# Texturing injection molds using microelectronics techniques

Louise Burdin, Anne-Catherine Brulez, Radoslaw Mazurczyk, Jean-Louis

Leclercq, Stéphane Benayoun

► To cite this version:

Louise Burdin, Anne-Catherine Brulez, Radoslaw Mazurczyk, Jean-Louis Leclercq, Stéphane Benayoun. Texturing injection molds using microelectronics techniques. Surface and Coatings Technology, 2025, 502, pp.131989. 10.1016/j.surfcoat.2025.131989. hal-05350523

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Submitted on 6 Nov 2025

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# Texturing injection molds using microelectronics techniques

Louise Burdin<sup>a,*</sup>, Anne-Catherine Brulez<sup>a,b</sup>, Radosław Mazurczyk<sup>c</sup>, Jean-Louis Leclercq<sup>c</sup>, Stéphane Benayoun<sup>a</sup>

## ARTICLE INFO

### Keywords:

Microtexturation

X38CrMoV5

{Cl₂/Ar} plasma etching

Cr/CrN coating

Microinjection molding

## ABSTRACT

In recent years, there has been growing interest in imparting specific properties to the surfaces of polymers. This functionalization can be achieved through microinjection molding. For the first time, this study introduces the use of {Cl₂/Ar} plasma etching on an injection mold made of X38CrMoV5. However, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) analyses revealed the presence of nonvolatile chlorine residues on the mold surface, which were oxidized upon exiting the reactor. Therefore, {H₂}, {N₂}, and {O₂} plasma post-treatments were performed to reduce chlorine formation. Among these, the {H₂} post-treatment was the most effective, delaying the onset of corrosion by 90 days. To further enhance this protection, a Cr/CrN bilayer was deposited on the mold surface. However, this coating was required to meet specific criteria to ensure its durability and accurate replication of the molded parts. Characterization analysis revealed that the coating uniformly covered the mold textures, demonstrated good mechanical properties, adhered well to the mold, and showed no degradation after 1000 injection cycles. Furthermore, injection tests demonstrated excellent replication of micrometric patterns on polypropylene (PP) parts.

## 1. Introduction

The texturation of polymers to impart specific properties has been the focus of numerous studies, as it can be applied in various fields such as solar cells [1,2], automotive industry [3,4], or biomedical devices [5,6]. In the case of large-scale production with short cycle times, it is preferable, for cost considerations, to create textures using the injection-molding process. However, designing surfaces with superhydrophobic [7], antireflective [8], or with plasmonic properties [9] requires the replication of textures with micron-scale textures. This makes microinjection, which operates on the same principle as injection molding, a viable option. The principle is based on the rapid filling of the cold cavities in a mold. During filling, the plastic material flows into all the mold cavities. The cold mold walls cool and solidify the material. After cooling and solidification, the shaped plastic piece is demolded. Various techniques exist to functionalize injection molds, such as femtosecond laser texturing [10]. However, this method does not control the patterns precisely, making photolithography and plasma etching superior alternatives. For example, in the creation of superhydrophobic surfaces, studies have emphasized the importance of pattern slopes, as

highlighted in Extrand's model [11]. According to his model, one of the conditions for the surface to remain in the Cassie-Baxter state [12] is as follows:

$$ \theta_{a,0} + \omega > 180^{\circ} \qquad (1) $$

where $\theta_{a,0}$ denotes the advancing contact angle on the smooth surface, and $\omega$ represents the slope of the asperity as shown in Fig. 1.

The advancing contact angle on a smooth PP surface is 95° [13]. Therefore, the texture slope must exceed 85° to maintain a Cassie-Baxter state. However, achieving such a slope is extremely challenging with femtosecond laser texturing.

