Beryllium thin films deposited by thermionic vacuum arc for nuclear applications

Applied Surface Science 481 (2019) 327–336

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Beryllium thin films deposited by thermionic vacuum arc for nuclear applications

T

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Vasile Tirona, Corneliu Porosnicub, Paul Dincab, Ioana-Laura Velicuc, , Daniel Cristead, Daniel Munteanud, Ádám Révésze, George Stoianf, Cristian P. Lungub a

Research Department, Faculty of Physics, Alexandru Ioan Cuza University of Iasi, Iasi 700506, Romania National Institute for Laser, Plasma and Radiation Physics, Magurele 077125, Romania c Faculty of Physics, Alexandru Ioan Cuza University of Iasi, Iasi 700506, Romania d Faculty of Materials Science and Engineering, Department of Materials Science, Transilvania University, Brasov 500068, Romania e Department of Materials Physics, Eötvös University, Budapest, Hungary f National Institute of Research and Development for Technical Physics, Iasi 700050, Romania b

A R T I C LE I N FO

A B S T R A C T

Keywords: Beryllium Mechanical properties Plasma diagnostics Thermionic vacuum arc Thin film

The aim of this paper is to investigate the influence of the thermionic vacuum arc (TVA) operation parameters (arc current, arc voltage and thermo-emission filament current) on the Be ion energy and plasma ionization degree, as well as on the topological, structural and mechanical properties of Be thin films. For this purpose, nanocrystalline Be thin films, with thickness of approximately 1 μm, were deposited by TVA on not intentionally heated silicon and stainless steel substrates by varying the arc voltage from 0.8 to 2.0 kV. Topological, structural and mechanical (hardness, Young's modulus, adhesion critical loads, and friction coefficient) properties of Be thin films were investigated using atomic force microscopy (AFM), X-ray diffraction (XRD), scanning electron microscopy (SEM), Rutherford backscattering spectrometry (RBS), nanoindentation and scratch tests. The mechanical behaviour and structural changes are discussed based on TVA plasma diagnostics results. Be ion energy and plasma ionization degree were measured using cold and emissive probes, energy-resolved mass spectrometer and a quartz crystal microbalance. The preferential crystallographic orientation, morphology and packing density are sensitive to the selected processing parameters which, in turn, control, over a wide range, the plasma ionization degree and Be ion energy. The film deposited using an arc voltage value of 1.5 kV exhibits dense structure with low lattice defect density, high hardness, low coefficient of friction, good wear-resistance, fracture toughness and adhesion to the substrate.

1. Introduction Often called “the miracle metal”, beryllium (Be) is known to be one of the lightest and stiffest metals, with huge potential applications in medicine, safety, aerospace, communications, national defence, transportation, homeland security, military and nuclear sectors. So widely spread, Be significantly improves the quality of our lives. Able to keep its shape and properties under extreme temperatures, with excellent conductivity and reflectivity, high melting point and high transparency to X-rays, Be was classified as a strategic and critical material by agencies in both the U.S. and European governments. In industry, Be is used in three forms: as pure metal (≈10% of the annual Be consumption), as beryllium oxides (BeO, ≈15% of the ⁎

annual Be consumption), and in alloys (≈75% of the annual Be consumption), especially with copper, magnesium, aluminium or nickel [1]. Metal usage of Be is widespread in nuclear and aerospace applications. Exploiting their high thermal conductivity and good electrical resistivity to give an effective heat sink, beryllia ceramics are used as electronic substrates, particularly in high power devices or high-density electronic circuits for high speed computers. Since BeO is transparent to microwaves and X-rays, it is also used as radomes and antennas in microwave communication systems and microwave ovens, as X-ray windows, as well as in high-power laser tubes [2]. Due to its remarkable properties (low atomic mass, low X-ray absorption cross section, and high neutron scattering cross section), BeO is very attractive for nuclear

Corresponding author at: “Alexandru Ioan Cuza” University of Iasi, Faculty of Physics, Blvd. Carol I, Nr. 11, Iaşi 700506, Romania. E-mail address: [email protected] (I.-L. Velicu).

https://doi.org/10.1016/j.apsusc.2019.03.096 Received 20 December 2018; Received in revised form 3 March 2019; Accepted 11 March 2019 Available online 12 March 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.

