Experimental characterization of the operation of
modular cascade plasma torches

Myriam Sleiman, Geoffrey Darut, Ralph Seulin, Marie Pierre Planche, Jean-Jacques Gonzalez, Francis Sambou, Armando Salito, Manfred Rösli, Pierre Freton

► To cite this version:

Myriam Sleiman, Geoffrey Darut, Ralph Seulin, Marie Pierre Planche, Jean-Jacques Gonzalez, et al.. Experimental characterization of the operation of modular cascade plasma torches. Surface and Coatings Technology, 2025, 515, pp.132529. 10.1016/j.surfcoat.2025.132529. hal-05356814

HAL Id: hal-05356814

https://hal.science/hal-05356814v1

Submitted on 13 Nov 2025

**HAL** is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
# Experimental Characterization of the Operation of Modular Cascade
Plasma Torches

**Myriam SLEIMAN, Geoffrey DARUT, Ralph SEULIN, Marie Pierre PLANCHE**

UTBM, CNRS, ICB, F-90010 Belfort cedex, France

corresponding author, @: [myriam.sleiman@utbm.fr](mailto:myriam.sleiman@utbm.fr)

**Jean-Jacques GONZALEZ, Pierre FRETON, Francis SAMBOU**

LAPLACE, UMR 5213 CNRS-INP-UPS, Toulouse, France

**Armando SALITO, Manfred RÖSLI**

Gulhfi AG, Wohlen, Switzerland

## Abstract

Industries typically use the plasma spraying process because of its efficiency and flexibility. It involves a plasma jet made up of an arc column that originates at the cathode tip and that ends at the anode's surface. The industry has historically preferred a two-electrodes plasma torch configuration with a single part anode. On the other hand, voltage instabilities have been noted to be problematic, affecting the physical characteristics of particles in the plasma (velocity, temperature, ...). New generations of plasma torches with segmented or cascade configurations have been developed to overcome this problem. The Larmor torch is a modular cascade plasma torch. Different nozzle structures (length and inner diameter) can be used. This study investigates four different configurations of this torch and their effects on the spraying of alumina particles. Diagnostic instruments are employed to examine plasma length, voltage fluctuations, particle characteristics (temperature and velocity) and spraying efficiency to comprehend how various
torch combinations operate. The results demonstrate that increasing the nozzle length increases especially particle velocity, while increasing the diameter leads to a decrease in both velocity and temperature. Additionally, several plasma spraying parameters, such as argon and hydrogen flow rates, the arc current are examined for alumina material to determine the range of operating parameters. The results showed that the parameter that has the most influence is the arc current. An increase in this parameter causes an increase in the velocity and temperature of the alumina particles.

## Keywords

Cascade torches; atmospheric plasma spraying; particles characterization; alumina coatings

## Introduction

Thermal spray coatings offer a versatile range of materials, including metals, ceramics, polymers, alloys, and composites [1]. These coatings enhance the functionality of components by providing protection against wear, corrosion, thermal extremes, and electrical conductivity [2–6]. Plasma spray technology is a prominent method for applying these coatings. It uses a high-energy plasma jet to melt and deposit material onto a targeted surface. Feedstock material, often in powder form, is injected into an argon-based plasma. Additional gases like hydrogen, helium, or nitrogen can be introduced to optimize thermal conductivity or flow characteristics. The molten or semi-molten particles are then deposited onto a prepared substrate, resulting in a coating with a microstructure desired to the application [4]. Atmospheric Plasma Spraying (APS), the most widely used process, utilizes a DC arc to generate the plasma jet within an ambient air environment.

Plasma generation takes place within the torch, formed by a cathode and a concentric anode, where the plasma gases (like argon, hydrogen) are introduced. The cold gas is injected at the cathode
through this chamber and is ionized by an electric arc, forming plasma. Temperatures at the hottest regions of the plasma exiting the anode can reach 15000K, with a velocity of approximately 2000m/s [3]. A plasma jet's arc column begins at the cathode tip (electron source) and ends at the anode surface. The termination point, called the arc root, marks where the plasma contacts the anode's inner surface. The most popular device for generating a plasma jet is the plasma torch. The conventional plasma torch utilizes a single cone-shaped tungsten cathode and an anode concentric to the cathode. But this configuration often leads to inconsistencies and instabilities in plasma generation [7]. The main challenge in plasma spraying is the instability caused by the free movement and attachment of the arc root on the anode wall. These fluctuations influence the arc voltage. Analyses of the in-flight particles from the spray show very different temperature and velocity characteristics depending on these voltage values, due to the jet instabilities.

To solve this problem, in particularity, to control arc voltage variations, new generations of plasma torches have been developed in the past years [9]. These so-called cascade/segmented torches address the issue of the free movement of the arc root by employing insulated rings along the anode. These torches utilize a segmented anode structure, consisting of stacked copper rings that form a tunnel (Fig. 1).