In the semiconductor industry, wet etching was the first method used for pattern transfer, and chemical plasma etching only started in the late 1960s, when Irving set out to remove photoresist films using an oxygen plasma [14]. Irving quickly recognized the potential of plasma etching for other materials, such as silicon oxide (SiO₂) and aluminum, using fluorine- or chlorine-based plasmas, respectively [15]. The major advantage of using plasma etching to pattern semiconductor devices was to minimize the chemical waste produced by wet etching methods [16]. In addition, silicon nitride emerged as the material of choice; however,

<footnote><sup>a</sup> Ecole Centrale de Lyon, CNRS, ENTPE, LTDS, UMR5513, 69130 Ecully, France</footnote>

<footnote><sup>b</sup> ITECH, 69130 Ecully, France</footnote>

<footnote><sup>c</sup> Univ Lyon, CNRS, INSA Lyon, Ecole Centrale de Lyon, Université Claude Bernard Lyon 1, CPE Lyon, INL, UMR5270, 69130 Ecully, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: louise.burdin@ec-lyon.fr (L. Burdin).</footnote>
<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /><figcaption>**Fig. 1.** Schematic representation of the advancing contact angle on a smooth surface ($\theta_{a,0}$) (a). $\theta_{r,0}$ corresponds to the receding contact angle on a smooth surface, and $\alpha$ represents the angle of substrate inclination. Schematic representation of pillars with associated topographical parameters (b).</figcaption></figure>

<table><caption>**Table 1**
Most common semiconductors plasma etched in {Cl₂/Ar} in an ICP reactor.</caption><thead><tr><th>Type</th><th>Material</th><th>Plasma used</th><th>Reference</th></tr></thead><tbody><tr><td>Type IV-IV semiconductor</td><td>Si</td><td rowspan="5">{Cl₂/Ar}</td><td>[27]</td></tr><tr><td rowspan="5">Type III-V semiconductor</td><td>GaN</td><td>[28]</td></tr><tr><td>GaAs</td><td>[29]</td></tr><tr><td>GaP</td><td>[30]</td></tr><tr><td>InP</td><td>[31]</td></tr><tr><td>X2CrNiMo18-15-3</td><td>[21]</td></tr><tr><td rowspan="2">Steel</td><td>X50CrMoV15</td><td>[21]</td></tr><tr><td>NiFe</td><td>[32]</td></tr><tr><td rowspan="2">Films</td><td>NiFeCo</td><td>[32]</td></tr></tbody></table>

no suitable wet etchant existed to pattern it¹⁶. Fluorine-based plasmas soon became the ideal solution for patterning nitride silicon without damaging the aluminum in the contact pads. Other studies have performed silicon etching using {CF₄} plasma for the fabrication of integrated circuits [17–20]. Under the influence of an electromagnetic field, CF₄ is dissociated into fluorine atoms, radicals, and ions. The fluorine atoms chemically react with silicon to form SiF₄. This product, highly volatile at room temperature, easily desorbs from the surface, constituting chemical etching. Simultaneously, physical etching occurs, with positive ions being accelerated toward the surface, causing ion bombardment.

Since then, the etching process has continuously evolved, particularly with the use of chlorine-based plasmas to etch group III-V semiconductors, which are used in the fabrication of transistors, diodes, lasers, and others [21]. Similar to {CF₄} plasma, chlorine plasma enables the combination of chemical and physical etching. Some studies have shown that using {Cl₂/Ar} plasma is more effective than pure {Cl₂} plasma [22,23]. Indeed, argon increases the ion energy and facilitates the desorption of chlorines formed on the substrate surface. However, certain materials comprising transition metals remain extremely difficult to etch owing to their low vapor pressure at temperatures below a few hundred degrees. Nevertheless, the microtexturing of metallic materials to provide new properties is of considerable interest not only for microelectronics [24] but also for applications in biology [25], optics [26], and electrical engineering [25].

However, to the best of our knowledge, no study has been conducted on the microtexturing of injection molds using plasma etching. Therefore, the aim of this study was to propose a method for etching injection molds using chlorine-based plasma to create microtextured parts. To achieve this, an X38CrMoV5 steel insert is first etched with a {Cl₂/Ar} plasma in an inductively coupled plasma (ICP) reactor. However, conventional materials used for such methods are semiconductor materials (Table 1), and only few studies have focused on steels. Furthermore, to the best of our knowledge, no studies have been conducted on plasma etching of X38CrMoV5. Therefore, this study proposes a protocol for etching such steel.

