# Elaboration of Printable Nanopowder Zirconia Suspensions for Vat Photopolymerization

Aymeric Jugan, Sylvain Marinel, Loïc Le Pluart, Théotim Marie, Romuald

Herbinet, Charles Manière

► To cite this version:

Aymeric Jugan, Sylvain Marinel, Loïc Le Pluart, Théotim Marie, Romuald Herbinet, et al., Elaboration of Printable Nanopowder Zirconia Suspensions for Vat Photopolymerization. Journal of Materials Engineering and Performance, In press, 10.1007/s11665-025-11338-8. http://05131528

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# Elaboration of Printable Nanopowder Zirconia Suspensions for Vat Photopolymerization

Aymeric Jugan<sup>a*</sup>, Sylvain Marinel<sup>a</sup>, Loïc Le Pluart<sup>b</sup>, Théotim Marie<sup>a</sup>, Romuald Herbinet<sup>b</sup>, Charles Manière<sup>a*</sup>

* Corresponding author:: aymeric.jugan@ensicaen.fr and charles.maniere@ensicaen.fr

(a) Université Caen Normandie, ENSICAEN, CNRS, Normandie Univ, CRISMAT UMR6508, F-14000 CAEN, France

(b) Université Caen Normandie, ENSICAEN, CNRS, Normandie Univ, LCMT UMR6508, F-14000 CAEN, FRANCE

## Keywords

Vat photopolymerization; Ceramics; Submicron powder; Suspension; sintering

## Abstract

This study focuses on preparing and optimizing a submicron zirconia (ZrO₂) suspension for vat photopolymerization (SLA) 3D printing. The suspension includes TZ-3YS-E zirconia powder, acrylate-based monomers (HDDA and TMPTA), a dispersant (Disperbyk-103), and a photoinitiator (TPO). It was prepared using a ball-milling process to ensure homogeneity and optimal dispersion. Rheological properties were assessed to determine the ideal dispersant concentration and milling time, aiming for a low-viscosity and stable suspension. Sedimentation tests were performed to evaluate suspension stability, identifying optimal parameters at 5 wt% dispersant concentration and 120 minutes of ball milling. Additionally, an original curing depth experimental tool was built to optimize printing parameters such as laser speed and power, revealing a maximum over-polymerization of 30% in the lower layer. Finally, the ZrO₂ suspensions with optimal parameters were used to print complex-shaped objects.
# 1. Introduction

3D printing technologies have emerged as an innovative processing method, unlocking new possibilities for creating complex structures [1–3]. Among the various 3D printing techniques adapted to ceramic materials, several technologies are available, including fused deposition modeling (FDM), robocasting, binder jetting, and vat photopolymerization (SLA). Each method has its own advantages and disadvantages [4–8]. Extrusion processes are rapid and well-established but are limited by the resolution of their printing nozzles, making them suitable for producing large parts with resolutions ranging from 100 µm to several millimeters. Binder jetting offers higher resolution, but a key challenge lies in applying layers of ceramic powder, which is less fluid than metal powder. As a result, coarser powders are often used for specific molding applications [9]. In this context, SLA stands out for its ability to produce complex structures with exceptional resolution down to 10 µm [5,10]. This high resolution originates from the precision of the UV laser and the ability to add thin layers of resin with fine ceramic particles. Such resolution can rival traditional processes like green machining and injection molding. However, a significant limitation is the lengthy debinding process required to remove 50-60 wt% of organics, which takes about a week and restricts wall thickness to 3-5 mm. Despite this, SLA is particularly suitable for advanced fields such as biomedical, aerospace, and electronics, where small, thin parts with complex shapes and high resolution are required [6,11].

One of the major challenges for successful vat photopolymerization printing is the formulation of the suspension and the identification of optimal printing conditions. Several physical parameters must be controlled to ensure proper ceramic powder dispersion, prevent sedimentation over extended periods (days or weeks), and achieve effective photopolymerization of the layers. These parameters include ensuring shear-thinning
rheological behavior for easy dispensing and layer scraping, adjusting the dispersant to avoid powder agglomeration and sedimentation, and fine-tuning the monomer and photoinitiator to ensure sufficient polymerization with a curing depth thick enough to facilitate layer cohesion [10,12]. To optimize these parameters, it is essential to adjust various processing factors, such as ball-milling time, dispersant quantity, layer thickness, and the laser's power and speed, which are among the most critical.

Another drawback of SLA is that only a few materials are commercially available, and most of them use coarse ceramic powder mixes, which are cheaper and easier to print. However, these powders often require higher sintering temperatures, leading to the formation of larger grains [13] with lower mechanical strength. The use of nano/submicron powders in SLA is challenging due to high UV light dispersion and reduced curing depth, a problem that is particularly pronounced with zirconia, which has a high refractive index [10]. Additionally, small-sized powders are prone to agglomeration, necessitating careful attention to milling and dispersion processes. To achieve fine-grained ceramics, it is crucial to produce our own zirconia-based paste. Therefore, the goal of this paper is to develop a zirconia paste suitable for SLA 3D printing, using 90 nm zirconia powder to enable the precise printing of zirconia parts with superior submicron microstructures and enhanced mechanical properties.

