Phase transformations during liquid phase sintering of porcelain stoneware tiles

Porcelain stoneware tiles are low porosity products with excellent technical performances, (mechanical, tribological, chemical and frost resistance). Their main standard requirement is a very low water absorption, being <0.1% in most products. However, a residual closed porosity is always present (usually from 2 to 8%) affecting significantly the product performances, especially mechanical strength and stain resistance. The sintering of porcelain stoneware is accomplished in roller kilns by fast single firing, resulting in a phase composition consisting of vitreous phase, mullite, and quartz, plus accessory minerals (feldspars, cristobalite). The phase transformations occurring during sintering (decomposition of clay minerals, formation of amorphous phases and mullite, partial melting of feldspars and quartz in eutectic compositions) were studied by hot stage microscope, x ray diffraction, and SEM investigation, determining amount and chemical composition of vitreos phase, its viscosity and surface tension at high temperature on both industrial and simplified bodies. The study is addressed to a better understanding of the sintering mechanisms, and of the chemico-physical evolution of the liquid phase formed at high temperature, in order to point out the critical aspects occurring in the densification process. During the sintering, many mechanisms are contemporarily active, due to a wide range of pore and particle size in the green compacts, making it difficult any actual design of the microstructure. The densification appears to be rate-controlled by the strong dependence of melt viscosity on temperature and by the solubility of solids in the liquid phase. The particle rearrangement and viscous flow are promoted by the fast development, accounting for most densification, of an abundant liquid phase, whose composition corresponds to feldspars-quartz eutectics. Nevertheless, in the final stage, coarsening and solubility of gases filling the closed pores become the most important phenomena affecting the microstructure.