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potential is hundreds of volts or even several kilovolts above the grounded chamber wall), so that the metal ions will be accelerated towards the vacuum vessel wall or growing film, gaining an energy proportional with the plasma potential against the ground. Another important feature of the thermionic vacuum arc (under suitable experimental conditions) is the presence of a strong double layer (DL) structure surrounding the anodic plasma, which separates the lowdensity plasma – formed downstream, within vessel wall region, from the high-density plasma – formed upstream, in the anode region [11]. The potential drop across DL is of the order of the anode potential (kV), so that the charged particles crossing DL may gain very high kinetic energy. The formation of a DL in TVA plasma allows avoiding the use of ion accelerating grids or substrate bias to control the ion energy. Apart from the kinetic ion energy control, the TVA enables to adjust the plasma ionization degree and deposition rate by means of a simple choice of the operating parameters [12]. Because the TVA plasma is generated only in the metal vapours and the growing film is bombarded with high-energy metal ions, nanostructured thin films with high purity, increased adhesion, low roughness and compact structure can be obtained [13]. In TVA deposition system, due to very large gradient of metallic vapours and open geometry of the discharge electrodes, the plasma species diffuse in almost all directions (radial losses) allowing very large surfaces to be coated. Furthermore, the TVA method was already employed for the deposition of Be based coatings in several fusion related studies [14–16]. This paper describes results of the optimization of TVA deposition process in order to obtain high-density Be coatings with enhanced mechanical properties, suitable for nuclear applications. The influence of the discharge voltage and current on the films' microstructure and mechanical properties was investigated using nanoindentation and scratch tests, X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM) and Rutherford backscattering spectrometry (RBS). The influence of the processing parameters on the mechanical behaviour and structural changes of the deposited thin films is discussed based on TVA plasma diagnostics results. This paper comes to expand the existing knowledge on this topic because, so far, insufficient information is available on the characterization of pure beryllium thin films, especially because special precautions must be taken when working with Be, due to its toxicity.

applications [3]. It was selected as a reflector or moderator in nuclear fission reactors. It has also been evaluated for use in high temperature gas cooled reactors. Be also possesses a unique combination of interesting mechanical and physical properties, being highly suitable for applications in engineering. Copper‑beryllium alloys are used for their good electrical and thermal conductivities and high corrosion and fatigue strength. Some of the most important current applications of beryllium are: the mirrors of Spitzer Space Telescope (launched in 2003) and the James Webb Space Telescope (planned to launch in 2021 to be operated at the second Sun-Earth Lagrange point to extend the discoveries of Hubble Space Telescope) [4], military fighter jets (F-35 Lightning II Joint Strike Fighter, F-22 Raptor, F-18 Super Hornet, F-16 Fighting Falcon, F-15 Strike Eagle, etc.), in optical systems of military helicopters, as vibration dampener for battle tanks, in emerging guided missile defence systems, etc. Concerning the metal usage of Be in the nuclear sector, the main aim of the ITER-like wall (ILW) project is to demonstrate fusion nuclear reactor operation with beryllium (Be) based plasma facing components (PFC) in the main chamber. Beryllium, due to its ability to improve the vessel conditions and plasma performances, was already successfully used as a limiter material in the ISX-B tokamak and as limiter and divertor in JET projects [5,6]. The PFC is exposed to high heat loads and particle fluxes coming from the fusion plasma. This interaction results in melting and sputtering of the inner-wall materials. In order to reduce the influx of high-Z metals, to lower the oxygen levels and eliminate the chemical sputtering of C-based PFC, the inner wall of the reactor's main chamber was made from bulk Be tiles and Be coated inconel marker tiles [7]. The electrical, thermal and structural properties of the Be coating must match, as closely as possible, those of bulk Be. In addition, in order to ensure good thermo-mechanical compatibility of the system, a very good adhesion of the Be coating on the Inconel tiles is required. Research on plasma processing of materials is especially focused on the control of plasma parameters for their direct correlation to the microstructure and properties of the deposited films. In the plasmaassisted deposition technology, the predominant factor controlling the structure and properties of the deposited thin films is the mobility of condensing species on the substrate. The mobility of the film forming species can be controlled through the substrate temperature (thermal diffusion) and/or through the energies and fluxes of the bombarding species generated within the plasma [8]. The thermionic vacuum arc (TVA) is a metal vapour plasma source characterized by highly energetic ions, high ionization degree and high deposition rate. These features can be fully controlled during the thin films' deposition process through the operating parameters (arc current, arc voltage, thermo-emission filament current, geometry and relative position of the electrodes) [9,10]. An important property of the thermionic arc is that the plasma bulk is highly positive (the plasma

2. Experimental details 2.1. Thin film deposition The experimental setup for the TVA deposition process consists of an ultra-high vacuum stainless steel chamber (inner volume of 1 m3), electrically grounded. In order to obtain high-vacuum condition before arc plasma ignition, a pumping system consisting of a turbo-molecular

Fig. 1. Schematic view of the discharge electrodes system (a) and side view of TVA plasma (blue colour) ignited in beryllium vapours (b). The bright white light comes from the incandescent filament. 328

