# Pneumatic dispensing of aqueous Al O suspensions via material extrusion additive manufacturing

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

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Flavie Lebas, Sylvain Marinel, Christelle Bilot, Romuald Herbinet, Loïc Le Pluart, et al. Pneumatic dispensing of aqueous Al O suspensions via material extrusion additive manufacturing. Next Materials, 2025, 8, pp.100750. 10.1016/j.nxmate.2025.100750. hal-05131410

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Submitted on 26 Jun 2025

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# Pneumatic dispensing of Aqueous Al₂O₃ Suspensions *via* Material Extrusion additive manufacturing

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

Material extrusion, also known as Robocasting or Direct Ink Writing (DIW) is an efficient and eco-friendly additive manufacturing method for fabricating large and thick ceramic components, such as alumina. While screw-based extrusion is the prevalent approach due to its precise control over printing parameters, pneumatic-based extrusion is gaining attention for its suitability in automated processes and reduced handling requirements. However, pneumatic extrusion faces challenges, primarily the narrower range of printable rheological behavior. Achieving a balance between extrudability and the self-supporting capacity of printed structures is critical. Moreover, bubble elimination, easily addressed in screw-based extrusion, remains a significant challenge in pneumatic systems. To overcome these issues, optimal concentrations of dispersants, ceramic loading, and additives were determined to achieve the desired shear-thinning rheological properties suitable for pneumatic extrusion. Additionally, this study, by employing a rapid one-step ball milling method for suspension preparation, is somewhat unique, providing a streamlined alternative to traditional processes. A degassing study was conducted to minimize bubble formation, ensuring print quality. Using the optimized parameters, including extrusion pressure and layer height, complex structures were successfully printed. After sintering at 1450 °C, the samples exhibited a 99.5% dense microstructure with an average grain size of 1.71 µm. The alumina demonstrated excellent mechanical properties, achieving a hardness of 2094 HV at 1450°C, which is well aligned with the literature.

## GRAPHICAL ABSTRACT

<figure><img src="image_2.png" /></figure>

<footnote n="1">¹Université 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>
**Keywords**

Al₂O₃ Ceramic, Additive Manufacturing, Material Extrusion, Rheology, Bubble Elimination, Ball Milling.

# 1. INTRODUCTION

Technical ceramics possess remarkable properties such as thermal stability, chemical inertness, excellent wear resistance, good mechanical strength, and high hardness [1]. Alumina (Al₂O₃) is a technical ceramic widely used in various industries, including aeronautics, bioengineering, dental applications [2], high-temperature filters, thermal barriers, fuel cells, and electrical systems, due to its versatile properties [1,3,4]. Conventional manufacturing methods (such as pressing, molding, injection molding) do not meet the requirements for designing complex shapes and high-precision manufacturing [1,3]. The development of 3D printing since the 2000s, along with the growing demand for technical ceramics with complex geometry, has significantly increased research into the additive manufacturing (AM) of ceramics. In contrast, AM technologies offer many advantages, including flexible design freedom, the ability to produce complex geometric products in a single print, and reduced waste generation [1]. In particular, AM can produce shapes with porous lattice structures that cannot be achieved through green machining or other previously mentioned processes [5,6]. AM, combined with topology optimization tools, represents a highly powerful approach for producing optimized, lightweight shapes [7–10]. There are many AM processes, and the choice of the appropriate process highly depends on the required specifications, such as density, surface finish, size, and geometrical complexity, as well as the nature of the ceramic [11,12].

