Among ceramic tiles, porcelain stoneware is the best product. Its relatively high density gives this product class special mechanical and wear resistance characteristics. This material consists mainly of a glassy matrix in which gas bubbles, developed during firing, are trapped and several crystalline phases are embedded. These microstructural singularities within the glassy matrix can lead to different toughening mechanisms. For example, the quartz crystals can be responsible for different microcraking toughening mechanisms (crack deflection, crack branching and crack stopping). In the toughening of the glassy matrix of porcelain stoneware material, the role both of the mullite content and the pores has been also considered. On the basis of these observations, porcelain stoneware tiles, if compared with others typologies of products, present a relatively high value of fracture toughness, very close in any case to the value of glass, in agreement to the glassy nature of its matrix. However, even if it has to take into account for this significant difference, the high sensitivity of the material to the stresses coming from the environment, keeps on representing a serious drawback. Scratches, cuts and large areas, from which the material has been removed, still visible on the working surface of the tiles after the polishing operation, can be attributed both to a not correct machining procedure and parameters and to the rather low fracture toughness, not sufficient to react to crack propagation. Although many efforts have been addressed to improve the mechanical strength of these materials, crack resistance has received, until now, little consideration.
The reinforcing mechanisms are based on control of flaws and toughening. While the first approach concerns the control of the critical flaws, by analysing the process and identifying their source, the second one, acting in the microstructural feature, is addressed to achieve materials characterised by a high reliability and enhanced fracture resistance. The control of critical flaws does not seem to be a viable method, in these materials. Regarding the second approach, several ways to modify the microstructure and toughening a ceramic matrix exist: (i) toughening transformation, (ii) in situ crystallisation of elongated second phases, (iii) addition of high strength ceramic particles, metal particles and short and long fibres and, (iv) direct metal oxidation.
In the present investigation, the role played by different fluxing raw materials (sodium feldspar, potassium feldspar, calcite and soda-lime glass) in the densification and crystallisation behaviour during firing was considered. The crystalline index was considered and correlated to the mechanical properties (fracture toughness, elastic modulus and flexural strength). In addition, the possibility of reinforcing a standard body mix for porcelain stoneware tiles, by the addition of alumina powders was investigated. The results obtained show that the increase of crack resistance, can be attributed to the increase in the crystalline index and, in particular to the presence of alumina particles, since the fracture toughness of these particles is higher than that of the glassy matrix.