Laourine [21] et al. demonstrated that {Cl₂/Ar} plasma etching can lead to corrosion on the surface of etched materials. They studied the influence of using {H₂} plasma post-treatment. Results are consistent

with those obtained by Jung et al. [32] Surfaces exposed to {H₂} plasma exhibited no signs of corrosion, even after several months of treatment. However, no precise quantitative data are provided.

The final quality of plastic parts depends on the polymer used, the shape of mold textures, and mold coating characteristics. Due to the micrometric sizes of the textures, replication quality is influenced by factors such as the coefficient of friction, surface energy, and the chemical interaction between the mold and the molten polymer [33]. Therefore, to improve replication quality, mold coatings must exhibit certain characteristics, such as a low coefficient of friction, good adhesion to the mold, low adhesion to the polymer, high wear resistance, and the ability to withstand high pressure [33]. In the plastic industry, metal nitride coatings are often used to improve the quality of part replication. Indeed, the adhesion between the mold and the melt polymer is a crucial parameter to consider. Vera et al. [34] noted that the adhesion work between the mold and the melt polymer needs to be significant to ensure good impregnation of the mold cavities. Furthermore, they demonstrated that a CrN-based coating, compared to TiN, improved the quality of replication.

Additionally, CrN-based coatings are widely used to protect injection molds against corrosion. Compared to TiN coatings, CrN coatings are softer, less brittle, and exhibit lower stress [35]. Furthermore, although TiN coatings present higher mechanical and thermal properties, they oxidize easily to form TiO₂ at temperatures above 500 °C, leading to cracks in the coating [36,37]. Alternatively, CrN coatings, offer better corrosion resistance and form a Cr₂O₃ layer on the surface that acts as a protection layer [38,39]. Currently, there are no specific requirements regarding the coating thickness. The thickness must be sufficient to protect the surface from corrosion but not too large as it could reduce the mechanical properties of the coating [40]. In many studies, coating thickness is ~3 µm [33,41–43]. Additionally, metallic films could be used as interlayers to improve the adhesion of the coating on the surface and enhance the corrosion resistance of the coating [44,45].

The main objective of this study is to develop a {Cl₂/Ar} plasma etching protocol for X38CrMoV5 steel. However, as previously mentioned, this type of etching generally induces corrosion. Therefore, different post-treatments are investigated, and the evolution of the oxygen percentage is quantitatively monitored several months after etching. One of the advantages of plasma etching is that it allows for the obtention of nearly vertical texture slopes, unlike other methods such as femtosecond laser texturing. This control over the slopes is crucial in certain applications, such as the design of superhydrophobic surfaces. Moreover, once the mold is textured, it enables the rapid and large-scale production of parts through injection molding.

Subsequently, a Cr/CrN bilayer is deposited on the mold surface to enhance its durability and replication quality. The adhesion and ability of the coating to withstand multiple injection cycles are analyzed. Injection tests are performed using PP. PP is frequently used in various fields, such as automotive, aerospace, and home appliances, due to its ease of processing and its highly desirable mechanical and thermal properties [46]. Finally, optical and topographical characterizations are conducted on the mold and the obtained parts to assess the replication
<table><caption>**Table 2**
Percentage of elements present in the X38CrMoV5 steel.</caption><thead><tr><td>Chemical element</td><td>Chemical composition in weight %</td></tr></thead><tbody><tr><td>C</td><td>0.37</td></tr><tr><td>Si</td><td>1.0</td></tr><tr><td>Cr</td><td>5.3</td></tr><tr><td>Mo</td><td>1.3</td></tr><tr><td>V</td><td>0.4</td></tr></tbody></table>

quality of the parts and the durability of the mold.

## 2. Materials and methods

### 2.1. Materials selection

The X38CrMoV5 steel, whose chemical composition is presented in Table 2, is known for its robustness and corrosion resistance. This steel is a material most commonly used in the manufacture of molds for injection molding.

Only the insert in the cavity that is textured is made of the X38CrMoV5 steel. This part is inserted into a stainless-steel insert, X105CrMo17 which is directly screwed onto the mobile half of the mold (Fig. 2). From an economic point of view, this allows machining only the insert to be textured. Functionally, this approach avoids the need to disassemble the entire tooling setup between each injection cycle. Additionally, it is not possible to place large parts in the equipment used for mold etching.

