Mechanical Properties of 3Y-TZP Woodpile Scaffold Made by Extrusion 3D Printing

Abstract

Zirconia woodpile scaffolds (3Y-TZP/PL, n = 20), designed with null inter-filament spacing, were manufactured using the Direct Ink Writing (DIW), an extrusion 3D printing technique. A ceramic ink containing 40%v/v 3Y-TZP powder, 59%v/v PEG (Polyethylene glycol)/Laponite ink, and 1%v/v DBP (Dibutylphthalate) was used. For 3D printing, we used Ø 0.63 mm nozzles, a printing speed of 10 mm/s, a cross-layer deposition strategy, and no air gaps between filaments. The scaffolds were sintered at 1550 °C for 2 h. The mechanical characterization involved measurements of X-ray diffraction, scanning electron microscopy, Vickers microhardness, Vickers nanohardness, and modulus of elasticity and compressive strength. The sintered samples showed predominantly the ZrO₂-tetragonal phase and a microstructure characterized by a bimodal distribution of grain sizes. The samples had a relative density of 90.1 ±1.5%, a Vickers microhardness of 1172 ±45 HV, a Vickers nanohardness of 1608 ±78 HV, a modulus of elasticity of 203 ±16 GPa, and a compressive strength of 192 ±54 MPa. The results showed that DIW processing followed by proper sintering is a promising method for making zirconia scaffolds for biomedical applications.

Keywords:
Additive manufacturing; DIW; 3Y-TZP; bioceramics; mechanical properties

1. Introduction

Yttria-stabilized zirconia (Y-TZP) is a ceramic used as a biomaterial, especially in the manufacturing of dental prostheses. Y-TZP has a hardness of about 1250 HV, an elastic modulus of about 200 GPa, a flexural strength of about 800 MPa, and a fracture toughness of about 7 MPa·m^1/2. Y-TZP has toughening mechanisms due to the stress-induced phase transformation from tetragonal to monoclinic, which reduces crack propagation¹. Among the different zirconia compositions, 3Y-TZP (ZrO₂ - 3 mol.% Y₂O₃) has the highest mechanical strength due to a higher percentage of the tetragonal phase. The 5Y-PSZ zirconia (ZrO₂ - 5 mol.% Y₂O₃) offers better optical properties due to a more significant proportion of the cubic phase but lower mechanical strength. These characteristics make Y-TZP ceramics a viable option for dental prostheses and implant components²,³.

Additive Manufacturing (AM) is a technique based on material deposition in successive layers⁴, which enables the rapid production of parts. AM processes begin with the creation of a 3D model using CAD software. The 3D model is converted into a .stl file and sliced into individual layers by software. The data is then transferred to the AM equipment, which builds the 3D objects by adding successive layers⁵,⁶. Although AM initially involved only polymeric and metallic materials, advances in methodologies and equipment have made it possible to manufacture ceramic materials.

Research has shown that AM ceramics, when sufficiently densified within their limits, have properties that can meet the mechanical requirements for various applications, such as high compressive strength, making them suitable for special applications such as single-unit dentures, where the masticatory loading stresses are mostly compressive⁷-⁹. The possibility of developing products with antibacterial properties in dentistry has also been explored¹⁰-¹³.

Direct Ink Writing (DIW) is a material extrusion process useful for advanced applications involving complex geometries impractical with other manufacturing processes. High-quality DIW ink formulations require refined powders, suspensions with high solid content, and polymeric additives. Moreover, the processing and sintering parameters are crucial for improving the properties of the final product. It is noteworthy that the deposition of layers in an orthogonal orientation (woodpile) constitutes one of the most stable and prevalent forms of printing in this technique, commonly referred to as a scaffold structure¹⁴,¹⁵.

Studies have focused on the impact of pore size and porosity on the mechanical properties of scaffolds. The present study aims to manufacture and characterize ceramic samples based on 3Y-TZP.The specimens were produced using the DIW technique. The samples were subsequently sintered in an effort to achieve a solid structure with no air gaps between filaments. The effects of null inter-filament spacing on the mechanical properties of woodpile scaffolds were investigated. The evaluation of the samples included analysis of printing parameters and sintering conditions. The objective of this analysis was to obtain three-dimensional ceramic structures that exhibit mechanical properties suitable for applications in dental prostheses.

