# Experimental and numerical analysis of effects of supercritical carbon dioxide debinding on Inconel 718 MIM components

Olivier Dugauguez, Aboubakry Agne, Antonia Morales, José Manuel Torralba, Thierry Barriere

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Olivier Dugauguez, Aboubakry Agne, Antonia Morales, José Manuel Torralba, Thierry Barriere. Experimental and numerical analysis of effects of supercritical carbon dioxide debinding on Inconel 718 MIM components. Powder Technology, 2019, 355, pp.57-66. 10.1016/j.powtec.2019.07.011. hal-02867768

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# Experimental and numerical analysis of effects of supercritical carbon dioxide debinding on Inconel 718 MIM components

Dugauguez Olivier<sup>a,b</sup>, Agne Aboubabky<sup>a</sup>, Jimenez-Morales Antonia<sup>b</sup>, Torralba José Manuel<sup>b</sup>, Barriere Thierry<sup>a,*</sup>

Polymer extraction is a key step of the metal injection moulding (MIM) process. An innovative method for extracting an organic binder involves the use of supercritical fluid instead of a regular solvent. The supercritical fluid selected was carbon dioxide due to its affinity toward polyethylene glycol. The supercritical carbon dioxide parameters were optimised in order to complete the extraction of this polymer from the Inconel 718 superalloy feedstock. The effects on the surface quality of the components and on the microstructure of the densified component were also investigated. A finite element model, which combines the fluid model and structural mechanics, enabled the estimation of the stress distribution during the supercritical debinding step. The study was completed by a parametric analysis of the pressure variation inside the equipment with respect to the processing parameters for the MIM components.

## 1. Introduction

The properties of supercritical fluids were easily modified by modifying the pressure and the temperature of the fluid [1], making them excellent solvents for different applications. The solubility of polymers into supercritical fluids has recently attracted interest for biomaterial processing [2] but other main interests include the modification of the polymer viscosity for the oil industry [3], particle design for drug delivery [4], and medical device elaboration [5] for the pharmaceutical industry. The supercritical carbon dioxide is used most often in these applications due to its attractive properties. The supercritical parameters are easily attained: $T_c$ = 304 K and $P_c$ = 7.38 MPa. It is also safe to use as it is non-toxic, non-flammable, and easily removable from a system [6]. Moreover, a significant amount is absorbed by some polymers, mainly non-polar polymers, inducing plasticisation of the polymer [7]. Hence, it is seen as a green alternative in the industry to

organic solvents. Therefore, supercritical carbon dioxide has considerable potential as a solvent or a plasticizer [8].

Polyethylene glycol (PEG) is a water-soluble polymer that can be utilised to catalyse reactions, in the food, cosmetic, and pharmaceutical industries [9]. There is significant interest in its behaviour in the presence of supercritical carbon dioxide [10]. The main aim has been realizing a decrease in the viscosity of PEG at low temperatures without the use of organic solvents and degrading thermally sensitive materials. There have been several studies on the viscosity as a function of thermal and pressure parameters, but also on the effect of the molecular weights of PEG. Weidner et al. [11] studied the evolution of the solubility for different molecular weights, from 4000 to 35,000 g mol⁻¹. They observed a decrease in the melting temperature of the polymer between 1 and 10 MPa. Also, the solubility of the polymer was independent of its molecular weight, and was influenced by the pressure and temperature parameters of the carbon dioxide.

The metal injection moulding (MIM) process relies on shaping metal particles with the aid of a binder [12]. This method combines the advantages of powder metallurgy technology with the flexibility of the thermoplastic polymer injection moulding process. The density and mechanical characteristics of the final product can be compared to those components obtained by machining or forging [13]. There are four steps in the MIM process: the formulation and production of a feedstock by mixing a metallic powder with a polymeric binder, injection into a die cavity mould at a set pressure, solvent and/or thermal extraction of the organic components, and, finally, thermal treatment (sintering) of the shaped sample.

Abbreviations: MIM, Metal injection moulding; PEG, Polyethylene glycol; PP, Polypropylene; SA, Stearic acid; FSI, Fluid-structure interaction; $ρ_{fluid}$, Density; M, Dynamic viscosity; K, Thermal conductivity; $C_p$, Specific heat capacity; P, Pressure; T, Temperature; Q, Inlet flow; $P_o$, Atmospheric pressure; E, Young's modulus; N, Poisson's ratio; P, Density; $\alpha$, Linear expansion coefficient; $\sigma$, Stress tensor; $\sigma_o$, Initial residual stress; $\sigma_e$, Von Mises stress; $R_p$, Maximum profile peak height; $R_a$, Roughness average; $F_v$, Volume load.

