Influence of composition on mechanical behaviour of porcelain tile. Part III: Effect of the cooling rate of the firing cycle

This paper is the third part of a study focusing on determining the influence of the porcelain tile composition on mechanical behaviour of sintered bodies. Tile compositions were prepared according to a simplex-centroid mixture design set out in Part I of this research, in which the microstructural characterisation of sintered specimens was carried out. In Part II the influence of the starting composition on the mechanical properties of sintered porcelain tile was evaluated on the basis of the linear elastic fracture mechanics. Finally, in this last Part, ceramic bodies from seven compositions were subjected to fast cooling after firing, in order to reproduce the industrial cooling rates. The main objective was to analyze the influence of the mineralogical composition of the starting mixture on the development of macroscopic residual stress and growth of flaw size. When the pieces were subjected to fast cooling, flaw size was the main factor determining the variation of the mechanical strength. This increase in flaw size can be interpreted from the Weibull modulus, from 6 to 8 in those mixtures, with high deterioration of mechanical properties. The mullite hypothesis as a strengthening mechanism in triaxial porcelains was clearly manifested when the samples are fast cooled. This mechanism was the main responsible for the strengthening, what contrasts with the increase in flaw size. The microscopic residual stress caused by the thermal expansion mismatch of the phases also acted as a reinforcement mechanism.