<figure><img src="image_2.png" /><figcaption>Fig. 1. Scheme of Larmor segmented plasma torch.</figcaption></figure>
These copper rings are electrically isolated from each other and are called neutrodes. The arc's motion is restricted to the anode ring, ensuring its stability within the design, Fig 1. This typical cascade design is more complex than conventional torches due to the presence of these neutrodes.

They are separated by ceramic rings, to ensure their electrical insulation and temperature resistance, to protect the O-ring seal (of the cooling water circuit) located behind them. In addition, to improve voltage stability, the aim of this design is to lengthen the electric arc in order to reduce the intensity of the arc current required (higher voltage to obtain the same net power) and thus extend the lifespan of the electrodes (not to mention the 2 to 7 times lower hydrogen flow rates required too).

When the torch is disassembled, these ceramic rings are prone to cracking, due to the repeated thermal stresses during torch use and the clamping pressure during torch assembly. More prolonged use of electrodes with parameters including lower arc current and very low hydrogen ratios can remedy the problem of maintaining electrode changeover.

This design significantly enhances arc stability by restricting its attachment to the final anode ring [10,11]. An alternative approach has been explored, the possibility of controlling the arc movement using an externally applied magnetic field to the torch [12,13]. This torch design facilitates the extension of the plasma column closer to the nozzle outlet (anode) providing two key advantages, enhancing torch efficiency by reducing heat loss in cooling water circuit and increasing the amount of energy available for particle treatment. R C. Seshadri et al. have shown that the cascade torch deliver a higher heat flux to the substrate compared to F4 torch (~ 1.5 times higher), leading to higher particle surface temperatures [14]. This could be due to a higher plasma temperature or a slower decrease in the temperature gradient at the nozzle outlet, because of the attachment of the arc near the surface of the final anode. More energy would be available to the injected particles, and perhaps less would be transferred to the water circuit in the torch because of the position of the arc root far downstream of the nozzle. Those improvements translate into enhanced coating
reproducibility. Additionally, to melt materials and produce coatings, a lower total power and secondary gas flow rates are required [15,16]. For example, Darut *et al.* [17] produced alumina coatings without hydrogen and only with an arc current of 400A (21kW net power related to 37kW total power) using cascaded torch, on the contrary an alumina coating is obtained with an F4 torch, the operating parameters of this torches are: 650 arc current, 40L/min argon flow rate and 12L/min hydrogen flow rate, the power obtained are :25kW net power related to 47kW total power. Again, hotter plasma gradients are potentially available at particle injection point (nozzle outlet) due to the proximity of the arc root. For the same amount of particles to be treated and the achievement of a similar deposit, conventional torches such as F4 or SG100, with an arc voltage between 30 and 70V operate at higher electrical power levels (up to 80kW) [4]. In contrast, the arc voltage of segmented torch range between 100 and 150V corresponding to lower electric power, between 20 and 60kW [18]. Obtaining the desired properties in coatings necessitates the successful completion of several steps or sub-tasks. Additionally, when the arc root movement is free (conventional torches), electrode evaporation does not occur and thus will not contaminate the coating, thereby preserving the coating chemical composition. This is because the arc attachment does not remain fixed at one location but instead establishes a continuous axial movement and circular rotation on the anode surface [19]. However, once an area has been eroded, the arc tends to stick to it systematically. This phenomenon, combined with high arc current and high hydrogen content, leads to spitting (Cu, W), which pollutes the coating. One of the advantages of increasing the length of the tunnel with cascade torches is that high arc voltages can be used. This results in lower current intensities than with standard torches, which limits anode erosion (450A max in this study).

This experimental work presents the study of different cascade plasma torch configurations in terms of assembled neutrode number (nozzle length) and tunnel and final anode diameter (8 or 10mm). The results concern the effect of operating parameters and geometry of the torch, on the
velocity and temperature of particles, therefore on the microstructure of alumina coatings in terms
of porosity, hardness, and deposition efficiency. These findings provide valuable guidance for
implementing modular cascaded plasma torches and optimizing high-performance coating
deposition.

# 1. Materials and processes

## 1.1. Feedstock

The feedstock used in the investigations was a commercially available Al₂O₃ Amdry 6062 powder
(Oerlikon Metco, Switzerland) with a particle size distribution ranged from 22 to 45µm. The
powder mass flow rate in the study was 30g/min.

## 1.2. Spray Process and Related Operating Parameters

This study employed a Larmor cascade plasma torch manufactured by Gulfhi AG (Wohlen,
Switzerland). Two torch configurations were investigated, Larmor 9 and Larmor 15. They differed
in the number of neutrodes, so in the cathode-anode distance (39.1mm for 9 stacks and 61.8mm for
15 stacks). Additionally, each configuration was tested with two combinations of nozzle/tunnel
diameters (8 and 10mm).

Table 1 summarizes the data variables. Plasma operating parameters, including arc current and
argon/hydrogen flow rates, were varied for each torch configuration. The specific range of these
parameters are also detailed in Table 1. The operating window of the torch, which defines voltage,
net power, and total power values, is presented in Figure 2(a). A specific code will be used to
indicate main operating parameters as follow I/Ar/H₂: arc current (A) / argon flow rate (L/min) /
hydrogen flow rate (L/min). The injection is performed perpendicular from above to the plasma
axis, as shown in Figure 2(b).
<caption>**Table 1**</caption>

Configurations of Larmor cascaded plasma torch and range of operating parameters investigated.

