# Debinding and sintering optimization of stereolithography based silicon nitride parts for attaining centimetric wall-thickness shapes

Théotim Marie, Zehui Du, Chee Lip Gan, Sylvain Marinel, Vijay Shankar

Sridharan, Charles Manière

► To cite this version:

Théotim Marie, Zehui Du, Chee Lip Gan, Sylvain Marinel, Vijay Shankar Sridharan, et al. Debinding and sintering optimization of stereolithography based silicon nitride parts for attaining centimetric wall-thickness shapes. Journal of the European Ceramic Society, 2025, 45 (2), pp.116911. 10.1016/j.jeurceramsoc.2024.116911. hal-04738296

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# Debinding and sintering optimization of stereolithography based silicon nitride parts for attaining centimetric wall-thickness shapes

Théotim Marie¹,²*, Zehui Du³, Chee Lip Gan³, Sylvain Marinel¹, Vijay Shankar Sridharan², Charles Manière¹

¹ CRISMAT, ENSICAEN, UNICAEN, Normadie Univ, 6 Bd Maréchal Juin, 14050 Caen Cedex 4, France

² School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore

³Temasek Laboratories, Nanyang Technological University, 637553, Singapore

**Corresponding authors***: Théotim Marie [theotim.marie@ensicaen.fr](mailto:theotim.marie@ensicaen.fr)

**Abstract** : Stereolithography of UV-curable ceramic suspensions is an additive manufacturing technique with high precision and great resolution to fabricate complex ceramic parts. While it widens the possibilities of applications, one of the drawback of this method is the low wall-thickness of the parts. The polymers forming the network structure upon cross-linking undergo pyrolysis in a step called debinding. During debinding, the gaseous compounds going through evacuation channels create internal pressures, often resulting in crack formation. So far, the critical wall-thickness where crackfree parts are obtained is located around 4 millimeters for silicon nitride. This paper reports the successful debinding of silicon nitride parts obtained by stereolithography. Thanks to an optimization of the debinding relying on TGA analysis, defectless parts with a wall-thickness of up to 11 mm were obtained, yielding parts of 9 mm after sintering. The mechanical properties were measured, showing values equivalent to silicon nitride obtained through conventional methods.

**Keywords** : Stereolithography; Silicon nitride; Debinding; Wall-thickness; Hot Isostatic Pressing

## 1. Introduction

Silicon nitride is a material that saw its development accelerated in the 1980s, when it was acknowledged that it could be used as an advanced structural ceramic[1]. Silicon nitride is a material that is used in a broad range of applications from medical field[1] to automotive parts[2], ceramic bearings and cutting tools[3] or even space applications[4]. It is also widely considered as very promising for future applications thanks to its thermal properties such as heat resistance, high thermal shock resistance, low coefficient of thermal expansion, but also due to its mechanical properties such as high hardness, high wear resistance, high creep
resistance, high fracture toughness or also high bending strength for a ceramic[4-6]. However, when using conventional machining methods, those mechanical properties make the fabrication of complex shapes arduous and require costly post-processing steps[7]. Therefore, the recent developments of additive manufacturing are quite appealing as a new method to manufacture complex-shape silicon nitride parts.

First introduced in the 1980s for model making and rapid prototyping, additive manufacturing has been largely developed as a flexible technology for Computer-Assisted Design (CAD)[8]. Especially in the ceramic field, stereolithography is one of the most promising additive manufacturing technique to fabricate structural parts with a high resolution[9-12]. In stereolithography, the first step is to import a 3D file in the device and set parameters such as layer thickness, laser power or exposure time. Then, a slurry containing ceramic powder, monomers, dispersant and photoinitiators is cured upon exposition of UV light. Thanks to monomers with different functionalities, a crossed-link network that serves as structure for the whole part is formed[12-14]. Therefore, parts obtained through stereolithography are made of thin layers of cured slurry on top of each other.

