# Role of crystallographic and morphological textures on wear resistance of an additive-manufactured Ni-based superalloy

Pierre-Nicolas Parent, Jean-Yves Paris, Joel Alexis, Christine Boher

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Pierre-Nicolas Parent, Jean-Yves Paris, Joel Alexis, Christine Boher. Role of crystallographic and morphological textures on wear resistance of an additive-manufactured Ni-based superalloy. Wear, 2025, 570, pp.205894. 10.1016/j.wear.2025.205894. hal-04957518

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https://imt-mines-albi.hal.science/hal-04957518v1

Submitted on 16 Jul 2025

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# Role of crystallographic and morphological textures on wear resistance of an additive-manufactured Ni-based superalloy

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Pierre-Nicolas Parent<sup>a,b, ID,*</sup>, Jean-Yves Paris<sup>b, ID</sup>, Joël Alexis<sup>b</sup>, Christine Boher<sup>a, ID</sup>

## ARTICLE INFO

Keywords:

Microstructure

Wear

Additive manufacturing

Ni-based superalloy

## ABSTRACT

This study employs the LPBF process to design components with specific scanning strategies, particularly emphasising the crystallographic texture. The present study aims to assess the relationships between the microstructural features and the tribological behaviour of a novel Ni-based superalloy developed explicitly for additive manufacturing. The tribological tests are conducted with a reciprocating ball-on-flat rig without lubrication at room temperature. The analysis of the as-built microstructures and wear tracks was primarily conducted using SEM and XRD techniques. A variety of scanning strategies are investigated to tailor the microstructures. The specific wear rate is sensitive to changes in microstructure, with values ranging from $2.82 \pm 0.58 \times 10^{-7} \text{mm}^3/(\text{N mm})$ to $4.75 \pm 0.43 \times 10^{-7} \text{mm}^3/(\text{N mm})$. However, it is observed that samples manufactured using different scanning strategies but with the same resulting crystallographic texture exhibit different wear rates. Consequently, the crystallographic texture does not significantly affect the wear rate. Cellular structure, in terms of size, morphology and heterogeneities among melt pools, and coverage rate by oxidised layers exert a more pronounced influence. The SEM observations of the wear tracks indicate that the primary main wear mechanisms are abrasive and oxidative wear.

## 1. Introduction

Tribology is a multi-scale and multi-disciplinary science [1]. Among the various approaches to assess tribological behaviour, the microstructure is particularly interesting, given its relationship to other material properties. The emergence of additive manufacturing technologies has enabled the fabrication of components with complex geometries and intricate microstructures, which are strongly influenced by the parameters employed during the manufacturing process. These process parameters can be regrouped into two main groups. The first regroup the laser parameters (power, speed, ...), and the other one the scanning strategies related to the laser beam path and the building direction (BD) of the parts in the building chamber.

In their work on Inconel 718 produced using the LPBF process, Gokcekaya et al. [2] generated various crystallographic textures, ranging from a texture resembling a single-crystal structure oriented <101> //BD to a polycrystalline structure. Nadammal et al. [3] demonstrated the influence of hatch length on crystallographic texture. Modifying the hatch length allowed them to adjust the intensity of the texture without changing the crystallographic texture plane. Xu et al. [4] studied the influence of the laser scanning strategy on the microstructure of Inconel 738 parts produced via LPBF. They identified a bimodal grain structure

for rotations between layers of 67° and 90°, consisting of a mix of columnar and equiaxed grains. For the 90° rotation, the texture was primarily along the <001> direction. In contrast, parts without layer rotation exhibited a structure mainly composed of large columnar grains oriented along <001> on one vertical surface (parallel to the build direction), and multiple columnar grains oriented along <101> on the other vertical surface. The horizontal surface (perpendicular to the building direction) consisted of a mix of columnar grains oriented along <101> and fine equiaxed grains oriented along <100>. Other authors also observed these crystallographic textures and grain structure variations due to laser scanning strategy modifications [5-7]. Thus, the laser scanning strategy allows for the generation of a wide diversity of microstructures, and is the process parameter used to generate the different microstructures in this study.

However, the relative density of the parts is sensitive to this parameters [8], and a low relative density leads to reduced wear resistance [9]. Therefore, the studied material is the ABD-900AM, a Ni-based superalloy developed for additive manufacturing. Its specificity is that it is printable over a wide range of process parameters without defects, allowing us to keep the parts' chemical composition constant. Since the parts have a high relative density, we can only work on the influence

<footnote><sup>a</sup> Institut Clément-Ader (ICA) ; Toulouse University ; CNRS, IMT Mines Albi, INSA, ISAE-SUPAERO, UPS, Campus Jarlard, 81013 Albi, France</footnote>

<footnote><sup>b</sup> Laboratoire Génie de Production (LGP), UTTOP, Toulouse University, 47 avenue d'Azereix, 65000 Tarbes, France</footnote>

<footnote>* Corresponding author at: pierrenicolas parent@uttop.fr (P.-N. Parent).</footnote>

<footnote>E-mail address: pierrenicolasparent@uttop.fr (P.-N. Parent).</footnote>
of the microstructure.

