Scientific African (Mar 2021)
Influence of residual stress on the mechanical behavior of ceramics with various quartz sizes
Abstract
Ceramics are semi vitreous materials that are made of kaolin clay, feldspar and quartz. These materials are fired at a temperature greater than 1000°C. Due to low sinterability during firing, quartz remains embedded in a glassy matrix. During cooling, a mismatch in the thermal expansion coefficient of quartz and its surrounding creates cracks as a result of residual stress. This study therefore explores the effect of quartz particle size on the magnitude of residual stress. The results are later compared with the mechanical behavior of the samples.Test samples were pressed at 40 MPa and then fired at a peak temperature of 1300°C at a rate of 60°C/min. Their microstructure was studied using a scanning electron microscope (SEM). The X-ray diffraction method was used to measure residual stress, calculation was based on [101] and [110] quartz planes. In addition, the measurement of flexural strength and hardness was limited to three point loading and Vickers indentation, respectively.The study revealed that the residual stress on quartz decreases with particle size over a range of 45–200 µm. The decrease was attributed to cracks formed in the microstructure of fired samples. The SEM micrograph of bodies with 45 µm showed a crack free glassy phase. As a result, a high value of flexural strength (33±1 MPa) and hardness (5.8±0.2 GPa) was exhibited by these samples. These results further point out that flexural strength is comparable to ISO 13006 standards of ≥ 35±2 MPa. The samples with 90 µm particle size exhibited strength of 25±1 MPa and hardness of 5.4 ± 0.1 GPa. Due to severe cracks triggered by residual stress effect, samples with 200 µm exhibited strength of 15±1 MPa and hardness of 4.6±0.1 GPa. Therefore, to minimize cracks due to stress effect, quartz size milling of less than 90 µm is encouraged.