Although stereolithography and additive manufacturing in general enable the fabrication of complex shape parts, a downside of stereolithography is the limitation of the wall thickness of those parts. Stereolithography is considered a near net-shape technique, meaning there is almost no machining of the parts required after printing. However, it is not free of post-processing. In order to obtain a fully dense ceramic part, the organics are removed during a thermal treatment called debinding, before the final step of sintering. During debinding, the binders are pyrolyzed and are evacuated as gaseous compounds through the creation of interconnected channels[15]. This has the effect of creating an internal pressure within the parts, resulting in the formation of cracks. The thicker a part is, the more likely it is to observe cracks after debinding. Consequently, a maximum wall thickness must be respected to ensure the achievement of defect less ceramic parts. While the minimum wall-thickness has been widely studied with reports of wall thicknesses decreased down to 250 µm for silicon nitride[16,17] and 150 µm for other ceramics[18,19], the amount of studies regarding the maximum wall thickness is not as broad. Sun et al. have achieved the fabrication of high-quality translucent alumina ceramic by using stereolithography method, but they limited themselves to parts with wall thickness no higher than 2 mm to avoid unnecessary cracking[20]. Xu et al. have observed cracking on hollow cubes made of silica of a size of 25 mm on the edge and a wall thickness of 4 mm[21]. There are reports stating that the maximum wall thickness is approximately 10 mm for alumina and silica and that the debinding process already spans over several days for this size[22,23]. An emphasis has to be put on the fact that non-oxide ceramics inevitably require long debinding treatment, with slower heating rate and longer holding times, as described by Zhou et al. [24]. Wang et al. have fabricated Si₃N₄ standard sample strip by Digital Light Processing (DLP) for testing bending strength. They have tested several heating rates for the debinding of their parts and have found that 0.1°C/min was the only heating rate that did not yield cracks. In addition, they have also stressed that further refinement of the debinding temperature program should be done for large, complex geometries of green parts[25]. As of now, the
maximum wall thickness of silicon nitride is yet to be studied. This could be due to the high refractive index (n = 2.4) of silicon nitride, causing the curing depth of the slurry to be quite low. By rending the stereolithography fabrication of silicon nitride difficult, the amount of reports of silicon nitride with this technique is still low[5]. The other consequence of this parameter is the likeliness of having residual uncured monomers in the printed part, which affects the debinding. There has been multiple studies on the presence of uncured monomers, demonstrating that it evaporates quickly during the early stage of debinding, therefore increasing the risk of crack formation[26-29]. As silicon nitride is used for structural applications, the low wall-thickness achievable by stereolithography is a limiting factor for the use of such technology. Therefore, increasing the wall thickness of ceramic parts with good properties would expand the fields in which the advantages of stereolithography can be brought to produce silicon nitride parts.

In this paper, we present a method to optimize the debinding thermal profile, based on the study of thermogravimetric analysis. Silicon nitride parts fabricated by stereolithography, with centimetric wall-thickness were successfully debound. These parts were sintered and their microstructure was investigated. The mechanical properties such as hardness, fracture toughness and bending strength were measured. Complex shapes like honeycombs, gears and lattice structures were fabricated, successfully debound and sintered.

## 2. Materials and methods

### 2.1 Ceramic slurry and stereolithography

The ceramic-filled slurry was provided by 3Dceram, which is designed for their C-900 FLEX stereolithography device that was used to fabricate the green parts. The silicon nitride powder used to make the slurry contains additives : Al₂O₃ and Y₂O₃. This slurry is a paste formulation, with a solid loading of about 50 vol%. The size distribution of silicon nitride particles is bimodal. The first mode is centered at a size of 1.1 µm and the second one is centered at a size of 11.5 µm (Fig. 1). Nanometric particles help the densification during sintering. Micrometric particles are also necessary to obtain suitable rheological properties for stereolithography, hence the use of a bimodal distribution. In order to study the critical wall-thickness, cubic parts with wall-thicknesses ranging from 5 to 15 mm were designed and printed. To measure the flexural strength, bars of a size of 62.5x7.5x3.75 mm³ were printed. This size was chosen to obtain specimens of a size of 50x6x3 mm³ to perform 4-point bending test, by taking into account the final shrinkage after debinding and sintering. The 3D file included 0.15mm-deep chamfers to avoid machining on sintered samples.
<figure><img src="image_2.png" /><figcaption>Fig. 1 Grain size distribution of silicon nitride particles used for the paste</figcaption></figure>