<table><thead><tr><th>Plasma torch</th><th colspan="4">Larmor</th></tr></thead><tbody><tr><td>Stack number or</td><td colspan="2">9</td><td colspan="2">15</td></tr><tr><td>cathode/ final Anode distance (mm)</td><td colspan="2">39.1</td><td colspan="2">61.8</td></tr><tr><td>Tunnel/Anode diameter (mm)</td><td>8</td><td>10</td><td>8</td><td>10</td></tr><tr><td>Arc current (A)</td><td colspan="4">From 350 to 450</td></tr><tr><td>Argon flow rate (l/min)</td><td colspan="4">From 35 to 55</td></tr><tr><td>Hydrogen flow rate (l/min)</td><td colspan="4">From 0 to 4</td></tr></tbody></table>

<figure><img src="image_3.png" /><figcaption>**Fig. 2.** (a) Control panel of the system, (b) Placement of the injector.</figcaption></figure>

The coatings were sprayed on steel plates of 40*20mm and 2mm thick. The plates were previously grit blasted with F36 corundum at a pressure of 2.5bar to achieve a surface roughness of about 3-3.5µm. The spray distance is about 110mm from the exit of the torch to the surface of the plates.

The following parameters presented in Table 2 were held constant for all deposition experiments.
<table><caption>**Table 2**

Constant parameters for spray operation.</caption><tr><td>Powder Mass flow rate (g/min)</td><td>28</td></tr><tr><td>Injector diameter (mm)</td><td>2</td></tr><tr><td>Substrate surface area (mm²)</td><td>800</td></tr><tr><td>Carrier gas flow rate (L/min)</td><td>4</td></tr></table>

### 1.3. Characterization techniques

Velocity and temperature of particles were measured by the DPV 2000 device (Tecnar, Canada).

This optical system detects the light radiation of in-flight particles and thus allows the online measurement of the velocity, size, and temperature of particles during the spray. The distance applied to the measurement is shown in Figure 3.

<figure><img src="image_4.png" /><figcaption>**Fig. 3.** Image of the DPV 2000 equipment.</figcaption></figure>
The detector was positioned 110mm from the exit of the torch on the vertical position where the particle jet is maximum, and 100mm from the plasma jet corresponding to the lens focusing distance.

Detailed microstructural analysis of the as-sprayed coatings was conducted using field-emission scanning electron microscopy (FE-SEM) (JSM-7800F, JEOL, Japan) on polished cross-sections. Specimens were prepared via diamond saw sectioning and resin mounting, followed by meticulous grinding with SiC paper and diamond paste polishing to achieve a mirror finish.

The thickness of the deposited layer was initially measured using a capillary for preliminary assessment, and then with an optical microscope EPIPHOT (NIKON) with a magnification of x200 (average of ten thickness measurements).

Microhardness measurements were conducted on polished cross-sections using a Vickers indenter (Leitz, RZD-DO, Germany) with a 300g load and 15s holding time. Ten indentations were randomly made within different regions of each sample.

ImageJ software [21] was used to estimate porosity by analyzing five randomly acquired SEM images (x200 magnification) across the entire cross-section, excluding edge regions of the sample.

## 2. Results

### 2.1. Influence of the arc current

The influence of the increase of the arc current on the velocity and temperature of alumina particles is presented in Figure 4 (a and b respectively) for a fixed flow rate of argon and hydrogen gases (45/4 L.min⁻¹).
<figure><img src="image_5.png" /><figcaption>Fig. 4. Evolution of particle a) velocity and b) temperature as a function of arc current for Larmor (9 and 15) with tunnel inner diameter (8 and 10 mm) - (Ar/H2 = 45/4 L/min).</figcaption></figure>

The particle velocity increases with the arc current, especially for torch configuration with the smallest tunnel diameter of 8mm. In fact, the slope of the trend lines for the measurement points is steeper for the smaller diameter. As already demonstrated by Planche *et al.* [22] and E. Bourdin *et al.* [23], this can be attributed to the increase in the gas flow velocity. When the arc current increases, the power supplied to the arc increases. Therefore, the momentum of the jet and correlatively the velocity increases. For a 10mm larger diameter, the results are different. In the case of the Larmor 15, there is still an increase in velocity with the arc current, but of lesser importance (reduced slope of the trend line). A bigger tunnel seems to have a minor influence on the plasma velocity, due to its profile enlargement. But for a shorter tunnel as Larmor 9 tunnel 10, the trend is reversed with a decrease in velocity with the arc current (decreasing slope). For this torch configuration, the plasma behaves differently to the 3 others configurations. One hypothesis could be a change in the plasma regime, which would become more turbulent as the tunnel increases in diameter and decreases in length. This more turbulent regime would lead to a lower plasma velocity [24], which would have an impact on the particles. The radial and axial profiles of the plasma temperature and velocity vary according to the different operating conditions. The
associated effects are interrelated, complex and non-linear. Only appropriate modelling can provide
a good explanation to confirm this hypothesis. To explain this, a model of the plasma in the torch
is needed to understand its kinematic and thermal properties. Models for SinplexPro plasma
cascade torch exist [12,25].