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pump backed by a rotary vacuum pump was used. This pumping system achieves inside the deposition chamber a residual pressure below 10−7 Torr. The anode used for the deposition process was a Be rod (2.5 cm in diameter and 3 cm in height), placed in a tungsten holder (Fig. 1a). The discharge cathode consists of a tungsten filament, loopshaped (1 cm in diameter), surrounded by a Wehnelt cylinder (5 cm in diameter and 5 cm in height) used for thermo-electrons focus. This geometry of the discharge electrodes is different from that used in references [7–11] and the entire system is known in the literature as TVA system with circular cathode configuration. The tungsten wire of the electron gun is electrically heated using a high current and low voltage power supply. One end of the filament together with the Wehnelt cylinder is electrically grounded. For a convenient distance (approximately 1 ÷ 2 cm) between the anode and the filament and a sufficiently high positive potential (1 ÷ 2 kV) applied between the anode and the grounded filament, the thermo-electrons are accelerated towards the anode and focused on the anode surface by the Wehnelt cylinder, heating up the bulk beryllium. In this stage, the arc discharge operates at low current values (< 200 mA) limited by the electron gun thermo-emission current. During this thermal heating process, the bulk Be melts and then evaporates leading to a local increase of the Be metal vapours pressure. This new pressure condition together with the fast-electron collisions with neutrals creates optimal conditions for a bright arc plasma ignition (Fig. 1b), localized above the anode region. The large concentration gradient of metallic vapours, produced above the anode surface, leads to the formation of a bright plasma with quasi-spherical shape, whose volume depends on the processing parameters. After TVA plasma ignition, the anode draws a large arc current (0.5–2 A), whose value can be controlled either by the discharge current value, using a current limited supply, or by the filament current. TVA plasma is sustained by both electrons emitted by the hot filament cathode and those originating from the plasma itself. The TVA plasma becomes stable when the metal vapours loss rate is compensated by the thermal evaporation rate. In order to investigate the influence of processing parameters (discharge voltage and current) on Be thin film properties, several Be coatings (denoted here as S1, S2, S3, and S4) were deposited on silicon (Si) and polished stainless steel substrates, at different electrical parameters of the TVA discharge. The substrates were fixed on an electrically grounded substrate holder, positioned at 26.5 cm above the Be anode. During the deposition process, the substrates were unintentionally heated. The substrate temperature was monitored using a Ktype thermocouple probe, which was positioned on the substrate holder. According to the thermocouple probe measurement, the substrates temperature did not overcome 50 °C. Therefore, we can assume that the deposition temperature is low enough to not affect the structural properties of the deposited thin films. The filament current (If) value was adjusted between 33.9 and 35.5 A, while the discharge voltage (U) value was ranged between 0.8 and 2.0 kV. In order to keep the discharge power (P) constant (P = 1 kW), the arc current value was varied between 0.50 and 1.25 A. A quartz crystal microbalance (QCM) and a disc-shaped probe, positioned near the substrate holder, were used to monitor the deposition rate and ion flux, respectively. The processing parameters values of the TVA discharge together with the values for the film's thickness (estimated from cross-sectional SEM images) and deposition rates are listed in Table 1.

Table 1 Summary of electrical parameters of TVA discharge (discharge voltage (U), current (Ia), power (P), and filament current (If)), films' thickness (t), and deposition rate (S). Sample

U (kV)

Ia (A)

P (kW)

If (A)

t (nm)

S (nm/s)

S1 S2 S3 S4

0.8 1.0 1.5 2.0

1.25 1.00 0.67 0.50

1 1 1 1

35.5 34.9 34.5 33.9

975 965 1050 900

3.8 4.2 4.6 5.1

X-ray diffraction (XRD) measurements (Shimadzu LabX XRD-6000 diffractometer, Cu Kα source with λ = 0.15406 nm, in Bragg-Brentano geometry) were performed in order to analyse the structure of the deposited Be films. The X-ray diffraction patterns were evaluated by the Convolutional Multiple Whole Profile (CMWP) fitting algorithm [17]. In the CMWP procedure, the whole measured X-ray diffractogram is fitted directly by the sum of ab initio theoretically constructed size and strain profile functions and a baseline. In brief, these profile functions are calculated for each Bragg reflection for each crystalline phase as the inverse Fourier transform of the product of the theoretically elaborated size and strain Fourier coefficients [18]. The CMWP fit directly provides microstructural parameters, i.e. average coherent crystallite size, median and variance of crystallite size distribution, type, density and distribution of lattice defects of various kinds of materials [19]. Scanning electron microscopy (Carl Zeiss Crossbeam Neon 40ESB FIB/SEM Microscope) was used to observe the differences in the morphology of the Be films deposited under different TVA processing parameters. The thickness of the films, evaluated from cross-sectional SEM images, was used to estimate both deposition rate and film density. For the latter, the areal atomic density, estimated by Rutherford backscattering spectrometry (RBS), using 2.549 MeV 4He2+ ions from a 3 MV Tandetron™, was also used. [20]. The RBS spectra were simulated using SIMNRA code [21]. Mechanical properties (Young's modulus (E), hardness (H), adhesion critical loads (LC) and friction coefficient (μ)) were investigated by nanoindentation and microscratch tests, in ambient atmosphere, using a Nanoindentation Tester (NHT2, equipped with a three-sided diamond pyramidal Berkovich indenter tip, tip radius of about 100 nm) and a Micro-Scratch Tester (MST, equipped with a diamond Rockwell type indenter, tip radius of about 100 μm), (Anton Paar, CSM Instruments). The maximum indentation value was set in such a way to limit the indentation depth to no more than 15% of the films' thickness in order to minimize the substrate's influence on the indentation data. Each data point was determined from the mean of at least thirty measurements, for statistical relevance. The nanoindentation data was analysed using the Oliver-Pharr model [22,23]. During microscratch tests, the load was applied progressively, from 0.03 N to 20.00 N, with a loading rate of 10 N/min, on a length of 3 mm. At least eight tracks were made on each sample and the values for the critical loads were averaged. The critical load values were obtained after optical analysis of the scratch tracks. The critical loads are defined as follows: LC1 – the load necessary for the emergence of the first cracks in the film; LC2 – the load corresponding to the first delamination of the film; LC3 – the load responsible for the delamination of more than 50% of the film from the scratch track. From the scratch behaviour, on the initial region of the tests, which did not exhibit failures, the dynamic friction coefficient was extracted. An emissive probe was used to measure the plasma potential distribution along the normal axis to the anode surface. A detailed discussion of using the emissive probe to examine the plasma potential distribution in TVA is presented in a previous paper [12]. A disc-shaped cold probe, positioned near the substrate holder, was used to measure the Be ion flux towards the growing film. The time-averaged energy distribution function of Be ions (9Be+) was characterized using an