Material extrusion AM, also known as Direct ink writing (DIW) or robocasting, is an additive manufacturing (AM) technique that uses a syringe to extrude a filament of suspension or “ink” to create green parts. The process builds these parts layer by layer, guided by a Computer-Aided Design (CAD) model that has been transformed by slicing software. The advantage of material extrusion includes its compatibility with highly viscous aqueous ceramic suspension with low organic content (less than 2%), and the ability to use different nozzle sizes (0.4-2mm) [13]. While this method enables the rapid production of large components, it offers lower resolution (400µm-1mm) compared to other technologies, such as vat photopolymerization (10-100µm) [14]. This simple, cost-effective, efficient one-step extrusion technology shows significant promise for producing parts with varying thicknesses and sizes. Additionally, due to its low content of organic compounds, which reduces the need for extensive debinding, material extrusion is considered one of the most environmentally sustainable AM processes, contributing to reduced carbon footprint. This characteristic also makes material extrusion particularly efficient for producing large and thick parts compared to vat photopolymerization, where debinding cracks limit wall thickness to about 3 mm [15].
Finally, material extrusion offers great flexibility in the materials that can be printed, such as alumina, clays and silicon nitride, as demonstrated in the review [13].

The difficulty of material extrusion lies in the preparation and properties of the suspensions. The suspensions must be homogenous, free of agglomerates and air bubbles, and exhibit a printable rheological behavior. The latter must be shear-thinning with a threshold stress to ensure proper flow through fine printing nozzles (once the threshold is surpassed) and to retain filament shape immediately after printing (once the stress is removed) [1,16–18]. Aqueous ceramic suspensions are generally highly loaded with ceramic material. The organic content must be low (<2%), with low toxicity, and should ensure particles dispersion, binding after drying while preventing drying cracks, and enable rapid debinding [19].

In this study, we develop and utilize a one-step planetary ball milling process, in which all suspension components are mixed in a single ball milling step. In contrast, the literature often describes lengthy conventional processes that require multiple mixing stages with different mixers, often separating the gel and powder preparations, making them time-consuming [20–23].

In the literature, the DIW has been widely used for manufacturing both porous alumina parts [24–26] and dense parts [17, 20, 27, 28]. Many professional equipment manufacturers choose material extrusion via screw-based dispensing (e.g., Wasp, StoneFlower, Rapidia, Lutum-Vormvry, Lynxter, 3Dminerals) as it ensures better control over material delivery and effectively eliminates bubbles. In comparison, pneumatic extrusion (which uses gas pressure) is technologically simpler in terms of loading, cleaning, and cost but presents significant challenges in controlling flow discontinuities. Figure 1 summarizes these two technologies. In direct pneumatic dispensing, the availability of professional equipment is more limited (e.g., 3Dceram, EazaoBio). Additionally, punch dispensing (e.g., Hyrel3D, 3Dpotter) serves as an intermediate solution in terms of flow control challenges. The main issues are the elimination of bubbles in the suspension and the very restricted rheological printing conditions, which result in challenging pressure control and monitoring. In screw-driven extrusion, the intense shearing often comes with a system capable of eliminating large bubbles, unlike pneumatic dispensing methods, where the suspension is directly inserted into the syringe. In both methods, the best approach is to eliminate bubbles at the suspension preparation stage using centrifugal, vacuum, or other methods [20, 29, 30]. The aim of this work is to develop our own Al₂O₃ suspension and a very simple pneumatic syringe process to print defect-free parts. In this study, the rheological behavior of clay is used as a reference for printability. Clay possesses a layered structure, which provides good plasticity [31] and makes it an excellent suspension for material extrusion. Some researchers and organizations, such as 3D Minerals, ceramic artist and other with kaolin [32–36], have already explored this. After achieving the desired rheological behavior in the Al₂O₃ suspension, the study focused on optimizing printing
parameters like pressure and layer height to achieve optimal printing results. The challenge lies in adapting the suspension for pneumatic extrusion. The ultimate goal of the work is to propose a ready-to-use suspension fabrication procedure that allows for printing similar parts as easily as with screw-based processes, which often suffer from longer, mostly manual preparation steps, including material feedstock loading, cleaning and maintenance.