The selection of suspension components involves choosing various monomers with active functionalities (typically acrylates), a photoinitiator that interacts with the UV laser to initiate radical-free polymerization, and a dispersant to ensure proper dispersion of the ceramic particles [10–12]. Regarding the monomer selection strategy, multifunctional monomers are used to create a three-dimensional network. Combining different monomers with varying numbers of functional groups (such as di- and tri-acrylates) has been shown to help form a stable resin with minimal polymerization shrinkage [10,13]. The diacrylate HDDA (1,6-
Hexanediol diacrylate) and the triacrylate crosslinker TMPTA (trimethylolpropane triacrylate) are commonly used in ceramic resin compositions. They are employed in varying ratios to achieve low viscosity and ensure a stable resin [13,14]. The photoinitiator is selected based on its compatibility with the UV laser's wavelengths [15]. Finally, the dispersant is used to prevent powder agglomeration and sedimentation. In the SLA process, the ceramic resin in the reservoir tank must remain stable enough to prevent sedimentation during printing (4-24 hours) and ideally throughout the entire printing campaign (approximately a week). For aqueous suspensions, dispersants with carboxylic or amino ionizable groups attach to the charged oxide powder surface, providing electrostatic stabilization to maintain dispersion [16]. In a non-aqueous acrylate monomer suspension, the solvent mixture is less polar, and the dispersion mechanism is primarily steric, utilizing long-chain dispersants like Disperbyk-103 [10] [10.1111/ijac.13321] [10.1021/ma060577m]

This article investigates a monodispersed, submicron zirconia powder suspension designed for printing zirconia green specimens with submicron microstructures that exhibit improved properties compared to most multimodal, suspensions in the literature [10.1016/j.msea.2019.138768] [10.1016/j.matdes.2019.107960]. The study encompasses the entire process, starting from raw powder and monomers to the fabrication of the ceramic-loaded suspension, establishment of suitable printing conditions. One challenge is adapting the suspension for use with a professional 3DCERAM Sinto® device. The study will examine the effects of dispersant quantity and ball-milling time on the rheological behavior of the suspension, as well as the impact of laser power and speed on the stability and curing depth of the monolayers. Finally, intricate honeycomb shapes will be printed to validate the developed suspension and printing conditions.
## 2. Experiment and method

### 2.1. *Printing device*

The aim of this study is to formulate a ZrO₂ suspension suitable for printing on the professional 3DCERAM Sinto® C100 EASY LAB printer. This printer supports laboratory-scale suspension quantities (~500 ml) and operates on a build platform with dimensions of 100x100 mm². It uses a UV laser with a wavelength of 405 nm. The printer's "top-down" printing strategy allows for layer thicknesses down to 10 µm, where the build platform moves down during printing and the layer and lasing are applied from the top. This approach eliminates the need for a transparent build platform and permits operation at high laser power. The conditions on this device are similar to those of the larger 3DCERAM Sinto® C3601 ULTIMATE, which features a 600x600 mm² platform, facilitating the transition from laboratory-scale to fabrication-scale applications.

The ceramic powder used is Tosoh® TZ-3YS-E zirconia with an average particle size of 90 nm. This powder has a low sintering temperature and exhibits high sintered mechanical properties, including a hardness of 12.5 GPa, when processed by cold uniaxial pressing [17]. The objective is to achieve similar mechanical properties in the 3D-printed and sintered ceramics as those observed in the cold uniaxial pressed samples.

From the articles [10,11,14,18], two types of acrylate monomers were utilized: Trimethylolpropane triacrylate (TMPTA, TCI_T0949, 1.1 g/cm³) and 1,6-Hexanediol diacrylate (HDDA, TCI_B2936, 1.01 g/cm³). A previous study [19] identified that a 4:1 ratio of HDDA to TMPTA results in a dense 3D network with optimal rheology for zirconia suspensions. The same ratio of the two monomers is then selected here. The photoinitiator employed was Diphenyl (2,4,6-trimethylbenzoyl)-phosphine oxide (TPO, TCI_D3358, 1.17 g/cm³). This photoinitiator has an effective absorption peak ranging from 360 to 410 nm [20],
which aligns well with the 405nm laser of the 3DCERAMSinto Ceramaker100. A dispersant, Disperbyk-103 (Byk®), has been chosen as it demonstrates an effective dispersion of zirconia powder [21]. The proportion of this dispersant strongly depends on the effective specific surface of the powder after milling. Consequently, a parametric rheological analysis is needed to adjust its quantity. No diluent was utilized in this study because the HDDA/TMPTA mix with the dispersant and powder displays the expected viscosity and shear thinning rheological behavior. The MDL III model emits a 405 nm laser beam with an electrical power of 300 mW, operating in continuous wave mode with diode pumping. Its beam diameter of 0.7 ± 0.1 mm ensures high precision for various optical applications.