Increasing particle velocity leads mathematically to decrease the residence time of the particles in the jet which should go against the increase in their heat treatment. Nonetheless as shown in Figure 4(b), the temperature of alumina particles increases with the increase of the arc current. This increase is due to the fact that enthalpy of the plasma is proportional to the arc current as mentioned by Mavier [26] and also by R. C. Seshadri *et al.* [14].

Note that when increasing the arc current, temperatures rise. To maintain the flow rate, the jet velocity and, consequently, the particle velocity may increase. However, keeping particle injection parameters constant, such as the carrier gas flow rate, results in varying particle trajectories within the jet. Thus, fewer particles reach the hottest regions at higher velocities.

In this study, particle detection was focused on a specific location. If the study had considered the overall thermodynamic state of the particles, while some would have higher temperatures, the total quantity would be limited.

## 2.2. Influence of the torch configuration

Particle velocity vs. temperature window was drawn with different plasmas (Ar/H₂, arc current) for the 4 torch configurations (diameter and tunnel length).

### 2.2.1. Influence of the tunnel diameter (8 or 10 mm)

First, based on Figure 5, the velocity of alumina particles using a Larmor 15 tunnel 8 seems to reach a maximal velocity higher than Larmor 15 tunnel 10. In addition, the maximum velocity
appears to correlate with a maximum temperature. The increasing of the tunnel diameter implies
the decrease of the particle velocity. Different reasons can explain this evolution:

- the particle velocity is related to the plasma gas flow velocity which is lower using the 10mm tunnel due to a wider velocity profile.

- the smaller the diameter of the tunnel, the more constricted the plasma jet. It is caused by the decrease in the gap between the plasma column and the inner surface of the nozzle. In these conditions, the momentum and velocity gradients are more significant as observed by Betoule [27].

<figure><img src="image_6.png" /><figcaption>Fig. 5. Comparison of particle velocity as a function of particle temperature for Larmor 15 with tunnel inner diameter (8 and 10 mm).</figcaption></figure>

The authors observed no real temperature variations which is in good agreement with the results
(from 2750 to 3100°C for tunnel 8 and from 2750 to 3050°C for tunnel 10).

Increasing the diameter can lead to a small decrease in particle temperature (Tmax= 3100°C for
diameter 8mm compared to 3000°C for diameter 10mm). As the plasma jet expands, its overall
characteristics change. This can lead to a reduction in the energy density within the jet, consequently affecting the heat transfer to the particles and resulting in a lower particle temperature.

### 2.2.2. Influence of the number of neutrodes (tunnel length)

Figure 6 presents the particle velocity vs. the temperature for a same tunnel diameter (8mm) and two numbers of neutrodes (Larmor 9 and 15).

<figure><img src="image_7.png" /><figcaption>Fig. 6. Comparison of particle velocity as a function of particle temperature for Larmor 9 and 15 with a tunnel inner diameter of 8 mm.</figcaption></figure>

Whatever the number of neutrodes, the results show a similar interval of temperature approximately from 2700 to 3150°C for Larmor 15 and from 2750 to 3150°C for Larmor 9. It seems that the number of neutrodes does not affect the temperature of in-flight particles for the range of parameters tested as much as it affects their velocity. A previous study presented the influence of plasma parameters on arc voltage and total and net power [28]. The number of electrodes influences
the arc length and therefore the voltage, interval 120-140V for Larmor 9 tunnel 8 and 150-170V for Larmor 15 tunnel 8. Consequently, the power involved is not the same, between 44 and 63kW for Larmor 9 tunnel 8, and 56-78kW for Larmor 15 tunnel 8. Moreover, the applied current remains independent of the number of electrodes. It is therefore quite normal to find that the longer the arc, the higher the jet temperature. However, the number of electrodes also affects the power loss through water heat exchange. The calculation of total and net power is explained in a previous study [28]. The ratio of the net power on total power is ranged from 48 to 55% for Larmor 9 tunnel 8 and 42 to 52% for Larmor 15 tunnel 8. The more the electrodes, the greater the losses. Therefore, after a certain point, the net power values will be approximately similar, 22-36kW for both torches. Consequently, the temperature difference is not noticeable. Other effects can affect the particle temperature such as the plasma temperature, the particle microstructure and shape, associated to drag and dwell time within the plasma, which are strongly coupled with plasma gas velocity. Also, electromagnetic pump (Lorentz force), defined as J*B, related to the interaction between the current density (J in A/m²) and a magnetic field (B in Tesla), plays a significant role by accelerating the plasma and affecting particle drag and dwell time. An uncontrolled J*B effects can trigger plasma instabilities, so the particles [29].