<footnote><sup>a</sup> Univ. Bourgogne Franche-Comté, FEMTO-ST Institute, CNRS/UFC/ENSMM/UTBM, Department of Applied Mechanics, 25000 Besançon, France</footnote>

<footnote><sup>b</sup> Universidad Carlos III de Madrid, Avda. Universidad 30, 28911, Leganés, Madrid, Spain</footnote>

<footnote>* Corresponding author at: Univ. Bourgogne Franche-Comté, FEMTO-ST Institute, CNRS/UFC/ENSMM/UTBM, Department of Applied Mechanics, 25000 Besançon, France.</footnote>

<footnote>E-mail address: thierry.barriere@femto-st.fr (B. Thierry).</footnote>
In most feedstocks, it is common to use a volume loading rate of 60 vol% of the metallic powder (40 vol% of the binder). The binder composition depends on the chosen powder. It must be fluid enough during mixing to result in a homogeneous mixture with the powder [14]. It also ensures the cohesion of the components during the manipulation of the component after injection and get enough mechanical and chemical properties to challenge the process without damages. Binders are generally composed of kind of polymers, one to get the mechanical resistance, the second to promote a low viscosity during mixing and injection, and, finally, a dispersive agent to achieve homogeneity [15,16]. Polymer extraction in the MIM industry can be performed using different methods, depending on the selected formula. The main methods include the use of organic solvents [17,18] or water [19], catalytic reactions [20], or thermal degradation [21,22]. The development of PEG-based feedstocks for water solubility progressed with industrial requirements for cost-reduction and environmental regulations. The major drawback of these feedstocks is the long degradation time of PEG in water [23]. In the MIM research field, only a few studies have explored the application of supercritical fluid extraction. The main research was performed by Chartier et al. [24,25], Kim et al. [26], and Shimizu et al. [27] on ceramic/wax feedstocks. They obtained excellent extraction rates without damaging the components. Yannig et al. [28] performed supercritical extraction on TiO₂/wax systems and observed the effects of the supercritical fluid combined with organic solvents. The extraction rates were improved but the use of the organic solvent was detrimental to the powder particle structure.

This work focuses on the application of supercritical carbon dioxide extraction on Inconel 718 feedstocks composed of polypropylene (PP), PEG with a molecular weight of 20,000 g mol⁻¹, and stearic acid (SA). The principal focus of this study is on the effects of the supercritical fluid parameters and the binder volume on the extraction efficiency. The PEG-based feedstocks, debinded by water and solvents, have already been investigated [29,30]. The interest in the replacement of solvent debinding with supercritical fluid debinding is mainly because of the possibility of faster extraction. However, the extraction of the binder should not damage the component. Agne et al. [31] modelled the process using finite element method with this binder on Inconel 718 component, the polymer extraction mechanism, and their numerical results by taking into account the shape of the MIM component and the diffusion coefficient of the soluble binder. Their optimal conditions, in terms of the temperature and pressure of supercritical debinding, were obtained in this study. During supercritical debinding, three phenomena may induce dilatation or shrinkage and stress inside the component: the swelling due to polymer extraction, the effect of the high pressure of the supercritical fluid on the component, and the thermal expansion. The shrinkage measured after supercritical debinding is negligible (less than 1%) [31]. The swelling behaviour due to the binder diffusion can also be neglected. The effects of the high pressure of the fluid on the mechanical structure and the thermal expansion during the debinding process generate several stresses on the component and are described using the fluid-structure interaction corresponding to the coupling fluid flow model and structural mechanics. The thermal stress during the process can be treated as the initial residual stress since the binder is a viscoelastic material [32]. Moreover, the plasticization effect of the dissolved soluble polymer assists the relaxation of the component.

The fluid parameters depend on the temperature and the pressure [33]. Molecular simulations are being increasing applying employed to predict thermo-physical properties of supercritical fluids [34]. This methodology can be used under a large number of physical conditions in terms of temperature and pressure. Many studies focused on supercritical carbon dioxide [35,36], and the thermo-physical properties studied include the viscosity and heat capacity at constant volume [37].

The effects of the processing method and associated parameters on the quality of the components are investigated to realize the best performance of the material using the supercritical carbon dioxide extraction

method with two different powder volume loadings (62% and 71% in volume). The optimisation of the debinding conditions is realized in order to achieve complete extraction of PEG at the highest possible rate. The results obtained during these tests will help to determine the influence of the parameters necessary for performing the numerical simulation. The debinded components will also be compared to those obtained by water debinding throughout the MIM process until the sintering steps, in terms of quality of the final part (e.g., in terms of geometry) and the microstructure obtained will be discussed. The effect of the supercritical fluid on the component is described using the fluid-structure interaction, which combines fluid flow and structural deformations. The fluid parameters depend on the temperature and the pressure. In this study, classification of the 2-D axisymmetric problem coupled with the fluid-structure interaction will be performed using the Comsol Multiphysics finite element® software platform with a stationary solver to predict the stress distribution, the fluid flow, and the deformation of the cylindrical component composed of Inconel 718 powder inside the supercritical fluid cylindrical reactor. The numerical simulation is performed to analyse how the supercritical fluid and the temperature affect the specimen during the process and if the stress may cause defects in the debinding stage. Moreover, the pressure variation of the supercritical carbon dioxide inside the reactor was investigated in order to characterize the mechanical effect on the component.

## 2. Materials and methods

### 2.1. MIM components

The Inconel 718 superalloy powder was provided by Sandvik Osprey Ltd. (Sandvik Materials Technology, Sandviken, Sweden). Fig. 1 exhibits the SEM images of this gas-atomised powder offers a perfectly spherical shape. The particle size distribution was measured by laser granulometry and the diameter values corresponding to the cumulative volume fractions D₁₀, D₅₀, and D₉₀ are 3.53 µm, 6.24 µm, and 10.97 µm, respectively, with a standard deviation of 0.86 µm. The selected binder is composed of PP, PEG, and SA purchased from EMD Millipore Corporation (Billerica MA, USA). PEG has a molecular weight of 20,000 g mol⁻¹. The compatibility of this binder with the fine-particle size distribution also aids the injection process. Two powder volume loadings were selected, respectively, of 62 and 71% in volume in order to evaluate the influence of the powder volume on the polymer debinding performance.