The printability of the suspension rheology has been compared with existing commercial alumina and silicon nitride suspensions from 3DCERAM Sinto®. These commercial suspensions represent two extremes: alumina, which is easy to print, and silicon nitride, which has a low curing depth and a lengthy printing process. Our submicron suspension is expected to fall between these two extremes.

## 2.2. Suspension preparation

The ZrO₂ ceramic slurries were prepared using a planetary ball mill (Pulverisette 6, Fritsch, Germany) with zirconia bowls and balls. For testing various conditions, a multiple-bowl setup for four compositions was used in the exploratory phase (see Fig 1). Subsequently, a larger 500 ml zirconia bowl was employed to produce printable suspensions in greater volumes. Monomer weighing was conducted under controlled conditions to protect the suspension from UV light. The suspension preparation involved first mixing the organic components and then performing planetary ball milling to achieve the final ceramic suspension. The preparation process followed the steps detailed below.
<figure><img src="image_2.png" /><figcaption>Fig 1 Preparation, from raw powders to the suspension.</figcaption></figure>
<figure><img src="image_3.png" /></figure>
## 2.3. Characterization

All experiments were conducted at room temperature (22°C) and were shielded from UV light to prevent premature polymerization. The rheological properties of the slurry were evaluated using a rotational plane-plane rheometer (TA Instruments Rheometer AR1000), with measurements performed in steady mode over a shear rate range from 0 to 700 s⁻¹. The gap between the plates is 1mm. To compare the rheological behavior of our in-house slurry with that of printable commercial options from 3DCERAM, we tested two commercial resins: alumina resin (3DCERAM Sinto, AL-ref), known for its easy printability (low viscosity and clear color), and silicon nitride resin (3DCERAM Sinto, SN-ref), which is more challenging to print (high viscosity and grey color).

Good stability is essential to prevent the separation of monomers and particles during printing, which can lead to a non-homogeneous suspension and result in defects such as density variations or particle inclusions after debinding and sintering. The ceramic resin must remain stable throughout the entire printing campaign, which lasts about a week. To evaluate this stability, sedimentation tests were conducted. Each suspension was poured into 5 ml test tubes with a diameter of 15 mm and left to settle for extended periods, up to 50 days or more. Over time, a clear sedimentation front developed, separating from the suspension. The height of this sedimentation front (H1) was measured and compared to the initial height (H0) of the suspension. A higher H1/H0 ratio indicated lower stability. The goal is to determine the optimal amount of dispersant to prevent sedimentation over the course of a week. To minimize sedimentation, the suspensions were continuously agitated using a jar roller (Bottle/Tube Roller, Thermo Scientific, China).

Optimizing the printing conditions involves fine-tuning the laser power/speed and scraping/dispensing settings, which are highly sensitive to the suspension's rheology. If the
rheology of the suspension is comparable to that of commercial resins, the primary focus shifts
to aligning the printing layer height with the curing depth, which is influenced by the laser
power and speed.

To characterize the curing depth, an experiment was conducted in which nine small
containers were loaded by the suspension and placed on the printing platform to test monolayer
printing under nine different conditions (see Fig 5). Nine different laser speeds were evaluated,
and each printed monolayer was measured with a digital micrometer (Micromètre d'extérieur,
TESA SA, Switzerland) to determine the curing depth. This test also helps identifying issues
such as light diffusion or warping at slower laser speeds and provides a qualitative assessment
of the printed layer. The objective is to achieve a monolayer curing depth that is at least 10-
35% greater than the layer thickness, without visible defects. Additionally, a balance between
laser speed and layer thickness must be considered to minimize printing time.

**3. Results and discussion**

3.1. *Effects of dispersant concentrations*

Achieving high-resolution printed parts requires using a homogeneous and well-dispersed
suspension. Understanding various destabilizing phenomena, such as particle agglomeration, is
crucial. Fine powders, such as nanoscale ZrO₂, with their high specific surface area, are prone
to aggregation. This aggregation can lead to defects during printing, including a rough surface
finish, structural weaknesses, and visual imperfections [9].

The goal is to prevent particle agglomeration by adding a dispersant in appropriate
quantities. Moreover, the optimal dispersant content should help reducing viscosity while
increasing the maximum solid fraction of the suspension, which can exceed 50 vol% in some
cases [20]. In our study, we have limited the solid fraction to 40 vol% because our printed was
designed from cream like suspension rather than paste.
The dispersion mechanism can be steric, electrostatic, or electro-steric. In aqueous media, zirconia particles can be dispersed using any of these three mechanisms. In acrylate monomers, the dispersion mechanism remains under investigation, but steric dispersion is expected to play a significant role. However, an optimal amount of dispersant must at least cover the entire surface of the particles. As observed in aqueous solutions, an excess of dispersant can lead to interactions between dispersant chains, which increases the viscosity of the suspension [10,16]. Therefore, the objective is to optimize the quantity of Disperbyk-103 [21] in a 40 vol% zirconia-filled solution with an HDDA/(HDDA+TMPTA) mass ratio of 80:20 and a TPO concentration of 2 wt% (based on organic weight).