However, an increase of particle velocity is observed when increasing the number of neutrodes for a same tunnel diameter. A maximum velocity of 400m/s is achieved with Larmor 9 compared to 500m/s with Larmor 15. This increase in velocity can be attributed to the longer constricted jet with Larmor 15 resulting in higher jet velocity consequently in higher particle velocity. This case of comparison is the originality of this work, to conclude the effect of number of neutrodes on all other parameters. The greater number of neutrodes leads to higher arc voltage and consequently increased power input. This enhanced energy: modifies plasma characteristics (temperature,
enthalpy), intensifies gas expansion and acceleration and results in higher particle velocities. Also to reduce the electrodes wear, and so be more economical.

## 2.3. Particle Velocity and Temperature vs. Coating Microstructure

Graph of the velocity vs. temperature for the 4 torch configurations is presented in Figure 7.

<figure><img src="image_8.png" /><figcaption>**Fig. 7.** Evolution of particle velocity as a function of particle temperature for Larmor (9 and 15) with tunnel inner diameter (8 and 10 mm).</figcaption></figure>

Please note that error bars are not shown in the figure to improve clarity. The error bars are typically within 2-5% for temperature measurements and around 10-15% for velocity measurements. The Larmor 9 tunnel 10, is related to the smallest particle velocity and temperature, as the Larmor 15 tunnel 8 is related to the largest values of these two parameters. Moreover, the Larmor 15 tunnel 10 and Larmor 9 tunnel 8 have the same range of particles velocity but different particle temperature. As explained previously, the tunnel diameter plays a key role in both particle temperature and velocity. A smaller diameter, like the Larmor 9 tunnel 8 configuration, confines
the particles within a tighter space, leading to two effects: increased particle temperature and enhanced particle velocity. On the other hand, the Larmor 15 tunnel 10 configuration has a larger diameter. While the increased number of neutrodes in this torch might tend to boost velocity, this effect is counteracted by the wider diameter. The wider space spreads out the plasma flow, reducing its ability to accelerate the particles significantly. Consequently, the Larmor 15 tunnel 10 configuration might achieve a similar velocity to the Larmor 9 tunnel 8, despite having a longer cathode-anode distance.

Regions where alumina particles have the same velocity and temperature (zones 1, 2, and 3) for different torch configurations are studied. The analysis of these zones is based on the microstructure of the obtained deposits (see properties in Table 3). Coating microstructures are shown in Figure 8, and the operating parameters are displayed below each microstructure image in Figure 8. For each of these three zones, the microstructures for the different torch configurations should be the same since they have similar particle velocities and temperatures. In all three zones, the torches that exhibit identical particle velocities and temperatures are Larmor 15 tunnel 8 and 10, and Larmor 9 tunnel 8. For zone 1, the particle velocity and temperature are approximately 350m/s and 2750°C, respectively. The porosity of the deposits ranges from 0.7 to 1.2% (see Table 3).

<table><caption>Table 3
Properties of alumina coatings in the three different zones for different Larmor torch configurations.</caption><thead><tr><th rowspan="2"></th><th rowspan="2">Tunnel<br>(mm)</th><th colspan="3">Porosity (%)</th><th colspan="3">Hardness (HV0.3)</th></tr><tr><th>Zone 1</th><th>Zone 2</th><th>Zone 3</th><th>Zone 1</th><th>Zone 2</th><th>Zone 3</th></tr></thead><tbody><tr><td>Larmor<br>9</td><td>8</td><td>0.8 ± 0.1</td><td>0.9 ± 0.2</td><td>0.6 ± 0.1</td><td>1051 ± 84</td><td>1029 ± 124</td><td>1030 ± 81</td></tr><tr><td>Larmor<br>15</td><td>8<br>10</td><td>1,2 ± 0.1<br>0.7 ± 0.2</td><td>0.8 ± 0.1<br>1 ± 0.1</td><td>1.1 ± 0.2<br>0.7 ± 0.2</td><td>970 ± 100<br>1007 ± 26</td><td>1052 ± 100<br>1036 ± 36</td><td>1081 ± 35<br>1118 ± 75</td></tr></tbody></table>

The hardness values are between 970 and 1050 HV₀.₃. For zone 2, the particle velocity and temperature differ, with values of 320 m/s and 2840°C, respectively. The porosity is similar between 0.8 and 1%, and the hardness values slight increase between 1029 and 1086 HV₀.₃. Finally, for zone
3, the particle velocity is 400m/s, and the particle temperature is approximately 3000°C. The porosity ranges from 0.6 to 1.1%, and the hardness values are between 1030 and 1118 HV₀.₃. So analyses in these zones (details in Table 3) revealed similar porosity content (0.6-0.8%) and a microhardness ranged from 970 to 1118HV₀.₃) for the alumina coating. This finding aligns with the established principle that the microstructure is directly influenced by both the velocity and temperature of the impacting particles. For all zones, coatings have a dense microstructure with a porosity below 1% and a hardness exceeding 1000HV₀.₃.

<caption>**Table 4**</caption>

Torch and operating parameters used to obtain extreme in-flight particle velocities and temperatures.