<figure><img src="image_3.png" /><figcaption>Fig. 1. Screw-driven and pneumatic dispensing technology</figcaption></figure>

## 2. MATERIALS AND METHODS

### 2.1 Raw materials

The alumina powder used is from Baikowski, (BWP grade). This powder is trimodal, and contains spherical agglomerates ranging in from 1 µm to 100 µm. The elementary alumina particle size ranges from 0.1 µm to 1 µm [37]. The particle size was checked using laser granulometry (Mastersizer 2000, Malvern, UK) and scanning electron microscope (SEM) (JSM-7200F, JEOL, Japan). The powder was ground to produce a more uniform particle size distribution and to break down larger agglomerates, facilitating the suspension preparation and printing process. The grinding was carried out using attrition for 3h by mixing 400 cm³ of powder with 600 cm³ of zirconia balls in distilled water medium. The zirconia balls and powder are mixed in a large container using a paddle. The mix (water and powder) is then collected, and the water is evaporated to obtain the dry powder. The particle size distribution of the final
product was measured using laser granulometry and observed with scanning electron microscopy (SEM).

One of the most important organic additives is the dispersant. It is used for stabilizing the suspension by increasing the repulsive forces between particles. This prevents particle agglomeration and sedimentation, which could occur in the syringe. There are different types of particle-particle repulsion mechanisms, including electrostatic and steric repulsion. The dispersant functions allow both electrostatic and steric repulsion [13]. The amount of dispersant must be optimized to ensure that the surface of the particles is fully covered, providing adequate repulsion and preventing agglomeration. However, if the content of dispersant is too high, it can lead to agglomeration resulting in high viscosity [38–40]. A compromise in terms of dispersant content must be found as illustrated in Fig. 2. In our study, Dolapix CE 64 (Zschimmer & Schwarz, Germany) was selected because it is well established that it efficiently disperses alumina powder [41–44]. The potassium hydroxide (KOH) (Sigma-Aldrich-Merck, Germany) was added to stabilize the suspension (at around pH 9).

<figure><img src="image_4.png" /><figcaption>Fig. 2. Impact of particle coverage on electrostatic repulsion and agglomeration</figcaption></figure>

The cohesion and strength of the green parts after drying are ensured by the binder. Binders usually consist of long polymer chains that are adsorbed into the particle surfaces, and act as viscosifying agents. It helps to regulate viscosity and shear-thinning behavior by orienting the molecules during the shear gradient [38,45]. In this study, the polyvinyl alcohol (PVA) (OTAPIX PA 4G, Zschimmer & Schwarz, Germany) was used [44]. In addition to binders, plasticizers can be added to increase the flexibility and handling of green parts during drying, thereby preventing the formation of cracks. In this perspective, Poly(ethylene glycol) (PEG) (Mn=400) (Sigma-Aldrich-Merck, Germany) was added to prevent the formation of cracks during drying [38,45]. Carboxymethylcellulose (CMC, Sigma-Aldrich-Merck, Germany) is a multifunctional additive (binder, viscosifying agent, etc..) [46,47], used to significantly
increase viscosity and stabilize the suspension. Therefore, CMC is used as a strong viscosifying agent, and its content is calculated along with the water content.

## 2.2 Preparation and rheological measurements of suspensions

Inks with shear-thinning behavior and yield stress are important for ensuring a smooth flow through fine printing nozzles while maintaining filament shape immediately after deposition. Additionally, these properties allow the lower layers to be strong enough to support the weight of the upper layers during printing [16].

All powders, including the CMC powder, were weighed, placed in a 100 mL agate bowl and manually mixed before adding the water and additives (Dolapix CE64, PEG, PVA, KOH), which had been previously weighed in a beaker. The mixture was manually mixed before the addition of agate balls. The bowl was then put in a planetary mono mill grinder (Pulverisette 6, Fritsch, Germany) for 30 min, rotating at 350 rpm. At the end of this unique mixing stage, the suspension is ready and can be collected with a Maryse and stocked in a plastic bottle. The mixing often involves separate gel and powder milling and then final mixing [23,29]. We will show the single ball milling can at the same time break powder agglomerates, and homogenize the liquid phase viscous agents. The suspension was finally mixed in Speedmixer® (DAC 150.1 FV, Hauschild GmbH&Co.KG, Germany). This step allows for degassing using Dual Asymmetric Centrifugation (DAC) technology (Dual Asymmetric Centrifugal), which rotates at 3500 rpm for 10 seconds [48]. We will see later that this speedmixer was removed as it can generate micro-bubbles.