The ZrO2 slurry contained 40 vol% TZ-3YS-E solid, 80 (wt%) HDDA/(HDDA+TMPTA), 20 (wt%) TMPTA/ (HDDA+TMPTA), and 2 wt% total photoinitiator (TPO). The dispersant contents (wt % powder) were 3, 4, 5, 6, 7, and 8, and the milling time was 120 min.

<figure><img src="image_4.png" /><figcaption>Fig 2 (a) Viscosity vs. shear rate for six dispersant concentrations, compared to the commercial printable Si₃N₄ suspension. (b) Viscosity vs. dispersant concentration at a fixed shear rate of 65 s⁻¹.</figcaption></figure>
The primary objective of the conducted tests was to determine the optimal dispersant concentration. Achieving the lowest possible viscosity is crucial for several reasons. Lower viscosity facilitates the uniform distribution of ceramic particles, resulting in parts with improved dimensional accuracy and smoother surfaces. It also reduces the probability of air bubbles and imperfections during printing, enhancing the quality of the finished part. To achieve this, we performed a series of rheological tests by varying the dispersant amount from 3 to 8 wt%.

The results of these tests are presented in the **Erreur ! Source du renvoi introuvable**. A, where the viscosity is plotted against the shear rate. **Erreur ! Source du renvoi introuvable**. shows the plot of viscosity versus the dispersant concentration for a shear rate fixed at 65 s⁻¹. The main objective of the dispersion study is to minimize sedimentation. Consequently, a low shear rate of 65 s⁻¹ is selected, as it allows for a clear distinction of the effect of dispersant content. At this shear rate, all the suspensions viscosities are similar to the printable Si3N4 suspension, also demonstrating their printability.

From Fig 2 B, it has been identified that a minimal viscosity is obtained with 4 to 5 wt% dispersant in the suspension. This probably corresponds to a state where the molecules effectively repel the particles, making the suspension as fluid as possible. At a lower concentration, i.e. 3 wt% the non-total coverage provided by the Disperbyk-103 molecules on the particles results in less interparticle repulsion. Conversely, at higher concentrations (6, 7, 8 wt%), the adsorption of Disperbyk-103 molecular chains on particle surfaces reaches saturation, leading to greater aggregation and therefore, an increase of the viscosity. This content can be further optimized, by combining ball milling and sedimentation tests on compositions close to 5 wt% of Disperbyk-103.

### 3.2. *Effect of ball-milling time*
The duration of the ball-milling process significantly impacts the particle size distribution, homogeneity, stability, and rheological properties of the ceramic suspension. If the ball-milling duration is too short, dispersion will be incomplete, leading to high viscosity. Conversely, if the ball-milling process is too long, it can reduce particle size excessively, which may increase agglomeration and viscosity [14]. To assess the compatibility of the zirconia ceramic suspension with our printer, rheological tests were conducted. Several ball-milling durations (2 minutes in a mortar, and 10, 30, 60, and 120 minutes) were applied to suspensions with dispersant contents ranging from 3 to 6 wt%. Table 1 presents the ball-milling test plan, outlining the various experimental conditions tested.

<table><caption>Table 1 Ball-milling test plan for each tested dispersant concentrations</caption><thead><tr><th>TZ-3YS-E<br>Solid loading<br>(vol%)</th><th>HDDA/<br>(HDDA+TMPTA)<br>Wt%</th><th>TMPTA/<br>(HDDA+TMPTA)<br>Wt%</th><th>Dispersant<br>content<br>(wt%powder)</th><th>TPO content<br>(wt%total)</th><th>Ball milling<br>(mins)</th></tr></thead><tbody><tr><td rowspan="4">40</td><td rowspan="4">80</td><td rowspan="4">20</td><td>3</td><td rowspan="4">2</td><td>0</td></tr><tr><td>to</td><td>30</td></tr><tr><td>8</td><td>60</td></tr><tr><td></td><td>120</td></tr></tbody></table>
<figure><img src="image_5.png" /></figure>
<figure><img src="image_6.png" /><figcaption>Fig 3 Viscosity variation of ZrO2 suspensions with varied ball-milling times for different disperbyk-103 concentrations (a) 3 wt%, (b) 4 wt%, (c) 5 wt%, (d) 6 wt%; the lower graph represents the viscosity value at a 65 s⁻¹ shear rate.</figcaption></figure>
<figure><img src="image_7.png" /></figure>
The rheological curves from the ball milling study are reported in Fig 3. The rheological curve for commercial Si₃N₄ (3DCERAM) is shown in green, demonstrating the targeted printable behavior. From a general perspective, these graphs first show that hand-milling with a mortar for 2 minutes is insufficient (except for 4 wt% of Disperbyk-103). For 3 and 4 wt% of Disperbyk-103 (Fig 3A and B), increasing the ball-milling duration significantly reduces viscosity. However, the ball milling effect on viscosity is no longer observed for 5 wt% of Disperbyk-103 (see Fig 3C, indicating that the dispersant plays a significant role in breaking the agglomerates, and longer milling times do not improve dispersion. For 6 wt% of Disperbyk-103 (see Fig 3D, the milling effect is no longer present (indicating efficient deagglomeration), but viscosities slightly increase. This suggests that the excess dispersant molecules interact with each other, leading to a slight increase in viscosity. Based on this, it is clear that the optimal dispersant concentration is 5 wt% (7.14 mg/m²).

