# Versatile aqueous alumina suspensions formulations by one step ball milling for robocasting from fine to coarse powders

Flavie Lebas, Sylvain Marinel, Christelle Bilot, Romuald Herbinet, Loïc Le Pluart, Charles Manière

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## ► To cite this version:

Flavie Lebas, Sylvain Marinel, Christelle Bilot, Romuald Herbinet, Loïc Le Pluart, et al.. Versatile aqueous alumina suspensions formulations by one step ball milling for robocasting from fine to coarse powders. Materials Today Chemistry, 2025, 45, pp.102647. 10.1016/j.mtchem.2025.102647. hal-05131426

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https://hal.science/hal-05131426v1

Submitted on 26 Jun 2025

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# Versatile aqueous alumina suspensions formulations by one step ball milling for robocasting from fine to coarse powders

Flavie Lebas<fnref n="1" />*, Sylvain Marinel<fnref n="1" />, Christelle Bilot<fnref n="1" />, Romuald Herbinet<fnref n="2" />, Loïc Le Pluart<fnref n="2" />, Charles Manière<fnref n="1" />*

## ABSTRACT

Direct Ink Writing (DIW), or robocasting, is a rapid extrusion-based additive manufacturing technique offering significant potential for fabricating complex ceramic technique components as alumina, due to its exceptional mechanical strength, thermal stability, and wear resistance. This study explores the preparation of aqueous alumina suspension with different alumina powder with a minimal organic content to optimize suspension preparation for robocasting. Three alumina powders, including expensive fine grain powder to cheaper and coarser powder (BMA15, P172LSB and APA-0.5), were used to develop suspensions via a one-step ball milling process, aiming at replicating the properties of a reference suspension of the finest powder while minimizing the amount of organic additives. The suspensions were designed to exhibit shear thinning behavior with defined yield stress for smooth extrusion and post deposition shape retention. The developed suspensions, containing less than 5 vol% of organic additives have demonstrated excellent stability, homogeneity, and absence of agglomerates. Successful printing of two distinct parts with these three suspensions confirmed their capability to maintain high yield stress, enabling the stacking of multiple layers and fabrication of complex geometries. The parts made from the finest alumina powder exhibit a microstructure and mechanical properties (BMA15: 1874 ± 48 HV, 2.78µm of grain size) comparable to those conventionally manufactured ceramic. The two other suspensions, with coarser powders, exhibit similar mechanical properties (P172LSB: 1794 ± 30 HV and 3.3µm of grain size, APA-0.5: 1718 ± 60 HV and 2.76 µm of grain size).

## GRAPHICAL ABSTRACT

Protocol for Preparing Alumina Suspensions with Powders of Varying Particle Sizes for Direct Ink Writing Applications

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## Keywords

Ceramic additive manufacturing, Robocasting, Alumina, Suspensions

<footnote n="1">¹ Université Caen Normandie, ENSICAEN, CNRS, Normandie Univ, CRISMAT UMR6508, F-14000 CAEN, FRANCE</footnote>

<footnote n="2">²Université Caen Normandie, ENSICAEN, CNRS, Normandie Univ, LCMT UMR6507, F-14000 CAEN, FRANCE</footnote>

<footnote>*Corresponding authors: flavie.lebas@ensicaen.fr, charles.maniere@ensicaen.fr</footnote>
# 1. INTRODUCTION

Direct Ink Writing (DIW), also known as robocasting, is an additive manufacturing (AM) technique that involves the extrusion of a paste filament by a syringe to create green parts. Ceramic parts are built layer by layer according to a Computer-Aided Design (CAD) model and a slicing software. Robocasting is highly versatile, as it can accommodate a wide range of materials for printing, as highlighted in recent review [1]. Its flexibility makes it a promising solution for addressing the growing demand for technical ceramics with complex geometries, which cannot be produced by traditional manufacturing methods (such as pressing, molding, and injection molding) [2,3].