2. Experimental Procedure

2.1. Processing

This study used the ceramic powder of 3Y-TZP zirconia (G2DY-020OO, SINOCERA, China). Table 1 summarizes the main characteristics of this powder, including chemical composition, density, particle size, and specific surface area. To improve the extrudability and structural stability of the ink, 7.5% v/v of Laponite XLG (BYK/Colormix Ltda., Brazil) a nanosilicate powder, rheological modifier, and gel former, was added to an aqueous solution of 44.4% v/v of Polyethylene Glycol (PEG) 400 (380-420 g.mol^-1, Dinamica Ltda., Brazil) in deionized water. Dibutyl phthalate (DBP with 278.35 g.mol^-1, Synth, Brazil) was also used at 1% v/v as a rheological modifier in the ink. The remaining volume (40% v/v) was filled by incorporating ZrO₂ ceramic powder.

Table 1
Summary of the characteristics of the raw powders used in this study, as provided by the manufacturers.

Powder characteristics (wt.%)	Laponite XLG (BYK)	3Y-TZP (SINOCERA, G2DY-020OO)
Y 2 O 3	-	3.40 - 3.70
SiO2	59.5	-
Na2O	2.8	-
Li2O	0.8	-
MgO	27.5	-
ZrO2	-	balance
Specific surface area (m2/g)	370	9.00 -11.5
Density (g/cm3)	2.53	6.05
Melting temperature (°C)	900	2715

Figure 1 shows the development of the PEG-Laponite ink and a flowchart of the preparation and 3D printing of the samples, including sintering and characterization of the final product. The gel base, containing the same additives (PEG-Laponite-DBP) previously mentioned, had its rheology investigated in earlier studies¹⁶,¹⁷. The present work replaced only the zirconia source used in the ink preparation.

Figure 1
Flowchart for the development of 3Y-TZP/PL ceramic ink and subsequent steps.

2.2. 3D printing by extrusion

The fabrication of ceramic prototypes was carried out using a DIW-type 3D printer equipped with a two syringes extrusion head (3D iCan X dual, ICAN, Brazil). The 10 mL disposable syringes with Luer lock connections (Descarpack, Brazil) were fitted with conical nozzles of 630 μm diameter (20G, Nordson EFD, USA). The printing process occurred at room temperature (25 ±2 °C), with a relative humidity of 60-65%. The prismatic structures (10.7 x 10.7 x 17.6 mm) were built layer by layer, totaling 32 layers with 18 filaments per layer and no air gaps between the filaments. The deposition strategy involved perpendicular cross-layer stacking. More details on the geometry and printing strategy can be found in Figure 2.

Figure 2
Realistic layout of the layers and printed parts with 3Y-TZP/PL woodpile scaffolds.

After printing, the samples were heated in an oven at 40 °C for 48 hours. Once dried, the green-printed structures had enough strength for manual handling and were transferred from the printing platform to the furnace. They underwent pre-sintering at 1100 °C for 1 h, with heating and cooling rates of 1 °C/min and 5 °C/min, respectively. This step aimed to remove the binders and organic additives, minimizing sample deformation and cracking risk. Subsequently, the samples were sintered according to the following thermal cycle:

• heating from room temperature to 600 °C at a rate of 10 °C/min;

• additional heating to 1550 °C at a rate of 5 °C/min, maintaining this temperature for 2 h;

• cooling from 1550 °C to room temperature at a rate of 5 °C/min.

2.3. Characterizations

Using the Archimedes method in distilled water at room temperature, the apparent density was measured following ASTM B962 (2017)¹⁸. The correlation between the measured apparent density and the real density of the samples was used to estimate the relative density. The real density of the sintered 3Y-TZP-laponite ceramics was determined by helium-pycnometry using an Ultrapyc 1200e, V5.04 pycnometer. Sintered samples were fragmented, sieved (<25μm), compacted (Ø20mm x 3mm height), and then measured in duplicate for ten cycles per specimen.