## 2.2 Debinding and Sintering

The debinding was performed in N₂ atmosphere in a tubular furnace which allows very low heating rate (Carbolite, STF 15/450, UK). The debinding experiments were performed on a “line of cubes” which refers to a line of 6 cubes of different size edge : 5 ; 6 ; 7 ; 9 ; 11 and 15 mm. Then, the parts were placed in a BN crucible and were sintered by Hot Isostatic Press method, using AIP10-30H (American Isostatic Presses, Inc, USA) also in N₂ atmosphere.

## 2.3 Materials characterization

Granulometric distribution was measured using a laser granulometer (Malvern Mastersizer 2000). Thermogravitational Analysis and Differential Scanning Calorimetry (TGA and DSC) were performed on TA Instruments Q500 (TA Instruments, USA) in N₂ atmosphere. Density of as-sintered parts was measured by using Archimede's method. Theoretical maximum density was measured with a Micromeritics Accupyc 130 Gas Pycnometer (Micromeritics Instrument Corporation, USA) by grinding debound samples. Observations under optical microscope were performed using an Olympus TH4-200 (Tokyo, Japan). Morphology and microstructure observations were done with a field emission scanning electron microscope (FESEM, Thermo Fisher Scientific, Helios 5 DualBeam, USA). Due to their
extreme brittleness, polishing could not be performed on debound samples. Sintered samples were ground on successively finer SiC papers, from 320 to 2500 grit sizes at a speed of 180 rpm. They were then polished on fine cloth, with diamond solution from grit size of 3 down to 1 µm. The Vickers hardness was determined by indenting the samples with a Wilson VH1150 (Buehler, USA) at a 98N load for 15 seconds. The toughness was calculated by measuring the length of the cracks at the tip of the indentations. 5 samples were tested in the build direction and 5 others in the build plan. Each sample was indented 20 times to obtain an average value of hardness and toughness. Since (l+a)/a ≤ ≈ 3 the crack system is considered as Palmqvist[30]. The equation used to calculate the fracture toughness was the equation from Niihara[31] :

$$K_c = 0.0122 * \left(\frac{E}{H_v}\right)^{2/5} \frac{P}{al^{1/2}} * 10^{-6} \quad (1)$$

Where Kc is the fracture toughness (MPa.m¹/²), E the Young modulus (GPa), Hv the Vickers hardness (GPa), P the load (N), 2a the diagonal length of the indentation (m) and l the crack length (m).

4-point bending test was carried out with a Universal Testing Machine Instron 5567 (Instron, USA) in ambient conditions as per the configuration B described in ASTM C1161-18[32] except for the crosshead speed that was set to 0.2mm/s to make sure the failure would not happen too quickly. 9 specimens were tested. The specimens for this test were ground by hand due to warping after debinding and sintering. SiC paper of a 320 grit size was wrapped around a rubber and 20 strokes were given on each side of the long face of the specimens. This step was then repeated with a SiC paper of a 640 grit size.

The flexural strength was calculated with the following equation :

$$\sigma = \frac{3FL}{4BH^2} \quad (2)$$

Where σ is the flexural strength (MPa), F the load (N), L the span length (mm), B the width (mm) and H the thickness (mm).

With the purpose of improving the observation under Scanning Electron Microscopy (SEM) of the morphology and microstructure, the samples were etched using Reactive Ion Etching method (RIE) in a Plasmalab80plus (Oxford instruments, UK). The samples were then coated with platinum to avoid the usual quick charging of silicon nitride.