The polymer selected for this study is PP (ISPLEN® PR 290 X9M, RESPOL). This thermoplastic presents excellent processability and properties, which are presented in Table 3. The drying of the plastics before processing is essential. The PP is placed in a vacuum oven at 80 °C for at least 2 h.

As discussed previously, no study has been conducted on the plasma etching of X38CrMoV5. Therefore, it appears to be more appropriate to focus on simple patterns, such as grooves. However, this type of pattern creates anisotropic surfaces. Therefore, the selected patterns are

cylindrical pillars with a diameter of 10 µm and spacing of 10 µm.

### 2.2. Plasma etching

The entire etching process described in this section was developed in

<table><caption>**Table 3**
PP datasheet from the supplier.</caption><thead><tr><td>Properties</td><td>Method</td><td>Unit</td><td>Value</td></tr></thead><tbody><tr><td>Melt flow rate (230 °C; 2.16 kg)</td><td>ISO 1133</td><td>g/10 min</td><td>25</td></tr><tr><td>Density</td><td>ISO 1183</td><td>g/cm3</td><td>0.905</td></tr><tr><td>Flexural modulus</td><td>ISO 178</td><td>MPa</td><td>800</td></tr><tr><td>Heat Deflection Temperature</td><td>ISO 75/B</td><td>°C</td><td>60</td></tr></tbody></table>

<table><caption>**Table 4**
Polishing steps.</caption><thead><tr><th>Step</th><th>1</th><th>2</th><th>3</th><th>4</th><th>5</th></tr></thead><tbody><tr><td>Support</td><td>Abrasive paper P320 (Presi)</td><td>Abrasive paper P600 (Presi)</td><td>Woven support TOP (Reflex PAD-MAG, Presi)</td><td>Woven support RAM (Reflex PAD-MAG, Presi)</td><td>Woven support NT (Reflex PAD-MAG, Presi)</td></tr><tr><td>Lubricant</td><td>Water</td><td>Water</td><td>Diamond suspension 9 µm (Reflex LDP, Presi)</td><td>Diamond suspension 3 µm (Reflex LDP, Presi)</td><td>Diamond suspension 1 µm (Reflex LDP, Presi)</td></tr><tr><td>Force (daN)</td><td>2.5</td><td>2.5</td><td>2</td><td>2</td><td>2</td></tr><tr><td>Plate speed (rpm)</td><td>300</td><td>300</td><td>150</td><td>150</td><td>150</td></tr><tr><td>Head speed (rpm)</td><td>150</td><td>150</td><td>135</td><td>135</td><td>135</td></tr><tr><td>Time (s)</td><td>30</td><td>60</td><td>180</td><td>120</td><td>90</td></tr></tbody></table>

<figure><img src="image_6.png" /><figcaption>**Fig. 2.** Diagram of the mold system and the molded part.</figcaption></figure>
<table><caption>**Table 5**
Experimental conditions for plasma etching.</caption><thead><tr><th>Step</th><th>1</th><th>2</th><th>3</th></tr></thead><tbody><tr><td>ICP power (W)</td><td>300</td><td>400</td><td>800</td></tr><tr><td>RF power (W)</td><td>120</td><td>25</td><td>170</td></tr><tr><td>Pressure (Pa)</td><td>0.3</td><td>2</td><td>0.67</td></tr><tr><td>Ar flow (sccm)</td><td>30</td><td>-</td><td>9</td></tr><tr><td>CHF<sub>3</sub> flow (sccm)</td><td>9</td><td>-</td><td>-</td></tr><tr><td>H<sub>2</sub> flow (sccm)</td><td>4</td><td>-</td><td>-</td></tr><tr><td>O<sub>2</sub> flow (sccm)</td><td>-</td><td>50</td><td>-</td></tr><tr><td>Cl<sub>2</sub> flow (sccm)</td><td>-</td><td>-</td><td>21</td></tr><tr><td>Temperature (°C)</td><td>20</td><td>20</td><td>200</td></tr></tbody></table>

