Determinação da energia de fratura em materiais cerâmicos em altas temperaturas.
Abstract
In this work, the main objective was to determine the energy of fracture in
ceramic materials, focused on refractories, at high temperatures and to inquire
the interference of the creep on this measurement. Therefore, castable
containing white fused and calcined aluminas, hydrated alumina as a binder,
with and without, a glass of SiO2-CaO-Al2O3-B2O3-MgO was designed. This
material can simulate the real situation of refractory castables that present
possible glassy phase formation due to the presence of cement, silica fume and
impurities of raw materials. In addition to this material two commercial
refractories were also characterized. Physical characterizations, refractoriness
under load, modulus of rupture, Young's modulus and coefficient of linear
thermal expansion were evaluated. The energy of fracture was measured
through stable crack propagation in three-point bending tests. The creep curves
were obtained in three-point bending and compression tests. The concept of
rate of energy of creep deformation was used to evaluate creep semiquantitatively
as well as to consider a description for the energy of fracture at
high temperatures. The results indicate that a maximum in the energy of
fracture measured through stable crack propagation under three-point bending
tests occurs as a function of temperature. For this case it will exist a range of
temperature at which the material will suffer a transition from brittle to ductile
behaviour due to the presence of glassy phase in the microstructure. In the
transition, bridges of glassy phase contribute for the increase of the average
energy for crack propagation. Consequently, these results have strong influence
on the thermal shock damage resistance parameters.