### 3.3. Sedimentation

We typically aim for good stability over a period of at least 5 days, which corresponds to the approximate duration of a printing campaign. A well-dispersed and stable suspension, indicated by minimal sedimentation, is crucial for ensuring dimensional accuracy, particularly in vat photopolymerization. In most cases, achieving an optimal dispersant concentration [10] is closely linked to obtaining a stable suspension [14].
<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /><figcaption>Figure 4 (a) Sedimentation tests at dispersant concentrations of 3%, 4%, 5%, and 6% after 120 minutes of ball milling, observed over 5, 15, 30, and 50 days, (b) sedimentation stability for 5% of disperbyk-103.</figcaption></figure>

For the sedimentation tests, we selected four concentrations near the viscosity optimum, with a ball-milling duration of 120 minutes. This duration represents the optimal milling time for 3-4 wt% suspensions and is close to the optimum for 5-6 wt% suspensions.

In our observations (see Fig 4 A), we noted very low sedimentation for these 4 concentrations, a phenomenon mainly attributed to the use of nanometric ZrO₂ powder. The nanometric size of the particles results in a considerably reduced mass, which prevents ZrO₂ powder from sedimentation, at least, when the agglomeration is not taking place. Despite their size, these particles have a large specific surface area, which increases the interactions between the particles, such as electrostatic forces, chemical bonds and van der Waals forces. These
electrostatic interactions generate surface charges on the particles, which interact with the
functional groups of Disperbyk-103, strengthening the electrostatic repulsion between particles
and thus preventing both agglomeration and sedimentation.

According to the sedimentation tests, the least stable suspension is the 3wt% suspension
after only 5 days. This result was expected as this lowest dispersant content suspension has the
highest viscosity among the four suspensions. The second suspension to start to sediment is the
4wt% suspension after 15 days, followed by the 5wt% suspension after 30 days, and finally,
the 6wt% suspension after 50 days. As mentioned earlier, our goal is to ensure at least a 5-day
stability for printing purposes. Therefore, the 4wt%, 5wt%, and 6wt% suspensions are suitable
for our application. To minimize sedimentation risk, the 5wt% suspension is preferred (see Fig
4 B), as sedimentation is very limited until 15 days. In conclusion, the 5wt% suspension is
preferred over the 6wt% suspension due to its lower viscosity, as determined in the previous
section. Thus, for the continuation of this study, we have selected a 5wt% dispersant content
suspension ball-milled for 120 minutes.

3.4. *Curing depth and lasing parameters.*

Vat photopolymerization uses a suspension of ceramic particles mixed with photosensitive
monomers that polymerize when exposed to light at a specific wavelength. Our printer uses a
laser beam with a 405 nm wavelength which is compatible with de photoinitiator wavelength
spectrum. The activated free radicals then initiate a chemical reaction with the monomers
(HDDA and TMPTA), triggering the formation of a polymer network and solidifying the
suspension.

The accuracy of the printed part is also influenced by various printability parameters, such
as layer thickness, which is controlled by lowering the platform. In practice, the 405nm beam
must penetrate both the current layer and slightly into the underlying one [22] (see Fig 5A).
Insufficient depth of polymerization can result in defects like incomplete layers, delamination, rough surfaces, and increased porosity, all of which decrease the resolution thus the overall quality of the printed part. Conversely, excessive polymerization can heighten the risk of warping, unnecessarily extend the printing time, and cause issues with the recoating blade impacting and detaching the part during the printing process.

<figure><img src="image_10.png" /><figcaption>Fig 5 (a) Process parameter in vat photopolymerization inspired from [23], (b) parametric printing exploration device using a 3D-printed structure with 9 containers loaded by the suspension, (c) each container is exposed to a 2x2 mm monolayer printing with different lasing speed; the effective curing depth is then determined by measuring the cured specimen thickness.</figcaption></figure>

Adjusting certain parameters can help resolving problems related to the suspension. Specifically, the speed and power of the laser have a significant impact on the printing process and the mechanical properties of the printed part. Finding the right balance for these parameters is crucial.

In our study, we conducted experiments to optimize these factors for our printing process. According to the literature, polymerization depth is influenced by the ceramic volume fraction, particle size, differences in refractive index (which cause light scattering), and light absorption. [10,22] As the number of ceramic particles increases, the increased scattering/reflections make it more challenging for UV light to penetrate, consequently reducing polymerization depth.

Moreover, there is an indication of a relationship between polymerization depth and milling time. Specifically, a longer milling time leads to a decrease in particle size, causing an increase in polymerization depth, which also depends on the level of agglomeration [24]. The intention
here is not to manipulate these parameters to establish their accuracy but to incorporate them
into our study [25].

