Queima estendida de glazeamento em cerâmicas de corte duro: cicatrização de defeitos e efeito sobre as propriedades físico-mecânicas

Abstract

This thesis evaluated an extended glaze firing (EG) versus the conventional glaze firing (G, manufacturer-recommended) for finishing of densely sintered ceramics indicated for machining in CAD-CAM (Computer Aided Design; Computer Aided Machining) systems. Initially, the effects of both firings on ceramic surface crack healing, residual stresses, optical characteristics and crystalline structure were evaluated. Rectangular specimens (≈ 14 x 12 x 1,5mm) obtained from the section of feldspathic (FEL), leucite- (LEU), lithium disilicate- (DIS), and zirconia-reinforced lithium silicatebased (ZLS) prefabricated ceramic blocks were divided into groups according to the applied firing (n=5): G, EG (15min at dwell time, following slow cooling) and C (control/no heat treatment). Defect generated by Vickers indentation was analyzed in an electronic microscope before and after firing (n=1) to evaluate its healing process. Residual stresses were determined by indentation technique. Specimen color differences (ΔE) and variations in translucency (ΔCR) were measured after firing, respectively, by CIEDE2000 method and contrast ratio. Stability of crystalline microstructure was analyzed by X-ray diffraction (n=1). Regardless of the material, EG had greater ability to heal defects when compared to G and also produced compressive residual stresses, while G generated tensile stresses. Color differences after EG were imperceptible in FEL and LEU ceramics, perceptible/acceptable for DIS, and clinically unacceptable for ZLS. G did not perceptibly change the color of the materials. After EG, ZLS ceramic was perceptibly less translucent. Crystalline phase of all the ceramics remained stable after G and EG. Therefore, in a second moment, we intended to investigate the effect of EG and G firings on the flexural fatigue strength (FFS) of LEU and DIS ceramics. For this, ceramic blocks were machined in disc-shaped specimens using CEREC inLab MC XL and were divided into six experimental groups (n=20) according to the ceramic – LEU or DIS – and the firing – G, EG or C. Specimens were submitted to fatigue using staircase test design in water (piston-on-three ball, 500.000 cycles, 20Hz and sinusoidal loading). Surface roughness of the discs was measured using a contact profilometer, before and after firing. Fracture origin was analyzed by scanning electron microscopy (SEM). Means and the standard deviation of FFS were submitted to analysis of variance (ANOVA-1 factor) and Tukey's test (α = 0.05). The most appropriate statistical tests for pared comparisons of the roughness data were selected based on normality/homoscedasticity results. For all the tested materials, EG achieved the highest FFS, statistically higher than G and C groups. The surface roughness of the LEU and DIS ceramics did not change after G and EG. Fractographic marks led to the identification of fracture originating from the tensile surface. Thus, it was concluded that the EG cycle promoted greater defect healing in relation to G, developed tolerable residual stresses, and did not alter neither the optical properties beyond the clinical threshold, nor the microstructure of the FEL, LEU and DIS ceramics; in addition, it have improved the flexural fatigue strength values for LEU and DIS machined specimens. Therefore, it seems to be a promising alternative for finishing such materials.