<figure><img src="image_2.png" /><figcaption>Fig. 1. SEM image of Inconel 718 superalloy powder.</figcaption></figure>
<table><caption>**Table 1**

Composition in volume of the two different binder formulations.</caption><thead><tr><th></th><th>Inconel 718</th><th>PP</th><th>PEG</th><th>SA</th></tr></thead><tbody><tr><td rowspan="2">Compositions (vol%) of the feedstocks</td><td>62</td><td>15.20</td><td>20.90</td><td>1.90</td></tr><tr><td>71</td><td>11.60</td><td>15.95</td><td>1.45</td></tr></tbody></table>

The compositions (vol%) of the feedstocks are shown in Table 1. The mixing of the binder with the metallic powder was performed with a twin-screw mixer Brabender Plastograph EC provided by Brabender GmbH & Co. KG (Duisburg, Germany) with a 50 cm³ mixing volume. The elaborated feedstock was then milled to get pellets and were injected to obtain cylinders by using an Arburg 220S hydraulic injection moulding press provided by ARBURG GmbH & Co KG (Lossburg, Germany) equipped with a cylindrical mould cavity with an injection volume between 3 and 12 cm³. The injection temperatures were set to ensure homogeneous powder and polymer mixture. The injected cylindrical components were 15 mm in length with a diameter of 10 mm.

## 2.2. Experimental procedures

The optimised process of the Inconel MIM samples is composed of different steps. The injected parts first underwent to solvent debinding. They were introduced into the distilled water at around 308.15 K for 48 h in order to remove PEG. They were then dried in an oven at 348.15 K for 2 h. The supercritical fluid debinding, in replacement of the water debinding, was performed with an equipment developed by the Separex® company (France), as shown in Fig. 2.

As presented in Fig. 2, the carbon dioxide in a gaseous state was extracted from a bottle at the beginning of the process, then was cooled to a liquid state (1). The pressure was then increased via a pump (2). When the pressure required for the supercritical state (up to the critical point) was reached, the carbon dioxide was heated by a heat exchanger (3). At the supercritical temperature and pressure, the CO₂ was directed toward the circular reactor (5), regulated by thermocouples, which contained the specimens (4). The carbon dioxide was recovered by a valve (6), cooled down, and was directed toward a separator (7) and a recycling line (6–9) or was removed from the circuit (6–8). Before and after the debinding, the components were weighed to estimate the mass loss due to the PEG elimination. Moreover, three successive tests were performed for each debinding condition for repeatability testing.

The supercritical fluid debinding tests depend on the pressure, temperature, holding time and flow rate of supercritical carbon dioxide. These parameters can be defined in a programmed cycle of the

<figure><img src="image_3.png" /><figcaption>**Fig. 3.** Typical pressure and temperature evolution vs supercritical debinding time.</figcaption></figure>

controlling software of the equipment (Fig. 3.). Under increasing pressure, a constant rate of 1 MPa min⁻¹ was applied until the desired pressure was attained. Also, it is important to take into account the change in state and density of the supercritical fluid into the gas state. To avoid damage occurring on the components, the pressure was decreased at two rates: at 2 MPa min⁻¹ until 9 MPa and at 1 MPa min⁻¹ until the optimal debinding pressure. The temperature of the system was maintained throughout the test until the end of the holding time.

PP was eliminated by thermal debinding at 773.15 K for 2 h under an Ar atmosphere. A heating rate of 2 K min⁻¹ was applied throughout the cycle to avoid overpressure inside the sample during degradation of the polymer. Once the thermal debinding was complete, the sample was sintered at a temperature of 1563.15 K for at least 3 h at a heating rate of 5 K min⁻¹. Cooling was performed at a rate of 10 K min⁻¹. The thermal treatments (classical sintering experiments) were accomplished inside a vertical dilatometer (SETSYS Evolution TMA) from Setaram© (Setaram, Caluire, France).

The complete infrared spectroscopy system (IR-spectrometer Nicolet™ iS™ 10) was developed by Thermofisher Scientific® (Waltham, Massachusetts, USA). This equipment combines a Smart OMNI-Transmission Nicolet iS10 infrared spectrometer with a Haake Mars rheometer. For the coupling function of the two primary parts of equipment, the attenuated total reflection (ATR) was performed. The maximum test temperature reached 400 °C, which is suitable to investigate the chemical and thermal behaviour of polymers, binders, and feedstock. Working in the mid infrared range from ~ 4000 to 500 cm⁻¹. In our current study, the IR-spectrometer was used to observe the influence of the supercritical fluids on the binder.

The extraction rates were obtained by comparing the mass of the components after the tests and were confirmed by TGA analysis via the SETSYS Evolution TMA from Setaram© (Setaram, Caluire, France).

The carbon and oxygen levels of the MIM components were obtained via a LECO CNS-2000 analysis equipment (Lakeview Avenue, Saint Joseph, USA). The main feature of the analyse is based through high-temperature combustion test followed by infrared and thermal conductivity detection. The effectiveness of automated dry combustion instrument to quantify total carbon and oxygen levels in samples was determined by comparison with measurements by standard single-element methods and with reference materials. The analysis of the test was accomplished using the standard test method ASTM C 1408 for the carbon and oxygen levels.