Robocasting is a simple, rapid, and cost-effective extrusion-based technology that offers significant potential for manufacturing parts of various sizes and thicknesses. The nozzle size can easily be adjusted, ranging from 0.2 to 2 mm, which allows for the quick production of large parts. However, the resolution (ranging from 400 µm to 1 mm) is generally lower than the values usually observed on other AM technologies, such as vat photopolymerization (with resolutions of 10-100 µm) [4]. Despite this limitation, robocasting is considered as one of the most environmentally friendly AM processes. This is due to the low organic content in the ceramic pastes used (<2%), reducing the need for extensive and tricky debinding process. It simultaneously allows to minimizing the carbon footprint [1,5].

One of the key materials of interest for robocasting is alumina (Al2O3), a widely used technical ceramic known for its outstanding properties, such as high mechanical strength, good thermal stability, and high wear resistance [2]. Alumina is extensively employed in various industries, including aerospace, bioengineering, dental applications [6], high-temperature filtration, thermal barriers, fuel cells, and electrical systems [2,3,7].

The challenge of robocasting primarily resides in the preparation and characteristics of the suspensions. These suspensions need to exhibit a specific rheological behavior: shear-thinning with a defined yield stress that allows for smooth flow through fine printing nozzles (once the stress threshold is exceeded) and ensures the filament maintains its shape immediately after deposition (once the stress is relieved) [8,9]. In addition, to achieve the desired rheological properties for printing, these suspensions must be stable, homogeneous, and free from agglomerates and air bubbles. The robocasting is an AM technology that utilizes aqueous high loading ceramic suspensions with minimal organic additives, such as dispersants, binders, and plasticizers, to ensure particle dispersion, prevent cracks during drying, and facilitate efficient debinding after printing [5]. The viscosity of these suspensions is significantly influenced by the particle size distribution [10–12]. Fine particles increase viscosity due to their higher specific surface area, while coarser particles reduce viscosity. In any case, it requires careful stabilization against agglomeration and sedimentation using dispersant,
added according to the nature of the powder and its level of agglomeration. The preparation of stable, homogenous suspensions depends on the grinding and milling conditions. The grinding energy and its duration must ensure good particles dispersion to avoid agglomeration. Techniques such as ball milling are particularly effective, especially for high solid-loading suspensions [13–15].

As reported in the literature, many conventional processes are time-consuming and require multiple milling stages with various processes, such as attrition and planetary milling for instance [16–18]. In this study, we develop a one-step planetary ball milling process and a simple protocol to produce alumina suspensions using three distinct powders from fine to coarser submicronic particles (BMA15, P172LSB and APA-0.5). The goal is to replicate the rheological properties of a reference suspension, made from a fine grade of alumina powder [19]. Otherwise, this work aims at enabling cheaper and coarser powders suitable in robocasting process. In the same time, the objective is to reduce proportions of organic additives to reduce the carbon footprint of our suspensions (fast debinding and sintering). The protocol will be to validate through the successful printing and sintering of complex shapes.

## 2. MATERIALS AND METHODS

### 2.1. Raw powders

Three following alumina powders were used in this study: BMA15 (Baikowski, France) with a BET specific surface area of 16 m²/g and a D₅₀ of 0.12 µm. This powder is the finest and is used to test the different milling protocols and their efficiency. The P172LSB (Alteo, France) and APA-0.5 (Sasol, USA) powders are both with a BET specific surface area of 8.2 m²/g and with a D₅₀ of 0.4 and 0.29 µm, respectively. The latter are considered as the two coarser and cheaper powder to adapt from the BMA15 optimized milling protocol.

