Carla Castiglia Gonzaga

Effect of processing induced particle alignment on the fracture toughness and fracture behavior of multiphase dental ceramics

OBJECTIVE To investigate the processing induced particle alignment on fracture behavior of four multiphase dental ceramics (one porcelain, two glass-ceramics and a glass-infiltrated-alumina composite). METHODS Disks (Ø12 mm x 1.1mm-thick) and bars (3 mm x 4 mm x 20 mm) of each material were processed according to manufacturer instructions, machined and polished. Fracture toughness (K(Ic)) was determined by the indentation strength method using 3-point bending and biaxial flexure fixtures for the fracture of bars and disks, respectively. Microstructural and fractographic analyses were performed with scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction. RESULTS The isotropic microstructure of the porcelain and the leucite-based glass-ceramic resulted in similar fracture toughness values regardless of the specimen geometry. On the other hand, materials containing second-phase particles with high aspect ratio (lithium disilicate glass-ceramic and glass-infiltrated-alumina composite) showed lower fracture toughness for disk specimens compared to bars. For the lithium disilicate glass-ceramic disks, it was demonstrated that the occurrence of particle alignment during the heat-pressing procedure resulted in an unfavorable pattern that created weak microstructural paths during the biaxial test. For the glass-infiltrated-alumina composite, the microstructural analysis showed that the large alumina platelets tended to align their large surfaces perpendicularly to the direction of particle deposition during slip casting of green preforms. SIGNIFICANCE The fracture toughness of dental ceramics with anisotropic microstructure should be determined by means of biaxial testing, since it results in lower values.

Mechanical properties and porosity of dental glass-ceramics hot-pressed at different temperatures

The objective of this work was to evaluate biaxial-flexural-strength (σf), Vickers hardness (HV), fracture toughness (KIc), Youngs modulus (E), Poissons ratio (ν) and porosity (P) of two commercial glass-ceramics, Empress (E1) and Empress 2 (E2), as a function of the hot-pressing temperature. Ten disks were hot-pressed at 1065, 1070, 1075 and 1080 °C for E1; and at 910, 915, 920 and 925 °C for E2. The porosity was measured by an image analyzer software and sf was determined using the piston-on-three-balls method. KIc and HV were determined by an indentation method. Elastic constants were determined by the pulse-echo method. For E1 samples treated at different temperatures, there were no statistical differences among the values of all evaluated properties. For E2 samples treated at different temperatures, there were no statistical differences among the values of σf, E, and ν, however HV and KIc were significantly higher for 910 and 915 °C, respectively. Regarding P, the mean value obtained for E2 for 925 °C was significantly higher compared to other temperatures.

Effect of cement space on stress distribution in Y-TZP based crowns.

OBJECTIVE To evaluate the stress distribution in bi-layered Y-TZP based crowns, according to the occlusal internal spacing between coping and abutment. METHODS Twelve premolar shaped Y-TZP copings were made by a CAD/CAM system and seated on an abutment to evaluate the internal fit at the occlusal third using micro-CT images. Considering the fitting range obtained experimentally, two 3D finite element models, consisting on bone tissue, a titanium implant, a zirconia abutment, cement layer and a bi-layered Y-TZP ceramic crown were constructed based on the micro-CT images, one corresponding to the thinnest cement space and other representing the specimen with the thickest cement space obtained experimentally. A 250N axial load was applied at the center of the occlusal surface of the crown (≅0,8mm2 area) and the first principal stress distribution was plotted and analyzed. RESULTS The greatest maximum principal stress occurred within the veneer ceramic right below the site of loading. The thickest cement model showed higher stress concentration at the center of occlusal surface of veneer and the center of occlusal internal surface of coping. SIGNIFICANCE Knowledge of stress distribution in ceramic crowns with different cement thicknesses will help clinicians to properly adjust crown fit, in seeking to avoid porcelain fractures.

Variables that Affect the Indentation Fracture Testing (IF) of a Dental Porcelain

The aim of this study was to investigate the influence of the indentation load, the period waste between indentation and measurement, and the gold coating on the fracture toughness (K Ic ) of a feldspatic porcelain (d.Sign, Ivoclar) measured by the Indentation Fracture Technique (IF). Porcelain beams (∼3x4x20mm 3 ) were fabricated by sintering green bodies at 870 C under vacuum with rapid heating and cooling rates. The samples presented microstructure composed of a glassy matrix, second phase particles and residual pores. Vickers hardness tester was used to produce indentation patterns on mirror polished surfaces. The diagonal of indentation (2a) and the length of crack (2c) were measured in an optical microscopy with better resolution than the hardness tester. The increase of indentation load from 0,2 to 5kg caused: i) increase of c/a ratio from 1,8 to 3,3; ii) almost no variation of Vickers hardness number; and iii) constancy of K Ic value from 0,3kg load. It was observed no influence of a gold coating on K Ic measurement. K Ic suffered significant reduction with time, mainly in the first hour.