Measurement and modeling of residual stress in porcelain tiles formulated from different quartz particle sizes in Uganda
Quartz plays a significant role in physicomechanical properties of tiles, because it is the most abundant crystalline phase present in a fired product. The use of quartz particle size that exceeds 32 µm can be detrimental to the properties of tiles. This is attributed to cracks caused by the presence of residual stress. This study therefore, aims to explore the effect of quartz particle size on the magnitude of residual stress, and compare stress to mechanical behavior of the samples. The samples were formulated from kaolin, feldspar and quartz in the ratio of 5:3:2 by weight, respectively. They were pressed at 40 MPa and fired from 1150-1350 C at 60 C per minute. Microstructure analysis of the fired samples was carried out using a scanning electron microscope (SEM). The X-ray diffraction method was used to measure quartz residual stress, based on crystal planes , , ,  and . The finite element method was adopted to model residual stress, and it was implemented using ABAQUS software. In addition, flexural strength was measured by three-point loading, whereas hardness was limited to Vickers indentation. The results revealed an optimum firing temperature of 1300 C, and firing to 1350 C resulted in bloating corroborated by SEM. At optimum temperature, there was a fall in the magnitude of residual stress with quartz particle size. This was triggered by the severity of cracks in the body. The experimental results exhibited a good agreement with the finite element model. From the model, a high stress concentration was predicted at the sharp corners and between areas of close quartz proximity. It indicates that these points are more susceptible to cracks. Comparing residual stress and mechanical behavior, the results indicate that they follow a negative correlation with quartz particle size. This was associated with the crack and pore morphology present in the fired samples. Flexural strength reduced from 33 ±1 to 15 ±1 MPa with particle size, and 5.8 ±0.3 to 4.6 ±0.2 GPa for Vickers hardness. This points out that sample with fine quartz particles are within the ISO 13006 standards for porcelain floor tiles, that is 35 ±2 MPa or more. In addition, samples with fine and medium quartz particles had values comparable to 5.5±0.6 GPa reported as the hardness of commercial tiles. However, samples with coarse quartz particles exhibited properties below standard. This study therefore, agrees with the ideal of fine quartz particle milling (≤90 µm).