During the solidification of metals, the cooling conditions at the solid/liquid (S/L) interface dictate the resulting structure of the material. These cooling conditions can be analysed through the temperature gradient (G) and the solidification rate (R), where their ratio determines the structure formed during cooling. At the same time, the product G x R influences the finess of the microstructure [10]. The solidification mode can be planar, cellular, columnar dendritic, or equiaxed dendritic [10]. Research in the literature shows that the structure developed during solidification in the LPBF process is cellular. Depending on the laser parameters and scanning strategy, this cellular structure can exhibit a columnar morphology, equiaxed morphology, or a mixture of both [11]. A secondary phenomenon during solidification is the micro-segregation of alloying elements into intercellular regions. This phenomenon arises from the instability of the planar S/L interface, which tends towards cellular solidification by radially pushing solutes into the liquid between the cells [10,12]. This is typically referred to as a cellular or cellular-dendritic structure. The solidification of this solute-rich liquid phase leads to micro-segregation, forming the Laves phase at this scale.

For nickel-based superalloys, the cells can be defined according to two approaches regarding the cell walls. The first approach defines the cell walls as regions with a chemical composition different from the cell core. The second approach defines cells as walls with a high density of dislocation entanglements referred to as "dislocation cellular patterns" (DCP). The mechanical behaviour and formation mechanism of DCPs is different from those of dislocation cells induced by plastic deformation [13]. Most of the dislocations in the as-fabricated state are accumulated in DCPs. Several researchers have explained the formation mechanism of DCPs using thermal history, which relates to the thermal stresses induced by heating and cooling cycles [14]. The presence of zones enriched in alloying elements helps to block the movement of dislocations and fix them in the intercellular regions, contributing to the final morphology of DCPs. This cell type, where a dislocation wall and a zone enriched in solute atoms overlap, is specific to the LPBF process. The size of the cells is an important parameter to consider as it creates the characteristic spacing between two barriers for dislocation movement. However, determining this size is challenging because cell size is directly related to the cooling rate, which is heterogeneous in a melt pool. For example, Tao et al. [15] measured cellular spacings ranging from $27.6 \times 10^{-2}$ µm to $147.3 \times 10^{-2}$ µm, which can result in large standard deviations. The geometric characteristics of the cells are microstructural parameters whose influence on tribological behaviour and mechanical properties has been discussed by various authors [16–20]. In their study on Ni20Cr, Joshi et al. [20] showed that cells are responsible of 43% of the yield stress, and that strain hardening is controlled by cells. Furthermore, Yan et al. [16] studied the tribological behaviour of Inconel 625 parts manufactured with LPBF process with various building directions, and linked the wear resistance to the cellular size and morphology within melt pools. However, the cellular size has not been quantified.

This study investigates the influence of various multi-scale microstructures on tribological behaviour. To do so, the LPBF process is used because it can modulate the microstructure of the parts. The modified process parameter is the scanning strategy related to the laser beam direction. The microstructural features analysed are the crystallographic texture and the cellular structure (morphology and size). The crystallographic texture is determined using X-ray diffraction (XRD) measurements, while the cellular structure is analysed using image processing techniques. The microstructural characterisations will first be presented according to the different scanning strategies, and then the tribological behaviour of the parts will be analysed. Finally, the end of the article focuses on understanding the links between the various elements involved in part wear.

<table><caption>**Table 1**
Laser parameters.</caption><thead><tr><td>Laser power</td><td>Laser speed</td><td>Hatching distance</td></tr></thead><tbody><tr><td>275 W</td><td>760 mm/s</td><td>120 µm</td></tr></tbody></table>

## 2. Materials & methods

### 2.1. Alloy studied: the ABD-900AM

The powder used to manufacture the parts is a nickel-based superalloy called ABD-900AM. It is a powder marketed by Aubert & Duval®, and the supplier specifies its physicochemical characteristics. The powder is manufactured using the argon gas atomisation process. Its characteristic diameters, obtained following ASTM B822-2020, are $D10 = 18.6$ µm, $D50 = 31.8$ µm and $D90 = 54.3$ µm. The chemical composition of the powder is, in wt%, Ni = 49.8, Cr = 17.0, Co = 20.4, Mo = 2.1, W = 3.1, Al = 2.1, Ti = 2.3, Ta = 1.5, Nb = 1.8, C = 0.049, O = 0.011, N = 0.010 and B = 0.004.

The 3D printing machine used for this work is the SLM® 125HL from SLM Solutions GmbH, equipped with a 400 W continuous laser. During printing, the production chamber is hermetically sealed and flushed with neutral argon gas to prevent oxidation, maintaining oxygen levels below 0.2%. The main laser parameters are given Table 1.

The laser parameters for all production runs were set to a laser power of 275 W, a scanning speed of 760 mm/s, a hatching distance of 120 µm, and a layer thickness of 50 µm. These parameters were chosen based on recommendations from the powder supplier, who recommends using the Inconel 718 laser parameters given by the printing machine manufacturer. To reduce residual stresses during fabrication, the build plate was preheated to 200 °C. The laser utilised a bi-directional scanning strategy without contouring steps. In addition to the samples for this study, other samples with variable geometries along their height were also printed on the same build plate. This could result in varying laser exposure times across layers, potentially affecting the microstructure and properties due to changes in local thermal conditions. To mitigate this issue, an inter-layer time of 63 s was implemented. This time was calculated using Materialise Magics© software, which optimises the layout of the samples on the build plate. The software calculates the total manufacturing time, determining the time per layer when all parts are placed on the platen.

The components were produced in a vertical orientation, and Fig. 1 illustrates the four scanning strategies under consideration.

In the first three strategies, the parts are manufactured without interlayer rotation. The key parameter under investigation is the angle between the laser scanning direction and the surface of interest. The three angles considered are 0°, 45°, and 90°, and these strategies will be referred to as Ang0, Ang45, and Ang90, respectively. The final scanning strategy involves a 90° rotation between layers and is designated as Rot90. Fig. 1 provides a schematic representation of these four scanning strategies, highlighting the friction plane in grey. This friction plane also serves as the plane for microstructural characterisations for each scanning strategy.