### 2.2. Determination of the most appropriate grinding process (attrition vs ball milling)

In traditional suspension fabrication procedure, the attrition process is used to pre-grind and disperse the powder afterward a subsequent planetary ball milling is used to mix the ceramic powder, the solvent, additives and homogenize the suspension. This procedure ensures a safe dispersion and a paste mixture homogenization [19,20]. However, this procedure is time consuming and the planetary ball milling can also be used to disaggregate
and homogenize the powder saving a considerable processing time [21]. One of our first investigation will be to test this possibility by comparing the granulametric distributions of alumina after attrition or after ball-milling. This exploration is conducted on the BMA15 powder, which is the finest, and then the most challenging with respect to agglomeration.

The attrition consists of preparing an aqueous slurry made from 400 cm³ of ceramic powder, 600 cm³ of zirconia balls (2 mm). The milling is done in a PTFE container with stirring blades during 3h. The planetary ball milling consists of using agate balls (20 mm) in an agate container (250 cm³) with a pulverisette 6 planetary mono mill (Fritsch, Germany). A rotation speed of 350 rpm for 2 hours is applied during ball milling/ and the aqueous suspensions is loaded with 40 vol% of alumina powder. The milling configuration consists of 2/3 void and 1/3 paste and balls. The material to ball volume ratio is 3:2. This setup maximizes grinding efficiency and promotes more effective particle size reduction. This method has indeed shown its efficiency in terms of particle size refining and homogeneity of the paste. However, it limits the maximum paste volume capacity, which is not an issue for our study but would require adaptations for scale-up. All grounded powders were subsequently analyzed by laser granulometry (Mastersizer 2000, Malvern, UK) and observed by scanning electron microscopy (SEM) (JSM-7200F, JEOL, Japan) to characterize and compare their particle size distributions. After optimization of the grinding/mixing procedure with the BMA15 powder the selected procedure is applied to the coarser powders (APA-0.5, P172LSB) and their raw and milled granulometry is characterized.

## 2.3. Suspensions organic additives

Along with the selected grinding process, various suspensions of alumina were prepared using the following additives: the Dolapix CE 64 (Zschimmer & Schwarz, Germany) as dispersant, the polyvinyl alcohol (PVA) (OTAPIX PA 4G, Zschimmer & Schwarz, Germany) as binder, the Poly(ethylene glycol) (PEG) (Mn=400) (Sigma-Aldrich-Merck, Germany) as plasticizer and the carboxylmethylcellulose (CMC, Sigma-Aldrich-Merck, Germany) as viscosifying and binding agent. First, the BMA15 was used to find out the best experimental conditions for the preparation of the suspension. Afterward, those conditions will be adapted to the coarser powders to evaluate the robustness of our process in a view of being able to print a wide range of alumina powder.

## 2.4 Preparation, suspension additive optimizations and rheological measurements

### 2.4.1. Suspensions preparation for viscosity measurement

The procedure is crucial for developing inks with shear-thinning behavior and high stress, ensuring smooth flow through fine printing nozzles while maintaining filament shape
immediately after deposition. These properties also help the lower layers supporting the weight
of the upper layers during printing [16]. The protocol of preparation was inspired by ref [19]
method and used for the three powders. The optimal amount of dispersant is determined for
each powder. Since ball milling refines the powders during the grind milling (use to elaborate
the suspension) and narrows their particle size distribution, it is essential to use powders
processed by this method. The optimal dispersant content was studied by testing the viscosity
of suspensions containing 40% vol. ceramic powder grind-milling 2H content with varying
amounts of Dolapix CE64 (0–4 wt%, based on total solids content) without any other organic
additives.

2.4.2.Suspensions preparation ready to use for robocasting

The suspensions were prepared by weighting and placing in a 100 mL agate bowl, the powder and the CMC powder and mixed manually both before adding water and additives (the optimal amount of Dolapix CE64 corresponding to the powder, PEG, PVA, KOH), which had been pre-weighed in a same beaker. The mixture was further stirred manually before adding agate balls. The bowl was then placed in a planetary mono mill grinder (Pulverisette 6, Fritsch, Germany) for 2.5 hours, rotating at 350 rpm. The milling time is slightly increased because the paste quantity is higher. After mixing, the paste was collected using a "Maryse" spatula and stored in a plastic bottle.