The optimal content of dispersant was studied by testing the viscosity of suspension containing 22.5% vol of ceramics with different amounts of Dolapix CE64 (0 to 5 wt% based on total solids content). To develop a suitable suspension for material extrusion, its viscosity must be optimized to match the rheological properties of printable clay suspension. The contents of PVA, PEG and KOH were kept constant in the composition of the suspensions.

Solid loading was initially adjusted by preparing suspensions containing 22.5% vol, 30% vol, 35% vol, 40% vol and 45% vol of ceramic powder. All suspensions were then characterized. Thereafter, different amounts of carboxylmethylcellulose were tested (0 wt%, 1 wt% and 2 wt% based on the amount of water). Many compositions were tested to find an optimal balance between the amount of solid loading and the amount of CMC.

A rotational rheometer (AR1000, TA instruments, USA) was used to measure the viscosity of suspensions. Measurements were taken with a shear rate ranging from 0.1 - 100 (1/s) to 10-100 (1/s), using a plate-plate configuration (Fig. 5), at room temperature for a short duration (~2m30) to prevent the slurry from drying during the measurement. Indeed, different measurements were conducted over various time intervals for a suspension containing 40% vol
of solid loading to determine the optimal measurement time that avoids drying, which can
cause significant noise in the data.

**2.3 Exploration of suspension bubbles eliminations**

The pneumatic dispensing approach is particularly sensitive to the presence of bubbles, which
can negatively impact the final product quality, as demonstrated in the results section during
the layer height study. To address this, samples of the final suspension were taken and
sintered after each processing step (planetary mono mill and speedmixer) to characterize the
evolution of bubble formation by measuring bubble-induced porosity using optical microscopy.
Following the speedmixer stage, a degassing step (commonly used in some studies [30]) was
introduced to remove any remaining bubbles and prevent the drying of the suspension. This
step is particularly important, as the speedmixer generates heat during mixing, which could
accelerate solvent evaporation and affect suspension stability. The suspension was then
placed in a vacuum chamber, where a pump created a vacuum, causing bubbles to rise and
be eliminated over the course of 20 minutes. After the degassing process, the suspension was
reprocessed using the speedmixer to ensure uniform consistency, as some bubbles might
migrate to the surface without being fully eliminated. Thus, the study systematically explores
bubble formation and elimination at different stages: after ball milling, after speed mixing, after
vacuum degassing, and following an additional speed mixer stage.

**2.4 Printing of parts by material extrusion**

The alumina parts were printed using the Eazao Bio 3D printer (Eazao, China) shown
in Fig.3.a). This printer operates with pneumatic dispensing system, utilizing a syringe
controlled by a manometer and a switch. The suspension held in a 30 cm³ syringe, is extruded
through a 1 mm diameter nozzle at speed of 20mm/s. Additionally, the printer can be equipped
with an air control unit (Fig.3.b)) to enable non-continuous flow printing [49]. The extrusion flow
rate was optimized by adjusting the pressure *via* the manometer, ensuring a uniform and
continuous deposition. A test was systematically performed before printing each part.
<figure><img src="image_5.png" /><figcaption>Fig. 3. Eazao Bio a) Printer, switch and manometer, b) the air controller unit, c) the CAO draw, d) the filling simulation, e) the printing</figcaption></figure>

The parts were designed (Fig.3.c)) and exported as an STL file using the FreeCAD software. To configure printing parameters, including layer height and infill strategy, Cura Ultilmaker slicing software was employed. This software also provides a visualization of the layer-by-layer construction and infill patterns, as shown in Fig.3.d). The GCode file generated was saved into a SD card, which was subsequently inserted into Eazao Bio printer for production.