From our formulated solution determined in the previous experiments, the goal is to achieve
high-resolution printing. To achieve this, we established an experimental plan to assess the
impact of power and speed on polymerization depth and apparent resolution. For this purpose,
we decided to introduce our suspension into 9 containers and polymerize a single layer of
2x2mm in each (see ** ERROR ! Source du renvoi introuvable. ** and C) The experimental plan
was to compare our ZrO₂ suspension with two commercial suspensions of Si₃N₄ and Al₂O₃ that
work on our printer.

The Fig 6 shows polymerization depth as a function of laser speed for three suspensions.
We have two commercial suspensions, Al₂O₃ and Si₃N₄, provided by 3DCERAM, which are
compatible with our printer. Through various tests, we identified an Al₂O₃ speed of
approximately 12500 mm/s at 50mW, while for Si₃N₄, a speed of around 250 mm/s at 200mW;
i.e. a speed 50 times slower for a power 4 times higher compared to Al₂O₃. In our ZrO₂ case, at
laser powers below 200mW, polymerization was only measurable at speeds below 200 mm/s.
Thus, to avoid excessively long printing times, we decided to use a laser power of 200mW, the
same power used for the Si₃N₄ suspension.
<figure><img src="image_11.png" /><figcaption>Fig 6 (a) Study of the curing depth as a function of the laser speed at 200mW (b) Surface aspect of 2x2 mm squares printed via vat photopolymerization.</figcaption></figure>

The Fig 6 A clearly shows that lower lasing speeds result in greater polymerization depth. It also highlights significant differences between Al₂O₃ and Si₃N₄ suspensions. To ensure strong cohesion between layers, we aim to over-polymerize the top layer onto the bottom one. Therefore, speeds of 750 and 850 mm/s are not compatible with printing layers that are 50 µm thick.

The Fig 6 B indicates that lower speeds increase the residual light and scattering which drastically reducing the surface resolution. Thus, surface resolution speeds above 450 mm/s are acceptable. Our range is therefore between 450 and 650 mm/s.

### 3.5. *Printing tests*

To confirm our results, we conducted 3D tests printing 3x3x3 mm cubes at various speeds. The system uses a homemade ZrO₂ suspension with a laser power of 200 mW and a scanning speed ranging from 50 to 850 mm/s. These tests showed that at speeds below 550 mm/s, the blade interfered with the part during printing, causing damage to the prints or detachment of the part from the build plate (see Fig. 7A), ultimately leading to the print being stopped due to recoater blade interference.
Conversely, as previously explained, speeds of 750 and 850 mm/s resulted in poor cohesion and incomplete parts.

<figure><img src="image_12.png" /><figcaption>Fig 7 (a) 3D printed cube at a speed of 550 mm/s. The red arrow points to the ridge created by the blade's friction. (b) 8 cm honeycomb structure printed with homemade ZrO₂ suspension.</figcaption></figure>

Therefore, only speeds of 550 mm/s and 650 mm/s allowed for the successful production of high-resolution cubes, which corresponds to a polymerization depth of approximately 30% of the lower layer. To optimize printing time, we chose a speed of 650 mm/s.

This choice of a laser speed of 650 mm/s at a laser power of 200 mW with a platform descent of 50 µm (layer height) between each layer allowed us to print a lot of parts. To illustrate the effectiveness of these parameters, we chose to print a honeycomb part over 8 cm in length (Fig 7 B). This intricate shape has been successfully printed with a shape accuracy.

### 3.6. Debinding and Sintering

The debinding and sintering cycle of the zirconia ceramic suspension was designed based on the results obtained from DSC analysis conducted at a heating rate of 1K/min (see Fig. 8). The TGA graph reveals that the main mass loss occurs between 100°C and 530°C. The DSC
shows a significant positive peak, indicating the high exothermicity of these decompositions.

In some methods, debinding can be carried out in a nitrogen atmosphere to avoid thermal runaway and crack formation during the process [10.1016/j.jeurceramsoc.2024.116911].

However, this approach is very time-consuming and requires a specialized furnace. Since zirconia is an oxide, we chose a slow debinding rate of 0.1K/min in air up to 530°C, followed by 2K/min to 1200°C to presinter the samples.

<figure><img src="image_13.png" /><figcaption>Fig 8 (a) DSC analysis in air of the zirconia ceramic suspension at a heating rate of 1°C/min and (b) the associated debinding/presintering cycle.</figcaption></figure>

Microscopic observations of sintered zirconia at low magnifications (see Fig. 9A) reveal that the ceramic structure exhibits very few defects. Slight 'ridges' are visible on the surface, which are areas prone to the initiation of small cracks. However, these cracks stop after 100 µm in depth, preventing significant propagation and preserving the overall integrity of the structure.