<table><caption>**Table 6**
Experimental conditions for Cr/CrN coating.</caption><thead><tr><th></th><th>Cr</th><th>CrN</th></tr></thead><tbody><tr><td>Ar flow (sccm)</td><td>50</td><td>20</td></tr><tr><td>N<sub>2</sub> flow (sccm)</td><td>-</td><td>5</td></tr><tr><td>Pressure (mbar)</td><td>5.10<sup>-3</sup> mbar</td><td>5.10<sup>-3</sup> mbar</td></tr><tr><td>RF power applied to the substrate (W) (Bias (-V))</td><td>50 (481)</td><td>50 (484)</td></tr><tr><td>DC power applied to the target (W) (Bias (-V))</td><td>350 (355)</td><td>350 (400)</td></tr><tr><td>Target-substrate distance (cm)</td><td>9.7</td><td>9.7</td></tr><tr><td>Time (min)</td><td>17</td><td>40</td></tr></tbody></table>

<figure><img src="image_7.png" /><figcaption>**Fig. 3.** Diagram of the obtained part (dimensions are expressed in mm).</figcaption></figure>

the cleanroom at the Institut des Nanotechnologies de Lyon.

Before etching, the samples were mirror polished using an automatic polishing device (Mecatech 250 SPC, Presi). The different polishing steps are presented in Table 4. The samples were then cleaned in de-ionized water, dried with compressed air, and subsequently coated with a 3-µm-thick SiO₂ mask fabricated by plasma-enhanced chemical vapor deposition. All samples were patterned using standard ultraviolet photolithography. A ~ 4.5-µm SPR 220-4.5 photoresist was spin-coated over the samples before exposure and development in MF26-A developer. The samples were subsequently etched via ICP (ICP Etch System from SENTECH Instruments GmbH) in three steps under the conditions presented in Table 5. Each etching step performed until the SiO₂ mask was completely etched.

After etching, the insert is coated with a 1-µm-thick layer of Cr and a 2-µm-thick layer of CrN by magnetron sputtering under the conditions presented in Table 6. The Cr target has a diameter of 101 mm and a purity of 99.95 %.

## 2.3. Injection molding

Injection molding was performed at the Institut Technique et Chimique de Lyon on a 30 T electric Fanuc (Roboshot α-S30iA) industrial equipment with a 22-mm screw diameter. Fig. 3 shows the parts obtained during the injection-molding trials. The part is a dog-bone specimen with a thickness of 1.5 mm, a width of 30 mm and a length of 114.8 mm. The textured zone, measuring 78.5 mm², is located in the center of the part. The injection parameters are presented in Table 7.

<table><caption>**Table 7**
Injection parameters.</caption><tr><td>Plastic temperature (°C)</td><td>240</td></tr><tr><td>Mold temperature (°C)</td><td>60</td></tr><tr><td>Injection velocity (mm/s)</td><td>150</td></tr><tr><td>Holding pressure (bar)</td><td>350</td></tr></table>

## 2.4. Samples characterization

The textured injection molds and obtained parts were qualitatively and quantitatively analyzed using a SEM instrument (MIRA 3, Tescan) and an interferometer (GT-K1, Bruker). SEM analyses were performed using a secondary electron detector with an accelerating voltage of 15 kV. Furthermore, SEM observations were completed with EDX, particularly to monitor the evolution of the oxygen percentage on the surface.

As mentioned previously, topography was studied quantitatively by optical interferometry in vertical scanning interferometry mode using white light illumination at 50× magnification (Mirau Interferometer) and a 0.5× lens. All the obtained interferometric measurements were then post-processed with MountainsMap software (version 7.4.8076, Digital Surf) [47].

Daimler-Benz Rockwell-C adhesion tests [48] were performed to characterize the adhesion between the Cr/CrN coating and the substrate. The principle involves using a diamond cone indenter into a coated surface to induce plastic deformation of the substrate and fracture of the coating. The sample is then observed using an optical microscope, and based on the defects observed (Fig. 4), it is possible to determine whether or not the coating adheres to the substrate.