### 2.2. Microstructural characterisations

The samples were mounted and polished to achieve an OPS (Oxide Polishing Suspension) finish to facilitate microstructural observations. The microstructure was revealed through electrochemical etching using a 10% oxalic acid solution at 3 V DC. Observations were conducted with an Olympus PMG3 optical microscope and a ZEISS EVO HD15LS scanning electron microscope (SEM) for detailed microstructural analysis.

Cell size measurements were performed using the Local Thickness plug-in in ImageJ, based on electron micrographs captured using secondary electrons. The steps involved in image processing are illustrated in Fig. 2 and described as follows.
<figure><img src="image_4.png" /><figcaption>**Fig. 1.** A schematic representation of the scanning strategies employed in this study. The grey surface represents the surface on which microstructural characterisation and tribological tests are conducted.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Image processing steps to measure cell thickness. (a) Secondary electron micrograph, (b) Noisy binarised image, (c) Binarised image, (d) Image after application of the "Local Thickness" module.</figcaption></figure>

The first step in processing the image (Fig. 2b) is binarisation, followed by noise reduction after the conversion (Fig. 2c). To minimise noise, secondary electron images are blurred before binarisation. Once binarised, noisy particles are removed by measuring their size, then inverting the binary value of any particle whose size is equal to or smaller than the noise particles to match the binary value of the matrix. The size of noise particles and the level of binarisation can vary depending on sample preparation and SEM settings, so it must be adjusted for each sample.

In the final step, the Local Thickness module is applied. This module fits a sphere with the largest possible diameter inside the measured object, within the boundaries defined by the Laves phase (Fig. 2d). The diameter of this sphere is the measure of cell thickness. The image is

8-bit, meaning each pixel can represent 256 values, with smaller cells shown as dark spheres and larger ones as light spheres; the colour of the spheres is, therefore, relative to each image. The colour-coded mappings generated by the Local Thickness module will not be shown to avoid confusion. Instead, binarised images will be presented to assess the cellular structure and mitigate the impact of heterogeneities.

For SEM image processing, the analysed zone measures 120 × 150 µm². The width (120 µm) matches the hatching distance, covering the region from the centre of one melt pool to another. This accurately represents distinct zones within a melt pool while preserving surface proportions between adjacent zones.

This measurement method's limitation is that it cannot account for the cell form factor, as it only measures cellular thickness. However, the
<table><caption>**Table 2**
Tribological test parameters.</caption><thead><tr><td>Normal load</td><td>Stroke length</td><td>Frequency</td><td>Test duration</td></tr></thead><tbody><tr><td>30 N</td><td>10 mm</td><td>1 Hz</td><td>3600 s</td></tr></tbody></table>

process is automated, allowing for reliable measurement of many cells. An advantage of this technique is its ability to measure cell thickness repeatedly along a single cell. For example, a thick and long cell will be measured multiple times along its entire length, capturing spatial dimensions across the entire analysed surface.

X-ray diffraction (XRD) texture analysis were performed using a Panalytical X'Pert MRD diffractometer, equipped with a cobalt tube ($K_{α1}$ = 1.78901 nm, 35 kV and 45 mA). To reconstruct pole figures of samples with a face-centred cubic (FCC) structure, two to three families of hkl planes must be analysed. For this study, the (111), (200), and (220) planes were selected. Measurements were taken at $\chi$ angles between 0° and 70° and $\phi$ angles between 0° and 360°, in 5° increments. The resulting diffraction data were processed using MTEX software, which calculates the orientation distribution functions (ODFs) and reconstructs the pole figures.

## 2.3. Tribological tests

To study the tribological behaviour, dry sliding tests were conducted at room temperature using a Bruker TRIBOLAB UMT with ball on plane configuration of contact in alternating motion. These tests were performed three times for each scanning strategy. The tribometer has sensors that measure both the normal load ($F_N$) and the tangential load ($F_T$) at an acquisition frequency of 200 Hz, the normal load being controlled digitally. The tribometer software calculates the coefficient of friction (COF) over time. The balls used in the tribological tests are made of alumina ($Al_2O_3$) and have a diameter of 10 mm. The tribological parameters are provided in Table 2.

We chose a load to have a corresponding Hertz pressure higher than the pressure required to initiate the first plastic flow. The load calculated is 8.7 N for the material pair used. Before conducting the tribological tests, the sample and the ball were cleaned in ultrasonic tanks for 5 min each, first with acetone and then with ethanol. As previously mentioned, the sliding direction was parallel to the building direction.

The wear indicator chosen for the superalloy plates is the wear rate $k$, which is calculated according to the following formula (1):

$$k = V/(L * F_N) (\text{mm}^3/(\text{N mm})) \quad (1)$$

$V$ is the wear volume, $L$ is the total sliding distance, and $F_N$ is the normal load. To calculate the wear volume ($V$) of the plates, the cross-section of the wear track was measured using an extended field confocal microscope (AltiSurf520 from Altimet) equipped with a white light source. The measured cross-section was then integrated over the length of the wear trace to determine the total wear volume.

