Journal of the European Ceramic Society
Volume 41, Issue 8,
, Pages 4666-4670
Author links open overlay panel, , , ,
Stereolithography has been proven as a feasible approach to make crack-free ceramic macrostructure with customized designs, but the microstructure, especially pore structure remains to be tailored more precisely for better performance, where the sintering protocol and related densification characteristics could play a vital role as the slurry preparation and debinding protocol do. Herein we report a phenomenon named “asynchronous densification”, that is, the surface region of zirconia ceramics formed by stereolithographic additive manufacturing would be densified prior to the bulk at 1200°C during the conventional pressureless sintering in air. The cause of this asynchronism is unclear but supposed to be correlated with low packing density, high sintering activity, poor thermal conduction of ceramics and impurities. Early densification of the surface may have negative effects towards ceramic components with more homogeneous microstructure, suppressed pore coalescence and limited grain growth, and therefore needs to be better controlled through optimization in sintering protocol.
Slurry-based stereolithography has become a vital, perhaps the most widely discussed and studied additive manufacturing technique specialized for ceramic materials [1,2]. After the decisive progress in commercialized facilities early in the last decade , consistent efforts have been made to manufacture ceramic components with performance meeting criteria in corresponding application fields. Among them, one of the major and remarkable advances recently is the success in realizing crack-free bulk components via optimization in slurry preparation and debinding protocol [, , , ], therefore it becomes reasonable and realistic now to evaluate a densified component formed by stereolithographic additive manufacturing as an integrated monolith for performance issues. However, for this target the microstructure of ceramics formed by stereolithographic additive manufacturing still needs to be further optimized to achieve better functional performance including but not limited to mechanical properties. For example, the relative density of the printed components was reported to reach 98% or even 99% [5,6], but these zirconia components still presented a pale-white appearance. Since the presence of only 0.1 vol% porosity is sufficient to turn an otherwise transparent ceramic into translucent or opaque in theory [8,9], tailoring features of the remaining 1 vol% porosity could be one of the critical factors to make zirconia fabricated by stereolithographic additive manufacturing practical and competitive in the most-discussed application of dental restoration, especially for aesthetic restoration in anterior teeth against other monolithic zirconia restorations already on the market .
Therefore, an ideal approach of densification should be able to achieve nearly the theoretical density while preventing pores and grains from significant growth. Besides homogeneous and stable slurry containing well-dispersed nanopowders and controllable, sufficient and efficient removal of organic composition during debinding , sintering protocol would also play a key role in the effort towards better densification. In this communication, we report a phenomenon named “asynchronous densification” observed when sintering zirconia ceramics formed by stereolithographic additive manufacturing, that is, rapid densification of surface at 1200°C while the bulk remains under-densified under our experimental conditions. We believe that this phenomenon should not be ignored as the early densified surface would influence the following densification and pore elimination progress in the bulk, thus hinder the effort in pursuing superior microstructure of additively manufactured bulk ceramics. It may also bear general implication to the sintering of ceramics with homogeneously packed well-dispersed fine particles in a broader perspective.
Commercial 3 mol% yttria-doped tetragonal zirconia polycrystal (3Y-TZP) nanopowders (D50 = 210 nm) were used in preparing photocurable hybrid slurry for stereolithographic additive manufacturing experiments. Other precursors used in the slurry were the same as those reported in our previous studies, including photocurable acrylic resin-based monomer, dispersant agent, plasticizer and photoinitiator [5,12]. The slurry was prepared via mixing, ball-milling and degassing to achieve a solid loading
Fig. 1 shows the polished cross section of zirconia specimens sintered at different temperatures. It is obvious that densification is promoted and fewer pores remain with higher sintering temperature in general, along with some other specific features in microstructure evolution. First, a layered structure is vaguely visible in specimens sintered at 1200°C, 1300°C and 1450°C, indicated by those parallel dashed lines in Fig. 1a, d & g, respectively. Spacing of each layer is about 20 μm, which is
Hypothesis on the cause of asynchronous densification
Though the exact cause of this asynchronous densification phenomenon is yet unknown, it is speculated to be correlated with the nature of zirconia green bodies formed by stereolithographic additive manufacturing that need to be further evaluated in the future. One hypothesis is that rapid densification on the surface could be favored by the loose but homogeneous packing of well-dispersed nanoparticles that increases the sintering activity of stereolithography fabricated zirconia green bodies.