To develop a suitable paste for robocasting, viscosity must be optimized. The target
printable viscosity domain is based on the rheological BWP alumina suspension developed in
ref [19]. First of all, the composition of the suspensions keeps the same proportions as the
BWP suspension. However, in second time the composition is adjusted (Solid loading and
CMC amount) as a function of the alumina powder. As demonstrated in BWP study, solid
loading and CMC content both help increasing viscosity.

A rotational rheometer (AR1000, TA Instruments, USA) was employed to measure the
suspension viscosity within a shear rate range of 0.1–100 (1/s) to 10–100 (1/s), using a plate-
plate configuration at room temperature. To prevent drying, measurements were conducted
for a brief period (~2 minutes and 30 seconds).

**2.5 Printing of parts by robocasting**

The printing process used the Eazao Bio (Eazao, China) robocasting machine
(Fig.1.a)), equipped with a pneumatic dispensing system controlled by a manometer to
maintain a consistent extrusion flow. The paste was extruded through a 0.8 mm nozzle at a
speed of 20 mm/s, with optimized pressure to produce uniform filaments. Design files created
in FreeCAD and configured in Cura Ultimaker slicer (Fig.1.b)) were saved as GCode and
transferred via an SD card.

<figure><img src="image_4.png" /><figcaption>Fig. 1. a) Eazao Bio, b) The printing simulation for one thickness and two thickness</figcaption></figure>

With BMA 15, P172LSB, and APA-0.5 suspensions, vertical geometry parts such as a
"flower" and a cylinder shell, composed of only a few layers, were printed. These initial prints
were specifically produced to verify for each suspension the adhesion of the first filament layer
to the support surface and to assess extrusion quality, particularly when printing curved
sections. The layer-by-layer superposition allowed for a detailed inspection of print quality and
enabled the detection of potential issues with layer support, especially if the applied stress
threshold of the different suspensions was insufficient. The success of layer stacking relied
heavily on the viscosity of the suspensions; if the viscosity was too low, stacking the layers
would not be feasible. Thus, the viscosity of the suspensions was regularly compared with
regards to the print quality, allowing adjustments to be made as needed to optimize stability
and adhesion. The suspensions had to be of high quality (free from agglomerates, bubbles,
and properly bound) to avoid printing defects and ensure the successful printing of complex
shapes without structural collapse. The success of the printing of these parts by a suspension
validates the suspension printability. Finally, all printed parts were left to dry for at least one
night at room temperature to maintain consistency and stability in the printed structures.

## 2.6. Sintering

The parts were sintered using a conventional approach and inspired from the
dilatometry study of I.Cornu et al [22]. The heating ramp is set at 100°C/h, the dwell time
remains 3 hours, and the cooling rate is set to 600°C/h. This cycle is kept constant for all
samples, with only the holding temperature varying. Various sintering temperatures were
evaluated, specifically at 1300°C, 1400°C, 1450°C, 1500°C and 1550°C. Due to the low
organic content (less than 3 wt%), the debinding process was integrated directly into the
sintering thermal cycle.
## 2.7. Characterization of parts

The sintered parts microstructures were observed by SEM (JSM-7200F, JEOL, Japan), with a backscattered electron detector (BSE). The hardness was characterized by Vickers micro-indentation (MMT-X7B, Matsuzawa, Japan) using 1kgf load. Approximately 9 hardness measurements were taken at the core of the parts for each sintering temperature. The microhardness is determined by an average and a standard deviation calculated using Student's law. The porosity and grains size of the various parts were measured by image analysis using an open-source software for image processing (Image J - National Institute of Mental Health, USA). The average grain size was assessed using the linear intercept method, yielding an average value that is multiplied by 1.56 [23] to account from the transition from 1D to 3D stereology, representing the 3D grains average value. For porosity measurement, threshold image processing was employed to determine the proportion of porosity within the parts. From these, the area fraction of pores was determined. To achieve this, three SEM images at a magnification of 5000× were selected to represent the overall microstructure.

