Resistance to deep abrasion of porcelain stoneware tiles: key factors

The resistance to deep abrasion of porcelain stoneware tiles (ISO 10545-6) corresponds to a model of tribological behaviour for three-body abrasion, based on mechanical characteristics (hardeness and fracture toughness) and wear conditions (aalpied load, contact area and size of abrasive particles). The mechanical properties are affected by the total porosity (2-8%) of porcelain stoneware; thus there is a certain dependence of the material removed by abrasion on the pore volume, that however does not explainthe entire variance of the measurements of wear-resistance. The phase composition plays a fundamental role and a simple predictive model has been developed, indicating that the larger is the content of mullite, zircon and quartz, the higher is the resistance to deep abrasion. In particular, mullite and zircon ensure tha main contribution to the tribological properties, owing to both their hardness and a toughening mechanism originated by differences of thermal expansion and elastic moduli between gassy and crystalline phases, inducing a compression at grain boundary. Other crystalline phases, such as quartz and corundum, have a less relevant effect on the wear resistance, for the unfavourabledifferences of thermal expansion with the vitreous matrix. To improve the resistance to deep abrasion of tiles, it is possible to act on the body formulation, in order to increase the amount of “hard” phases, mailnly mullite and zircon, without altering the amount of residual porosity, that influences heavily both hardness and fracture toughness of porcelain stoneware.