Finally, the sintered components were characterised by optical microscopy (OM) and scanning electron microscopy (SEM) (Evo Ma15, from Zeiss) observations. The SEM analyses were performed with the standard test method ASTM E2809 (Standard Guide for Using Scanning Electron Microscopy). The surface quality was analysed via a roughness analysis performed with a non-contact optical 3-D surface metrology device (Alicona Infinite Focus) based on focus-variation. Measurements achieve a vertical resolution of up to 10 nm with a minimum measurable roughness of 1.5 × 10⁻² µm developed by Alicona™ (Graz, Austria).

<figure><img src="image_4.png" /><figcaption>**Fig. 2.** Diagram of the supercritical fluid extraction equipment.</figcaption></figure>
## 2.3. Model definition, fluid flow and structural mechanical formulations and numerical simulation conditions

During the supercritical debinding process, the component was placed in a temperature-pressure controlled reactor with a carbon dioxide (CO₂) flow entry. In our case, the reactor had a cylindrical shape with a length of 300 mm with a diameter of 100 mm, as shown in Fig. 4 (a). The cylindrical component was 15 mm in length with a diameter of 10 mm, and was composed of 62% volume Inconel 718, PP, PEG, and SA as the binder (Table 1). It was positioned at a distance of about 80 mm in the Z direction from the top, as shown in Fig. 4 (b).

An investigation on the two-dimensional problem was considered and is shown in Fig. 5 (a). The circular reactor was modelled by taking into account the 2-D axisymmetric problem, as in Fig. 5 (b), and the reactor (fluid domain), which contains the components, was modelled as a rectangle with a height of 80 mm and a width of 50 mm. Under these conditions, the specimen had a height of 15 mm and a width of 5 mm, as shown in Fig. 5 (b). The axisymmetric problem also considers the geometry of the reactor and the component.

The fluid-structure interaction (FSI) is a combination of the fluid flow model and the mechanical properties that determine the physical phenomena occurring between a fluid and a deformable solid and how they affect each other [39–41]. It models both the fluid domain and the solid domain and includes a predefined condition for interaction at the fluid-solid boundaries. An arbitrary Lagrangian-Eulerian method [42] was used to combine the fluid flow, formulated using a Eulerian description; the spatial frame, with the structure formulated using a Lagrangian description; and a material reference frame.

The fluid used in this study was carbon dioxide in a supercritical state at 423 K and 40 MPa. The different thermo-physical fluid properties used in the numerical simulations, in terms of density, dynamic viscosity, thermal conductivity, heat capacity, and pressure, are presented in Table 2 [33,37,43–45]. The carbon dioxide inlet flow was equal to 25 L h⁻¹. The outlet surface was set as the inlet flow, equal to 25 L h⁻¹. The inner lines of the reactor were modelled as walls.

The carbon dioxide flow inside the reactor is described by the incompressible Navier-Stokes Eqs. [47] for the velocity field $u_{fluid}$ and the pressure $p$ in the spatial moving coordinate system for a stationary study, as follows:

$$ 
\begin{aligned}
\nabla \cdot \left[ -p\mathbf{I} + \mu \left( \nabla u_{fluid} + (\nabla u_{fluid})^T \right) - \frac{2}{3}\mu \left( \nabla \cdot u_{fluid} \right) \mathbf{I} \right] + F = 0 \\
\nabla \cdot (\rho_{fluid} u_{fluid}) = 0
\end{aligned}
\quad (1) $$

<figure><img src="image_5.png" /><figcaption>Fig. 4. Images of (a) the supercritical reactor and (b) the cylindrical component location inside the circular reactor during supercritical debinding.</figcaption></figure>

where $I$ denotes the unit diagonal matrix and $F$ the volume force affecting the fluid. There were no gravitation or volume forces, so $F$ is null. The fluid pressure was assumed to be constant and equal to 40 MPa during the numerical simulation.

The structural mechanic problem was solved using an isotropic linear elastic formulation [48] applied to the cylindrical component, as the powder and the binder were supposed to solidify during the process. This model takes into account the thermal expansion of the component. For a plane-strain problem, the relationship between stress and strain is given by:

$$ \varepsilon = D^{-1}(\sigma - \sigma_0) \quad (2) $$

where $\sigma$ is the stress tensor and $\sigma_0$ is the initial residual stress and, which can be neglected in this study.

$D$ is the elastic stiffness matrix defined for an isotropic material as:

$$ \mathbf{D} = \frac{E}{(1 + \nu)(1 - 2\nu)} \begin{bmatrix} 1 - \nu & \nu & \nu & 0 & 0 & 0 \\ \nu & 1 - \nu & \nu & 0 & 0 & 0 \\ \nu & \nu & 1 - \nu & 0 & 0 & 0 \\ 0 & 0 & 0 & 1 - 2\nu & 0 & 0 \\ 0 & 0 & 0 & 0 & 1 - 2\nu & 0 \\ 0 & 0 & 0 & 0 & 0 & 1 - 2\nu \end{bmatrix} \quad (3) $$

where $E$ is Young's modulus and $\nu$ is the Poisson ratio.