# 3. RESULTS & DISCUSSION

## 3.1. Powders and grinding procedure optimization

The granulometry curves (Fig.2) present the BMA15 particles size distribution for various milled conditions. Two populations (0.18 µm and 1 µm) are visible for the raw commercial BMA15 (black line). A similar granulometry was found in the work of I.Cornu *et al* [22]. As seen in the SEM image (Fig.2.b)) more or less spherical agglomerates composed the BMA15 which are made from fine particles (Fig.2.c)). As seen in the dashed line in graphic (Fig.2), after attrition, the mode of the distribution is still centered at 0.18 µm as the commercial BMA15 and the volume fraction of agglomerates of 1 µm size has significantly decreased but some large agglomerates having a size bigger than 10 µm appear. These agglomerates result from the process but they can be avoided by the use of a dispersant during the mixing. The SEM image of particle (Fig.2.a and c) are similar. The attrition process is relatively time consuming, and is not suitable to produce a large amount of materials. In addition, these granulometric distributions show that this attrition step is not effective to significantly decrease the particle size for the BMA15 powder compared to the BWP powder [19] where it was highly needed. As the additional planetary milling stage is needed to mix all the ingredients, it is
worthy to evaluate the possibility to not use this attrition step but instead, only use the planetary milling to mix and desagglomerate the powder.

The granulometry analysis of the BMA15 with sole planetary ball milling (green dash line) show that the process reduce to a size inferior to 0.1 µm and grind the population agglomerated at 1 µm. In addition, the SEM image Fig.2.d) clearly shows that the size of the agglomerates has decreased after ball-milling, without significantly decreasing the elementary particles size (Fig 2.e)). In Fig.2 graphic, the green dash line indicates also two bigger populations at 3.80 and 20 µm. Those peaks likely correspond to agglomerates which are formed due to the ball-milling process that breaks down the agglomerates. To evidence the fact that this apparent bigger agglomerates are soft agglomerates, the granulometry analysis of BMA15 planetary-milled were repeated using the optimal concentration of dispersant that has been optimized later in the article. The new curve (green curve) (Fig.2) clearly shows the elimination of the two populations at 3.80 and 20 µm. This confirms that they are soft agglomerates due to the absence of dispersant in the pre-grinding study. These evidence that one planetary ball milling step is suitable and enough to both destroy agglomerates and homogenize the suspension.

Therefore, it appears that the preliminary stage of attrition can be avoided. This is also evidenced by the fact that the final suspension is easily extrudable on 0.4 to 0.8 mm nozzle while 20 µm hard agglomerates would have prevent a fluid extrusion. The fact that the BMA 15 powder is the finest may also explain why these soft agglomerates forms while this phenomenon is less present in the two coarser grade detailed afterward. In addition, in terms of elementary particle size and morphology, it can be clearly seen that ball-milling or attrition lead to similar results, as testified on higher magnification SEM images recorded on both powder (Fig 2.f)).
<figure><img src="image_5.png" /><figcaption>Fig. 2. Graphic: Particle size distribution of BMA15, SEM images:a) particle size of BMA15 attrited, b) and c) BMA15 commercial, d) and e) BMA15 planetary-milling 2h, f) Particle size and morphology of BMA15 attrited and planetary milling</figcaption></figure>
The particle size distribution of the P172LSB and APA-0.5 are presented in graphic a) and b) Fig.3. They exhibit a single mode centered respectively at 0.36 µm and 0.55 µm. After 2h of planetary-milling, particle sizes are slightly decreased. Therefore, the determination of the optimized dispersant amount must be carried out on the ground powder. As previously mentioned, some soft agglomerates are observed on both ball-milled powder (see the peaks at aroufd 10 µm). Those agglomerates are not a cause of concern as it was shown that by using an appropriate amount of dispersant, those agglomerates are removed. A comparison of the SEM image (Fig.3.a) and c) of these two commercial powders show that the P172LSB is a coarser powder and even after the grind milling (Fig.3.b) and d)).