# 3. Results and Discussions

## 3.1 Debinding atmosphere

In order to study the decomposition process during thermal debinding, TGA is the most efficient technique to do so. Before focusing on temperature decompositions, the choice of the atmosphere used during debinding is the first parameter to set. Therefore, a
TGA/DSC was performed at a heating rate of 10°C/min under nitrogen and air flow (Fig. 2). The DSC has revealed that the decomposition under air flow was an exothermic process, while the decomposition under nitrogen flow was an endothermic process. The debinding is a critical step that relies on controlled diffusion. Therefore, using an atmosphere in which the process occurs through an endothermic process will always be preferable over one where the process occurs through an exothermic process[33]. Thus, this analysis has demonstrated that the use of a nitrogen flow was preferable over an air flow for debinding. That was also confirmed experimentally. Fig. 3 depicts two lines of cubes that underwent the first step of debinding, until 150°C at a heating rate of 0.1°C/min. The experiment was run under an air flow for the upper line of cubes while it was run under a nitrogen flow for the bottom line of cubes. As shown by these pictures, the amount and the size of cracks are much higher when run under air than nitrogen.

Although nitrogen was the preferable atmosphere for debinding, it can be noticed that the final weight loss was slightly higher under air. Under nitrogen, the mass loss was 27.89% whereas under air, the mass loss was 28.86%. This is explained by the fact that the presence of oxygen is necessary to eliminate the last remaining of polymers, achieving a complete removal of carbon compounds.

Consequently, debinding experiments were conducted under nitrogen flow. Nevertheless, during the dwelling at the maximum temperature of the debinding, the nitrogen flow was replaced by an air flow during two hours to obtain carbon-free ceramic parts. The sample weight in our first experiment have shown no oxidation despite this short period of air flow during the last dwelling. Also based on the sample weight after debinding, we can confirm it allowed us to achieve a complete removal of carbon compounds. This step under oxygen flow must be done to ensure there is no pollution of residual carbon during the subsequent sintering. The complete removal of carbon compounds is mandatory for silicon nitride. Otherwise, silicon carbide could be formed during sintering if there was any residual carbon [34].
<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /><figcaption>Fig. 2 ATG/DSC of an as-printed sample performed at 10°C/min under a) nitrogen flow b) air flow</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Fig. 3 Two lines of cubes which underwent a thermal treatment until 150°C at a heating rate of 0.1°C/min.
Upper line in air atmosphere and bottom line in nitrogen atmosphere.</figcaption></figure>

**3.2 Debinding temperature profile**

Mass loss curves of the TGA (Fig. 4) were thoroughly studied to develop the debinding profile. The first TGA that was performed was run at a heating rate of 10°C/min (Fig. 4, black curve). The derivative of the mass loss was plotted, representing the mass loss rate (Fig. 4, black dashed curve). The decomposition occurred in two different mass losses. A first mass loss was detected at low temperature, starting at T = 126°C and reaching the highest loss rate at T = 154°C. The second mass loss was observed at high temperature, starting slowly at T = 317°C, with the loss rate increasing quickly from T = 395°C and reaching the maximum loss rate at T = 455°C. A first debinding profile was created based on those mass losses observed (Fig. 5, black curve). Two dwellings were set, at T = 150°C and 200°C for 4 hours each. The heating rate was set at 0.2°C/min. Likewise, as the interconnected channels are created during this first mass loss and the loss rate is really low between T = 200°C and 317°C, the heating rate used could be set at 0.2°C/min. Regarding the second mass loss, a total of 4 dwellings of 6 hours were set with a heating rate of 0.1°C/min between those dwellings. The first one happened at T = 320°C when the mass loss was beginning according to the TGA. The 3 others were set at T = 380 ; 430 and 480°C. The strategy was to place dwellings before the crucial instants of the decomposition such as maximum loss acceleration and maximum loss rate. By doing so, the mass loss could start earlier and at lower rate, reducing the internal pressures within the parts, thus reducing the risks of cracking or delamination.
Therefore, a dwelling at 380°C was placed due to the inflexion point of the derivative curve at 395°C. With the maximum decomposition rate at 455°C, a dwelling was placed at 430°C. The last dwelling for this section of debinding was placed at 480°C, just after the decrease of loss rate. This dwelling ensures stability before ramping up to 600°C at a heating rate of 0.2°C/min, where the final dwelling of the cycle occurred. This dwelling lasted 6 hours. As mentioned above, air introduction is necessary to obtain a full removal of carbon compounds. Therefore, an air flow was introduced for 2 hours, at the middle of this dwell. The samples were then cooled down to room temperature at a heating rate of 0.5°C/min under nitrogen. One may notice that no dwelling was set up at the exact temperatures where the loss rates are the highest. Instead, two dwellings were placed at temperatures slightly lower and higher than this point. This was done with the aim of smoothing out the mass loss: it was assumed that having two medium mass losses would decrease the risk of delamination or crack formation compared to one large mass loss, where the built-up internal pressure would have been higher. Unfortunately, it was not possible to place the dwellings as such for the first mass loss as the furnace was unable to stop at such a low temperature and would always overshoot by a large margin. 150°C was the minimum temperature for the first dwelling. To verify that this first step could provide an early stage without creating any defect, a partial debinding at a heating rate of 0.1°C/min that was stopped after the first dwelling at 150°C was tested. This experiment has showed that no defect were observed except for the 15 mm cube (Fig. 3, bottom line). While the critical wall-thickness was previously 4 mm, crackfree cubes of up to 7 mm were obtained (Fig. 6 (a)).

