Reinforcement of single-firing ceramic glazes with the addition of polycrystalline tetragonal zirconia (3Y–TZP) or zircon

The addition of fine-grained polycrystalline tetragonal zirconia (3%Y2O3–TZP, or 3Y–TZP) to conventional, single-firing, ceramic glazes was studied with the aim to enhance their hardness, fracture toughness and wear resistance for floor tile applications. The stability of the added tetragonal zirconia towards solution in the glaze and/or zircon crystallization was found strongly dependent on glaze composition, firing treatment, and on the way of preparing the samples (uniaxially-pressed pellets or thin-layer coatings). 3Y–TZP remained stable in fast-fired pellets, but partial crystallization of prismatic zircon microcrystals (2–3 μm) occurred in samples prepared as thin-layer coatings. Addition of 10–30 wt.% of 3Y–TZP to a conventional single-firing glaze that was industrially enameled (thin films) onto low-porous floor tiles, promoted an important enhancement of Vickers microhardness (Hv from 6.0 to 8.4 GPa), fracture toughness (KIC from 1.35 to 2.23 MPa m1/2), and wear resistance (PEI abrasion number from 2 to 5). The achieved reinforcement was higher with 3Y–TZP than with zircon addition, and must be attributed to the stress-induced transformation-toughening mechanism promoted by the undissolved stabilized tetragonal zirconia, and also to partial zircon crystallization. The possibility of obtaining ceramic coatings mechanically reinforced with 3Y–TZP additions, while also maintaining a glossy aspect, was also confirmed.