<figure><img src="image_6.png" /><figcaption>Fig. 3. Graphic: a) Particle size distribution of P172LSB b) APA-0.5, SEM images: a) P172LSB commercial, b) P172LSB grinding 2h, c) APA-0.5 commercial and d) APA-0.5 grinding 2h</figcaption></figure>
To conclude on the three alumina powders, the attrition is not necessary, the planetary ball milling is very efficient for both elaborate the suspension and disaggregate the powder in one step.

## 3.2 Rheology of the slurries

In this part, the rheology of the new alumina grades suspensions is studied. The rheology of the printable BWP suspension previously developed is used here as a target. The optimal dolapix CE64 dispersant concentration needs to be evaluated to achieve a stable suspension by evaluating the lowest viscosity across a range of dispersant levels in a suspension loaded with 40% vol solids (close to the final printable ceramic charge ~50 vol%). As suggested by the zeta potential analysis for different alumina grades, the pH must be set to slightly basic zone, typically 9-10 [11]. Figure 4 illustrates viscosity as a function of dispersant content at various shear rates: 10 and 100 s⁻¹, representing the printability limits, and 30 and 50 s⁻¹, corresponding to shear rates in 1.2 mm and 0.6 mm diameter nozzles, respectively.

Although the required amount of dispersant generally depends on the specific surface area of the powder, the three powders (despite differences in specific surface area and particle size distribution) require nearly the same amount of dispersant, to get the minimum value of viscosity. The three curves (Fig.4) show that a minimum viscosity is obtained around 2 wt% of dispersant (at the beginning of the dwell), suggesting that this concentration is optimal. Viscosity often increases with excess dispersant [24] due to dispersant-dispersant interactions. However, in this case, the excess of dispersant does not significantly increase the suspension viscosity, which improves the stabilization tolerance of the suspensions.
<figure><img src="image_7.png" /><figcaption>Fig. 4. Determination of optimal concentration of dispersant for a) BMA15, b) P172LSB c) APA-0.5</figcaption></figure>

Figure 5.a) shows the rheological properties of suspensions prepared with the three different alumina grades. The rheological behavior of these suspensions was measured and compared to the successful BWP alumina suspension which represents the printable zone, as illustrated in Figure 5.a) in red line.

All three new suspensions exhibit shear-thinning behavior; however, their viscosity is too low, as they fall below the rheological curve of the BWP suspension. This inadequate viscosity prevents successful 3D printing of parts, as the suspensions lack the necessary viscosity and yield stress to support printed structures. Although the component ratios were kept the same across all powders, the BMA 15 powder differs significantly from the P172LSB and APA due to particle size as explained in articles [10,25].

In Figure 5.b), the adjusted compositions are presented, achieved through parameter adjustments including increasing the solid loading, modifying the amount of CMC (viscosifying
agent), and extending the mixing time (2h30). These adjusted suspensions exhibit viscosities higher than that of the BWP suspension, enabling the successful printing of complex structures. Further adjustments to the formulation could enhance the yield stress, potentially allowing for the production of larger, more stable printed parts.

Finally, an optimized suspension was tested without the addition of PVA and PEG for the following reasons. The composition already includes CMC, a multifunctional additive [26,27] that acts as a binder and viscosifying agent, among other roles. Thus, CMC can potentially replace the functions of PVA and PEG. Moreover, removing PVA and PEG reduces the total organic additive content, simplifying the overall composition. Figure 5.c) illustrates the rheological properties of the simplified final composition (without PVA and PEG) using the three different powders. Notably, the removal of PVA and PEG did not significantly affect the viscosity. To conclude, despite differences in powder characteristics, the rheological properties of the suspensions are remarkably similar, demonstrating that adjustments in formulation can minimize the impact of powder variability.