2.2. Be thin films characterization Surface roughness and morphology were characterized by atomic force microscopy (AFM) using a NT-MDT Solver Pro-M system. All the AFM measurements were carried out in contact mode, in air, at room temperature, using the same cantilever (NSG03 from NT-MDT). AFM scans over multiple random 3 × 3 μm2 areas were performed for each sample during one imaging session. The AFM images were processed using Nova software from NT-MDT. 329

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electrostatic quadrupole plasma analyser (EQP 1000, from Hiden Analytical Ltd). The mass spectrometer's nozzle (orifice diameter of 0.1 mm) was positioned sideways to the TVA plasma, at 10 cm distance from the discharge axis and 10 cm above the discharge anode. Plasma diagnosis was performed in order to understand the correlation between the TVA processing parameters, ion energy and ion-to-neutral flux ratio, and the corresponding structure and properties of the deposited Be thin films. 3. Results and discussions 3.1. TVA plasma diagnosis Extensive studies have demonstrated that ion energy and flux play an important role on the deposition and films' growth processes, properties and microstructure evolution. The controlled ion bombardment of the growing film is a key parameter and it can be used to modify and improve its structure and properties. Although the TVA system with circular cathode configuration was intensively used in the last decade to deposit thin films for nuclear application [24–27], the influence of the processing parameters on the films' structure and properties needs further investigations. Plasma diagnosis helps us to understand the correlation between the processing parameters, ion energies and ion fluxes, and the corresponding structure and properties of the films obtained in a TVA system. It is also necessary to determine if the energetic ions are favourable or detrimental in obtaining highquality coatings.

Fig. 2. Spatial distribution of plasma potential measured along the discharge axis at different electrical parameters of the TVA discharge. The zero axial position corresponds to the upper part of the anode.

filament. The plasma potential distribution approaches the potential distribution reported in previous studies [10,13] and we can assume that the open symbols from Fig. 2 indicate the values of the bulk TVA plasma potential, which represent 3/4 of the arc voltage. By taking into account the small voltage drop across the bulk plasma [10,13], we can state that approximately 1/4 of the arc voltage drops on the anode fall. Previous studies [10,13] have shown a nearly linear relationship between the maximum ion energy and the cathode potential drop. For this reason, we can assume that, in our case, the maximum ion energy corresponds to the cathode potential drop. The maximum bulk plasma potential values were registered close to the discharge anode and they are strongly dependent on the anode voltage. For each processing parameters set, the maximum plasma potential value is about 75% from the applied anode voltage value. Consequently, an effective way to increase the ion energy and to enhance the ion bombardment in TVA is to increase the arc voltage. The increase of the arc voltage with decreasing arc current, for a fixed input power, can be obtained by reducing the filament current. By decreasing the filament current, the electron flux coming from the hot filament decreases and, in order to keep the same value for the discharge current, the voltage drop across the electrodes increases, leading to a higher production rate of metal ions. For a fixed arc current, the intense plasma extends spatially with increasing the arc voltage. The ions generated in TVA plasma are accelerated in the large plasma potential gradient and gain energy continuously during their travel from the anode region towards the walls (and subsequently towards the substrate) due to the potential difference between the plasma and the grounded walls. A grounded substrate placed at a certain distance from the arc source receives ions with energies according to the local plasma potential where they were produced if the collisions of heavy particles are neglected. Under certain specific operation conditions, in the anode vicinity, the metal vapour pressure can be very high, and the charge exchange collisions must be considered. From the deposition rates presented in Table 1, a vapour pressure of about 100–133 Pa in the inter-electrode space can be estimated. With a BeeBe cross section of about 3 × 10–16 cm2, a mean free path of 1–1.4 mm can be estimated. Due to resonant charge transfer, some ions may gain energy corresponding only to a part of the full cathode voltage drop since they are neutralized through these collisions. Also, the initial neutral atoms may be accelerated in the partial cathode potential drop as soon they become ionized by charge exchange collisions. For this reason, we can