After each tribological test, the wear tracks on the samples were examined with a scanning electron microscope (SEM) FEG EVO HD15LS from Zeiss, equipped with a Backscattered Electron (BSE) detector to identify elementary wear mechanisms and to assess the percentage of the surface covered by oxidised layers through image processing. Fig. 3a shows the image acquired at the centre of the wear track, while Fig. 3b shows the corresponding binarised image in which the oxidised layers are black. To confirm that these interfacial layers are oxidised, Energy-dispersive X-ray spectroscopy (EDS) elemental analysis was carried out on one of these layers. The layer analysed is shown in Fig. 3c, and the corresponding analysis is shown in Table 3. The measurement shows a high oxygen content, confirming that these layers are oxidised.

# 3. Results & discussion

## 3.1. Microstructure characterisations

The microstructure for the different scanning strategies is shown in Fig. 4.

For the Ang0 strategy (Fig. 4a), the microstructure consists of broad, banded melt pools with a homogeneous cellular structure. The Ang45 and Ang90 (Fig. 4b,c) strategies exhibit vertically oriented melt pools. The Ang45 and Ang90 strategies show a coarser cellular structure between melt pools. The Ang90 strategy contains a higher density of columnar cells than other strategies. The Rot90 strategy (Fig. 4d) features vertically aligned melt pools, similar to Ang90, and melt pools that are elongated across the width of the micrograph, akin to Ang0.

High-magnification electron microscope analysis (Fig. 4e) reveals that the microstructure primarily comprises cells and Laves phase in the intercellular regions, resulting from micro-segregation during solidification. Additionally, the microstructure includes submicron round particles dispersed mainly in the intercellular regions. These particles are MC-type carbides, as identified in the study by Tang et al. [21] on ABD-900AM.

Fig. 5a-d presents binarised images of the Ang0, Ang45, Ang90, and Rot90 scanning strategies to assess cell shape.

The images show that the cell structure exhibits an equiaxed morphology when employing the Ang0 and Ang45 scanning strategies. For the Ang0 strategy, this equiaxed structure is homogeneous across the entire surface. In contrast, the Ang45 strategy results in a coarser cellular structure between the melt pools. The cellular structures observed in the Ang90 and Rot90 parts show variable morphologies, ranging from equiaxed to columnar. The Ang90 strategy demonstrates a similar size heterogeneity as seen in the Ang45 strategy but with a columnar morphology between the melt pools. The Rot90 strategy, on the other hand, features a finer equiaxed cell structure between the melt pools compared to inside them. These observations are consistent with those shown in Fig. 4.

A single-pass test conducted under the same experimental conditions allowed us to identify the contact width, which is 193 µm, and that the first pass does not cause any damage to the ball. This implies that the contact width spans nearly the width of two melt pools at the initial friction stage. Consequently, the heterogeneities in the cellular structure can influence the results, particularly at the beginning of the tests.

Cell size measurements for each scanning strategy are presented in Table 4.

The average cell sizes for the Ang0, Ang45, Ang90, and Rot90 strategies are 0.61 ± 0.13 µm, 0.77 ± 0.32 µm, 0.83 ± 0.41 µm, and 0.75 ± 0.34 µm, respectively. While the cell size increases with the laser scanning angle, the high standard deviations prevent us from concluding that these differences are statistically significant. Binarised images further indicate that the difference between Ang0 and the other strategies is more pronounced. The standard deviation is highly sensitive to changes in the laser scanning angle, with the part manufactured using a 0° laser angle (Ang0) showing a standard deviation three times lower than that of the part with a 90° laser angle (Ang90). The Ang45 and Rot90 strategies exhibit intermediate standard deviations compared to Ang0 and Ang90. This variability in standard deviations suggests that cell populations exhibit differing levels of size homogeneity depending on the scanning strategy used.

## 3.2. XRD texture analysis

The pole figures of the strategies studied are shown in Fig. 6.

The pole figure for the Ang0 strategy (Fig. 6a) reveals two primary textures. In the pole figures (200) and (220), two distinct populations are identified, as denoted by the orange and grey arrows. In the (200) pole figure, the texture indices of the points highlighted by the
<figure><img src="image_6.png" /><figcaption>**Fig. 3.** Determination of the percentage of surface covered by oxidised layers on the plate's wear track (Ang0). (a) SEM-BSE image of the wear track, (b) binarised image corresponding to the red rectangle in (a) and (c) high magnification view in BSE of an interfacial layer on the plate, the orange cross indicates the point where the measurement was taken. The oxidised layers appear in black in the figure (b).</figcaption></figure>

<table><caption>**Table 3**
EDS analysis of the interfacial layer in the plate's wear track, corresponding to Fig. 3c.</caption><thead><tr><th>Elements</th><th>O</th><th>Ni</th><th>Co</th><th>Cr</th><th>Al</th><th>Ti</th><th>Mo</th><th>W</th><th>Nb</th><th>Ta</th></tr></thead><tbody><tr><th>at.%</th><td>60.1</td><td>17.9</td><td>7.9</td><td>7.8</td><td>2.6</td><td>1.2</td><td>0.6</td><td>0.5</td><td>0.4</td><td>0.2</td></tr></tbody></table>

<table><caption>**Table 4**
Average cell size of Ang0, Ang45, Ang90 and Rot90 strategies. Measurements are performed with the "Lo-cal Thickness" module.</caption><thead><tr><th>Scanning strategy</th><th>Cell size (µm)</th></tr></thead><tbody><tr><td>Ang0</td><td>0.61 ± 0.13</td></tr><tr><td>Ang45</td><td>0.77 ± 0.32</td></tr><tr><td>Ang90</td><td>0.83 ± 0.41</td></tr><tr><td>Rot90</td><td>0.75 ± 0.34</td></tr></tbody></table>

arrows are identical, suggesting that these two populations are in equal proportions. The grain family indicated by the orange arrows shows a texture in (200), though it is unclear whether this texture is cubic or fibrous. The grain family marked by the grey arrows exhibits a (220) texture, with the [001] crystallographic axis oriented perpendicular to the building direction.

