No abstract

This work investigated the effect of adding ceria-stabilized tetragonal zirconia (Ce-TZP) on the fatigue behavior of alumina-based ceramic composites. Alumina powder (control group) and mixtures containing 5 wt.% (group A) and 20 wt.% (group B) of a commercial m-ZrO 2 /Al 2 O 3 /CeO 2 powder mixture were milled/homogenized, compacted, sintered at 1600°C-2h, and submitted to hydrothermal degradation. The samples were characterized by relative density, microstructure, crystalline phases, and static mechanical properties. The cyclic fatigue strength was determined using the modified staircase method in 4-point bending tests. The results indicate that adding the m-ZrO 2 /Al 2 O 3 /CeO 2 powder mixture to the Al 2 O 3 -matrix increases the tetragonal-ZrO 2 grains (Ce-TZP) content, presenting 2.9 wt.% of Ce-TZP and 11.9 wt.% of Ce-TZP for group A and group B, respectively. Furthermore, the addition of Ce-TZP improves densification (98.5% → 99.1%) with a slight reduction in hardness and modulus of elasticity and a significant K IC increase of the composite (K IC = 6.7 MPa.m 1/2 , group B) when compared to monolithic alumina (K IC =2.4 MPa.m 1/2 ). The fatigue strength limit of the control group was around 100 MPa, while the composites (groups A and B) presented the values of 279 MPa and 239 MPa, respectively. The results indicated that the incorporation of Ce-TZP significantly improves the fracture toughness of alumina-based ceramics. On the other hand, regarding the fatigue behavior, there was an increase in fatigue resistance in group A, resulting from the benefits of the t→m Ce-TZP grains transformation, which occurs during cyclic loading, producing a zone shielding that involves the tip of the crack, slowing its growth. The increase in the amount of Ce-TZP (group B) leads to an increase in the internal residual stresses between the phases due to anisotropy and difference in the thermal expansion coefficients, which accelerates the phase transformation and formation of microcracks at grain boundaries, reducing the fatigue strength of composites of group B.

This work investigated the effect of a composite of tetragonal zirconia with alumina-reinforced Ceria (ATZ) previously synthesized, on the properties of alumina-based ceramics (Al 2 O 3 ). Monolithic alumina powder and Al 2 O 3 powder mixtures containing 5, 10, 15 and 20 wt.% of Ce-TZP/Al 2 O 3 were processed by high energy milling, compacted and then sintered at 1600 °C -2 h. Sintered pure alumina and composites were characterized by relative density, scanning electron microscopy, X-ray diffraction and surface roughness. Then, the elastic modulus, the Vickers hardness, the fracture toughness and the 4-point flexural strength were evaluated. The results indicated an increase in relative density as a function of the addition of ATZ, with values between 94.3 ± 0.8% and 98.9 ± 0.7%. The observed microstructure after sintering showed tetragonal ZrO 2 grains with average sizes of 0.6 μm well dispersed in the Al 2 O 3 matrix, which presents average grain sizes of around 1.5 μm. The crystalline phases identified in the composites were ZrO 2 -tetragonal and Al 2 O 3 hexagonal. The addition of the composite (ATZ) in the alumina matrix generates a gradual reduction in the elastic modulus (398 ± 15 GPa ~366 ± 34 GPa) and in the hardness (20.5 ± 1 GPa ~17.3 ± 0.7 GPa) of the sintered ceramics. On the other hand, the addition of this same composite (ATZ) in the alumina matrix considerably increases the materials fracture toughness, reaching values of approximately 6.7 ± 0.9 MPa.m 1/2 . The same trend was observed in the flexural strength results which ranged from 258 MPa (5wt.% Ce-TZP/Al 2 O 3 ) to 316 MPa (20wt.% Ce,Y-TZP/Al 2 O 3 ). The Ce-TZP reinforcement acts as a toughening agent of the Al 2 O 3 matrix due to the coupled mechanisms of toughening by zirconia phase transformation, residual stresses due to the difference in thermal expansion of the crystalline phases and differences in microstructural morphologies.