The layer height ($H_l$) is an important parameter that affects the adhesion of the filaments to the support, their adhesion to each other, and ultimately on the density of the part. The layer height must be inferior or equal to 70 % of the nozzle diameter ($Ø_{nozzle}$) to obtain a dense part [1, 13, 50, 51]. Indeed, the layer height depends on the nozzle diameter, which significantly affects the surface quality of the printed structure. When the layer height is equal to the nozzle diameter, the filament is not compressed. There are also inter-rod pores that lead to porous parts. On the other hand, when the layer height is 70% or less of the nozzle diameter, the filaments are compressed, which helps reduce the inter-filament pores.

Bars were printed with different layers to determine the optimal layer height for our suspension. After the bars were sintered, a cross-section of each bar was observed by optical microscopy (BX53M, Olympus, Japan).

With the proper settings applied, different parts were printed. A hexagonal twisted tower was printed to assess the adhesion of the first filament on the support, as well as the threshold stress and viscosity of suspension. Indeed, this part involved superimposing many layers onto a single base layer. At the same time, the twisting of the filaments superposition was slightly staggered. A second type of part was printed: a double gear (Fig.3.e)). This complex shape was used to validate the ability of our suspension to print thick parts. The filling was chosen to allow to first gear can support the second gear above. All parts were left dried at least one night at room temperature.
**2.5. Sintering**

Parts were sintered using conventional sintering according to the thermal cycles previously designed by Jérémy Croquesel et al. [37]. Different sintering temperatures were tested: 1350°C, 1450°C, 1550°C. The debinding stage is included in the sintering thermal cycle because the organic content is low (<3wt%).

**2.6. Characterization of parts**

The sintered parts microstructures were observed by optical microscopy (BX53M, Olympus, Japan) and SEM (JSM-7200F, JEOL, Japan), with a backscattered electron detector (BSE). Energy Dispersive Spectroscopy (EDS) (X-Flash 6-60, Bruker, Germany) was used to map the chemical elements which are present. The hardness was characterized by Vickers micro-indentation (MMT-X7B, Matsuzawa, Japan) using 1kgf load. The fracture toughness is measured using Vickers indentation at a 1 kgf load, following the Lankford model [52]. Approximately 9 hardness measurements were taken at the core of the parts for each temperature sintering. 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 with a stereological factor of 1.56 [52]. For porosity measurement, threshold image processing was employed to determine the proportion of porosity within the parts.

# 3. RESULTS & DISCUSSION

**3.1. Powders and grinding**

Fig.4 shows the particle size distribution of the BWP powder. Three modes were detected. The largest mode centered at 70 µm is too large as it could clog the extrusion nozzle. Indeed, SEM microstructures shown in Fig 4.a), reveal that the BWP alumina contains spherical agglomerates, some of which are coarse enough to obstruct the nozzle during printing. The finest particles, which form agglomerates, are shown in Fig4.b). Their size is submicronic. After attrition, the particle size distribution is presented Figure 4 (green curve). This grinding step allowed for the breakdown of agglomerates, resulting in a single-mode distribution centered at approximately 0.185 µm.
<figure><img src="image_6.png" /><figcaption>Fig. 4. Particle size distribution of alumina BWP and SEM observation (a) Agglomerates (b) Particles</figcaption></figure>

## 3.2 Rheology of the slurries

When using the plate-plate rheology test, in situ drying presents a significant challenge. This issue is exacerbated when the solid content of the suspension is high, as a higher solid loading accelerates the drying process. To assess the optimal measurement duration on the shear rate range 0.1-100 s⁻¹, different measurement durations have been tested, including 10 min, 5 min and 2min30s. As shown in Fig. 5.a), decreasing the measurement duration helps minimizing measurement noise (with no noise at 2 min 30 seconds), without affecting the viscosity curve.