The part produced using the Ang45 scanning strategy displays a prominent (111) texture, as illustrated in Fig. 6b. In this case, the pole figure (200) shows a ring structure with two additional points of higher intensity, suggesting a strong (111) texture with fibre characteristics and the potential development of a cube texture in some grains.

The Ang90 strategy's pole figures (Fig. 6c) indicate a pronounced (200) texture. Intense points in the (111) and (220) pole figures suggest that the texture is predominantly cubic, with the [001] crystallographic direction oriented at ±45° relative to the building direction.

Fig. 6d shows the pole figures for the Rot90 strategy, which demonstrate a texture identical to that of the Ang90 strategy. The pole figures display comparable points, with only minor differences in the texturing intensity of the (200) plane.

Overall, the four strategies reveal three distinct textures, with the Ang90 and Rot90 strategies exhibiting the same crystallographic texture on the analysed planes.

### 3.3. Tribological behaviour

The results and analyses of the tribological tests are organised as follows. First, we will present the wear characteristics of the samples, including a study of macroscopic parameters derived from sensor measurements. Next, we will discuss the wear tracks, focusing on measuring the coverage rate of the wear track by oxide layers and describing the main wear mechanisms identified through SEM observations.
<figure><img src="image_7.png" /><figcaption>**Fig. 4.** Microstructure at the melt pool scale for different building strategies, observed with an optical microscope: (a) Ang0, (b) Ang45, (c) Ang90, and (d) Rot90. Panel (e) shows SEM observations of the fine microstructure. The grey surface in the schematic representation of the scanning strategies indicates the area where microstructural characterisations were conducted. The red lines mark the boundaries of the melt pools.</figcaption></figure>

### 3.3.1. *Wear and friction results*

The results of the tribological tests, which indicate the extent of wear, are presented in Fig. 7a.

The wear rates of the samples, shown in Fig. 7, for the Ang0, Ang45, Ang90, and Rot90 scanning strategies are $2.8 \pm 0.6 \times 10^{-7} \text{ mm}^3/(\text{N mm})$, $3.8 \pm 0.7 \times 10^{-7} \text{ mm}^3/(\text{N mm})$, $4.8 \pm 0.4 \times 10^{-7} \text{ mm}^3/(\text{N mm})$, and $2.7 \pm 0.1 \times 10^{-7} \text{ mm}^3/(\text{N mm})$, respectively. The data indicate that the Ang0 and Rot90 strategies have the lowest wear rates under these loading conditions, followed by the Ang45 and the Ang90 strategies. Additionally, the Rot90 strategy exhibits the best repeatability in wear results. The width of the wear tracks is approximately 2 mm, which is nearly equivalent to 17 melt pools when referring to the Ang45, Ang90, and Rot90 structures, which feature vertically aligned melt pools. The structure identified as the most homogeneous, produced with the Ang0 strategy, has a similar standard deviation to that of Ang45 and Ang90, suggesting that structural homogeneity does not correlate with the repeatability of the tribological tests.

As illustrated in Fig. 8, wear and friction curves are analysed systematically.

The analysis of these curves revealed the presence of multiple friction and wear regimes, which can be described as follows:

**Initial Regime:** This phase is characterised by a relatively high wear kinetics compared to other phases. It features significant fluctuations in the coefficient of friction (COF) and frequent changes in the wear slope.

**Second Regime:** Marked by a lower wear kinetics than the initial regime, and a COF that begins to rise. This phase is also characterised by intermittent spikes on the COF curve, occurring over several friction cycles. The amplitude of the fluctuations decreases compared to the initial regime.

**Third Regime:** This phase is distinguished by a low wear kinetics, close to zero, favouring a reduction in COF and possibly leading to its stabilisation. In this phase, the intermittent spikes seen in the second regime are less present. It is noteworthy that the third regime was not observed in every test. It is hypothesised that tests not exhibiting the third regime could reach this phase with a longer friction duration.

The friction curves of the different scanning strategies show that the third regime is reached for each scanning strategy at least one time, and the transition between the second and the third regime is indicated with an arrow. However, this third regime is not reached in every test, making it difficult to gather any statistic value on this third regime. It appears, however, that the Ang0, Ang45, and Ang90 strategies reach the third regime in two out of three tests. For these tests, the Ang0 strategy reaches the third regime the fastest, followed by the Ang45 strategy, and finally Ang90. For the Rot90 strategy, the third regime is reached only once. Among the four sanding strategies, Ang90 is the strategy whose friction curves show the best repeatability from one test to another.

Key parameters to assess the evolution of tribological behaviour relative to the laser scanning strategy include the duration of each wear regime, wear rates, average COF values for each regime, and
<figure><img src="image_8.png" /><figcaption>Fig. 5. Binarised micrographs of strategies (a) Ang0, (b) Ang45, (c) Ang90 and (d) Rot90. The curved black lines indicate the boundaries of the melt pools.</figcaption></figure>

the dissipated energy. During tribological tests, energy is primarily dissipated as heat and particle flow within the contact zone [22]. Some studies have emphasised the energetic approach to friction and wear, showing that wear rate has a linear correlation with dissipated energy [22,23]. Dante et al. [24] also demonstrated that dissipated energy values can indicate transitions from mild to severe wear.