The sintered samples and as-received powders were analyzed using X-ray diffraction (XRD). The experiment used a Panalytical (Empyrean model, Malvern PANalytical, UK) with Cu-Kα radiation. Data was analyzed using X’pert-Highscore (Phillips) software and the ICDD database. The experiment was conducted over a 2θ range of 10° to 90°, with a step size of 0.01°/s and a scan rate of 100 s.

Scanning Electron Microscopy (SEM) (Mira 3 XMU model, Tescan, Czech Republic) was used to analyze the surface microstructure. The samples were prepared by grinding and polishing with a 1 μm diamond solution. The samples were then heat-treated at 1400 °C for 15 min and sputter-coated with Au. Surface images were acquired at 20 kV, and the microstructural features were treated using Image-J software.

The Vickers microhardness (HV) was measured on ceramic samples using a Durometer (HMV-G model, Shimadzu, Japan) with a Vickers micro-indenter. The procedure involved applying a 9.8 N load for 30 s and conducting 20 indentations on the sample (ASTM C1327-15)¹⁹. The nanohardness was investigated through nanoindentation of the polished surface of the samples. An ultra microdurometer (DUH – 211S model, Shimadzu, Japan) was used, with loads of 1000 mN and a maximum penetration depth of 10 μm. Young’s modulus was calculated based on the Oliver and Pharr model²⁰,²¹. The values represent the average of 10 nanoindentations for each selected load, with the error expressed as a standard deviation.

Compression testing was conducted on 20 samples using a universal testing machine (DL10000, EMIC, Brazil) with a 20 kN load cell and a 1 mm/min rate. The top and bottom surfaces were ground and polished with diamond paste to 6 μm while maintaining the parallelism of the surfaces. The dimensions after sintering were 8.1 x 8.1 x 13.4 mm, with a slight height reduction of about 0.3 mm. The area for compressive stress calculation was 65.6 mm². The results were analyzed using Weibull statistics, a tool to model the probability distribution of failure and estimate stress values with high reliability for ceramic scaffolds.

3. Results and Discussion

3.1. Characterization of sintered samples

After sintering, the average relative density of the 3D-printed samples was 90.1 ± 1.5%. These results reflect the porosity profile of scaffolds built with cross-layer filament deposition. Figure 3 shows details of the filament layers in the green-printed samples. In these simulations, it is possible to observe that extensive macroscopic void formation occurs even with no gap between filaments. The voids were not eliminated during sintering, resulting in a residual porosity of 7.9%. The voids have a homogeneous distribution throughout the layers and among filaments.

Figure 3
(Yellow) layout of the printed ceramic ink layers; (Blue) simulation of the distribution of interconnected voids among the layers.

Figure 4 shows the XRD results. The 3Y-TZP/PL samples are characterized by three main polymorph phases: 7% m-ZrO₂, 66% t- ZrO₂, and 27% c+t'- ZrO₂. Notably, the cubic (c) and pseudocubic (t') phases exhibit similar behavior and peak intensities.

Figure 4
XRD pattern of the printed sample sintered at 1550°C for 2 h.

Figure 5 shows the SEM images of the samples. The samples showed a bimodal grain size distribution. The scaffold had a grain size higher than expected for zirconia grains. The grains are surrounded by a matrix of ZrO₂ grains smaller than 1 µm. The 3Y-TZP/PL larger grains had more than 5 µm. During the sintering process, the constituents of the Laponite nanosilicate fuse during heating, resulting in an intergranular glassy phase composed of Si, Mg, Na, Li, and O. This phase did not change the zirconia stability during the pre-sintering process at 1100 °C because these elements remain in an amorphous state. With the heating at temperatures above 1450 °C, the viscosity of the glass resulting from the Laponite fusion decreases, intensifying chemical interactions between the intergranular vitreous phase components and the zirconia matrix. When the samples were heated to 1550 °C, some components of this residual liquid phase contributed to the destabilization or migration of Y₂O₃ present in the ZrO₂ grains. This can be attributed to increased diffusivity in these grains, which facilitates migration to the preferential grains, transforming small tetragonal grains into larger cubic phase grains. This phenomenon has also been discussed in the infiltration of 3Y-TZP ceramics with silicate glasses²², supporting this hypothesis.

Figure 5
SEM images of the sample sintered at 1550 °C for 2 h.