<figure><img src="image_6.png" /><figcaption>Fig. 4 TGA used to build debinding profiles. Black full and dashed line, respectively weight loss and derivative of weight loss, 10°C/min. Red full and dashed line, respectively weight loss and derivative of weight loss, 0.2°C/min.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Fig. 5 Debinding profiles. Black line : profile based on the study of the TGA at 10°C/min. Red line : optimized profile based on the study of the TGA at 0.2°C/min.</figcaption></figure>

To further improve the debinding cycle and to increase the size of defectless cubes after debinding, a strategy was developed using an unconventional way of using TGA : setting up a low heating rate, 0.2°C/min in this case, a heating rate that is close to the one used during debinding (Fig. 4, red curve). Interestingly, shifts of decomposition temperature were observed compared to the TGA at 10°C/min. These shifts show that the decomposition process occurring in the furnace during debinding is much different than the process showed by the TGA at 10°C/min. These observations mean the previous profile could be further improved to increase the critical wall-thickness.

With this second TGA, it can be seen that the first mass loss starts at T = 64°C with the loss rate steadily increasing until reaching a maximum at T = 81°C. By letting the sample at these low temperatures for a lot more time than the TGA at 10°C/min, the mass loss also dramatically increased, from 2.58% to 5.05%. Then, the quick loss rate of the second mass loss also starts earlier, at T = 344°C instead of 395°C. However, it takes place through a broader range of temperature. While in the TGA at 10°C/min this mass loss occurred in a range of 75°C, between 395 and 470°C, it now occurs in a range of 91°C, starting from 344°C and ending at 435°C. This shows again that debinding is a process requiring a high amount of time to be done properly, as it takes hours for the decomposition to be complete when using a low heating rate. Finally, the loss rate reaches its maximum at 426°C compared to 455°C with the quicker TGA.

According to the TGA recorded at low heating rate, a new debinding profile was created (Fig. 5, red curve), with adjustments as follows :
As mentioned above, the first dwelling temperature could not be lowered despite the clear difference showed by this slow TGA. Therefore, to try and compensate for this limitation, the heating rate until the dwelling was reduced to 0.1°C/min and the time of the dwelling was increased from 4 to 6 hours. Afterwards, the second dwelling at T = 200°C was deemed useless, as the first mass loss was already completed according to the low heating rate TGA. 5 dwellings were programmed for the second mass loss, with the same heating rate of 0.2°C/min to reach the first dwelling and 0.1°C/min between the subsequent dwellings. The first one was set at T = 310°C for 4 hours, when the loss rate starts ramping up. The four other dwellings were set using the same strategy of positioning them slightly before and after the maximum loss rate. Several experiments were carried out with small tweaks between each of them. The best result was obtained for dwellings set at T = 360 ; 385 ; 410 and 450°C for 6 hours, except for the one at 450°C that lasted 4 hours instead. The final step to reach 600°C was unchanged and the air introduction as well as the cooling remained the same. Thanks to this optimized profile, the debinding of defectless parts with a wall-thickness of up to 11 mm was achieved (Fig. 6 (b)). This was obtained on four lines of cubes during three different runs, it is then safe to assume this is a repeatable result. These results are summarized in Table 1. After debinding, a low shrinkage was observed on so-called brown bodies : 2.33% in build plan and 1.16% in build direction.