<table><caption>**Table 5**</caption><thead><tr><th>Particle state</th><th>Cold and Fast (A)</th><th>Hot and ultra-fast (B)</th><th>Hot and Fast (C)</th><th>Hot and Slow (D)</th></tr></thead><tbody><tr><td>Torch</td><td>Larmor 15</td><td></td><td>Larmor 9</td><td></td></tr><tr><td>Tunnel (mm)</td><td>8</td><td></td><td>8</td><td>10</td></tr><tr><td>Operating parameters</td><td>350/45/2</td><td>450/45/4</td><td>450/35/2</td><td>450/35/2</td></tr></tbody></table>

Properties of alumina coatings for center and extreme in-flight particle velocities and temperatures.

<table><thead><tr><th rowspan="2">specific point</th><th colspan="2">Larmor 9</th><th colspan="2">Larmor 15</th><th rowspan="2">Center (E)</th></tr><tr><th>Tunnel 8</th><th>Tunnel 10</th><th>Tunnel 8</th><th>Tunnel 10</th></tr></thead><tbody><tr><td>(I/Ar/H₂)</td><td>Hot and fast (C)<br/>450/35/2</td><td>Hot and slow (D)<br/>450/35/2</td><td>Cold and fast (A)<br/>350/45/2</td><td>Hot and ultra-fast (B)<br/>450/45/4</td><td>350/35/4</td></tr><tr><td>Porosity (%)</td><td>0.7 ± 0.1</td><td>1 ± 0.1</td><td>1 ± 0.2</td><td>3 ± 0.4</td><td>0.9 ± 0.1</td></tr><tr><td>Hardness (HV₀.₃)</td><td>1090 ± 55</td><td>1083 ± 119</td><td>1023 ± 95</td><td>1156 ± 67</td><td>1020 ± 72</td></tr></tbody></table>
<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /><figcaption>Larmor 9 tunnel 8
350/45/2</figcaption></figure>

<caption>Larmor 15 tunnel 8
350/45/4</caption>

<figure><img src="image_11.png" /><figcaption>Larmor 15 tunnel 10
350/45/2</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>Larmor 9 tunnel 8</figcaption></figure>

<caption>Larmor 15 tunnel 8
Paramètres opératoires (I/Ar/H₂) **Zone 2**</caption>

<caption>Larmor 15 tunnel 10</caption>
<figure><img src="image_13.png" /><figcaption>**Fig. 8.** Image of coating cross sections depending on in-flight particle characteristics (Zones 1 to 3).</figcaption></figure>

To further explore the relationship between particle velocity and temperature on coating microstructures, an additional set of coating manufacturing was conducted. Spray parameters corresponding to the extreme particle conditions were chosen, represented by the blue circle in the Figure 7 and listed in Table 4. These extremes (denoted as A, B, C and D in Figure 7), include “cold and fast” (A), “hot and ultra-fast” (B), “hot and fast” (C), and “hot and slow” (D). This investigation aims to determine the range of necessary conditions to achieve a high-quality alumina coating. Coating microstructures are shown in the Figure 9, the operating parameters are displayed below each microstructure image in Figure 8. Under “hot and ultra-fast” (B) settings, the particle velocity reaches almost 500m/s. Compared to the “hot and fast” (C) scenario, which has a similar temperature but a lower velocity (around 380m/s), a significant increase in porosity is observed (see Table 5). This suggests that for too high velocities, main of the particles don't have a sufficient
residence time in the plasma jet to completely melt and form a good lamellar structure, resulting in higher porosity.

<figure><img src="image_14.png" /><figcaption>Fig. 9. Image of coating cross sections depending of in-flight particle properties (a) Hot and fast, (b) Hot and slow, (c) Cold and fast, (d) Hot and ultra-fast.</figcaption></figure>

By comparing “hot and fast” (C) and “cold and fast” (A) conditions, similar velocities and consequently similar porosity values (around 0.7% and 1% respectively) are obtained. However, a slightly higher hardness is observed for the “hot and fast” (C) scenario (1080 compared to 1020 HV<sub>0.3</sub>). This can be attributed to a better melting of the particles due to the higher temperature. In both cases, however, low-porosity coatings are obtained. Finally, comparing “hot and fast” (C)” to “hot and slow” (D) conditions reveals no significant difference beyond the expected velocity gap (400 versus 200m/s respectively).
Please note that high-quality particles can be produced through the current treatment process, even though the quantity is limited. A significant increase in the number of particles is possible, but this may result in lower deposition quality due to the non-optimization of the injector diameter and carrier gas flow rate.

## Conclusion

This study focuses on the characterization of a modular cascade plasma torch whose tunnel length (number of cascades) and inner diameter can be modified. Two main objectives are addressed: first, to investigate how the geometry of the torch, particularly the number of neutrodes (which affects the cathode-anode distance), influences the velocity and temperature of alumina particles. Second, to explore how operating parameters like arc current, argon and hydrogen flow rates affect particle properties and ultimately influence the microstructure of the resulting alumina coating in the range of the specific studied parameters.