The following relation (4) gives the equivalent von Mises stress $\sigma_e$ describing the distortion energy and corresponds to the plasticity yield criterion [49,50]:

$$ \sigma_e^2 = \frac{1}{6} \left[ (\sigma_x - \sigma_y)^2 + (\sigma_y - \sigma_z)^2 + (\sigma_z - \sigma_x)^2 \right] + \tau_{xy}^2 \quad (4) $$

where $\sigma_x, \sigma_y$, and $\sigma_z$ are, respectively, the stresses in the x, y, and z directions, and $\tau_{xy}$ is the shear stress in the xy plane.

The different physical properties of the components used for the numerical simulation are presented in Table 3. Young's modulus $E$ was calculated using the mixing law. The density was estimated using the mass-volume ratio, and the linear expansion coefficient was identified from experimental dilatometer tests.

The numerical simulations were performed using the Comsol Multiphysics® finite element software platform version 5.2 developed by Comsol Inc. (MA, USA) in 2015. The main objective was to obtain the stress distribution and the shrinkage of the MIM component during supercritical carbon dioxide debinding using the 2-D axisymmetric problem. The fluid-structure interaction, described above, was used to simulate the effect of the fluid on the component. Moreover, the initial velocity of the fluid was assumed to be null. In this study, the following hypotheses were assumed for the numerical simulation:

* The component is continuous;

* The fluid parameters depend only on the pressure and the temperature;

* There was no temperature variation during the process;

* The material is isotropic;

* The temperature did not affect the polymers properties (the binder was solid).

The cylindrical specimen inside the reactor, as shown in Fig. 5 (b), was modelled using the 2-D axisymmetric problem by the finite element method (FEM) and simulated using the fluid-structure interaction model. A stationary solver was used because this study is focused on parameters depended only on the temperature and the pressure. The Navier-Stokes equations (reactor domain) were solved using a freely-moving deformed mesh, which constitutes the fluid domain. The axial displacement on the bottom of the component was fixed during the numerical simulation. The 2-D component domain was discretised with 82 triangular elements with three nodes and the 2-D fluid domain was
<figure><img src="image_6.png" /><figcaption>**Fig. 5.** Presentation of (a) the circular reactor and the cylindrical component studied in the 2-D configuration of the equipment during supercritical debinding and (b) the 2-D axisymmetric problem at $r = 0$ used for fluid-structure modelling.</figcaption></figure>

discretised with 615 quadrangular elements with four nodes. A convergence test validates the mesh quality.

The bottom of the component was fixed in the axial direction (Z) and free in the radial direction (r), $u_z = 0$ and $u_r \neq 0$ (Fig. 5 (a)). All other boundaries of the modelled component experienced a load of the supercritical carbon dioxide given by the following equation:

$$F_T = -n \cdot \left( -pl + \mu \left( \nabla u_{fluid} + \left( \nabla u_{fluid} \right)^T \right) \right) \quad (5)$$

where $F_T$ is the sum of the pressure and the viscous force applied to the outer surfaces of the component and $n$ is the normal vector to the boundary.

The effect of the carbon dioxide pressure inside the circular reactor on the component during the binder extraction can be modelled as a volume load $F_v$ applied to the component. The applied load $F_v$ is function of the supercritical fluid pressure and the dimensions of the reactor. The following table (Table 5) gives the values of the calculated volume load $F_v$ following radial ($F_{v,r}$) and axial ($F_{v,z}$) direction function of the pressure inside the reactor from 10 MPa to 50 MPa:

### 3. Results and discussion

Fig. 6 shows the IR-spectrum results for each of the three basic ingredients composed of the binder (PP, PEG and SA). The injected component and the debinded component by supercritical fluid with a 62% powder loading volume. The debinding parameters were adjusted for 423.15 K and 40 MPa during 4 h. The PP and PEG spectrums can be identified inside the injected component. The SA is present in a small ratio in

the composition of the binder, indicating why that it is not seen in the binder spectrum. After the supercritical debinding cycle, there were no more signs of the presence of PEG. Moreover, only the PP spectrum can be identified, signifying that all PEG was completely removed and that PP did no undergo chemical degradation. The CO₂ in the supercritical state only interacted with PEG and did not interact with PP that were removed in the previous step by thermal degradation. The supercritical CO₂ may also react the SA during debinding. The intensity of the IR peaks is dependant of the quantity of the material introduced in the binder. In the case of the SA, the quantity is too low compared to the PP and the PEG, and cannot be clearly identified on the binder spectrum for the SC debinded spectrum. The spectrum for the SC debinded binder is also not showing a clear presence of the SA. As there is no clear proof of interaction between SA and the SC fluid, only the PEG will be considered for the weight loss observed during the tests.

Fig. 7 (a) shows the results obtained during the optimisation of the supercritical fluid parameters. The optimal pressure was reached with a temperature of 323.15 K. This temperature was chosen as it is at the beginning of the supercritical state of the CO₂. It appears that a pressure of 40 MPa gave the best results at 323.15 K but the quantity of PEG removed from the components is still under 40%. It can also be noticed that if the pressure increased to 50 MPa, the quantity of PEG removed decreased. The temperature optimisation was performed with the pressure that resulted in the best results; 40 MPa, as shown in Fig. 7 (b). The evolution of PEG removal as a function of the temperature slightly increased from 323.15 K until 373.15 K. It was only until the temperature of 423.15 K was reached that most of PEG was removed from the component, confirming the conclusions observed by Chartier et al. that extraction is easier near to the melting temperature of the binder, which needs to be reached in order to obtain the best debinding rate. Similar results were also obtained with paraffin wax [24,26].