In summary, the phenomenon of asynchronous densification at a relatively low sintering temperature in zirconia specimens formed by stereolithographic additive manufacturing was investigated in this study, which may have negative impact on acquiring densified ceramic components with more homogeneous microstructure, suppressed pore coalescence and limited grain growth. An optimized sintering protocol is thus the key to better tailor this phenomenon, while further improvement in powder
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
The authors would appreciate Dr. Mirva Eriksson (Department of Materials and Environmental Chemistry, Stockholm University, Sweden) for help in preparing some of cross section-polished samples and corresponding SEM characterization, and Prof. Martin Trunec and Dr. Vaclav Pouchly (CEITEC BUT, Brno University of Technology, Czech Republic) for help in dilatometry and mercury-intrusion porosimetry measurement. This work is supported by National Key R&D Program of China, Ministry of Science and
- Q. Lian et al.
Accurate printing of a zirconia molar crown bridge using three-part auxiliary supports and ceramic mask projection stereolithography
- M. Dehurtevent et al.
Stereolithography: A new method for processing dental ceramics by additive computer-aided manufacturing
- M. Saâdaoui et al.
X-ray tomography of additive-manufactured zirconia: processing defects – strength relations
J. Eur. Ceram. Soc.
- V. Prajzler et al.
Rapid pressure-less sintering of fine grained zirconia ceramics: explanation and elimination of a core-shell structure
J. Eur. Ceram. Soc.
- R. Kalousek et al.
Rapid heating of zirconia nanoparticle-powder compacts by infrared radiation heat transfer
J. Eur. Ceram. Soc.
- R. Moreno
Better ceramics through colloid chemistry
J. Eur. Ceram. Soc.
- X. Cui et al.
Esthetic appearances of anatomic contour zirconia crowns made by additive wet deposition and subtractive dry milling: a self-controlled clinical trial
J. Prosthet. Dent.
- K. Wang et al.
Study on defect-free debinding green body of ceramic formed by DLP technology
- J. Sun et al.
3D printing of zirconia via digital light processing: optimization of slurry and debinding process
J. Eur. Ceram. Soc.
- M. Zhou et al.
Preparation of a defect-free alumina cutting tool via additive manufacturing based on stereolithography – optimization of the drying and debinding processes
3D printing of ceramics: a review
J. Eur. Ceram. Soc.
Ceramic stereolithography: additive manufacturing for ceramics by photopolymerization
Annu. Rev. Mater. Res.
Stronger and tougher nanosized dense ceria-doped tetragonal zirconia polycrystals by sinter-HIP
2023, Journal of the European Ceramic Society
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 Ce0.75Zr0.25O2 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·m1/2, 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.
Tribological behaviour of transparent ceramics: A review
2022, Journal of the European Ceramic Society
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 MgAl2O4, 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.
Microstructures and mechanical properties of biphasic calcium phosphate bioceramics fabricated by SLA 3D printing
2022, Journal of Manufacturing Processes
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.
Crystal structure and morphology of CeO<inf>2</inf> doped stabilized zirconia ceramics under high-frequency microwave field sintering
2022, Ceramics International
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.
2023, International Journal of Applied Ceramic Technology
2023, International Journal of Advanced Manufacturing Technology
Vacancy-ordered yttria stabilized zirconia as a low-temperature electronic conductor achieved by laser melting
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.
Volume shrinkage and conversion rate of Al2O3 ceramic stereolithography suspension polymerised by ultraviolet light
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.
Research into the mechanical properties, sintering mechanism and microstructure evolution of Al2O3-ZrO2 composites fabricated by a stereolithography-based 3D printing method
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·m1/2 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.
Gelcasting of zirconia-based all-ceramic teeth combined with stereolithography
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 m1/2. Zirconia-based all-ceramic teeth were finally fabricated based on the optimal parameters by using combined stereolithography with gelcasting.
Sintering kinetics involving densification and grain growth of 3D printed Ce–ZrO2/Al2O3
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 Al2O3, indicating only grain boundary diffusion-controlled densification mechanism could be explained for the un-doped Al2O3, 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/Al2O3.
Fabrication of dense zirconia-toughened alumina ceramics through a stereolithography-based additive manufacturing
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 5.72MPam1/2, respectively. These properties are comparable to those for ceramics obtained through conventional ceramic processing.
These authors contributed equally.
© 2021 Published by Elsevier Ltd.