# 3. Results and discussion

## 3.1. Plasma etching of X38CrMoV5 insert

A {Cl₂/Ar} plasma etching protocol for X38CrMoV5 steel was developed using the conditions presented in the previous section. However, as shown in Fig. 5a, particles on the order of a few micrometers formed on the surface after etching. EDX analyses were performed on the areas indicated in Fig. 5a to determine the chemical nature of the particles. As shown in Fig. 6, zone 2 presents a high amount of oxygen and chlorine, along with a reduction in the amount of Fe compared to zone 1. As mentioned previously, {Cl₂/Ar} plasma etching can result in the formation of nonvolatile chlorine residues. Therefore, the particles visible in Fig. 5a may correspond to chlorines that were oxidized upon exposure after exiting the ICP reactor.

This result agrees with the findings of Laourine's study [21] where the etching of X2CrNiMo18-15-3 and X50CrMoV15 steel was performed. To prevent corrosion, the influence of a hydrogen plasma post-treatment was studied. EDX analysis confirmed no trace of chlorine. This result was obtained by Jung et al. [32] when they studied the effect of a hydrogen plasma after {Cl₂/Ar} plasma etching of thin films of NiFe and NiFeCo in an electron cyclotron resonance reactor.

Herein, three post-treatments are studied: {H₂}, {N₂}, and {O₂}. SEM images after each of these treatments, along with the associated EDX analysis, are presented in Figs. 5 and 6. As shown in Fig. 5c and d, particles are present on the surface of the insert after etching. EDX analysis shows that zones 5 and 7 exhibit a vital percentage of oxygen and chlorine. Moreover, zones 4 and 6, which do not include these particles, show a higher percentage of oxygen than zone 1. Thus, {N₂} and {O₂} post-treatments do not prevent corrosion; on the contrary, these treatments increase it. However, for the {H₂} post-treatment, the percentage of oxygen on the surface is extremely low (Fig. 6). Indeed, hydrogen radicals react with chlorine residues, leading to their removal through the formation of HCl during exposure to the {H₂} plasma [32,49]. Similar results were obtained in Laourine's study [21]. Nonetheless, Laourine [21] does not propose corrosion monitoring for the
<figure><img src="image_8.png" /><figcaption>Fig. 4. Schematic representation of the Daimler-Benz Rockwell-C adhesion test principle, including the different types of induced indentations [48].</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Fig. 5. SEM images of {Cl₂/Ar} plasma etching of X38CrMoV5 followed by no post-treatment (a), H₂ (b), N₂ (c), and O₂ post-treatment (d). The numbers correspond to the zones where the EDX measurements were obtained.</figcaption></figure>

months following the etching. Given that the insert will be used in an injection molding press, it must not corrode for cost and maintenance reasons. Detailed corrosion monitoring is therefore necessary to ensure optimal durability of the insert. The graph in Fig. 7 shows the evolution of oxygen percentage on the hydrogen plasma post-treated surface over 90 days after etching.

According to Fig. 7, although the oxygen percentage remains low (<10 %), it still tends to increase. Therefore, the {H₂} post-treatment does not indefinitely protect the surface. To enhance the durability of the insert, an initial layer of 1 µm of Cr followed by a 2-µm-thick layer of CrN is deposited on the surface. However, this coating must meet certain specifications, such as uniform thickness. Although the thickness cannot be exactly the same in all the textures, it must still be sufficient to protect the insert. To measure the coating thickness in the textures, the insert is

immersed in liquid nitrogen for 30 min and then fractured to obtain a cross-sectional view, as shown in Fig. 8.

## 3.2. Coating characterization

Using SEM images (Fig. 8), it was possible to measure the coating thickness, which is 3.07 ± 0.19 µm on the surface of the insert (1), 2.98 ± 0.11 µm at the bottom of the textures (2), and 2.34 ± 0.14 µm on the sides of the textures (3). The coating covers all the textures of the insert with sufficient thickness.

