Influência de moagem de alta energia nas propriedades físicas de uma porcelana de ossos

Abstract

Porcelain made from bovine bone ashes, known as bone china, is whiter, lighter, stronger and more translucent than ordinary porcelain. The strength of a bone china can reach up to twice the resistance of common porcelain. Another benefit of using this material, also considering environmental concern, is the recycling of bones. Bone china is composed of about 50 % bone ash, 25 % kaolin and 25 % feldspar. These values may vary, depending on the desired properties. The translucency of porcelains is, among other factors, determined by the composition, thickness of the molded parts, particle diameter, etc. In this work, bone china samples were produced using two different milling methods: low energy ball milling (LEBM) for 3 and 24 hours, and a high energy ball milling (HEBM), for 5 minutes. After pressing, the samples were burned at high temperatures for sintering (1100 °C, 1125 °C, 1150 °C, 1175 °C, 1200 °C and 1250 °C). The samples were characterized and their physical properties were determined to verify the effects of the milling process. The results show that high energy ball milling (HEBM) gives bone china a specific surface area (BET) 49.3 % higher than low energy ball milling (LEBM), demonstrating a higher efficiency in particle comminution. From the X-ray diffraction pattern (XRD) it was possible to observe that the kaolinite and muscovite phases decreased in intensity as they were milled in a more energetic milling (HEBM). This effect is also verified by thermogravimetric (TG), differential scanning calorimetry (DSC) and dilatometry tests, by the anticipation of the thermal events of surface adsorbed water evaporation, kaolinite dehydroxylation and sintering. The highest density was obtained in samples milled in HEBM 5 minutes (2.64 g/cm³), fired at 1200 °C, being 4.3 % higher than the highest density measured in LEBM (2.54 g/cm³) which, consequently, also resulted in a 6.3 % Young modulus (E) increase. Finally, it was verified that the milling method influences the translucency of a bone china until the beginning of the liquid phase formation during sintering (around 1150 °C), being more translucent in HEBM 5 minutes and LEBM 24 hours than in LEBM 3 hours. The presence of this liquid phase reduces porosity and, consequently, light scattering. In addition, as a positive aspect for bone china, the refractive indexes of its constituent phases are similar to each other, regardless of the milling method used. Therefore, at firing temperatures above 1150 °C, the milling process doesn’t directly influence the translucency of bone china.