The optimal dispersant concentration is first determined to achieve a stable suspension. This was done by measuring the viscosity versus shear rate for a 22.5% vol solid loading suspension with different amounts of dispersant. In Fig. 5.b), it can be seen that, regardless the amount of dispersant, the suspensions exhibit shear thinning behavior (i.e., viscosity decreases with increasing shear rate). Fig. 5.c) shows the viscosity versus dispersant content for different shear rates. The shear rates of 10 and 100 s⁻¹ represent the extreme values of printability range, while 20 s⁻¹ and 50 s⁻¹ respectively correspond to, the shear rate in the 1.2 mm and 0.6 mm diameter nozzles. These curves show that a minimum viscosity is obtained with 2 wt% of dispersant (at the beginning of the dwell), suggesting that this concentration is optimal. Viscosity often increases with excess dispersant [41] due to dispersant-dispersant interactions. However, in this case, the excess dispersant does not significantly increase the suspension, which helps stabilizing the suspensions.
Fig.5.d) illustrates the viscosity as a function of the shear rate for suspensions with varying solid loadings and a quantity of dispersant of 2wt%. The suspensions exhibit shear-thinning behavior, regardless of the ceramic loading content. It is observed that higher solid loading results in increased viscosity. The suspension with a solid loading of 40%vol exhibits rheological properties very similar to those of clay. Therefore, the formulation was optimized with 45%vol. To achieve the target viscosity, a compromise must be found between the amount of solid loading and the CMC content. As shown in Fig.5.e), viscosity increases with the amount of CMC. To reach printable conditions similar to those of clay while ensuring a stable suspension, 2wt% of CMC (relative to the total water content) was selected.

<figure><img src="image_7.png" /><figcaption>Fig. 5.a) Influence of measurement time on viscosity measurement, b) Viscosity of suspension for different dispersant concentration, c) Determination of optimal concentration of dispersant, d) Impact of solid loading on viscosity, e) Effect of CMC on the viscosity</figcaption></figure>
The final composition of Al₂O₃ suspension suitable for material extrusion with 2,83wt% of organic is presented in **table 1**. We clearly show that the single ball milling step can be used to efficiently homogenize all suspension components without the need to separate the gel preparation and the powder.

<table><caption>Table 1. Detailed composition of the final suspensions</caption><thead><tr><th>Materials</th><th>Alumina powder</th><th>Water</th><th>Dolapix CE 64</th><th>PVA</th><th>PEG</th><th>KOH</th><th>CMC</th></tr></thead><tbody><tr><th>(vol%)</th><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></tr></tbody></table>

### 3.3. Print Settings and success of printing parts

Once the final suspension composition was found, the printing parameters need to be established. The first parameter is the pressure, which depends on both the suspension properties and the nozzle size. Various gas pressure tests were conducted to identify the optimal pressure for achieving continuous filament extrusion with a consistent width. As seen in Fig.6, area 2 was printed with the optimal pressure value, resulting in a homogeneous and continuous filament. If the flow rate is too high, it results in over-extrusion, while a flow rate that is too low leads to discontinuous deposition.

<figure><img src="image_8.png" /><figcaption>Fig. 6. Gas pressure calibration tests to adjust the extruded flow.</figcaption></figure>

By printing rectangular parallelepiped, referred to as bars, different layer heights (H<sub>L</sub>) were tested with 1 mm nozzle diameter: 0,5 mm (50% of Ø<sub>nozzle</sub>), 1 mm (= Ø<sub>nozzle</sub>) and 0.7 mm (70% of Ø<sub>nozzle</sub>). The height of 0.5 mm is unsatisfactory (Fig.7.a), because the filament is overly compressed, which compromises the build but we can get around the problem by adjusting the pressure and consequently the flow. In the case of 1 mm (Fig.7.b), the filament was continuous with a consistent width. However, many layers are not connected to each other, creating inter-filament pores, as observed under the microscope (Fig.7.b). In the final case, where H<sub>L</sub>=0.7 mm, the bar was printed optimally. The filaments are pressed just enough to eliminate diamond-shaped pores without causing overflow, which could compromise the resolution of the parts. The chosen layer height was 0.5 mm (50% of Ø<sub>nozzle</sub>) to ensure optimal
printing while allowing for flow adjustments to prevent overflow. In these studies, both the gas pressure, which controls the extruded flow, and the layer height are interrelated parameters that need to be adjusted carefully.