As observed in Fig. 8, the coating exhibits surface irregularities with a span of a few microns. This phenomenon is well known and has been reported several times in the literature [43,50]. These surface defects can lead to irregularities on the top of the pillars of the plastic parts,
<figure><img src="image_10.png" /><figcaption>Fig. 6. Atomic percentage of the different atoms present in each zone.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>Fig. 7. Evolution of oxygen percentage over days following etching and hydrogen plasma post-treatment.</figcaption></figure>

resulting in a double scale of roughness. However, it is possible to adjust certain parameters to achieve a smoother surface. For example, Zairi [51] showed that increasing the thickness of the CrN coating could reduce roughness. Similarly, an increase of the nitrogen flow could result in a decrease in the surface roughness [51]. However, this study focuses only on the design and obtention of microtextured parts rather than on the optimization of the coating process, which can be conducted in future studies.

The coating must adhere to the surface and not peel off during various injection cycles. A 1-µm-thick layer of Cr followed by a 2-µm-thick layer of CrN, was deposited on a surface made of X38CrMoV5 steel by magnetron sputtering. The surface was then subjected to the Daimler-Benz Rockwell-C indentation test. Results are shown in Fig. 9. The initial sputtering tests were conducted in an equipment that did not allow for substrate polarization. However, as shown in Fig. 9a, after indentation of the conical diamond, the coating presents numerous cracks and delamination. Therefore, tests were performed in equipment that could

<figure><img src="image_12.png" /><figcaption>Fig. 8. SEM image of a cross-sectional view of textured X38CrMoV5 insert coated with a Cr/CrN bilayer (where 1 is the thickness on the surface of the textures, 2 is the thickness at the bottom of the textures, and 3 is the thickness on the side of the textures).</figcaption></figure>

polarize the substrate under the conditions presented in Section 2.2. This time, the defects observed after indentation are acceptable (Fig. 9b) and allow us to conclude on the good adhesion between the Cr/CrN coating and the substrate.

Finally, the coating must not be damaged by the plastic material during various injection cycles. As shown in Fig. 10a, a defect divided into three zones on the insert was observed before injection. These zones were selected because they are easily identifiable during SEM analysis, and, due to defects, they are likely to deteriorate first. The area of each zone is calculated after multiple injection cycles to observe whether there is any degradation of the Cr/CrN bilayer. Results are presented in Fig. 10b. According to Fig. 10b, the area of each zone did not change drastically and is approximately the same after 1000 injection cycles. In conclusion, the Cr/CrN bilayer coating adheres extremely well to the insert and does not deteriorate after 1000 injection cycles.
<figure><img src="image_13.png" /><figcaption>Fig. 9. Daimler-Benz Rockwell-C adhesion test results for Cr/CrN bilayer obtained by magnetron sputtering: without substrate polarization (a); with substrate polarization (b).</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>Fig. 10. SEM images of defects on X38CrMoV5 insert textured and covered by a Cr/CrN bilayer before injection (a). Evolution of the area of each zone as a function of the number of injection cycles (b).</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>Fig. 11. Comparison of the topographical parameters of the mold and PP parts after 100 injection cycles (a). Interferometric image of the injection molded PP part (b).</figcaption></figure>

### 3.3. *Texture replication quality*

The aim of this study was to obtain microtextured plastic parts. Therefore, the topography of the insert and PP parts is characterized to conclude on the quality of the replication. Different parameters are retained: diameter of the pillars (D), height of the pillars (H), and period between the pillars (P), i.e., the diameter plus the spacing between the pillars, as shown in Fig. 1. The graph in Fig. 11a compares the

topographical parameters of the textures obtained by plasma etching on the mold with those of the plastic parts after 100 injection cycles. Results reveal considerably good replication of the plastic parts.

Nevertheless, the pillars obtained show irregularities at their tops, as shown in Fig. 11b. These irregularities are mainly due to the Cr/CrN coating, which is not smooth. As mentioned previously, certain parameters can be adjusted to reduce the roughness of the coating, depending on the desired application. Indeed, for the development of
<figure><img src="image_16.png" /><figcaption>Fig. 12. Evolution of the topographical parameters of the PP parts (a) and mold (b) after multiple injection cycles.</figcaption></figure>

<figure><img src="image_17.png" /><figcaption>Fig. 13. Topographic profile of PP parts obtained using MountainsMap software [47].</figcaption></figure>

superhydrophobic surfaces, numerous studies have highlighted the importance of a dual-scale roughness to promote the Cassie-Baxter state.