The dissipated energy ($E_d$) can be calculated using Eq. (2).

$$E_d = \int_{x1}^{x2} |F_T| dx \qquad (2)$$

Where $x1$ and $x2$ represent the bounds of the interval under study. Since the third regime is not always observable, potentially due to test duration, it is impossible to provide certain details from the second regime, such as duration or the corresponding coefficient of friction, as averages for each laser scanning strategy. Similarly, data from the third regime cannot be averaged due to issues with repeatability.

The recorded data, averaged over three tests for each scanning strategy, are as follows:

Duration of the first regime ($D_{R1}$): This measures how long the system remains in the initial phase, characterised by high wear kinetics. The ability of a sample to transition to the subsequent regime indicates a reduction in wear kinetics and potentially an overall lower wear rate. It also reflects the material's capacity to form interfacial layers and/or a third body that accommodates part of the velocity difference.

The average coefficient of friction in the first regime ($\mu_{R1}$) provides insights into the contact phenomena and the force required to enable sliding.

Dissipated energy during the first regime ($E_{dR1}$) quantifies the energy lost as heat and particle flow during the first wear phase.

Total dissipated energy ($E_{dtot}$): The total energy dissipated after 3600 cycles. Without reliable data from regimes 2 and 3, $E_{dtot}$ is an overall indicator for the entire test duration.

The characteristic values extracted from the curves for different scanning strategies are illustrated in Fig. 9.

As illustrated in Fig. 9a, the duration of the initial regime shows that the Ang90 strategy exhibits the longest duration. The Ang0 and Ang45 strategies display similar durations, while the Rot90 strategy has the shortest duration. Although the duration $D_{R1}$ appears to be related to the wear rate, this relationship is inconsistent for the Ang0 strategy.

Fig. 9b shows that the average friction coefficients during the initial regime are comparable for the Ang0, Ang90, and Rot90 strategies, with values of 0.92 ± 0.01, 0.93 ± 0.02 and 0.92 ± 0.01, respectively. In contrast, the coefficient of friction for the Ang45 strategy is 0.97 ± 0.05. This higher mean value and considerable standard deviation suggest that the tribological behaviour of the Ang45 strategy differs significantly from the other strategies, indicating instability during the initial testing phase.

The energy dissipated during the first regime follows a pattern similar to its duration. Notably, the Ang90 strategy exhibits the highest energy dissipation among the strategies. However, the total dissipated energy after 3600 cycles shows no significant variation between the different scanning strategies. The good repeatability associated to the Ang90 strategy may be linked to its good friction curves repeatability.

### 3.3.2. Surface analysis

Observations of the plate wear tracks were conducted to identify the elementary damage and wear mechanisms occurring on the surface. The wear track of each scanning strategy was analysed. However, no change was observed in the nature of the damage and wear mechanism. Therefore, one wear track is shown to identify these different mechanisms. The intensity of these mechanisms, which changes according to the scanning strategy, will be presented after their identifying.

The edges of the plate wear track were analysed to assess the deformation structures. The plate shows shear bands, located outside the wear track (Fig. 10). This phenomenon is due to the pressures induced in the contact, and shows that the material accommodates part of the deformation outside the contact zone.

So, the characterisation of the wear track centre reveals grooves aligned with the sliding direction (Figs. 11a and 11b), indicative of
<figure><img src="image_9.png" /><figcaption>Fig. 6. Pole figures from XRD analysis for the scanning strategies (a) Ang0, (b) Ang45, (c) Ang90 and (d) Rot90.</figcaption></figure>

abrasive wear. The ductile nature of the material suggests that these grooves are likely formed by harder particles, such as alumina or oxidised metal particles, that cause plastic deformation of the surface without necessarily forming new particles. Interfacial layers are also observed that are agglomerates of fine particles (black layers in Fig. 11a). The EDS analysis shown in Fig. 3 confirms that these layers are oxidised. The influence of these layers will be discussed later in this subsection.

Following the tribological tests and after cleaning with ethanol in an ultrasonic bath, a ball wear track was observed using SEM (Fig. 11c,d). Since the ball is non-conductive, the SEM observations were conducted in a low vacuum, with an accelerating voltage of 10 kV and a working distance of 8 mm. The overall view of the wear track (Fig. 11c) shows a large number of adherent metallic particles (appearing in white). Micrometer-sized alumina grains can be observed (Fig. 11d). These grains display sharp-edged facets and can act as abrasive particles, responsible for the grooves observed on the wear track of the planes.

The coverage rate of the wear tracks on the plates by oxide layers is presented in Fig. 12.

The measurements reveal that the coverage rates of the wear tracks by oxide layers for the Ang0, Ang45, Ang90, and Rot90 strategies are

35 ± 8%, 31 ± 5%, 25 ± 5%, and 30 ± 10%, respectively. While the standard deviations preclude a definitive conclusion, a trend suggests that the wear rate is inversely proportional to the coverage rate for the samples without interlayer rotation. Notably, the coverage rate for the Rot90 strategy is similar to that of the Ang45 strategy, but the difference in wear rate may be due to the cell structure, which will be discussed in detail later. This indicates that, under the given loading conditions, higher coverage rate of the wear tracks by oxide layers, that potentially provide lubrication properties, corresponds to a reduction in wear rate.

