Efeito do Tamanho do Cristal e da Fração Volumétrica Cristalizada nas Propriedades Mecânicas do 5BaO-8SiO2 e Curva-R em Vitrocerâmicas

Abstract

The stoichiometric glass 5BaO.8SiO2 (B5S8) and Li2O.2SiO2 (LS2) are among the few stoichiometric glasses that exhibit homogeneous nucleation in the volume. B5S8 is a glass

system that has received limited examination regarding its mechanical and microstructu- ral properties. This study concentrates in the mechanical properties of B5S8 glass-ceramic

materials, including the measurement of elastic modulus (E), hardness (H), variation in exural strength (σs) and fracture toughness (KIC) dependence on the microstruture. The evaluation of these properties was carried out while maintaining control over the size of the crystals and the volumetric fraction of spherulites. A preliminary study of the R-curve and toughening mechanisms for lithium disilicate glass ceramic (LS2) was conducted. We found that in B5S8, spherulites contain residual glass and approximately 48% internal crystallization for spherulites larger than 10 μm. The core of the spherulites consists of a silica-rich region. The values of E and H increase with f

0 and do not depend on the size of the spherulites. The fracture resistance of the B5S8 system experiences a reduction of approximately 15 % in the rst 20 % of the spherulite volume fraction for all spherulite sizes in comparison with the strength of the glass. When f 0 exceeds 30 %, the biaxial resistance for all spherulite sizes rises with the volume of the crystallized fraction. For highly spherulized samples with a volume fraction of f

0 over 40 %, strength decreases

proportionally to d −1/2 , where d represents the spherulite size. This nding suggests that the value of d determines the size of the critical defect for samples that have a high f0 value. The fracture toughness of B5S8 increases as the volume fraction of spherulites increases. However, when f > 20 %, the fracture toughness remains constant and the

spherulite size does not signicantly aect the toughness. When compared to other glass- ceramics that nucleate in bulk, the increase in KIC of B5S8 is minimal. Through modeling

based on surface topography measurements, we observed that the observed increase in

toughness is caused by crack deection. The LS2 glass-ceramic displayed R-curve beha- vior, which was not present in B5S8. Previous experimental data was used to study the

mechanisms behind the R-curve in LS2 glass-ceramics. All proposed mechanisms indica- ted a correlation between the increase in KIC, due to the R-curve proportional to √

f.d.

This correlation was conrmed for LS2 glass-ceramics. Moreover, the primary toughe- ning mechanisms involved in LS2 were analyzed. The crystallization of a phase with high

toughness and the formation of spherulites with a high degree of internal crystallization are crucial for the production of tougher glass-ceramics.