Development and characterization of 3Y-TZP/SrAl12O19 ceramic composites: a threshold between densification and toughening

Alumina-toughened zirconia (ATZ) composites combine the high fracture toughness and bending strength of zirconia with alumina's high hardness and wear-resistance. Usually, ATZ composites are based on equiaxial alumina grains. However, the use of Al 2 O 3 reinforcement phases containing high aspect ratio can impair better mechanical properties. In this work, ceramic matrices based on yttria-stabilized tetragonal zirconia (3Y-TZP) were reinforced with different concentrations of in-situ growth strontium hexaluminate (Sr-H6A), an alumina-based compound with platelet-like crystallographic habit. The Sr-H6A powder was synthesized by sol-gel route, which resulted in an amorphous equiaxial precursor powder (D 50 ≈ 370 nm). The composites were prepared by a wet route, considering different concentrations of reinforcement phase (0, 5, 10, 15, and 20 wt%). The samples were processed by uniaxially pressing and sintered at 1500 °C, 1550 °C or 1600 °C under isothermal holding time of 2 h. The synthesized powders reacted in-situ , resulting in monophasic Sr-H6A crystals with platelet-like shape of high aspect ratio (≈6:1). The sintered composites presented tetragonal zirconia (T-ZrO 2 ) as major crystalline phase with secondary phases of a distorted tetragonal zirconia phase (T′-ZrO 2 ) and Sr-H6A. However, by increasing the Sr-H6A reinforcement phase concentration, the densification of the composites is hindered, with the best densification being found at 1600 °C for the composite reinforced with 5 wt% of Sr-H6A (98.5 %). The mentioned group presented good bending strength (σ 0 ≈ 920 MPa) and improved fracture toughness (K IF ≈ 9.7 ± 0.3 MPa m 0.5 ) when compared with the control group (σ 0 ≈ 1000 MPa and K I F ≈ 6.9 ± 0.2 MPa m 0.5 ). Evidence of toughening mechanisms, e.g., crack-bridging and crack-deflection, could be related to the presence of Sr-H6A crystals of high aspect ratio.