Sintering mechanisms of porcelain stoneware tiles

The sintering of porcelain stoneware tiles - a glass-bonded material with excellent technical performances produced in slabs up to 1 m2 - is accomplished in roller kilns with fast cycles (<60 min) at maximum temperatures in the 1180-1240°C range. The main limit in further development of this product is its residual porosity, consisting of 2-8% closed pores, that is detrimental to its mechanical and tribological properties as well as the resistance to stains of polished tiles. Several industrial and experimental body formulations were studied by isothermal and constant rate optical thermodilatometry, XRPD, SEM and BET, in order to understand which mechanisms are active during the whole sintering path. In the initial stage, a strong reduction of specific surface occurs with negligible shrinkage, suggesting that surface diffusion is the leading mechanism below 1000°C. In the intermediate step, densification is accomplished with increasing rate from 1050 to 1200°C, being a viscous flow the basic mechanism. However, the maximum shrinkage at each temperature is not related to the amount of viscous phase, so densification seems to be mainly governed by the viscosity of the liquid phase. In the final stage, a more or less conspicuous coarsening is observed, essentially over 1175°C, with development of coarser pores. In this critical step, the amount of residual porosity depends on the contrasting effects of a decreasing densification rate against an increasing coarsening rate