Phase 1 focused on the impact of torch design, particularly the number of neutrodes. In-flight particle measurements reveal a direct effect of the number of neutrodes on particle velocity. Increasing the distance between the cathode and anode led to faster particles. This increase leads also to a raises in arc voltage, thereby decreasing electrode wear and improving cost efficiency. Interestingly, the diameter had a contrasting effect: if it increases particle velocity is reduced but also their temperature increases.

Phase 2 explored plasma operating parameters on particle properties. Arc current has the most significant impact, raising both particle velocity and temperature with increasing arc current. Contrary to expectations, hydrogen flow rate did not significantly affect particle properties in this study and in the range of parameters studied. Argon flow rate, however, played a role in influencing particle temperature. The temperature rises with the flow rate increasing, while it does not affect
the velocity. Indeed, increasing the argon flow rate from 35 to 55L/min does not affect the velocity, suggesting that this variation range may not be sufficient to observe this effect.

The investigation of the relationship between particle velocity and temperature on the final microstructure of alumina coatings was done. This analysis demonstrates that velocities exceeding 400m/s significantly increase coating porosity, often by a factor of three or more. Conversely, higher temperatures generally lead to denser coatings and improve hardness. Notably, low-porosity coatings (below 1%) with a hardness exceeding 1000 HV₀.₃ are obtained with particle velocities between 200 and 400m/s and temperatures between 2700 and 3100°C.

The outlook of these analyses and results will focus on the influence of the powder mass feed rate of alumina powder to evaluate the effect on coating microstructures.

## Acknowledgments

This project was made possible by the financial support of the Bourgogne-Franche-Comté region.
ICB is supported by the EUR-EIPHI Graduate School (Grant No. 17-EURE-0002).

## References

[1] P. Fauchais, Understanding plasma spraying, J. Phys. D: Appl. Phys. 37 (2004) R86-R108.
https://doi.org/10.1088/0022-3727/37/9/R02.

[2] P.L. Fauchais, J.V.R. Heberlein, M.I. Boulos, Thermal Spray Fundamentals: From Powder to Part, Springer US, Boston, MA, 2014. https://doi.org/10.1007/978-0-387-68991-3.

[3] R. Manich, J.P. Davim, Thermal Sprayed Coatings and Their Tribological Performances, Engineering Science Reference, 2015.

[4] L. Pawlowski, The science and engineering of thermal spray coatings, J. Wiley and sons, Chichester, West Sussex, 2008.

[5] S. Matthews, B. James, Review of Thermal Spray Coating Applications in the Steel Industry: Part 1—Hardware in Steel Making to the Continuous Annealing Process, Journal of Thermal Spray Technology 19 (2010) 1267-1276.

[6] J.M. Matějíček, P. Chráska, J. Linke, Thermal Spray Coatings for Fusion Applications—Review, Journal of Thermal Spray Technology 16 (2007) 64–83.

[7] Z. Duan, et al., “Arc Instabilities in a Plasma Spray Torch”, J. Therma. Spray Technol., Vol. 11, No. 1 (2002), p. 44-51, doi.org/10.1361/105996302770348961, (n.d.).

[8] F. Bocklisch, G. Paczkowski, S. Zimmermann, T. Lampke, Integrating human cognition in cyber-physical systems: A multidimensional fuzzy pattern model with application to thermal
spraying, Journal of Manufacturing Systems 63 (2022) 162-176.
https://doi.org/10.1016/j.jmsy.2022.03.005.

[9] R. J. Molz et al., U.S. Patent 2014/0326703 A1, n.d.

[10] J. Colenares-Angulo, R. Molz, D. Hawley, R.C. Seshadri, Thorium-Free Versus Thoriated Plasma Gun Electrodes: Statistical Evaluation of Coating Properties, Journal of Thermal Spray Technology 25 (2016) 622-630.

[11] J. Schein, M. Richter, K.D. Landes, G. Forster, J. Zierhut, M. Dzulko, Tomographic Investigation of Plasma Jets Produced by Multielectrode Plasma Torches, Journal of Thermal Spray Technology 17 (2008) 338-343.

[12] R. Zhukovskii, C. Chazelas, A. Vardelle, V. Rat, Control of the Arc Motion in DC Plasma Spray Torch with a Cascaded Anode, Journal of Thermal Spray Technology 29 (2020) 3-12.

[13] H. Saito, H. Matsumoto, T. Fujino, Dynamic Behavior of Plasma in a Cascade Plasma Torch with External Magnetic Field, Journal of Thermal Spray Technology 29 (2020) 333-343.

[14] R.C. Seshadri, S. Sampath, Characteristics of Conventional and Cascaded Arc Plasma Spray-Deposited Ceramic Under Standard and High Throughput Conditions, Journal of Thermal Spray Technology 28 (2019) 690-705.

[15] D. Chen, R. Rocchio-Heller, C. Dambra, Segmented Thermal Barrier Coatings for ID and OD Components Using the SinplexPro Plasma Torch, Journal of Thermal Spray Technology 28 (2019) 1664-1673.

