Zirconia ceramics provide unique properties in the field of biomedical applications. They demonstrate a low affinity to bacterial plaque, limit inflammatory infiltration, and provide good soft-tissue integration. However, compared to metals and polymers, high-strength ceramics such as zirconia are relatively difficult to process into intricate and complex structures via subtractive machining. Additive manufacturing (AM) fabricates three-dimensional (3D) objects based on computer-aided design (CAD) models by depositing printable materials layer by layer. The AM process can produce a variety of complex and intricate geometrical shapes simultaneously in a single workflow, without wasting machining tools and production time. With all these benefits, additively manufactured ceramic products are available using various technologies such as binder jetting, powder bed fusion, material extrusion, and vat polymerization [1
]. Stereolithography (SL) or digital light processing (DLP) projector-based vat polymerization has especially high production accuracy and an excellent surface finish [4
In vat-polymerization-based AM processes using ceramics, a high solid loading is required to fabricate fully dense ceramic objects, causing the aggregation of ceramic particles in the slurry and resulting in a high viscosity [9
]. High viscosity produces challenges in maintaining accurate layer thickness and proper degassing. Although viscosity can be reduced by the use of diluents or temperature control, this is limited, as diluents can have adverse effects on the final density and produce high sintering shrinkage [12
]. In addition, the recoating of the ceramic layer material during the printing recoating requires slurry of a low viscosity [18
]. The homogenous temperature control of the whole vat area may help control the viscosity, though this is a challenging process [22
Another major issue in vat polymerization with ceramics is light scattering by ceramic particles, resulting in the unwanted overgrowth of features that not only affects cure depth but also causes unexpected curing in bilateral directions [15
]. Furthermore, the green printed state of ceramic material is fragile and lacking in the strength required to absorb the stresses involved in printing, making it susceptible to breakage during printing [28
]. The presence of detached cured layers or partially failed samples in the vat poses hindrances for further printing, making the remaining slurry potentially non-reusable. Overall, vat polymerization faces many challenges in the slurry dispensing system, which cannot accommodate a higher viscosity without wasting slurry and processing time [29
]. As an alternative method to vat polymerization, tape-casting-based additive manufacturing [22
] can be utilized to build up ceramic materials layer by layer. It uses a casting head that acts as both a slurry reservoir and a material dispensing system. It is usually comprised of a single doctor blade that shears the slurry into a thin film onto a tape passing below the blades, providing a continuous film with uniform thickness [28
]. It can also accommodate relatively higher-viscosity ceramics [28
The purpose of this study was to produce high-density zirconia prototypes using a tape-casting type DLP printer with a continuous film supply by the optimization of processing parameters, and to evaluate the recycling efficiency of the system. The zirconia ceramic slurry used in this AM was characterized and processed for the complete printing workflow. The viscosity, film thickness, slurry recycling efficiency, thermogravimetry, shrinkage rate, density, and microhardness were assessed to evaluate the performance of the printing system.
2. Materials and Methods
2.1. Printer Design and Processing
A modified version of a Onestage 6500 (Illuminaid Inc., Seoul, Korea) printer with a DLP projector was utilized for this research. The printer provides a continuous film of slurry to be printed in layers and is referred to as the “continuous film supply (CFS) system” throughout this study. The schematic design for this printing machine is shown in Figure 1
. It mainly consists of a transparent hydrophobic tape conveyor coated with silicone material that moves from one roller to the other, passing through a casting head with dual doctor blades, a build platform, and a recycling blade. The film recoating thickness is controlled by physically controlling the double doctor blades and rolling speed. The projector installed in the DLP has an LED light source with a maximum irradiance of 5.19 mW/cm2
at a peak wavelength of 405 nm.
The system worked in sequences running successively, one after another. The first sequence initiated the rollers to move the tape for a set distance at a controlled speed, such that the slurry-recoated film arrived just under the build platform. The second sequence moved the build stage down to a set distance at a controlled speed, and decelerated close to its destination on the glass plate. The third sequence controlled the curing times for each designated set of layers of the whole process. The fourth sequence moved the platform up and the glass plate down, and finally, the manufacturing process repeated from the first sequence again. After hardening the selective area of the layer on the platform, the unused slurry was left on the tape. The unused slurry was peeled off from the tape using the recycling blade, where slurry flowed downward into the recycled-slurry collector after passing through a mesh of 100 µm to remove any agglomerations produced during printing, such as detached layers or damaged samples.
2.2. Slurry Preparation
A solvent-free photocurable ceramic suspension was formulated, containing high-purity ceramic powder dispersed in an organic resin composed of two multifunctional acrylate monomers, two photoinitiators, and some processing additives. A 3 mol % yttria-stabilized zirconia powder (TZ-3YS-E, Tosoh Corp, Tokyo, Japan) with an average particle size of 90 nm was used for this slurry. The zirconia ceramic powder was coated with a dispersant by ball milling in ethanol for 5 h and then dried in a vacuum oven at 30 °C. A nonvolatile hydroxy-functional ester-based dispersant was chosen for the zirconia ceramic suspension, based on the surface chemistry of the ceramic powder and resin. For this research, a ceramic suspension with a solid loading of 45 vol.% was prepared to balance the required density and printable viscosity. A suspension with a solid loading of 44.65 vol.% was chosen for the recycling experiment to ease the filtration and recycling process. The prepared ceramic suspension (slurry) was further homogenized before printing with a planetary centrifugal mixer (Thinky, ARE-310, Tokyo, Japan). The degassing process was then conducted in a −0.1 MPa vacuum for 5 min. The homogenization and degassing processes were also carried out for the recycled slurry.