<figure><img src="image_8.png" /><figcaption>Fig. 5. Rheological properties of the three suspension a) with the same composition of BWP b) with the adjusted composition and c) with the final composition</figcaption></figure>
All compositions are listed in the table 1, below.

<table><caption>Table 1: Detailed composition of initial, adjusted and optimized suspensions</caption><thead><tr><th></th><th>(vol%)</th><th>Powder</th><th>H<sub>2</sub>O</th><th>Dispersant</th><th>PVA</th><th>PEG</th><th>KOH</th><th>CMC</th><th>Milling time (h)</th><th>Tot Organic</th></tr></thead><tbody><tr><td></td><td>BWP</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td></td><td>BMA15</td><td>42.80</td><td>52.32</td><td>2.84</td><td>0.57</td><td>0.38</td><td>0.44</td><td>0.65</td><td>2h</td><td>4.88</td></tr><tr><td></td><td>P172LSB</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td></td><td>APA-0.5</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="3">Adjusted</td><td>BMA15</td><td>46.32</td><td>48.21</td><td>3.07</td><td>0.62</td><td>0.41</td><td>0.47</td><td>0.90</td><td rowspan="3">2.5h</td><td>5.48</td></tr><tr><td>P172LSB</td><td>47.23</td><td>47.23</td><td>3.13</td><td>0.63</td><td>0.42</td><td>0.48</td><td>0.89</td><td>5.55</td></tr><tr><td>APA-0.5</td><td>48.06</td><td>46.18</td><td>3.19</td><td>0.64</td><td>0.42</td><td>0.49</td><td>1.01</td><td>5.76</td></tr><tr><td rowspan="3">Optimized</td><td>BMA15</td><td>47.02</td><td>48.94</td><td>3.12</td><td>x</td><td>x</td><td>0.48</td><td>0.92</td><td rowspan="3">1.75h</td><td>4.04</td></tr><tr><td>P172LSB</td><td>46.66</td><td>47.89</td><td>3.18</td><td>x</td><td>x</td><td>0.49</td><td>1.04</td><td>4.69</td></tr><tr><td>APA-0.5</td><td>48.58</td><td>46.68</td><td>3.22</td><td>x</td><td>x</td><td>0.50</td><td>1.02</td><td>4.74</td></tr></tbody></table>

### 3.3. Success of printing parts and sintering parts

Fig.6. presents all printed parts produced from the three powder suspensions, spanning from the green parts state to the fully sintered state. For all parts, regardless of their composition, visible shrinkage occurred, with no cracks or collapses observed during sintering. The stacking of filaments remained visible, and the surfaces of the parts appear smooth and visually glossy. The optimized BMA15 suspension (Fig.6) (i.e., without PVA and PEG) performed as well as the adjusted BMA15 suspension. No cracks were observed after drying and printing in any of the suspensions even the absence of the PEG400, which is generally, use to prevent crack [27].
<figure><img src="image_9.png" /><figcaption>Fig. 6. Green parts and sintering parts of each suspensions</figcaption></figure>
### 3.4. Alumina sintering and characterization

Table 2 provides a detailed characterization of three materials sintered at various temperatures, highlighting key properties such as porosity, grain size, and hardness. The sintering temperatures were optimized for parts fabricated using the BMA15 suspension adjusted. Among the tested conditions, 1450°C emerged as the most suitable sintering temperature due to its balance of reduced porosity and improved mechanical properties. In contrast, sintering at 1300°C resulted in insufficient densification, while at 1500°C, grain growth became significant (reaching an average size of 3.24 µm), which detrimentally affected the mechanical properties despite porosity levels remaining largely unchanged. The fact that densification increases with sintering temperature is explained by an increase of the species diffusion with temperature. Sintering is indeed a thermally activated process as described in the R. M. German book [28]. When the relative density becomes higher than 92 % of the theoretical density, grain growth may happen and can reduce the densification rate.