<figure><img src="image_9.png" /><figcaption>Fig. 7. Influence of layer height on printing: case a) $H_L$= 50% of $Ø_{nozzle}$, b) $H_L$=100% of $Ø_{nozzle}$ c) $H_L$= 70% of $Ø_{nozzle}$</figcaption></figure>

The cross-sectional view of these bars reveals the presence of bubbles. These bubbles may originate from the composition, which includes various additives. They may also originate during the loading of the viscous suspension into the syringe. This issue is often observed in pneumatic extrusion processes, whereas screw-based extrusion processes are much more effective at removing bubbles. Addressing this issue represents a valuable endeavor as the pneumatic extrusion is a simple process that can be more easily automatized with less maintenance. Nevertheless, a dedicated study on degassing was conducted to minimize or, at least reduce, bubble size.

After mixing with the mono mill planetary grinder (Fig.8.a)), numerous large bubbles are clearly observed in the sample, with a measured porosity of 11 %. When using the speed mixer subsequently, more uniformly distributed bubbles are observed (Fig.8.b)). Additionally, the use of the speed mixer refines the size of the bubbles, but also leads to the formation of micro-bubbles. The porosity is uniform, decreasing to 6%. Afterward, the degassing step is effective as porosity decreases to 1% (Fig.8.c)). However, the degassing process is challenging because the suspension tends to dry out during it. As a result, the degassing step is difficult not fully reproducible, but it appears to be a promising strategy for further removing
bubbles. It is worthy noting that an additional speed mixer step does not help, as it produces micro-bubbles, as seen in Figure 8.d). A slight increase in porosity after degassing is even noticed.

<figure><img src="image_10.png" /><figcaption>Fig. 8. Crossed section of parts after different successive milling degassing process: a) After planetary mono mill, b) After speedmixer, c) vacuum degassing 20 min, d) after an additional speedmixer step.</figcaption></figure>

Despite the presence of some microbubbles, various complex parts were successfully printed. The hexagonal twisted tower shown in (Fig.9.a) demonstrates the ability to print thin and high monolayer hollow structures. As it can be seen, the filament shape is uniform, continuous and precise, while the layering of filaments, which are slightly staggered, is well-executed. The gear was printed quite precisely with a concentric filling (Fig.9.b)) to support the second gear above. Some filaments are visible outside the gear due to the discontinuous flow printing mode. It could be interesting to use the air controller unit to print in a non-continuous flow mode and stop the extrusion when the nozzle moves outside the part.
<figure><img src="image_11.png" /><figcaption>Fig. 9. Green part: a) Hexagonal twisted tower, b) concentric filling printing, c) double gear</figcaption></figure>

**3.4. Alumina sintering and characterization**

The results of our sintering part are presented and compared to the initial green part in Fig.10 Shrinkage is visible, and no cracks or collapses occurred during the sintering. The surfaces of the parts were smooth and visually very shiny.

<figure><img src="image_12.png" /><figcaption>Fig. 10.a) green part and b) sintered part</figcaption></figure>