[16] A. Dolmaire, E. Hartikainen, S. Goutier, E. Béchade, Vardelle, Benefits of Hydrogen in a Segmented Anode Plasma Torch in Suspension Plasma Spraying, (n.d.).

[17] G. Darut, M.P. Planche, H. Liao, C. Adam, A. Salito, M. Rösli, Study of the In-Flight Characteristics of Particles for Different Configurations of Cascade Plasma Torches, in: F. Azarmi, X. Chen, J. Cizek, C. Cojocaru, B. Jodoin, H. Koivuluoto, Y.C. Lau, R. Fernandez, O. Ozdemir, H. Salimi Jazi, F. Toma (Eds.), Virtual, 2021: pp. 499-507.
https://doi.org/10.31399/asm.cp.itsc2021p0499.

[18] X. Tu, B.G. Chéron, J.H. Yan, K.F. Cen, Electrical and spectroscopic diagnostic of an atmospheric double arc argon plasma jet, Plasma Sources Sci. Technol. 16 (2007) 803-812.
https://doi.org/10.1088/0963-0252/16/4/016.

[19] C. Ruelle, S. Goutier, V. Rat, A. Keromnes, C. Chazelas, É. Meillot, Study of the electric arc dynamics in a cascaded-anode plasma torch, Surface and Coatings Technology 462 (2023) 129493. https://doi.org/10.1016/j.surfcoat.2023.129493.

[20] G. Renouard-Vallet, Elaboration par projection plasma d'électrolytes de zircone yttriée denses et de faible épaisseur pour SOFC., (n.d.).

[21] ImageJ. Available online: http://imagej.nih.gov/ij/ (accessed on 3 May 2024), (n.d.).

[22] M.-P. Planche, Contributon à l'étude des fluctuations dans une torche à plasma. Application à la dynamique de l'arc et aux mesures de vitesse d'écoulement., PhD thesis (in french), University of Limoges, 1995.

[23] E. Bourdin, P. Fauchais, A. Vardelle, Les transferts de chaleur et de quantité de mouvement plasma-particules sans réactions chimiques., Revue Générale de Thermique, 1983.

[24] K. Cheng, X. Chen, W. Pan, Comparison of Laminar and Turbulent Thermal Plasma Jet Characteristics—A Modeling Study, Plasma Chemistry and Plasma Processing 26 (2006) 211-235. https://doi.org/10.1007/s11090-006-9006-6.

[25] R. Zhukovskii, C. Chazelas, V. Rat, A. Vardelle, R.J. Molz, Effect of Cathode-Plasma Coupling on Plasma Torch Operation Predicted by a 3D Two-Temperature Electric Arc Model, J Therm Spray Tech 32 (2023) 532-547. https://doi.org/10.1007/s11666-022-01501-1.
[26] F. Mavier, Procédé de projection plasma en mode pulsé associé à une injection jet d'encre synchronisée Mise au point, processus en vol et élaboration de dépôts céramiques, University of Limoges, 2017.

[27] O. Betoule, Influence des distributions de vitesse et température de jet de plasma d'arc et de particules sur les propriétés des dépôts d'alumine, 1994.

[28] M. Sleiman, G. Darut, R. Seulin, M.P. Planche, J.-J. Gonzalez, P. Freton, F. Sambou, A. Salito, M. Rösli, Effect of Operating Parameters and Anode Configuration on Plasma Generated by a Modular Cascaded Torch, J Therm Spray Tech 34 (2025) 875–884. https://doi.org/10.1007/s11666-024-01867-4.

[29] K. Bobzin, M. Öte, M.A. Knoch, I. Alkhasli, U. Reisgen, O. Mokrov, O. Lisnyi, Simulation of the Particle Melting Degree in Air Plasma Spraying, J. Phys.: Conf. Ser. 825 (2017) 012002. https://doi.org/10.1088/1742-6596/825/1/012002.
**Figure captions**

Figure 1. Scheme of Larmor segmented plasma torch.

Figure 2. (a) Control panel of the system, (b) Placement of the injector.

Figure 3. Image of the DPV 2000 equipment.

Figure 4. Evolution of particle a) velocity and b) temperature as a function of arc current for Larmor (9 and 15) with tunnel inner diameter (8 and 10mm) - (Ar/H2= 45/4 L/min).

Figure 5. Comparison of particle velocity as a function of particle temperature for Larmor 15 with tunnel inner diameter (8 and 10mm).

Figure 6. Comparison of particle velocity as a function of particle temperature for Larmor 9 and 15 with a tunnel inner diameter of 8mm.

Figure 7. Evolution of particle velocity as a function of particle temperature for Larmor (9 and 15) with tunnel inner diameter (8 and 10mm).

Figure 8. Image of coating cross sections depending on in-flight particle characteristics (Zones 1 to 3).

Figure 9. Image of coating cross sections depending of in-flight particle properties (a) Hot and fast, (b)Hot and slow, (c) Cold and fast, (d) Hot and ultra-fast.