Moreover, in Fig. 9B, which shows the center, only slight porosity defects in the form of lines, corresponding to a print layer, can be observed. The SEM microstructure of the sintered zirconia (Fig. 9B) appears to be well-densified, with a relative density of 99%, successfully preserving a low submicronic grain size of 267 nm. Vickers hardness tests conducted on the samples yielded consistent results, with an average hardness of 14 GPa. These tests were repeated 8 times at different locations, with a standard deviation of only 0.3 GPa, indicating good
uniformity of the microstructure and a reliable sintering process. In Fig. 9C, this hardness is compared to recent articles on zirconia SLA [10.1016/j.ceramint.2017.11.135] [10.3390/app12189088] [10.1038/s41598-023-36181-6] [10.1155/2021/6612840] [10.1016/j.ceramint.2021.12.181] [10.3390/ma16010402] [10.3390/ma14133446] where our properties are well-positioned in terms of both grain size and hardness.
<figure><img src="image_14.png" /><figcaption>Fig 9 (a) Optical microscopic observations of the sintered zirconia ceramic structure at the edge and center, with slight 'ridges' (highlighted in red) prone to crack initiation at the edge and some interlayer defects in the center related to the printing process; (b) SEM image of the microstructure; (c) corresponding Vickers hardness in GPa, compared with values from other literature.</figcaption></figure>

4. Conclusion

In summary, our study successfully formulated a nanometric ZrO₂ suspension optimized
for use with the 3DCERAM C101 printer, targeting applications requiring high precision and
performance. Through experimentation, we identified the optimal dispersant concentration and ball-milling time to achieve a stable, well-dispersed, and printable suspension.

The optimal dispersant concentration was determined to be around 5 wt% (7.14 mg/m²), which ensures minimal viscosity and prevents particle agglomeration, thereby enhancing the suspensions stability. The ball-milling duration of 120 minutes was found to be ideal for achieving a homogeneous suspension with appropriate rheological properties. Sedimentation tests confirmed the stability of this formulation, with minimal sedimentation observed even after extended periods (more than 5 days). The debinding and sintering cycle, based on DSC analysis, produced a dense zirconia microstructure with minimal defects. Microscopic observations showed slight surface ridges and small cracks, which did not propagate beyond 100 µm, preserving structural integrity. SEM analysis confirmed 99% density and a small grain size of 267 nm. Vickers hardness tests yielded an average of 14 GPa with low variability, demonstrating excellent process consistency.

Adjusting the laser speed and power was critical to achieving the desired curing depth and surface resolution. A unique parametric explorative device has been designed and produced to quickly optimize the lasing conditions. A laser speed of 650 mm/s at a power of 200 mW provided the optimal balance between polymerization depth and print quality. A maximum over-polymerization of 30% of the lower layer has been identified. The obtained suspension is printable on a professional vat photopolymerization device and can be used to produce zirconia parts with smaller grains.
## Acknowledgements

The help and support of 3DCERAM Sinto, Jérôme Lecourt and Théotim Marie is gratefully acknowledged. This work has been funded by Unicaen.

## References

[1] Y. Lakhdar, C. Tuck, J. Binner, A. Terry, R. Goodridge, Additive manufacturing of advanced ceramic materials, Progress in Materials Science 116 (2021) 100736. https://doi.org/10.1016/j.pmatsci.2020.100736.

[2] A. Velasco-Hogan, J. Xu, M.A. Meyers, Additive Manufacturing as a Method to Design and Optimize Bioinspired Structures, Advanced Materials 30 (2018) 1800940. https://doi.org/10.1002/adma.201800940.

[3] Q. Zhang, X. Yang, P. Li, G. Huang, S. Feng, C. Shen, B. Han, X. Zhang, F. Jin, F. Xu, T.J. Lu, Bioinspired engineering of honeycomb structure – Using nature to inspire human innovation, Progress in Materials Science 74 (2015) 332–400. https://doi.org/10.1016/j.pmatsci.2015.05.001.

[4] T. Chartier, A. Badev, Rapid Prototyping of Ceramics, in: Handbook of Advanced Ceramics, Elsevier, 2013: pp. 489–524. https://doi.org/10.1016/B978-0-12-385469-8.00028-9.

[5] Z. Chen, Z. Li, J. Li, C. Liu, C. Lao, Y. Fu, C. Liu, Y. Li, P. Wang, Y. He, 3D printing of ceramics: A review, Journal of the European Ceramic Society 39 (2019) 661–687. https://doi.org/10.1016/j.jeurceramsoc.2018.11.013.

[6] A. Zocca, P. Colombo, C.M. Gomes, J. Günster, Additive Manufacturing of Ceramics: Issues, Potentialities, and Opportunities, J. Am. Ceram. Soc. 98 (2015) 1983–2001. https://doi.org/10.1111/jace.13700.

[7] T. Chartier, V. Pateloup, C. Chaput, Élaboration de pièces céramiques par fabrication additive, Verres et céramiques (2018). https://doi.org/10.51257/a-v1-n4807.

[8] R. Chaudhary, P. Fabbri, E. Leoni, F. Mazzanti, R. Akbari, C. Antonini, Additive manufacturing by digital light processing: a review, Prog Addit Manuf 8 (2023) 331–351. https://doi.org/10.1007/s40964-022-00336-0.

