hypothesis that presumed increases in strength values would be associated with a reduction in surface roughness. A third hypothesis was that a possible decrease in strength would be associated with an increase in crystallite size and crystal phase changes.