3.1.1. Plasma potential distribution Both maximum value and spatial distribution of plasma potential depend on the TVA operating parameters (arc current, arc voltage, thermo-emission filament current), anode material, but especially on the geometry and relative position of the electrodes [10]. An important feature of the thermionic vacuum arc is that the plasma bulk's potential is highly positive, and its maximum value and spatial distribution can be easily controlled by the arc voltage [11,12]. As was stated in the Introduction section, a strong double layer (DL) structure develops in the TVA plasma only under suitable experimental conditions. Our previous studies [12] have shown that the metal ions produced in TVA gain an amount of energy proportional to the potential drop across the DL. Unlike the plasma potential distribution (DL structure) reported in Ref. 11 and 12, in this study, the smooth decrease of the plasma potential does not indicate the presence of a DL structure. This is largely caused by the specific operation parameters employed in this work which differ from those used in previous studies [11,12]. From all these parameters, the most significant change involves the arc current and filament heating current, as well as the relative position of the electrodes. In the present work, both arc current and filament heating current are at least twice higher, while the filament is positioned just above the anode. Therefore, a much higher evaporation rate it is expected in this case leading to a strong vapour density gradient in the inter-electrode space. A high vapour density in the inter-electrode space requires a low cathode fall in order to sustain the discharge and to maintain the arc current at a constant level. Since the cathode is connected to the grounded vessel, the potential drop between the interelectrode plasma and the vessel walls equals the cathode fall [13]. Consequently, the ions produced in the inter-electrode plasma arrive at the metallic walls of the vessel with just the energy represented by the cathode fall. Fig. 2 shows the spatial distribution of plasma potential measured along the discharge axis, for different electrical parameters of the TVA discharge with circular cathode geometry. The spatial distribution of the plasma potential shows an exponential decay with the axial distance from anode discharge towards vacuum vessel walls. The open symbols correspond to the plasma potential values measured in the inter-electrode space, at 0.5 cm above the anode surface and 0.5 cm bellow the 330

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voltage. The ion acceleration is caused by the plasma potential gradient along the expansion direction of the TVA plasma. Therefore, the metal ions are continuously accelerated along this direction, from the anode towards the wall or grounded substrate.

3.1.3. Ion and neutral fluxes Since high deposition rate values are important for productivity and profitability, the research activity in the field of thin film deposition technology is focused on the improvement of films' quality without diminishing the power normalized deposition rate. In the sputtering process, most often, the increase in the deposition rate is not necessarily accompanied by an improvement in films' quality because their properties strongly depend on the energy of the film forming species, which consist of both neutral atoms and ions. Commonly, the ionization degree of the plasmas used for material processing is low and the neutral atoms are thermalized, making the ion-to-neutral flux ratio a much more important parameter in the control of the films' properties. Previous research studies showed that the mechanical properties of the coatings are strongly affected by their preferential orientation and packing density, which, in turn, depend on the substrate temperature and orientation, gas pressure and composition, films' thickness, ion energy and incident ion-to-neutral flux ratio [28,29]. Fig. 4 highlights the influence of arc voltage on the deposition rate and ion-to-neutral flux ratio during TVA operation in Be vapours, for a constant input power of 1 kW. The deposition rate was measured in situ using a quartz crystal microbalance, while the ion-to-neutral flux ratio was estimated based on ion flux and total particle flux measurements. A common way to measure the ionized flux fraction of plasma species is to use a gridless quartz crystal microbalance (QCM) sensor [30]. In this work, due to very high kinetic energy of ions, the ion flux was measured using an electrostatic collector, while the total particle (Be neutrals and ions) flux was estimated from QCM measurements. We assume that the sticking probability for particles on the QCM sensor is 1. An extensive description of the ion-to-neutral flux ratio measurement technique can be found in [31]. Although the TVA operates at the same input power (1 kW), by varying the arc voltage in the range of 0.65–2.00 kV, the deposition rate increases more than three times. The plot of deposition rate as a function of arc voltage shows two linear regions (one in the range of 650–850 V and the other one in the range of 850–2000 V) with distinct slopes, indicating two different operation regimes of TVA. The (a) region corresponds to the case when the total flux of particles which arrive on the QCM sensor is dominated by the metal atoms obtained by thermal evaporation with electrons originating mainly

Fig. 3. Be ion energy distribution function recorded during TVA plasma (arc voltage of 1 kV and current of 1 A).

state that the resonant charge transfer is an important mechanism limiting the ion energy. However, due to the direct relationship involving the maximum ion energy, cathode fall and arc voltage, the TVA discharge offers the possibility to easily control the energy of the metal ions by controlling only the arc voltage, without using specific ion accelerating means, like grids or substrate bias. 3.1.2. Ion energy distribution Fig. 3 shows the Be ion energy distribution function (IEDF) for TVA plasma operating at arc voltage and current values of 1 kV and 1 A, respectively. The IEDF is very broad and presents a peak corresponding to an ion energy value of about 125 eV and a high-energy tail which extends up to 730 eV. The low-energy part of IEDF corresponds to the Be atoms evaporated by electron impact and ionized during their transport towards the mass spectrometer head. These electrons have different origins: thermo-electrons emitted by the hot filament, secondary electrons produced in the plasma volume (α-Thousand processes) and electrons produced at the surface of the inner walls by ion bombardment (γ-Thousand processes). Instead, the high-energy tail structure might be related to the Be ions produced close to the discharge anode and accelerated towards the grounded mass spectrometer head by the highly positive plasma potential. The intermediate part of the IEDF corresponds to evaporated atoms, ionized through electron impact in different places (with different plasma potential) of the plasma volume, or to Be ions produced close to the discharge anode, subjected to collisions during their transport towards mass spectrometer head, losing their energy. Most of the ions have energies ranging from 50 to 200 eV, the mean energy being estimated as 138 eV. As the extraction orifice of the mass spectrometer is not facing the molten surface of the anode, the acceptance angle for the energetic ions will be reduced, leading to a drop-in ion counting efficiency. Therefore, the contribution of the energetic ions to the IEDF should be more pronounced and, consequently, a higher ion mean energy is expected. As the Be ions are mainly produced by electron impact ionization processes of thermally evaporated atoms, we can conclude that the ion energy gain comes only from the cathode potential drop. By considering the estimated value of the metal vapour pressure and the measured values of the cathode fall (3/4 of the full arc voltage), the density of the evaporated neutrals increases 1.33 times, while the cathode fall increases 2.5 times as the arc voltage increases from 0.8 to 2.0 kV. Therefore, the limiting ion energy by resonant charge transfer can be overcome by the enhanced cathode fall. So, it is expected that the mean energy of the ions, at the substrate level, to increase with the arc