<figure><img src="image_8.png" /><figcaption>Fig. 6 lines of cubes that underwent debinding (a) Profile based on the TGA at 10°C/min. (b) Profile based on the TGA at 0.2°C/min</figcaption></figure>

<table><thead><tr><th></th><th>5 mm</th><th>6 mm</th><th>7 mm</th><th>9 mm</th><th>11 mm</th><th>15 mm</th></tr></thead><tbody><tr><th>Cycle based on 10°C/min TGA</th><td>No crack</td><td>No crack</td><td>No crack</td><td>Delamination</td><td>Delamination</td><td>◎</td></tr><tr><th>Cycle based on 0.2°C/min TGA</th><td>No crack</td><td>No crack</td><td>No crack</td><td>No crack</td><td>No crack</td><td>Cracks</td></tr></tbody></table>
### 3.3 Sintering and mechanical properties

*Sintering and Mechanical Properties.* Fig. 7 depicts a line of cubes after sintering, which underwent the optimized debinding profile. Two batches of sintering were performed, the temperature profiles were the same for both with the samples being sintered at 1800°C for 3 hours. The only difference was the gas pressure during the dwelling. In the first one, the maximum pressure of 5 MPa was reached at 1500°C and the pressure was kept the same during the dwelling. In the second one, an additional 5 MPa was added after 1.5h of dwelling, for a total pressure of 10 MPa (Fig. 8). It was found that this addition of pressure helps to completely eliminate porosities, as the density went from 2.96 in the first batch, to 3.24 in the second batch (91 and 99% with a theoretical maximum density of 3.25 according to the measurements with the gas pycnometer) (Fig. 9). The total shrinkage observed in build plane and in build direction were respectively 19.83% and 21.00% for the first batch and respectively 21.37% and 24.40% for the second batch. Between the two batches, the build plane shrinkage was increased by a factor of 1.08 while the build direction shrinkage was increased by a factor of 1.16. Thus, the additional pressure provided a higher increase of densification in the build direction than the build plane. This shrinkage occurred almost entirely during sintering, as the shrinkage post-debinding is very low. As it was mentioned before, the shrinkage in build plane and build direction were respectively 2.33% and 1.16% after debinding. As opposed to the total shrinkage after sintering, the shrinkage after debinding is higher in the build plane than the build direction. This is explained by the fact that the layer interface remains the same after debinding. The carbon compounds are eliminated while leaving the structure intact in the build direction, hence a lesser shrinkage in the build direction. It is then the sintering stage that closes the gaps between layers, by bringing the layers closer together.