2.3. Specimen 3D Printing
The 3D printing of each specimen was performed according to the sequence mentioned above. The layer thickness was kept at 50 µm for this research. An exposure energy of 20.76 mJ/cm2
was applied to print each layer except for the first five layers, to which 62.28 mJ/cm2
was applied [30
]. Other parameters used are mentioned in Table 1
. The wait time mentioned in the table refers to the time the build platform spent on the slurry film just before and after curing, without changing its position. A standard tessellation language (STL) file format was used as a CAD model (Figure 2
) with specimen dimensions of 7 × 7 × 4 mm3
scaled linearly in all dimensions, considering compensation for the overgrowth percentage and sintering shrinkage.
All the printed specimens were cleaned using isopropanol through a handheld atomizing spray. Debinding was performed using an atmosphere type 1 furnace (PT-17EF022, PyroTech, Gyeonggi-do, Suwon si, Korea) in an argon atmosphere with a heating rate of 10 °C/h until 600 °C, with a dwell time of 2 h. A stairway approach to 600 °C was used with several holding temperatures, following derivative thermogravimetry analysis data. The sintering process was conducted in air at 1550 °C for 2 h with a heating rate of 2.5 °C/h, shown in Figure 3
, using a Super Kanthal furnace (AJ-SKB6, Ajeon, Gyeonggi-do, Suwon si, Korea). The weight percent values of the solid loading of the specimens printed using the original ceramic slurry and the recycled slurry were measured from the weight difference between the specimens after debinding and sintering.
2.5. Viscosity Analysis
The viscosity of 2 mL of the ceramic slurry after degassing was measured at 30, 40, 50, and 60 °C using a rotational rheometer (TA Instruments, ARES-G2, New Castle, DE, USA).
2.6. Film Thickness Analysis of Slurry
The film thickness was measured using a micrometer (293-240-30, Mitutoyo, Kawasaki, Japan) with an accuracy of ±1 µm. The whole film above the projector was hardened and an average value was taken after taking measurements across the width. Measurement was repeated 5 times to calculate standard deviation and maintain a consistent slurry amount behind the blade.
2.7. Thermogravimetric Analysis
Simultaneous thermogravimetric–derivative thermogravimetry analysis (TG-DTG) was conducted using an analyzer device (SDT-Q600, TA Instrument, New Castle, DE, USA) on a printed cubic specimen with a size of 3 × 3 × 3 mm3, in argon atmosphere, and with a 1 °C/min temperature gradient from room temperature to 600 °C.
2.8. Density Analysis
The density of the printed specimen was characterized via the Archimedean principle using 10 replicates and an analytical balance (Adam Equipment, SAB 125i, Oxford, CT, USA) with readability of 10 µg. For recycled slurry, five replicates were used due to the limited amount of recycled slurry.
2.9. Microhardness Testing
The microhardness test value was measured by determining the Vickers hardness (Shimadzu, HMV-2, Kyoto, Japan) on mirror-polished cross-sections of the specimens, according to ASTM C1327-15 [32
]. A loading force of 9.81 N was applied for 10 s, and the diagonals were measured through a ×40 objective lens. A total of 10 specimens were tested with at least 5 indents each, and the average value for those indents was considered as the final hardness value of each specimen.
2.10. Microstructure Analysis
The microstructure analysis was performed through a field-emission scanning electron microscope (AURIGA, Carl Zeiss, Oberkochen, Germany) on platinum-coated specimens with a coating thickness of 5 nm. The analysis was carried out on the fine-polished cross-sections of both as-printed specimens and sintered specimens after thermal etching at 1450 °C for 40 min. The grain size was measured for 5 sintered samples, printed in separate batches, by the linear intercept method with a conversion factor of 1.56 [33
Based on this study, the tape casting DLP printing with CFS is a capable alternative to vat polymerization to print dense zirconia prototypes using high-viscosity slurry. An appropriate dispersant and working temperature were necessary to achieve printable viscosity. Consistency of film thickness on the tape, throughout the whole printing process, was imperative for the DLP with CFS. Dual doctor blades were found to produce a film thickness well above the overall set layer thickness. Using the DLP with CFS, a 45 vol.% solid loaded 3Y-TZP slurry was used to fabricate ceramic products of a homogenous microstructure, with a relative density of 99.02% ± 0.08% and a mean grain size of 644 ± 20 nm after postprocessing. The microhardness value of the ceramic product was 12.59 ± 0.47 GPa. Furthermore, the DLP with CFS can effectively recycle and reuse slurry, providing a consistent density for at least two instances of recycling, making it a competitive alternative to conventional processing techniques for the fabrication of dense ceramic products.