The microstructural evolution at different sintering temperatures is illustrated in Figure 7, where the trends in porosity reduction and grain growth can be clearly observed to Fig.7.a) to Fig.7.e). The microstructure of parts sintered with the optimized BMA15 suspension at 1450°C is similar with the reference BMA15 adjusted sample, demonstrating the effectiveness of this temperature. Additionally, the absence of PVA and PEG additives was found to have no impact on the microstructural or mechanical properties. For the L172LSB and APA-0.5 materials (Fig.7.f) and g)), sintering was feasible and resulted in homogeneous microstructures with good mechanical properties. However, further optimization using a higher temperature than 1550°C (which is the limit of our furnace) is recommended to achieve optimal sintering conditions (such as reduce the porosity) and further enhance the material performance.

<table><caption>Table 2. Characterization of each suspension parts sintering for different temperatures</caption><thead><tr><th></th><th>T<sub>Sintering</sub> (°C)</th><th>Porosity (%)</th><th>Grain size (µm)</th><th>Hardness (HV<sub>1</sub>)</th></tr></thead><tbody><tr><td rowspan="4">BMA15 adjusted</td><td>1300</td><td>23.45 ± 2.52</td><td>0.07</td><td>972 ± 56</td></tr><tr><td>1400</td><td>1.60 ± 0.11</td><td>2.66</td><td>1897 ± 47</td></tr><tr><td>1450</td><td>0.35 ± 0.09</td><td>2.78</td><td>1892 ± 32</td></tr><tr><td>1500</td><td>0.43 ± 0.03</td><td>3.24</td><td>1805 ± 68</td></tr><tr><td>BMA15 optimized</td><td>1450</td><td>0.34 ± 0.06</td><td>2.78</td><td>1874 ± 48</td></tr><tr><td>L172LSB optimized</td><td>1550</td><td>1.6 ± 0.32</td><td>3.30</td><td>1794 ± 30</td></tr><tr><td>APA-0.5 optimized</td><td>1550</td><td>3.5 ± 0.31</td><td>2.76</td><td>1718 ± 60</td></tr></tbody></table>
<figure><img src="image_10.png" /><figcaption>Fig. 7. SEM microstructure of different sintering: a),b),c),d),e) BMA15, f) P172LSB and g) APA-0.5.</figcaption></figure>
## 4. CONCLUSIONS

In this article, three aqueous alumina suspension using low organic content (< 5 vol% of CMC binder and dispersant) were successfully developed using one step ball milling process. The straightforward protocol enables the preparation of alumina suspensions, even when using different alumina powders. Moreover, these suspensions are formulated with minimal organic additives (without binder and plasticizers), ensuring simplicity in their composition. The successful printing of two distinct parts demonstrated that all three suspensions exhibit sufficiently high yield stress, allowing the stacking of multiple layers. Furthermore, the printed parts revealed good homogeneity and stability, including the ability to fabricate complex curved geometries. The microstructure and mechanical properties of parts derived from BMA15 suspension are comparable to those produced through conventional ceramic manufacturing processes. While sintering of the other two suspensions was not fully optimized due to furnace limitations, the results are comparable to the BMA15 at same porosity level and could be comparable to those conventionally manufactured ceramics.

## ACKNOWLEDGEMENTS

The company Solcera® contributed to this work by providing the different powders used and by sintering the parts. We thank Eazao for this technical support.

## Funding:

This work as been funded by UNICAEN PhD funding number: "Allocations Etablissements 2023 (2023) 2023050211502900001220000257"

## Data

All data are given in the graph of the article

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