Vickers hardness and fracture toughness were respectively 13.94 ± 0.07 GPa and 9.00 ± 0.16 MPa.m$^{1/2}$ for the first batch and 15.57 ± 0.11 GPa and 10.27 ± 0.21 MPa.m$^{1/2}$ for the second batch. The Vickers hardness and fracture toughness values obtained for the first batch were already close to silicon nitride obtained through traditional means, but those of the second batch are what is usually found for fully dense silicon nitride[35,36]. These differences can be linked to the different densities and the sintering profile used. The samples within a single batch had all roughly the same density, hardness and fracture toughness, whichever debinding profile they were submitted to. As it was consistently observed during measurements, it should be mentioned that a slight anisotropy was observed. Bar a few exceptions, the build direction was a bit weaker than the two directions in the layer plane. In average, Vickers hardness and fracture toughness were about 1% lower when the measurement included the build direction. However, such a low difference should not really affect the use of the ceramic parts and the properties can be considered as isotropic.
The specimens for bending testing were included in the first batch and had a slightly higher density than the cubes of this batch, with an average of 3.03 (relative density of 94%). The flexural strength found was 407 ± 69 MPa. The failure behavior was satisfactory, as 8 out of the 9 samples failed in the section purely submitted to bending. Fig. 10 depicts one of those samples, which failed closed to the center. This value of 407 MPa is lower than the usual range of dense silicon nitride[4,35]. This can firstly be explained by the lack of densification as the density was 94% of theoretical density. In addition, some slight warping was observed on the specimens after sintering. Most importantly, the bending strength of additive manufacturing-based silicon nitride is generally lower than silicon nitride obtained by conventional means. Wang et al. and Dong et al. have respectively found a flexural strength of 162 and 323 MPa by using DLP and stereolithography [25,37].

<figure><img src="image_9.png" /><figcaption>Fig. 7 One of the line of cubes that went through the optimized debinding profile after sintering. The cubes that had a wall-thickness of up to 11 mm as-printed are all defectless.</figcaption></figure>

<figure><img src="image_10.png" /></figure>
Fig. 8 Hot-Isostatic Pressure sintering profiles. Red line : temperature profile. Blue dashed line : first batch pressure profile. Black dashed line : second batch pressure profile.

<figure><img src="image_11.png" /><figcaption>Fig. 9 Density versus cube size edge</figcaption></figure>

<figure><img src="image_12.png" /></figure>

Fig. 10 One of the bending test sample after failure. The failure occurred between the inner supports, as expected for a satisfying test.

## 3.4 Microstructure
*Microstructure post-debinding.* All the cubes were systematically observed under optical microscope after debinding. These observations have revealed that although smaller cubes looked unscathed, they had some so-called microcracks visible at their surface (Fig. 11(a)). Every cube displayed reminiscence of the layer upon layer printing principle. Fig. 11(b) features such aspect. This indicates that the evacuation channels were created along a preferential path due to a high amount of space between the layers, hence the higher shrinkage in the building plane than the building direction. This phenomenon can be observed on a cross-section of a layer of a partially debound sample (Fig. 12(a)). Evacuation channels can be seen as black diamond-shapes, showing that the carbon compounds diffuse between layers. EDS was performed on this sample (Fig. 12(b)) and confirmed those are indeed evacuation channels. Although EDS is not accurate for elements of low atomic mass, the carbon/nitrogen ratio can still give a good idea of the present elements. This ratio was equal to 0.5 in the matrix, while it was equal to 2 in the diamond-shapes. Thus, the presence of carbon is a lot higher in those shapes, meaning the polymers go through these places upon decomposition. These diamond-shaped evacuation channels are only formed at the surface of the layers, as the cross-section observation (Fig. 12(c)) does not display such feature. However, this figure shows some black lines, each separated by a height of 40 µm, exactly the thickness of the layers. It is likely that the carbon compounds are mostly evacuated at the interface between layers, rather than within a layer. Fig. 12(d) is a cross-section observation of the sintered sample, indicating that the layers disappear after sintering. Thus, the final microstructure does not suffer from these features, which is why the mechanical properties are within the range of silicon nitride fabricated by traditional means.

<figure><img src="image_13.png" /><figcaption>Fig. 11 Observation under optical microscope of debound samples a) an example of a microcrack that was often observed on samples. b) an area with the layer upon layer fabrication manner.</figcaption></figure>
<figure><img src="image_14.png" /><figcaption>Fig. 12 SEM observations. a) A longitudinal section of a layer on a partially debound sample. Evacuation channels are observed at the surface of the layer. b) EDS analysis performed on the channel and on the matrix next to it. c) A cross-section of a debound sample, showing 3 layers. d) A cross-section of a sintered sample, showing the layers have disappeared.</figcaption></figure>