The debinding time was ranged from 1 h to 5 h, for both feedstocks. Fig. 8 shows the results of the time optimisation. In the case of 62%

<table><caption>**Table 2**

Summary of the carbon dioxide thermo-physical properties at supercritical state (423 K and 40 MPa).</caption><thead><tr><th>Property</th><th>Name</th><th>Value</th><th>Unit</th></tr></thead><tbody><tr><td>Density</td><td>ρ<sub>fluid</sub></td><td>607.5</td><td>g l<sup>-1</sup></td></tr><tr><td>Dynamic viscosity</td><td>M</td><td>50.380 × 10<sup>-6</sup></td><td>Pa s</td></tr><tr><td>Thermal conductivity</td><td>k</td><td>0.070</td><td>W (m K)<sup>-1</sup></td></tr><tr><td>Specific heat capacity</td><td>C<sub>p</sub></td><td>1.735</td><td>J (g K)<sup>-1</sup></td></tr><tr><td>Pressure</td><td>p</td><td>40</td><td>MPa</td></tr><tr><td>Temperature</td><td>T</td><td>423</td><td>K</td></tr><tr><td>Inlet flow</td><td>Q</td><td>25</td><td>L h<sup>-1</sup></td></tr><tr><td>Atmospheric pressure</td><td>P<sub>a</sub></td><td>0.1</td><td>MPa</td></tr></tbody></table>

<table><caption>**Table 3**

Description of physical and mechanical properties of the component.</caption><thead><tr><th>Property</th><th>Name</th><th>Value</th><th>Unit</th></tr></thead><tbody><tr><td>Young's modulus</td><td>E</td><td>0.5</td><td>GPa</td></tr><tr><td>Poisson's ratio</td><td>ν</td><td>0.3</td><td>-</td></tr><tr><td>Density</td><td>ρ</td><td>0.013</td><td>g mm<sup>-3</sup></td></tr><tr><td>Linear expansion coefficient</td><td>α</td><td>1 × 10<sup>-4</sup></td><td>K<sup>-1</sup></td></tr></tbody></table>
<figure><img src="image_7.png" /><figcaption>**Fig. 6.** IR spectrum of all binder ingredients PEG (a), PP (b), SA (c), the injected component (mixed binder after injection) (d) and the binder after supercritical CO₂ debinding (e).</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>**Fig. 8.** Quantity of PEG removed as a function of the extraction time performed at 40 MPa and 423.15 K and for two powder loading volumes (62% and 71% in volume).</figcaption></figure>

Inconel 718 powder volume, it appears that 4 h was the necessary time to obtain the complete removal of PEG. However, it was not obtained with the higher powder loading volume of 71%. In this case, it was necessary to increase the debinding time. During the mixing stage, the optimization of the powder volume rate was investigated to obtain a homogeneous high-loaded feedstock. The critical volume of the powder into the binder was 72%. In order to be slightly below this critical powder loading, a feedstock with 71% powder volume was selected. Therefore, the distribution inside the MIM component between the powder and the binder was close to the limit. As a consequence, during the debinding stage, the space between the particles and the binder slowed the diffusion rate of PEG/CO₂. This could explain the longer time to completely eliminate the polymer, especially PEG present at the core of the component. This phenomenon was also observed during water debinding.

In the case of solvent debinding, Lin et al. [18] developed a numerical model based on a diffusion mechanism. These models were based on the unidirectional diffusion of a species from the centre to the exterior of a component at a constant diffusion rate. The starting condition is the initial concentration of the polymer. The results show that the higher the polymer concentration is, the higher is the extraction time. Chartier et al. [25] also used this model for a numerical simulation of supercritical debinding and obtained similar results from the numerical data and their experiment. However, they did not use such a high powder volume in their work. It seems that this affirmation reaches a limit when the critical powder loading of a feedstock is reached.

The supercritical fluid debinding enabled the complete removal of the PEG from the components. The next step was to check whether side effects were expected that could impact the rest of the MIM process. After thermal debinding, one of the components was used to perform a combustion analysis via the LECO equipment. The sample was introduced to a high temperature flame above 2000 °C and the resulting combustion was analysed with a mass spectrometer. Table 5 shows the

carbon and oxygen levels present inside the debinded component with water and the debinded component with CO₂ at supercritical state. The carbon and oxygen contents were very similar between the two methods. As the sintering after water debinding did not show any signs of carbon or oxygen pollution, the same was expected during the sintering of a component after supercritical fluid debinding.

Table 5 Carbon and Oxygen levels inside the powder after solvent and supercritical fluid.

Fig. 9 shows the OM images of sintered components obtained after supercritical CO₂ debinding and after water debinding. The sintered components have been realized at 1563.15 K over 4 h, after supercritical fluid debinding at 423.15 K under 40 MPa for 4 h (Fig. 9 (a) and (b)) or water debinding at 323.15 K over 72 h (Fig. 9 (c) and (d)). In the centre of both samples, signs of porosity or cavities from poor debinding cannot be found. However, on the surface of the sample, a different problem can be observed. The supercritical fluid debinding helped to maintain the roundness of the component with a neat shape while the water debinding left an irregular microstructure at the surface of the component. From this, it can be deduced that, the use of the supercritical fluid was less damaging to the surface of the component than the water debinding.