[9] M. Ziaee, N.B. Crane, Binder jetting: A review of process, materials, and methods, Additive Manufacturing 28 (2019) 781–801. https://doi.org/10.1016/j.addma.2019.05.031.

[10] I.L.D. Camargo, M.M. Morais, C.A. Fortulan, M.C. Branciforti, A review on the rheological behavior and formulations of ceramic suspensions for vat photopolymerization, Ceramics International 47 (2021) 11906–11921. https://doi.org/10.1016/j.ceramint.2021.01.031.

[11] S. Zakeri, M. Vippola, E. Levänen, A comprehensive review of the photopolymerization of ceramic resins used in stereolithography, Additive Manufacturing 35 (2020) 101177. https://doi.org/10.1016/j.addma.2020.101177.

[12] C.-J. Bae, A. Ramachandran, K. Chung, S. Park, Ceramic Stereolithography: Additive Manufacturing for 3D Complex Ceramic Structures, J. Korean Ceram. Soc 54 (2017) 470–477. https://doi.org/10.4191/kcers.2017.54.6.12.
[13] S.Y. Song, M.S. Park, D. Lee, J.W. Lee, J.S. Yun, Optimization and characterization of high-viscosity ZrO2 ceramic nanocomposite resins for supportless stereolithography, Materials & Design 180 (2019) 107960. https://doi.org/10.1016/j.matdes.2019.107960.

[14] G. Ding, R. He, K. Zhang, M. Xia, C. Feng, D. Fang, Dispersion and stability of SiC ceramic slurry for stereolithography, Ceramics International 46 (2020) 4720-4729. https://doi.org/10.1016/j.ceramint.2019.10.203.

[15] A. Bagheri, J. Jin, Photopolymerization in 3D Printing, ACS Appl. Polym. Mater. 1 (2019) 593-611. https://doi.org/10.1021/acsapm.8b00165.

[16] E. Peng, D. Zhang, J. Ding, Ceramic Robocasting: Recent Achievements, Potential, and Future Developments, Advanced Materials 30 (2018) 1802404. https://doi.org/10.1002/adma.201802404.

[17] Ceramic Powder - Zirconia | https://www.tosoheurope.com, (n.d.). https://www.tosoheurope.com/our-products/ceramic-materials (accessed August 29, 2024).

[18] A. Goswami, A. K. N. Balashanmugam, A.M. Umarji, G. Madras, Optimization of rheological properties of photopolymerizable alumina suspensions for ceramic microstereolithography, Ceramics International 40 (2014) 3655-3665. https://doi.org/10.1016/j.ceramint.2013.09.059.

[19] K. Zhang, R. He, G. Ding, C. Feng, W. Song, D. Fang, Digital light processing of 3Y-TZP strengthened ZrO2 ceramics, Materials Science and Engineering: A 774 (2020) 138768. https://doi.org/10.1016/j.msea.2019.138768.

[20] K. Zhang, R. He, C. Xie, G. Wang, G. Ding, M. Wang, W. Song, D. Fang, Photosensitive ZrO2 suspensions for stereolithography, Ceramics International 45 (2019) 12189-12195. https://doi.org/10.1016/j.ceramint.2019.03.123.

[21] X. Li, H. Zhong, J. Zhang, Y. Duan, H. Bai, J. Li, D. Jiang, Dispersion and properties of zirconia suspensions for stereolithography, Int J Applied Ceramic Tech 17 (2020) 239-247. https://doi.org/10.1111/ijac.13321.

[22] J. Tarabeux, V. Pateloup, P. Michaud, T. Chartier, Development of a numerical simulation model for predicting the curing of ceramic systems in the stereolithography process, Journal of the European Ceramic Society 38 (2018) 4089-4098. https://doi.org/10.1016/j.jeurceramsoc.2018.03.052.

[23] R.B.S. Gowda, C.S. Udayagiri, D.D. Narendra, Studies on the Process Parameters of Rapid Prototyping Technique (Stereolithography) for the Betterment of Part Quality, International Journal of Manufacturing Engineering 2014 (2014) 1-11. https://doi.org/10.1155/2014/804705.

[24] V. Sevriugina, D. Pavliňák, F. Ondreáš, O. Jašek, M. Štaffová, P. Lepcio, Matching Low Viscosity with Enhanced Conductivity in Vat Photopolymerization 3D Printing: Disparity in the Electric and Rheological Percolation Thresholds of Carbon-Based Nanofillers Is Controlled by the Matrix Type and Filler Dispersion, ACS Omega 8 (2023) 45566-45577. https://doi.org/10.1021/acsomega.3c05683.

[25] M.N. Cooke, J.P. Fisher, D. Dean, C. Rimnac, A.G. Mikos, Use of stereolithography to manufacture critical-sized 3D biodegradable scaffolds for bone ingrowth, J Biomed Mater Res 64B (2003) 65-69. https://doi.org/10.1002/jbm.b.10485.