**Microstructure post-sintering.** The microcracks were found to disappear after sintering. Cracks are usually amplified with sintering, however, most silicon nitride powders contain some additives such as Al₂O₃, MgO or Y₂O₃ for the most common. These additives and the SiO₂ layer at the surface of the grains react to form a liquid phase during the heating which allows the full densification of this ceramic[38]. Along densification, this liquid phase helps to close the gaps created by cracks, by filling them, leaving white lines at the surface of the samples. On some samples, this phenomenon also took place at a macroscopic scale for bigger cracks. Fig. 13 is an observation of a specimen that was used for 4-point bending tests where this phenomenon can be spotted. The literature on this crack-healing phenomenon in Si₃N₄ is rich [39–41]. More specifically, Hu et. Al [42] have stated that the crack-healing effect is considered to be triggered by oxidation. Sintering additives such as Al₂O₃ and Y₂O₃ react with the residual SiO₂ to form the liquid phase of aluminosilicate and silicon yttrium oxide, which flow into the crack to heal it. SEM observations were carried out on sintered samples. Coalescence of grains is observed, leading to rod-shaped grains, which is expected during the transformation from α to β phase during sintering (Fig. 14). XRD shows that this phase transformation is complete, as no α-phase is detected in the sintered sample (Fig. 15).
<figure><img src="image_15.png" /><figcaption>Fig. 13 Observation of a sintered specimen for bending testing under optical microscope. A large white line of precipitated additives can be observed after sintering (blue arrow). The onset is the same location on the sample before sintering.</figcaption></figure>
<figure><img src="image_16.png" /><figcaption>Fig. 14 SEM observation of a sintered sample with non-equiaxial β-grains.</figcaption></figure>

<figure><img src="image_17.png" /><figcaption>Fig. 15 XRD patterns of as-debound and as-sintered samples</figcaption></figure>
## 3.5 Complex shapes

As the main point of stereolithography and more generally additive manufacturing is the possibility to fabricate parts with a complex structure, various complex shapes parts were printed, debound and sintered. Fig. 16 displays these complex shapes parts after sintering, such as lattices, gyroids, honeycombs and gears. Every parts exhibited no defect and good finition surface, as expected with stereolithography. These parts show that the debinding cycle and the sintering cycle determined previously work well for complex shape parts.

<figure><img src="image_18.png" /><figcaption>Fig. 16 Some of the complex shapes that were obtained with the optimized debinding profile</figcaption></figure>

## 4. Conclusion

In this paper, a method to optimize the debinding of stereolithography-based silicon nitride was presented. It was found that analyzing thoroughly the TGA performed on as-printed sample and placing the dwellings accordingly during debinding helped to reduce the risk of cracking and delamination during this critical step. This method made possible the increase of critical wall-thickness from 4 to 11 mm, for which defectless parts can be obtained after debinding. During the sintering, the liquid phase fills microscopic cracks. Thus, a crack-healing phenomenon is observed for these small cracks. Hardness and fracture toughness
were measured and found in the expected range for silicon nitride obtained through
conventional methods, while flexural strength was lower. We believe the method presented
in this article could be applied to any ceramic parts fabricated by stereolithography. These
findings may enhance the way of using this fabrication technique for ceramic structural parts
by widening the possible applications due to a higher wall-thickness achievable. Optimizing
the thermal debinding is but one of the possibilities to obtain larger ceramic parts. Other
parameters such as paste composition, uncured monomers rate or layer thickness were not
studied in this work and could help to further increase the critical wall-thickness.

**Declaration of Competing Interest**

The authors declare that they have no known competing financial interests or
personal relationships that could have appeared to influence the work reported in this paper.

**Acknowledgements**

This work was supported by CNES (Toulouse, France) and Thales Alenia Space (Cannes,
France) thanks to a PhD allowance granted to "Experimental Study of the Additive
Manufacturing of Silicon Nitride". The authors wish to thank Creatz3D (Singapore) and
particularly Ye Pengcheng for printing the parts of this work. We also wish to thank Christelle
Bilot, who helped us with the SEM experiments of this paper.

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