Asynchronous densification of zirconia ceramics formed by stereolithographic additive manufacturing (2023)

Ceria-doped tetragonal zirconia polycrystal (Ce-TZP) ceramics possess significant toughness and aging resistance, while their strength is relatively limited due to the high tendency of grain growth, which has long hindered their versatile applications. This work demonstrates an effective method to combine stereolithography with pre-sintering and hot isostatic pressing (HIP) to obtain dense Ce-TZP ceramics with a reduced grain size of about 420nm. In addition to the contribution of smaller grains, the Ce-rich phase Ce_0.75Zr_0.25O₂ precipitates at the grain boundaries during HIP hinders grain growth and strengthens the grain boundaries, together leads to a transgranular-dominant fracture mode. As a result, the achieved Ce-TZP ceramic simultaneously reaches a high flexural strength of 770.90±76.06MPa and an excellent fracture toughness of 11.96±1.31MPa·m^1/2, which are 48% and 29% higher, respectively, compared to the pressureless-sintered samples. This method is expected to enable broader applications of pure Ce-TZP ceramics and to be further extended for designing and manufacturing densified and fine-grained components with complex structures.

Owing to superior properties, i.e. high hardness, high wear resistance, and weight reduction of transparent ceramics (TCs) over glasses, TCs have shown promising tribological potential for applications such as face shields, explosive ordnance visors, windows for aircraft, spacecraft and, re-entry vehicles, electromagnetic windows, laser igniter windows, screens for smartphones and more. Researchers globally have been attracted to explore more about TCs, considering the tremendously increasing demand over different other transparent materials. The optical quality of TCs is mostly characterized by the in-line transmittance, and the effect of various processing parameters on transmittance has already been studied by various researchers. In this review, the current research progress regarding tribological performance of TCs is compiled. TCs with potential in tribological applications include MgAl₂O_4, Al₂O₃, AlON, Lu₂O₃, c-BN, Y₂O₃, Si₃N₄, and SiAlON. The relevant strategies to improve the tribological properties, including microstructures and mechanical properties are comprehensively discussed. In addition, the mechanisms of material removal of different transparent ceramics are also presented. It is well observed that surface fracture comprising three stages is found as one of the dominant wear mechanisms during wear. This review aims to provide some meaningful guidelines for development of transparent ceramics with enhanced wear resistance, while identifying the wear mechanisms in particular wear conditions.

Porous biphasic calcium phosphate (BCP) bioceramics are considered to be the most promising bone repair materials in clinical medicine. Stereolithography (SLA) 3D printing can precisely fabricate bioceramic scaffolds with complex porous structure. During this process, how to obtain a suitable sintering procedure for BCP bioceramics by SLA 3D printing is the key to determine the microstructures and mechanical properties, but this is absent. In this present study, we prepared BCP bioceramics with superior densification and mechanical properties by SLA 3D printing, and mainly investigated the effects of sintering temperature and condition on the microstructures and mechanical properties of SLA 3D printed BCP bioceramics for the first time. At the optimized procedure of sintering temperature 1250°C for 2h, the 3D printed BCP bioceramics showed uniform shrinkage in all directions, and especially the mechanical properties were close to that of human cortical bone. Furthermore, complex porous BCP scaffolds with high porosity (51.49%) and compressive strength (8.14MPa) were successfully obtained. BCP bioceramics greatly promoted the proliferation of MC3T3-E1 cells by the release of Ca and P ion and presented excellent bioactivity. This work proves that SLA 3D printing technology can prepare BCP bioceramics with high mechanical and functional properties, and provides a new route for manufacturing high performance BCP bioceramic scaffolds with complex structure by SLA 3D printing for repairing bone defect in clinical.

Under high-frequency microwave irradiation, zirconia ceramics were prepared by sintering nano-CeO₂ (Ce=7mol%) doped zirconia powder. The different effects of temperature environment on the phase structure transformation, surface functional groups, microstructure, growth process, and density of doped zirconia were analyzed, and the optimized microwave sintering process for zirconia was determined. The experimental results reveal that the tetragonal phase of zirconia is positively correlated with the temperature when the temperature reaches about 1100°C in the studied range. The reason is that the grain grows with the increase of sintering temperature, and the surface energy of grain decreases, which leads to the fluctuation of tetragonal phase content. The density of zirconia reaches 98.03% at 1300°C, and the growth activation energy is 27.40kJ/mol. There is no abnormal growth of zirconia particles, and the phase transition temperature decreases, which is attributed to the efficient heating of microwave and the incorporation of nano-ceria stabilizer.

Journal of the European Ceramic Society, Volume 39, Issue 4, 2019, pp. 1374-1380

Laser melting is known to be capable in initiating thorough evolution in microstructure and bringing novel functional performance in metals. But realization of this potential in ceramics only reaches a preliminary stage that needs further investigation. Here we demonstrate zirconia, traditionally an insulative ceramic at low temperature, could be transformed into an electronic conductor with the conductivity on order of 10⁻³ S⋅cm^-1 at room temperature by a simple laser melting process without inducing metallic phases. Transmission electron microscopy and ab-initio simulation show that oversaturated oxygen vacancies, together with their ordered metastable distribution along <001 > , are introduced during this non-equilibrium process, and result in a clear defect level significantly narrowing bandgap to less than 1 eV, leading to the considerable electronic conductivity. These results identify a strategy of utilizing this non-equilibrium method in oxide ceramics to realize some unconventional performances determined by metastable structure thoroughly altered down to atomic level.