Fig. 4. The influence of arc voltage on deposition rate and ion-to-neutral flux ratio. 331

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from the hot filament. The (b) region corresponds to the case when the total flux consists of particles obtained by thermal evaporation with electrons originating from both hot filament and TVA plasma volume. In order to increase the arc voltage, without changing the value of the discharge power, the value of the current on the discharge must be reduced. This can be done either by reducing the filament current or by increasing the distance between the filament and the discharge anode. In our case, in order to decrease the discharge current value from 1.25 to 0.50 A (while maintaining the input power constant at 1 kW), the filament current value was reduced from 35.5 to 33.9 A. The filament current reduction leads to a decrease in the evaporation rate of Be atoms originating from thermal evaporation process (sustained by the electrons emitted from the hot filament). Therefore, in the second region of the plot ((b) slope in the deposition rate vs. arc voltage), the contribution of the electrons coming from TVA plasma to the enhanced evaporation rate is more significant. This statement is also supported by the particles flux measurements which indicate a linear dependence of the ion-to-neutral flux ratio on the arc voltage. By varying the arc voltage value in the range of 800–2000 V, the ion-to-neutral flux ratio increases up to 4.5 times, while the Be ion flux increases more than 7 times. The significant increase in the ion flux denotes that plasma density increases, which causes an increase of the electrons flux from the thermionic arc plasma towards the anode. The enhanced electron flux towards the anode leads to further increase of the Be metal evaporation and ionization rate. Accordingly, by operating the TVA discharge with high voltage, both the deposition rate and ion-to-neutral flux ratio increases. In addition, by considering the results presented in Subsections 3.1.1 and 3.1.2, a high arc voltage enhances both ion energy and ion flux towards the substrate. This may improve the quality of the deposited thin films. Therefore, a strategy of increasing both ion energy and ion flux is to increase the contribution of electrons coming from the plasma to the thermal evaporation process to the detriment of those produced by the hot filament.

crystallographic preferred orientation gradually changed from (002) to (101) by increasing the arc voltage value, while the grains size slightly increased from 38 to 44 nm. By increasing the arc voltage in the range of 1–2 kV, the (002) peak intensity decreases, while the (100) peak intensity gradually increases. For arc voltage values higher than 1 kV, a new orientation (101) appears in the coating structure. During plasma assisted thin film deposition process, films' orientation strongly depends on the processing conditions, such as: substrate temperature, substrate orientation, substrate bias, deposition rate, film thickness and composition, ion flux and ion energy [32–34]. In the present work, the Be deposited films have approximately the same thickness (~1 μm) and the substrate orientation (Si(111)), bias and temperature were almost the same for all the deposited samples. Therefore, the change in the films' orientation as the arc voltage was increased, could be related only to the increased ion energy and ion flux bombardment on the growing films. The film deposited at 0.8 kV received a relatively low ion energy and ion flux bombardment and the main parameter controlling the film growth is low surface energy. In the previous section, we showed that by increasing the arc voltage, both ion energy and flux are increasing. By increasing the arc voltage, we can assume that higher residual stress will be developed in the films due to the intense and energetic ion bombardment during deposition process, resulting in the change of the crystallographic orientations. Therefore, the competition between surface energy and strain energy leads to the crystallographic preferred orientation change. Based on the convolutional multiple whole profile (CMWP) evaluation method, we can conclude that each film exhibits a remarkably low lattice defect density (n). However, an increasing tendency (from 0.2 × 1014 m−2 to 0.9 × 1014 m−2, see Table 2) can be observed when the arc voltage is increased from 0.8 kV up to 2.0 kV. For all samples, the defects are randomly distributed and consist mainly of edge dislocations within the columnar grains. The excessive ion bombardment occurring during TVA operation at 2.0 kV arc voltage leads to the preferred orientation change in the film. The CMWP fits showed that the average crystallite size is in the nanometric domain for all the investigated films (38–44 nm). Cross-sectional SEM images of the Be films deposited at different arc voltage values show different microstructures (Fig. 6). Except the Be thin film deposited at an arc voltage value of 1.5 kV, all the other samples exhibit columnar structure. The film deposited at an arc voltage value of 0.8 kV exhibits porous columnar structure, developed under low ion energy and ion flux bombardment. By increasing the arc voltage, the film's microstructure changes from a porous columnar structure to a dense columnar structure. The film deposited at 1.5 kV exhibits a compact structure with enhanced packing density. This structure improvement is mainly due to the increased ion energy and ion flux bombardment which lead to an increased ad-atom mobility and diffusivity on the growing film surface, facilitating the filling of the gaps between columns. Therefore, the energetic enhanced deposition conditions lead to the formation of a denser microstructure. Based on the RBS and cross-sectional SEM measurements, the filmto-bulk density ratio (ρf/ρb) was calculated and summarized in Table 2. The density of the deposited thin films was estimated by measuring the areal atomic density (Ns) using Rutherford backscattering spectrometry and, independently, the film thickness (t) by SEM. For a single element film, the density (ρ) was calculated as [35]:

