Tratamentos térmicos em sistemas cerâmicos bicamadas: estudo das propriedades mecânicas, ópticas e tensões residuais

Abstract

This dissertation is divided in three parts, which investigated the effects of thermal treatments on the mechanical behavior of bilayer ceramic systems as well as identify the main influencing factors. The first part gathered the scientific literature available to evaluate the effect of thermal treatments (glaze, cooling protocol, number of firings, annealing) on the flexural strength and load to fracture of porcelain-veneered zirconia (PVZ) samples. Three databases were used for performing the search (PubMed/Medline, Scopus, and Web of Science). From 393 relevant studies, 21 were selected for full-text analysis, from which 7 failed to meet the inclusion criteria. The 14 remaining papers were included for the systematic review: 8 for meta-analysis and 6 restricted to descriptive analyses. The results showed that delaying furnace opening at a temperature below the porcelain’s glass transition temperature is advised for PVZ restorations, in order to improve their fracture resistance. Moreover, there is a lack of information about the effect of glaze firings on the studied outcomes. The second part evaluated the effect of dwell time (conventional or extended) and cooling protocol (fast or slow) of glaze firings on the flexural strength, resistance to crack propagation, and optical properties stability of a porcelain-veneered zirconia system. Bilayer disc-shaped samples were prepared using a zirconia framework veneered with porcelain, and divided according to the thermal treatment to be performed after sintering: glaze firing (1 min of dwell time) followed by slow cooling (furnace opening at 200ºC) or fast cooling (furnace opening at 600ºC), extended glaze firing (15 min of dwell time) followed by slow cooling or fast cooling, or no thermal treatment (control). Porcelain roughness (Ra and Rz) and translucency (TP00) were measured before and after firings, as well as the color difference (ΔE00) after the treatments. Crystalline phase was analyzed with X-Ray diffraction. Flexural strength was measured with the piston-on-three-ball test. Additional samples were prepared and subjected to Vickers indentations on the porcelain for crack length analysis. The extended glaze firing was observed to improve the flexural strength and the resistance to crack propagation. No clinically significant difference was observed regarding the optical properties stability. The third part studied the stress development in bilayer systems with zirconia or lithium disilicate (PVLD) frameworks. The effects of cooling rate and thickness ratio were evaluated using a Viscoelastic Finite Element Method. Bilayer crown models were generated using a finite element software (ABAQUS). The physical properties (coefficient of thermal contraction, thermal conductivity, specific heat, density and elastic modulus) of each material studied were experimentally measured as a function of the temperature. Two cooling protocols were simulated (Fast: ~300ºC/min, Slow: ~30ºC/min) from 70ºC above the softening temperature of the porcelains using a heat transfer, followed by the residual stress analysis. PVLD showed lower values of residual stresses than PVZ. The maximum tensile stresses in porcelain over zirconia framework were observed in the cusp area, whereas those in porcelain over lithium silicate were located in the central fossa. Therefore, slow cooling is preferable for both PVZ and PVLD, as well as using a thinner porcelain layer over zirconia when possible.