Materials Chemistry and Physics, Volume 267, 2021, Article 124661

The volume shrinkage and conversion rate are important factors determining the defect generation and accuracy control of green parts in ceramic stereolithography. In this study, a specific laser reflection method with coverslips and real-time Fourier-transform infrared (RT-FTIR) spectroscopy were used to determine the effects of the double-bond concentration, monomer ratio, photoinitiator, and incident light intensity on the real-time volume shrinkage and conversion rate. A slurry with a lower double-bond concentration obtained by using a solvent exhibited a smaller shrinkage. The shrinkage law for pure resin wasn't applicable owing to the ceramic particles. The mixing of ethoxylated pentaerythritol tetraacrylate (PPTTA) and 1,6-hexanediol diacrylate in suitable ratios promoted the conversion. In addition, the PPTTA could regulate the overall shrinkage. The photoinitiator had a small effect on the shrinkage. A high light intensity increased shrinkage significantly, while the conversion rate determined the shrinkage at a low light intensity.

Materials Chemistry and Physics, Volume 207, 2018, pp. 1-10

In this paper, a ceramic suspension with appropriate viscosity was prepared by optimizing the powder solid content and the dispersant content. An Al₂O₃-ZrO₂ ceramic green body was fabricated using the SLA technique, followed by the application of a liquid drying and two-step debinding process to prepare the defect-free Al₂O₃-ZrO₂ ceramic green body. The relative density, phase composition, microstructure, grain size, and mechanical properties of Al₂O₃-ZrO₂ samples sintered at different temperatures were compared. The main results indicated that the sample density increased with the sintering temperature until reaching a maximum density of 4.28 g/cm³ at 1600 °C. When the temperature further increased to 1650 °C, the density dropped instead. With increasing sintering temperature, both the number of grain boundaries and the number of pores located at the grain boundaries reduced greatly. The Vickers hardness of the samples first increased and reached its maximum at 1550 °C with a value of 17.6 GPa, and then decreased with further increases in sintering temperature. The fracture toughness increased with the sintering temperature and reached a maximum value of 5.2 MPa·m^1/2 at 1650 °C. A sintering kinetics window which could offer the relationship between the sintering temperature and the relative density & grain size was set and also the microstructure evolution of the sintered body was conducted to get a deeper understanding of the 3D printing Al₂O₃-ZrO₂ composites.

Ceramics International, Volume 44, Issue 17, 2018, pp. 21556-21563

A reliable method for fabricating zirconia-based all-ceramic teeth by stereolithography with gelcasting was demonstrated. The effects of ZrO₂ particle size were studied on the stability and rheological properties of the zirconia slurry. The effects of powder properties, slurry solid content and sintering temperature on the properties and microstructure of zirconia were also investigated. The results showed that optimal parameters were: 0.2 µm particle size, solid phase content 37 vol% of zirconia slurry and sintering temperature 1550 °C. Part density was up to 98.6%, flexural strength was 1170 MPa and fracture toughness was 19.0 MPa m^1/2. Zirconia-based all-ceramic teeth were finally fabricated based on the optimal parameters by using combined stereolithography with gelcasting.

Materials Chemistry and Physics, Volume 239, 2020, Article 122069

In this work, a systematic research is conducted into sintering kinetics of 3D printed Al₂O₃ and Ce–ZrO₂/Al₂O₃ to evaluate the effect of CeO₂/ZrO₂ co-doping on the densification and grain growth. The sintering window shows microstructure confinement and full densification could be both realized at 1550 °C for Ce–ZrO₂/Al₂O₃. The density curves versus time sintered at 1550 °C for the two material systems are very close and the highest relative density could be obtained at 1550 °C for both systems. The grain size exponent n is around 3 for all temperatures excluding 1550 °C for the un-doped Al₂O_3, indicating only grain boundary diffusion-controlled densification mechanism could be explained for the un-doped Al₂O_3, sintered at 1550 °C. The temperature dependent grain-growth constant K is one order of magnitude lower than that of the undoped Al₂O₃, which could fully manifest the grain growth retarding effect of Ce–ZrO₂ co-doping. The activation energies obtained for the undoped Al₂O₃ and Ce–ZrO₂/Al₂O₃ composites are 680.2 kJ/mol and 734.8 kJ/mol, respectively. The higher activation energy obtained for Ce–ZrO₂/Al₂O₃ composite could be attributed either to the enthalpy for defect formation or to the presence of a liquid phase at grain interfaces in Ce–ZrO₂/Al₂O_3.

Ceramics International, Volume 43, Issue 1, Part B, 2017, pp. 968-972

We report a novel approach to fabricate dense zirconia-toughened alumina (ZTA) ceramics with excellent properties via an additive manufacturing process based on stereolithography. The XRD patterns show the ZTA sample consists of α-Al₂O₃ and t-ZrO₂ with the dominance of α-Al₂O₃. The zirconia grain with the average size of 0.35µm is small enough to trigger the toughening behavior of zirconia in the ZTA. The prepared ceramics showed a density, Vickers hardness, bending strength, and fracture toughness of 4.26g/cm³, 17.76GPa, 530.25MPa, and 5.72MPam^1/2, respectively. These properties are comparable to those for ceramics obtained through conventional ceramic processing.