3.2. Be thin films characterization 3.2.1. Microstructure and surface morphology The XRD patterns of Be films deposited by TVA on Si substrates, with different arc voltages, are presented in Fig. 5. The corresponding arc voltage values involved during the deposition process were labelled at the right side of the figure. The Be film deposited at 0.8 kV exhibits a strong preferred (002) phase orientation, while the other films exhibit a polycrystalline structure with (100), (002) and (101) orientations. Therefore, an important feature of TVA discharge is that the arc voltage value has a significant influence on the film preferred orientations. The

ρ = (Ns ·M )/(t·NA)

(1)

where M is the atomic mass and NA is Avogadro's number. According to the experimental results, the film density gradually increases as the arc voltage increases, being approximately 10% lower than Be bulk at 0.8 kV and 5% higher at 1.5 kV. These results are consistent with the cross-sectional SEM images which show that the densest film is obtained at an arc voltage value of 1.5 kV. As the ion energy and ion flux in the plasma increase, the momentum transferred from the incident ions will increase the ad-atom mobility and highly mobile ad-atoms can

Fig. 5. XRD patterns of Be coatings deposited by TVA, at different values of arc voltage. 332

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Table 2 Estimated values of crystallite size (D), hardness (H), Young's modulus (E), H/E ratio, adhesion critical loads (LC), friction coefficient (μ), RMS surface roughness (RRMS), film-to-bulk density ratio (ρf/ρb) and lattice defect density (n) for nanocrystalline Be films deposited by TVA, on silicon and stainless steel substrates, using different arc voltage (U). U (kV)

0.8

1

1.5

2

D (nm) H (GPa) E (GPa) H/E F – LC1 (N) F – LC2 (N) F – LC3 (N) μ RRMS (nm) ρf/ρb n (1014 m−2)

38 4.3 ± 0.3 212.0 ± 17.3 0.020 – – 0.78 ± 0.17 0.112 ± 0.040 4.8 ± 0.2 0.90 0.2

38 4.9 ± 0.3 278.3 ± 15.2 0.018 0.80 ± 0.06 1.76 ± 0.13 6.69 ± 0.64 0.032 ± 0.011 22.4 ± 0.6 0.95 0.3

43 5.9 ± 0.2 266.2 ± 20.1 0.022 0.71 ± 0.04 1.56 ± 0.25 4.82 ± 0.26 0.027 ± 0.014 17.3 ± 0.5 1.05 0.2

44 5.2 ± 0.3 256.9 ± 20.1 0.020 1.89 ± 0.13 8.92 ± 0.39 13.45 ± 0.69 0.021 ± 0.012 31.6 ± 1.0 1.02 0.9

Under energetic and intense ion bombardment, the number of islands decreased, the size of the remaining islands increased and the secondary nucleation between existing islands was suppressed. As the arc voltage increased from 1.0 to 2.0 kV, the size of the features (the large diagonal of the pyramid's base) increased from approximately 300 nm to 500 nm. The high lattice defect density and preferred orientation change induced by the excessive ion bombardment, occurring at arc voltage 2.0 kV, will also affect the film surface morphology and surface roughness.

move or diffuse into the inter-grain voids, leading to a denser microstructure. However, the excessive ion bombardment occurring at arc voltage 2.0 kV can cause higher lattice defect density, high residual stress and displacement damage, hampering the further densification of the coatings. Two and three-dimensional surface morphologies of Be thin films deposited on Si substrates at different arc voltages were analysed by AFM (Fig. 7). The root mean-squared (RMS) surface roughness (RRMS) values are listed in Table 2. As it can be observed, the Be film deposited at 0.8 kV exhibits the lowest RMS roughness value of 4.8 nm. The surface roughness increases (22.4 nm) as the arc voltage reaches 1.0 kV and slightly decreases for 1.5 kV (17.3 nm). Further increase in the arc voltage (2.0 kV) results in an increase in the surface roughness up to 31.6 nm. Besides changing the roughness values, an increase in the arc voltage value leads to surface morphology changes in the films, due to an enhanced ion energy and flux. At low arc voltage value (0.8 kV), the surface topography mainly consists of very fine and uniform features (size diameter about 100 nm), homogenously distributed on the coatings surface. By increasing the arc voltage, the surface morphology evolves from fine grains to coarse islands, while the features' shape changes from dome to the rhombohedral pyramid-shape.