The SEM images of the obtained microstructure after both supercritical and water debinding are shown in Fig. 10 (a) and (b). The two sintered components analysed by EDS analysis are showing a similar microstructure. The zones 1 and 4 display the same elements as the initial powder corresponding to the γ matrix of the Inconel 718, see Fig. 10 (a). The zone 2 and 5 are mostly composed of chromium and the zone 3 and 6 are rich in niobium, see Fig. 10 (b). There were no signs of alteration of the microstructure chemistry between the usual water debinding and the CO₂ supercritical debinding.

As the supercritical debinding shows an impact on the surface of the component, roughness analyses were performed in order to obtain a

<figure><img src="image_9.png" /><figcaption>**Fig. 7.** Quantity of PEG removed vs pressure at 323.15 K (a) and vs temperature at 40 MPa (b), both over a period of 4 h for a feedstock with a 62% powder loading volume.</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>**Fig. 9.** OM images of components sintered at 1563.15 K over 4 h, after supercritical fluid debinding at 423.15 K, 40 MPa for 4 h (a and b) and water debinding at 323.15 K over 72 h (c and d).</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**Fig. 10.** SEM images of the obtained microstructure after both supercritical (a) and water debinding (b).</figcaption></figure>

reference for the surface quality. The optical analysis, by detecting the distance necessary to obtain a focus on the component, built a 3-D topographic model. It was made for two samples; one for the water debinded at 323.15 K over 72 h, and the other for the supercritical fluid debinded at 40 MPa, 423.15 K over 4 h. The tests were performed along the whole length of the cylinders.

The samples were analysed after each step of the MIM process (Fig. 11). In the case of water debinding, the obtained surface roughness doubled after applying the method and reached a $R_p$ of 9.27 µm and a $R_a$ of 3.77 µm ($R_p$ is the maximum profile peak height and $R_a$ is the roughness average). In the case of supercritical debinding, the roughness was limited to a $R_p$ of 6.03 µm and $R_a$ of 2.21 µm.

The 3D roughness profile was more regular due to a less strong debinding for the structure of the component obtained by sintering, see Fig. 12 (a) and (b). Yannig et al. [29] also observed this behaviour in the case of paraffin debinding. The possibility of obtaining a better surface quality could be interesting for industrial applications, as the need for treating the surface of the component is less important. The debinding of a soluble polymer inside of injected components occurs in two stages. The solvent first slowly diffuses into the binder, transforming the polymer into a gel, which may induce a swelling of the polymer. If the attraction between the polymer and the solvent is stronger than between the binder polymers, the disintegration of the soluble polymer will occur. The swelling of the component is dependant of the molecular interaction and the solubility reaction between the
<figure><img src="image_12.png" /><figcaption>**Fig. 11.** Roughness measured on the components obtained after the different steps of MIM treatment for water debinding and supercritical debinding.</figcaption></figure>

polymer and the solvent. The surface rugosity is less impacted by using CO₂ in supercritical state, meaning that the swelling was fewer than during the water debinding.

The Table 6 resumes the characteristics of the sintered components after both supercritical and water debinding. There are not a lot of differences in terms of behaviour after sintering with similar final densities (~ 98%) and shrinkages (~ 16%). The mechanical characteristics measured by nano-indentation are also very similar with nearly the same values of Young modulus and hardness. The supercritical debinding had only an influence on the surface quality of the components. Nevertheless, the mechanical properties of all sintering component values are highlighted superior in the case of supercritical process

compare with water debinding method with the Young modulus value equal to 65.4 GPa and the hardness equal to 268 HV, see Table 6.

Fig. 13 shows the finite element discretization of the two domains and the supercritical fluid velocity (arrows from bottom to the top) inside the reactor and the equivalent von Mises stress distribution applied to the component from the finite element simulation results under 40 MPa at 423 K. The obtained results indicate a mean velocity of the supercritical fluid equal to 9.4 × 10⁻¹ mm s⁻¹ inside the reactor and a mean equivalent stress of 2.6 MPa mainly due to the high volume load. The fluid flow was deviated by the component before going through the outlet. The component interacts as a wall with the supercritical fluid. There was a very low effect between the outer surfaces of the component and the supercritical carbon dioxide inside the reactor. This observation can explain the roughness profile obtained after sintering a sample debinded by supercritical carbon dioxide, Fig. 12 (a).

Moreover, the maximum stress value was observed on the bottom area due to the boundary condition. However, the mean equivalent stress was low despite the high temperature (423 K) and high pressure (40 MPa) of the fluid. The low stress was due to the no-constraint boundary conditions on the component (no sealed condition) and the elastic relaxation. Moreover, it was lower than the Young's modulus and did not affect the component because it was in the elastic domain. The mean stress on the interaction surfaces, around 2 Pa, was due to the low viscous force (Eq. (2)) applied by the supercritical fluid on the component.