Topographic measurements are performed after several injection cycles to ensure that the mold does not degrade (Fig. 12). Indeed, it is possible to imagine that plastic particles could remain stuck at the bottom of the textures.

According to Fig. 12a, the topographical parameters did not change drastically over the injection cycles, suggesting good replication of the parts even after 1000 cycles. Furthermore, Fig. 12b shows that topographical parameters of the mold before injection and after 1000 cycles are more or less the same, suggesting that the mold has not deteriorated.

Finally, as mentioned previously, one of the advantages of using the lithography process coupled with plasma etching is the control over textures, which can be extremely crucial in certain fields. As an example, to achieve superhydrophobic surfaces, steep slopes are sought to maintain a Cassie-Baxter state. Legrand [52] conducted femtosecond laser etching of injection molds to produce textured PP parts with micrometric cylindrical pillars. The resulting textures had the following dimensions: a diameter of 21.9 ± 0.6 µm, a spacing of 78.6 ± 1.2 µm, a height of 10.2 ± 0.3 µm, and a slope of 57 ± 8°. Therefore, laser texturing alone was insufficient to achieve superhydrophobic surfaces due to the inadequate texture slopes. In comparison, Fig. 13 shows the profile of PP parts obtained using an injection mold etched with {Cl₂/Ar} plasma followed by an {H₂} post-treatment. For an average height of 6.73 ± 0.14 µm, the slope (ω) is 80 ± 3° (Fig. 13). Thus, such a surface can be beneficial for designing superhydrophobic surfaces.

## 4. Conclusion

In conclusion, the main challenge faced in this study was the mold's etching due to its chemistry's complexity. As mentioned previously, to our knowledge, no studies have been conducted on the plasma etching of injection molds and even less on the plasma etching of X38CrMoV5.

Thus, the {Cl₂/Ar} plasma etching protocol was developed. However, this type of etching leads to the formation of nonvolatile chlorines that oxidize upon exiting the reactor. Several treatment methods were tested to limit the formation of chlorine residues. According to the results, {H₂} plasma provided the optimal protection for the surface against corrosion, even 90 days after etching. Nevertheless, the protection is not indefinite, and to reduce mold maintenance costs, the surface is protected by a Cr/CrN bilayer. Various characterizations of the coating demonstrated excellent thickness uniformity, even on the sides of the textures, strong adhesion to the steel, and good mechanical properties. No degradation was observed, even after 1000 injection cycles. Finally, the injection tests showed excellent and durable replication quality of the PP parts throughout the injection cycles. Thus, the etching method proposed in this study enables rapid and large-scale production of plastic parts. Additionally, the molds can be textured to impart specific functionalities, such as superhydrophobicity. In a future study, it would be interesting to characterize these functionalities.

Nevertheless, the injection tests were conducted with PP, a thermoplastic that is easy to process. To test the long-term durability of the mold and coating, it would be advisable to test other polymers, such as high-density polyethylene or polycarbonate, which have higher viscosities, or even greener polymers such as polylactic acid or recycled polymers. Moreover, only 1000 injection cycles were performed, which is already significant at the laboratory scale. However, to assess whether this project can be industrialized, testing on a far larger scale would be necessary.

## CRediT authorship contribution statement

**Louise Burdin:** Writing – review & editing, Writing – original draft, Visualization, Validation, Software, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **Anne-Catherine Brulez:** Writing – review & editing, Validation, Supervision, Resources, Project
administration, Methodology, Funding acquisition, Conceptualization.
**Radoslaw Mazurczyk:** Writing – review & editing, Validation, Super-
vision, Project administration, Methodology, Conceptualization. **Jean-
Louis Leclercq:** Writing – review & editing, Validation, Supervision,
Resources, Project administration, Methodology, Funding acquisition,
Conceptualization. **Stéphane Benayoun:** Writing – review & editing,
Validation, Supervision, Resources, Project administration, Methodol-
ogy, Funding acquisition, Conceptualization.

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.

Data availability

Data will be made available on request.

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