Development of bilayer zirconia ceramic blocks with distinct 3Y-TZP and 4Y-PSZ layers through double extrusion 3D printing

Abstract

In this study, bilayer zirconia ceramics blocks containing 3 mol.% and 4 mol.% of yttria were processed by an extrusion-based additive manufacturing process, using a double-extrusion 3D-system. The ceramic ink was prepared by a gel-like approach, using polyethylene glycol and Laponite® as gelling agents. Two gel-inks were prepared by adding 31 vol% ZrO₂ powders (3Y-TZP or 4Y-PSZ) and 2 vol% dibutylphthalate as rheological modifier. Disks and bars shaped samples were printed through cross-layer deposition at 10 mm/s printing speed and 0.63 mm nozzle diameter, within the first 4 layers of 3Y-TZP ink and another 4 layers of 4Y-PSZ ink. After drying, the samples underwent debinded at 1100°C-2h, followed by sintering at 1550°C-2h. XRD analysis with Rietveld refinement showed that both distinct layers of sintered material exhibited tetragonal and cubic ZrO₂ phases in different ratios. The sintered samples exhibited a relative density of approximately 91 %. The modulus of elasticity and hardness were measured using Vickers nanoindentation, with test loads varying between 1000 and 1960 mN. Microindentation Vickers test (9.81 N) also had been performed to obtain hardness and fracture toughness. Furthermore, 3-point bending test with respective Weibull's statistic was performed. In the 4Y-PSZ layer, the modulus of elasticity ranged from 131 GPa to 115 GPa, the hardness varied from 1229 HV to 924 HV by nanoindentation. For the 3Y-TZP layer, the modulus of elasticity ranged from 193 GPa to 182 GPa, while the hardness decreased from 1591 HV to 1272 HV. The microindentations results of 3Y-TZP layer reach 999.66 ± 88 HV while the 4Y-PSZ layer reach 908.89 ± 79 HV and fracture toughness of 5.25 ± 0.85 MPa m^1/2 and 4.07 ± 0.57 MPa m^1/2. The flexural strength ranging from 47 to 140 MPa, with a Weibull modulus of 5.1 and 10.9 for bottom face with 4Y-PSZ and 3Y-TZP respectively. Although the attained relative density is still below the desirable for structural applications, the results obtained here pave the way for the free-form processing of zirconia-based ceramics, with a compositional layered structure, using a material extrusion method.