The axial and radial displacements of the component inside the reactor due to the supercritical carbon dioxide and the thermal expansion

<figure><img src="image_13.png" /><figcaption>**Fig. 12.** 3-D Roughness profile of sintered samples debinded by supercritical CO₂ (a) and in water (b).</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>**Fig. 13.** Presentation of (a) the finite element discretisation by triangular and quadrangular elements using 2-D axisymmetric problem for FEM simulation of the supercritical debinding and (b) the numerical results of the fluid-structure interaction during supercritical debinding at 423 K and 40 MPa: the supercritical fluid velocity and the equivalent von Mises stress distribution.</figcaption></figure>
<figure><img src="image_15.png" /><figcaption>**Fig. 14.** The radial displacement (a) and the axial displacement (b) of the component from the numerical simulation results of fluid-structure interaction during supercritical debinding at 423 K under 40 MPa.</figcaption></figure>

<table><caption>**Table 4**
Evolution of the volume load applied to the component function of the pressure.</caption><thead><tr><th>Pressure (MPa)</th><th>10</th><th>20</th><th>30</th><th>40</th><th>50</th></tr></thead><tbody><tr><td>F<sub>Vr</sub> (N mm<sup>-3</sup>)</td><td>0.2</td><td>0.4</td><td>0.6</td><td>0.8</td><td>1</td></tr><tr><td>F<sub>Vz</sub> (N mm<sup>-3</sup>)</td><td>0.125</td><td>0.250</td><td>0.375</td><td>0.500</td><td>0.625</td></tr></tbody></table>

are shown in Fig. 14. The obtained results show a maximum radial dilation on the outer surfaces of 0.07 mm corresponding to 1.4% during the process and a maximum elongation (axial) on the top of 0.3 mm corresponding to a deformation of 2%. The values obtained from the numerical simulation show that there are several dilatations during the process that could be compensated by the shrinkage induced by binder extraction.

A parametric study of the pressure inside the reactor was performed to characterize the effect of the fluid pressure on the mean equivalent von Mises stress of the component. It was increased from 10 to 50 MPa using the volume load $F_v$, Table 4. The evolution of the mean equivalent von Mises stress function of the supercritical fluid pressure is shown in Fig. 15.

The results show a linear relationship between the fluid pressure and the mean equivalent stress on the component due to the elasticity law. The mean stress increases from 0.5 to 3 MPa despite the pressure variation until 50 MPa. The results show that the high fluid pressure has non-important effect on the component's mechanical

<figure><img src="image_16.png" /><figcaption>**Fig. 15.** Evolution of the mean equivalent von Mises stress of the component vs carbon dioxide pressure inside the reactor from 10 MPa to 50 MPa.</figcaption></figure>

<table><caption>**Table 5**
Carbon and Oxygen levels inside the powder after solvent and supercritical fluid.</caption><thead><tr><th></th><th>C (ppm)</th><th>O (ppm)</th></tr></thead><tbody><tr><td>Water Debinding</td><td>540</td><td>1200</td></tr><tr><td>CO<sub>2</sub> supercritical debinding</td><td>780</td><td>1000</td></tr></tbody></table>

<table><caption>**Table 6**
Comparison of mechanical properties of sintered samples debinded by supercritical CO<sub>2</sub> and in water.</caption><thead><tr><th>Property</th><th>Water debinding</th><th>CO<sub>2</sub> SC debinding</th></tr></thead><tbody><tr><td>Relative density (%)</td><td>97.6</td><td>98.8</td></tr><tr><td>Shrinkage (%)</td><td>15.7</td><td>16.1</td></tr><tr><td>E (GPa)</td><td>63.38</td><td>65.4</td></tr><tr><td>Hardness (HV)</td><td>245 ± 7</td><td>268 ± 5</td></tr></tbody></table>

characteristics. After the debinding step, there is no defect on the component due to the high pressure.

## 4. Conclusions

Supercritical fluid debinding using carbon dioxide to extract PEG can be considered very efficient. This method eliminates only the PEG and ensures integrity of the rest of the binder. In the case of PP, there were no signs of chemical degradation, as indicated by the infra-red spectrum. The debinding cycles were optimised in terms of pressure and temperature. The components composed of fine-particle size distribution were successfully debinded in a shorter time compared to the water debinding method. The optimised debinding cycle at 423.15 K and, 40 MPa enabled the complete extraction of PEG. The powder volume loading affected the debinding time it was near the critical powder loading. From the results of the metallographic analysis and the pollution tests performed by the LECO analysis, it can be concluded that the supercritical debinding method does not change the microstructure of the Inconel 718 in terms of porosity and carbon/oxygen pollution. The results of the roughness analysis, i.e., the roughness measurement and the 3-D profile analysis of the components after water debinding and supercritical fluid debinding showed an improvement in the surface quality.

The effect of the fluid was modelled using the fluid structure interaction that combines fluid flow and structural mechanics using the 2-D axisymmetric problem which reduced the calculation time. The results show that the mean stress inside the component was low. This low stress is due to the boundary conditions applied to the component. However, the total elongation during the process was around 1%, mainly due to the volume load by the fluid pressure and the thermal expansion. The pressure variation shows a low stress variation inside the component. However, the variation of the fluid velocity did not considerably affect the component due to the low viscosity applied, and the variation of the inlet flow rate may have considerably affected the binder extraction rate. Thermal dilation is probably the greatest source of strain and stress inside the component during the supercritical debinding. It can be seen that the high pressure and temperature of the carbon dioxide did not cause defects inside the component. It can be noted that the model used in this study did not take into account the melting temperature of the polymers inside the component, the debinding time, or the pressure variation, which may affect the results of the simulations. Nevertheless, the experimental results are encouraging for the development of this method in the MIM industry, due to the much faster processing time and improvement in the roughness of the components.

## Acknowledgements

The authors wish to thank the Regional Council of Franche-Comté and the FUI PROPIM project for the financial support and supercritical equipment within the framework of the MIFHYSTO Equipex platform.

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