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

Journal of the European Ceramic Society

Volume 41, Issue 8,

July 2021

, Pages 4666-4670

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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 [3], 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 [[4], [5], [6], [7]], 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 [10].

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 [11], 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.

Section snippets

Sample preparation

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

(Video) 3D Printed and implanted, the story of the first ceramic skull implant


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

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