Development of 3Y-TZP-SrAl₁₂O₁₀ Composites by Reaction Sintering: The Threshold between Sinterability and Toughening ^†

Alumina-toughened zirconia (ATZ) ceramics are composites that combine the excellent fracture toughness and bending strength typical of zirconia ceramics with the high hardness and wear-resistance of alumina (Al₂O₃) reinforcements. Usually, these ceramic composite materials contain equiaxial Al₂O₃ grains; however, ceramic composites reinforced with a high aspect ratio grain morphology usually present better mechanical properties. In this work, we synthesised and evaluated ceramic composites based on a matrix of Y-TZP doped with different amounts (5 to 20%wt) of strontium hexaaluminate (SrH6A), an alumina-based compound with platelet-like crystallographic habit. A simplified sol–gel route was employed for the SrH6A synthesis, which resulted in an amorphous equiaxial precursor powder. This powder was wet mixed with a commercially available 3Y-TZP powder at different concentrations (0, 5, 10, 15, and 20%wt). The obtained mixtures were uniaxially pressed (100 MPa) and sintered at different temperatures (1500, 1550, and 1600 °C) for 2 h. All samples were analysed by the Arquimedes method, X-ray diffraction, Rietveld refinement, scanning electron microscopy, Vickers hardness and indentation fracture toughness (IF-K_IC). The SrH6A addition hindered the full densification of the composites; nonetheless, by increasing the sintering temperature, the relative densities raised from 95 to 98.5% (15%wt SrH6A). SrAl₁₂O₁₉ (SrH6A) crystals, with an average length of 1–1.5 µm and an aspect ratio of 4:1, were formed in situ during the sintering process. The samples sintered at 1600 °C presented an improvement in the IF-K_IC data, from 7.8 to 11.3 MPa.m^1/2, as a function of the SrH6A concentration. Evidence of toughening mechanisms, e.g., crack bridging and crack deflection, could be related to the SrH6A crystals, suggesting that they are majorly responsible for improving the mechanical properties. The developed composites have a high potential to be employed as a biomaterial with structural behaviour, such as dental implants or prostheses for hip replacement.