The longitudinal section of the plate's wear track (Fig. 13) is shown to identify the formation of the oxide layers. The picture shows different layers. The first one is the oxide layers indicated with a blue arrow, which has been observed and characterised previously (Figs. 3 and 11). The second layer, marked with a green arrow, is a deformed layer in which the Laves phase is dissolved. A previous study by the same authors [25] showed that this layer corresponds to the Tribologically Transformed Surfaces (TTS). Finally, the last layer, indicated by an orange arrow, corresponds to the cellular structure. The TTS observed in this micrograph measures approximately 2 µm and is discontinuous
<table><caption>**Table 5**
Summary of the crystallographic texture measured by X-ray diffraction (XRD) as a function of the scanning strategy, and the associated wear rates.</caption><thead><tr><th>Scanning strategy</th><th>Texture</th><th>Intensity</th><th>Wear rate (10<sup>-7</sup>mm<sup>3</sup>/(N mm))</th></tr></thead><tbody><tr><td>Ang0</td><td>(200)/(220)</td><td>4.6/2.8</td><td>2.8 ± 0.6</td></tr><tr><td>Ang45</td><td>(111)</td><td>5.4</td><td>3.8 ± 0.7</td></tr><tr><td>Ang90</td><td>(200)</td><td>7.9</td><td>4.8 ± 0.4</td></tr><tr><td>Rot90</td><td>(200)</td><td>9</td><td>2.7 ± 0.1</td></tr></tbody></table>

<table><caption>**Table 6**
Summary of the cell characteristics along with the associated wear rates.</caption><thead><tr><th rowspan="2">Scanning strategy</th><th colspan="2">Cells characteristics</th><th rowspan="2">Wear rate (10<sup>-7</sup>mm<sup>3</sup>/(N mm))</th></tr><tr><th>Size (μm)</th><th>Morphology</th></tr></thead><tbody><tr><td>Ang0</td><td>0.61 ± 0.13</td><td>Predominantly equiaxed</td><td>2.8 ± 0.6</td></tr><tr><td>Ang45</td><td>0.77 ± 0.32</td><td>Predominantly equiaxed</td><td>3.8 ± 0.7</td></tr><tr><td>Ang90</td><td>0.83 ± 0.41</td><td>Equiaxe/columnar</td><td>4.8 ± 0.4</td></tr><tr><td>Rot90</td><td>0.75 ± 0.34</td><td>Equiaxe/columnar</td><td>2.7 ± 0.1</td></tr></tbody></table>

<figure><img src="image_10.png" /><figcaption>**Fig. 7.** Sample wear rates as a function of the scanning strategy.</figcaption></figure>

along the whole length of the wear track. However, as the TTS is between the cellular structure and the wear track, it is from the TTS that the particles used to form the oxide layers are emitted. The TTS is, therefore, a particle emission site.

### 3.4. Role of microstructural elements on the tribological behaviour of components

Table 5 resumes the correlation between the scanning strategy and the crystallographic texture. The elements of the crystallographic texture provided in this table group the textured crystallographic planes for each scanning strategy, along with the intensity measured from the associated pole figures.

When comparing the Ang90 and Rot90 strategies, it is observed that they have the same texture with similar intensities. It has also been shown that the pole figures for these two strategies are identical (Fig. 6c,d). However, the difference in wear rates is nearly a factor of two. Therefore, in this case, it is challenging to establish a clear link between the wear of the components and the cristallographic texture. The observations lead to the conclusion that, under the loading conditions for which the tribological tests were conducted, the crystallographic texture has a limited influence on the wear resistance of the components, and other microstructural elements must have a greater impact.

The cellular structure was analysed for different scanning strategies, and the main results are summarised in Table 6.

Analysis of the cellular structure shows that the Ang0 strategy has the finest and most homogeneous structure, predominantly composed of equiaxed cells. The Ang45 and Ang90 strategies exhibit coarser structures with size heterogeneities between the centre and the melt pool overlap areas, mainly consisting of an equiaxed structure for the Ang45 strategy and an equiaxe/columnar structure for the Ang90 strategy. However, the cell thickness difference is insignificant due to standard deviations. A noticeable trend is that a decrease in cell thickness is associated with better sample wear resistance. Size heterogeneities and cell morphology may also contribute to this trend. Indeed, size heterogeneities in the melt pool can lead to gradients in mechanical properties, making areas more prone to wear with higher wear kinetics. This could explain the higher wear rates of the Ang45 and Ang90 strategies. The difference in wear between the Ang45 and Ang90 strategies could be attributed to the difference in cell size. The predominantly equiaxed morphology of the Ang45 strategy might enhance its wear resistance. The Rot90 strategy, which exhibits the same cell size as the Ang45 strategy, has a fine equiaxed cellular structure between its melt pools, which could contribute to improved wear resistance and explain its lower wear rate.

According to the literature, if we consider cells as structural elements, their size effect can be related to a Hall-Petch type effect. Cells can also be described as dislocation cellular pattern (DCP), consisting of a cell wall overlapping with a dislocation wall. Current literature does not clearly distinguish between Hall-Petch or Taylor effects, with both being used in the literature, sometimes even in combination [26,27]. In their work, Liu et al. [27] showed that the overall strengthening of the cellular structure accounts for about 46% of the yield strength. In the same study, post-mortem TEM observations revealed that the deformation mechanism involves dislocation slip, which piles up at the cell boundaries. This piling up is facilitated by the existing dislocation network in the as fabricated state and by micro-segregation. In-situ tensile tests with TEM on 316L [28] demonstrated that plastic deformation is due to partial dislocations, whose movement is significantly but not completely blocked by the dislocation network. With increasing stress and strain, dislocations can cross this dislocation wall and move into an adjacent cell. According to the authors, atom segregation and the presence of phases that block dislocation movement, such as carbides, stabilise the dislocation network and maintain its size during plastic flow. The size of the dislocation network is maintained even at the final stage of plastic deformation when high plastic flow is achieved.