3.2. Mechanical properties of sintered samples

Figure 6 shows the microhardness, nanohardness, elastic modulus, and compressive strength results.

Figure 6
Mechanical Properties of 3Y-TZP Scaffolds Sintered at 1550 °C for 2 h.

The 3Y-TZP/PL scaffold had a Vickers microhardness of 1172 ±45 HV and a Vickers nanohardness of 1608 ±78 HV. Both values are slightly lower than those reported in the literature²³-²⁵.

As expected, printed ceramic scaffolds have higher nanometric hardness than micrometric hardness because pores have a reduced influence on the nanoscale. Therefore, it is recommended to perform measurements in regions with low porosity, away from pores, edges, and other indentations, to avoid measurement errors. This procedure is more difficult to achieve in microhardness tests, where the indentation size typically encompasses several micropores, reducing the recorded hardness.

Another property calculated through Vickers nanoindentation was the elastic modulus, which averaged 203.52 ±16.34 GPa. This value is close to that found in the literature for dense ZrO₂²⁶. Once again, due to the number and size of the pores in the prototype, no significant differences in elastic modulus were observed between the prototype with 92.1 ±3.4% relative density and pressed samples with higher relative densities.

The average compressive strength of the 3Y-TZP scaffolds (n = 20) was 191.5 ±53.7 MPa, with a Weibull modulus of 3.6 and a minimum compressive strength of about 120 MPa. The Weibull statistics and failure probability versus compressive stress graphs are presented in Figure 7. Due to their geometry, the scaffolds have a more fragile structure. The arrangement of the filaments and their circular shape lead to stress concentration points and interconnected voids throughout the volume, resulting in lower strength compared to dense parts of similar dimensions. The Weibull statistics graph (Figure 7a) indicates a distribution of results into two groups with distinct mechanical behavior and well-defined stress values, as highlighted in the circles in Figure 7b. This suggests heterogeneities in the distribution of residual porosity and/or defects in different internal regions of the scaffolds. The defects induced fracture strength variation. It is recommended that future studies investigate the causes of this behavior. Nonetheless, one group of samples with superior mechanical performance showed significantly higher performance, with values ranging between 200 MPa and 280 MPa, while the other group exhibited compressive strength between 120 MPa and 160 MPa.

Figure 7
Weibull Modulus and Failure Probability of 3Y-TZP scaffold samples.

4. Conclusions

Based on the experimental data analysis, it concluded that

1. The ceramic ink prepared with a mixture of deionized water, PEG, DBP, and Laponite as additives has good extrudability and potential for AM of 3Y-TZP zirconia.

2. Using selected DIW parameters and a layer deposition strategy, 3Y-TZP woodpile scaffolds sintered at 1550 °C for 2 h achieved an average densification of more than 92%.

3. The interaction of Laponite components, mainly Mg, with yttria-stabilized tetragonal zirconia grains, leads to an increase in the growth of the ZrO₂-cubic phase. Combined with some residual porosity at the filament junctions, this contributes to the observed degradation of mechanical properties. The compressive strength displayed an average value of 191.54 ±53.76 MPa with a Weibull modulus of 3.6, which, although lower than for fully dense samples, remains within an adequate range for various applications and demonstrates high reliability below 120 MPa.

Within this study's experimental limits, the results indicate the possibility of producing high-solid-content ceramic inks for 3D printing using the DIW method, as well as the ability to fabricate 3D ceramic scaffolds with interesting mechanical properties and potential biomedical applications using 3Y-TZP-based ceramics. However, further improvements in compound formulations and 3D printing strategies are needed to optimize densification and improve mechanical properties such as compressive strength, thereby increasing the reliability of the final product.

5. Acknowledgments

The authors thank Dr. Ivone Regina de Oliveira (UNIVAP, IP&D, S.J. Campos-SP, Brazil), for the He pycnometry analyses. Furthermore, the authors thank CNPq (Process No. 305532/2021-9 and 407680/2021-7) and FAPERJ (Process No. E-26/201.002/2022) for financial support.

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Publication Dates

• Publication in this collection
  09 May 2025
• Date of issue
  2025

History

• Received
  28 Oct 2024
• Reviewed
  05 Mar 2025
• Accepted
  16 Mar 2025