Fadiga e viscosidade de vitrocerâmicas de dissilicato de lítio e propriedades mecânicas de vitrocerâmicas do sistema MgO-Al2O3- SiO2/ZnO

Abstract

The bioceramic revolution has improved people’s life, however their wear affects their mechanical resistance with time. In face of this challenge, in this work the influence of the chemical environment (water and SBF solution) on the mechanical properties of glass and glass-ceramics with dental and bioactive applications was investigated. Weibull modulus, the dynamic fatigue parameter and mechanical strength of lithium dissilicate and bioglasses Biosilicate and 45S5 were studied in different environments using the ball-on-three ball test (B3B) with different loading rates. The test performed in 10% of relative humidity for the LS2 glass showed an average value of Weibull modulus of 4, while for the same glass tested in water was 5± 1. For the LS2 glass-ceramic with volume fraction of 96 ± 1% tested in water, the Weibull modulus found was 7± 1. The mechanical strength for LS2 glass-ceramics tested in 10% of relative humidity and in water increased with the loading rates and with the cristalized volume fraction. The Weibull distributions were 6 and 4 ± 1for the glass and glass-ceramic Biosilicate with 38% of volume fraction tested in SBF, respectively. The mechanical strength of the Biosilicate glass and glass-ceramic were 80 ± 1 and 77 ± 5 MPa, respectively, and for the 45S5 glass was 58 ± 10 MPa. The values of the fatigue parameter for LS2 tested in 10% of humidity varied from 15 ± 5 for the glass and increased with crystallized volume fraction, to 21 ± 9 for the volume fraction of 60 ± 5%. The fatigue parameter of the LS2 glass-ceramics tested in water varied from 14 ± 5 for glass to 12 ± 2 for the volume fraction of 67 ± 1%. The sample almost completely crystallized (96 ± 1%) showed the higher fatigue parameter, 30 ± 10. The mechanical strength for LS2 at 10% of relative humidity showed higher values when compared with tests performed in water. In both cases, the mechanical strength increases as the load rate increases. LS2 samples tested at 10% of relative humidity showed an increase in the fatigue parameters when compared glass, due to the test environment because it allows more time for crack growth before reaching a critical size. For samples tested in water, apparently there is a trend to decrease fatigue. For the system SiO2-Al2O3-MgO/Zno (MAS), the properties of sinterization, cristalization, mechanical properties and chemical resistance were investigated. It was observed that partial substitution of MgO by ZnO causes a decrease in Tg, a slight increase in the crystallization temperature and improves sinterability. Cordierite and enstatite phases are predominant in bulk glasses heat treated at 1000 °C. The addition of ZnO results in precipitation of phases cordierite, willemite and gahnite n bulk glasses, and in cristobalite, enstatite, ghanite and willemite for sintered powders. The glass and glass-ceramic with ZnO had the lowest hardness and elastic modulus, and all glass-ceramics had almost the same wear rate. Chemical durability tests showed that glass samples are more resistant to acid environment, while glass-ceramics CP2 and CP3 are more resistant to the basic environment. The effects of crystallization on the viscosity of LS2 glass-ceramics were investigated. The Volger-Fulcher-Tammann (VFT) e Mauro-Yue-Gupta-Adam (MYEGA) models fitted quite well to the glass and glass-ceramics with crystallized volume fractions of 3% and 14%. It was not possible to fit the model for 25 % and 40% due the concurrent crystallization of the samples during testing. The activation energies for viscous flow were 510 ± 20, 520 ± 30, 530 ± 10, 630 ± 70 and 910 ± 30 kJ/mol for the glass and 3, 14, 25 e 40% of volume fractions, respectively. Due to the fact that the crystals are not perfectly spherical, there is an additional energy dissipation during flow, increasing the activation energy as the crystallized fraction increases. These results are important for determination of the mechanisms that control the brittle-ductile transition in glass-ceramics materials.