Concerning our results, a smaller cell size combined with an equiaxed morphology implies a higher density of dislocation walls, and the hardening rate associated with dislocation movement and entanglement occurs more rapidly, which could contribute to reduced wear of the components. Cell size and morphology seem to correlate
<figure><img src="image_11.png" /><figcaption>Fig. 8. Time trends of (a) wear curves for the different scanning strategies and (b)(c)(d)(e) the COF curves of Ang0, Ang45, Ang90 and Rot90, respectively.</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>Fig. 9. Characteristic values extracted from wear and friction curves for samples with varying scanning strategies. The extracted values for the first regime are: (a) the duration $D_{R1}$, (b) the average coefficient of friction $\mu_{R1}$, and (c) the dissipated energy $E_{dR1}$, followed by (d) the dissipated energy after 3600 cycles ($E_{d3600}$).</figcaption></figure>
<figure><img src="image_13.png" /><figcaption>Fig. 10. SEM observation of the plate's wear track edge (Ang45).</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>Fig. 11. SEM micrographs of the wear tracks on the counterfaces. (a) Wear track of the plate (Ang0) in backscattered electrons, (b) higher magnification view in secondary electrons of the plate's wear track (Ang90), (c) wear track of the ball, with (d) a higher magnification view in backscattered electrons.</figcaption></figure>

with the samples' wear resistance, with a fine cellular structure and equiaxed morphology contributing to reduced wear of the components. A strong trend was also observable between the wear and coverage rates of the wear track. The contribution of the reinforcement of the parts due to the size of the cellular structure would contribute to maintaining this protective tribo-layer on the wear track and thus reduce the wear of the parts. The small size of the cellular structure between the melt pools of parts manufactured using the Rot90 strategy could explain why its wear rate is lower, despite its cellular structure size and coverage rate equivalent to the Ang45 strategy. Since TTS separates the cellular structure from the wear track, the following hypotheses are proposed to link the different elements.

The first hypothesis is that the dislocation network is maintained despite the dissolution of the Laves phase in the TTS. In this case, the

measurement of cell size, which reflects the distance between the dislocation walls, provides a direct link between the volume of wear and the size of the cell structure. However, Transmission Electron Microscope (TEM) analyses are needed to confirm or refute this hypothesis.

The second hypothesis is as follows. If we consider the TTS as a continuously stressed and renewed layer, a difference in its supply would stabilise the TTS and the oxide layers that protect the volumes of the antagonists. In this way, the difference would be made on a cellular scale and linked to the resistance to plastic flow of the cellular structure according to the scanning strategy. A fine structure would provide better resistance to plastic flow and stabilise the TTS and oxide layers. Since the high standard deviations of the cell structure size do not allow us to make a clear decision, one way of strengthening this hypothesis would be to print new samples with different laser powers
<figure><img src="image_15.png" /><figcaption>Fig. 12. Coverage rate of the plate wear tracks by oxide layers for each scanning strategy.</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>Fig. 13. SEM observation of the longitudinal section of the plate's wear track (Ang45).</figcaption></figure>

and/or speeds. For this, the Ang0 strategy seems the most suitable, as this strategy has a cell structure with a homogeneous size and an equiaxed morphology, which makes it possible to reduce the standard deviations and work on one cell morphology.

## 4. Conclusion

This study examines the relationships between reference microstructures obtained and their tribological behaviour. To do so, the LPBF process was employed for its ability to generate different multi-scale microstructures while keeping the laser parameters and material chemistry constant. To generate the different microstructures, the modified process parameters are the interlayer rotation and, for parts without interlayer rotation, the angle between the laser direction and the studied surface. These parameters are referred to as scanning strategies, as they do not involve a modification of the laser parameters (power, speed). The main conclusions are described as follows:

1. The variation of scanning strategy has allowed for the generation of different crystallographic textures. For the scanning strategies named Ang0, Ang45, Ang90, and Rot90, the developed textures are multi-texture (200)/(220), (111), (200), and (200), respectively.

2. The cellular structure developed inside the melt pools strongly correlates with the scanning strategies. An Ang0 strategy generates a homogeneous equiaxed structure with a smaller cell size.

The Ang45 strategy is also mainly composed of equiaxed cells, with cell sizes larger between melt pools. The Ang90 strategy exhibits a structure composed of columnar and equiaxed cells, with a larger size between melt pools. Finally, the Rot90 strategy is composed of columnar and equiaxed cells, with a smaller cell size between melt pools. The average cellular size for Ang0, Ang45, Ang90 and Rot90 is $0.61 \pm 0.13 \ \mu\text{m}$, $0.77 \pm 0.32 \ \mu\text{m}$, $0.83 \pm 0.41 \ \mu\text{m}$ and $0.75 \pm 0.34 \ \mu\text{m}$.

3. Wear results show that Ang0 and Rot90 are the strategies with the best wear resistance, followed by Ang45 and then Ang90. For the strategies built without interlayer rotation, the wear results show an inverse trend with the oxide coverage rate of the wear tracks. The size of the cellular structure shows the same trend. Therefore, the wear resistance of the parts is mainly associated to the coverage rate by oxidised layers. Depending on the size and the heterogeneities of the cellular structure, the parts maintain this protective layer more or less effectively.

## 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.
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