Fig.11 presents the characterization of the three materials sintered at different temperatures, including porosity, grain size, hardness and fracture toughness. Sintering at 1350°C is not suitable for producing a dense microstructure. As shown in the SEM image (a)
in Fig.12, the microstructure is porous, with a porosity of 19.13%. Therefore, the hardness value is low, achieving 1150 HV. This result clearly indicates that the densification is too low to provide adequate mechanical properties. The microstructure obtained after sintering at 1450°C (Fig.12.b)), reveals a nearly fully dense ceramic (porosity ~0.5%) that exhibits a high hardness value of 2094 HV. This high value can be correlated to the fine microstructure observed (grain size ~ 1.71 µm). If the sintering temperature is increased up to 1550°C (Fig.12.c)), the porosity slightly still decreases down to 0.4 %, but grain growth begins to occur (grain size ~ 3.13 µm). Simultaneously, the hardness slightly decreases to 1900 HV, as predicted by the Hall-Petch relationship applied to alumina [53]. An optimal microstructure consists of full densification with grains as small as possible. For grain size of 1.71 µm, the hardness value in the literature is around 1835 HV [54]. For hot isostatic pressing, a hardness of 1957 HV was reported [55], and 1886 HV for vacuum sintering [56]. Our value of 2094 HV suggests that zirconia pollution acts as a reinforcement. In comparison with other material extrusion techniques, reported hardness values of 1675 HV at 1450°C, 1880 HV at 1550°C[20], 1551 HV at 1500°C [28] and 1896 HV at 1550°C [19] have been obtained. Figure 11 clearly shows that, at 1450°C, our results exhibit a higher hardness than the Hall-Petch value (likely due to the zirconia reinforcement), while at 1550°C, our values are within the range reported in the literature for material extrusion, with only the HIPed hardness being superior. Regarding Vickers toughness, we obtained similar values of 4.1 MPa·m¹/² at both 1450°C and 1550°C. In the literature, a toughness of 2.5 MPa·m¹/² was reported at 1600°C with a hardness of 1700 HV [20], while a value of 3.31 MPa·m¹/² was recorded at 1500°C with a hardness of 1900 HV [19]. Therefore, our result is well within the expected range.
<table><thead><tr><td>T<sub>sintering</sub> (°C)</td><td>Porosity (%)</td><td>Grain size (μm)</td><td>Hardness (HV<sub>1</sub>)</td><td>K<sub>IC</sub> (MPa.m<sup>1/2</sup>)</td></tr></thead><tbody><tr><td>1350</td><td>19.13</td><td>0.36</td><td>1150 ± 28</td><td>x</td></tr><tr><td>1450</td><td>0.50</td><td>1.71</td><td>2094 ± 158</td><td>4.1 ± 0.7</td></tr><tr><td>1550</td><td>0.40</td><td>3.13</td><td>1900 ± 52</td><td>4.1 ± 0.6</td></tr></tbody></table>

<figure><img src="image_13.png" /><figcaption>Fig. 11. Table of characterization of Al₂O₃ sintering for different temperatures, b) Histogram of comparison of Vickers Hardness at different sintering temperatures: our research vs. the literature</figcaption></figure>
<figure><img src="image_14.png" /><figcaption>Fig. 12. SEM microstructure of different sintering: a) 1350°C, b) 1450°C, c) 1550°C and EDS analysis</figcaption></figure>

The composition of the white particles seen in all microstructure (Fig. 12) was determined using EDS. The x-ray mapping is presented in Fig. 12. White particles (pointed by orange circle) are identified as zirconium rich phase. The EDS spectrum of the orange circle area is presented in Fig. 12. The peak of zirconium is clearly detected and is present at 5,2% atomic. The presence of zirconium is likely due to zirconia contamination, as zirconia balls are used for attrition.

The final chosen sintering cycle is as follows: heating at 180°C/h up to 1450°C. The slow heating ramp ensures a smooth debinding step to prevent cracks. The dwell time was 3 hours, and a cooling ramp of 600°C/h was used to reach room temperature.
## 4. CONCLUSIONS

The transition from screw-based extrusion to pneumatic-based extrusion is challenging due to the need for suspensions with specific rheological properties (shear-thinning behavior with a threshold) that are free of bubbles and agglomerates, enabling rapid printing of large, thin, self-supporting complex parts. An eco-friendly aqueous alumina suspension with low organic content (<3 wt%) was developed using attrition milling to de-agglomerate the powder, followed by a one-step ball milling process to mix all components of the suspension. This suspension successfully printed parts using pneumatic extrusion, and by adjusting parameters (pressure and layer height), even in the presence of detected bubbles. A degassing approach reduced porosity from 11 % to 1 %. The resulting mechanical properties are comparable to, and slightly improved over, those of conventionally shaped and printed parts, with a hardness of 2094 HV and a dense microstructure (0.5 % porosity and grain size of 1.71 µm). The inherent zirconia contamination from ball milling appears to act as reinforcement.

## ACKNOWLEDGMENTS

We thank Eazao for its 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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