3.2.2. Mechanical properties The change in the film microstructure induced by the ion bombardment has a significant influence on the films' mechanical properties, as it will be further shown. The estimated values of indentation hardness (H), Young's modulus (E), H/E ratio, adhesion critical loads (LC) and friction coefficient (μ) for Be films deposited on stainless steel substrates, at different arc voltage values, are summarized in Table 2. The hardest Be film was found to be that deposited at 1.5 kV, while the best film-to-substrate adhesion was observed for the film deposited at 2.0 kV. The enhanced hardness of the film deposited at 1.5 kV is mainly due to structural improvement of the coating: it has the highest film-to-

Fig. 6. Cross-sectional SEM images for Be thin films deposited on Si substrates using different discharge voltage: a) 0.8 kV; b) 1.0 kV; c) 1.5 kV; d) 2.0 kV (the images are not on the same scale). 333

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Fig. 7. 2D and 3D AFM images for Be thin films deposited on Si substrates by TVA, using different discharge voltage values: a-b) 0.8 kV; c-d) 1.0 kV; e-f) 1.5 kV; g-h) 2.0 kV.

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Fig. 8. Optical micrographs of scratch tracks showing different stages of material failure.

The friction coefficient values of Be films decreased with the increase in the arc voltage, the film deposited at 2.0 kV revealing the lowest value (0.021) for this parameter. In this case, the crystallographic orientation of grains seems to be the main responsible factors for the improvement of coefficient of friction for the films deposited at elevated arc voltage. As the arc voltage increases from 0.8 to 2.0 kV, the crystallographic orientation of grains gradually changes from a preferred growth mode, with c-axis perpendicular to substrate surface, to a growth mode with a-axis perpendicular to the substrate surface. Fig. 8 shows the destructive events caused by the Rockwell diamond indenter, during the microscratch tests, for the samples deposited with arc voltage ≥ 1.0 kV. The micrographs from the left side represent the equivalent of the LC2 critical load, while the ones from the right side represent the equivalent of the LC3 load. The behaviour of the coatings can be divided in two: (i) sample S2, where relatively small delaminated regions appear, and discontinuous ductile perforation of the coating on the scratch track in conjunction with a higher degree of plastic deformation of the coating-substrate system, are clearly visible; (ii) samples S3 and S4, where gross interfacial spallation and high areas of delaminated substrate appear, with minimal plastic deformation. The behaviour of sample S2 could be explained if one looks at the H3/E2 ratio (0.0015, compared to the ones of sample S3 = 0.0028, and S4 = 0.0021). The H3/E2 ratio is an indicator regarding the material's resistance against plastic deformation. Lower values of this ratio signify a poor resistance to plastic deformation, a phenomenon clearly

bulk density ratio and a low lattice defect density. Also, the internal residual stress may contribute to the hardness improvement. An important and valuable parameter which estimates the tribological coating performance is the H/E ratio (elastic strain failure). A higher hardness together with a lower elastic modulus is a good indicator of improved toughness and tribological behaviour of the coatings because it allows the redistribution of the applied load over a large area, delaying the failure process of the film. In this study, the highest H/E ratio was also achieved for the Be film deposited at 1.5 kV. High packing density and low lattice defect density provide good wear-resistance and fracture toughness of the coatings. In the previous section, we showed that both ion energy and ion flux are increasing as the arc voltage increases. An increase in arc voltage will potentially improve the adhesion of the Be coatings to the stainless steel substrate. Although the excessive ion energy bombardment will result in increased defect incorporation in the films, hence increased residual stress, the bombardment during the first stage of the deposition process can knock off some atoms from the substrate and it may penetrate the substrate lattice to angstrom levels. Therefore, the intense ion bombardment causes the increase of the surface roughness of the substrate at atomic level, leading to the formation of surface features which could contribute to the mechanical anchoring of the coating to the substrate. These morphological changes at the stainless steel-Be interface are believed to be responsible for the adhesion behaviour of the coatings. 335

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exhibited by sample S2. Even if the H3/E2 ratio for the sample obtained using an arc voltage of 0.8 kV (S1 = 0.0017) is relatively close to the one exhibited by sample S2, thus suggesting a similar behaviour in terms of adhesion resistance to sample S2, the insufficient ion bombardment on the substrate surface cannot create the conditions for adequate adhesion between the coating and the stainless steel substrate, proven by the one order of magnitude lower LC3 value, compared to the remaining samples.

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4. Conclusions The thermionic vacuum arc process is a powerful technique for deposition of nanocrystalline beryllium thin films with improved topological, structural and mechanical properties for nuclear applications. Plasma ionization degree, ion energy and ion flux can be control through specific operation parameters like arc current, arc voltage and thermo-emission filament current. Based on TVA plasma diagnostics results, it was found that both ion energy and ion flux can be significantly increased by increasing the arc voltage. This processing parameter was used to control and to improve the Be thin film structure and properties. The spatial distribution of plasma potential measured along the discharge axis did not indicate the presence of a double layer for any of the selected operating parameters. Under these circumstances, the maximum ion energy corresponds to the cathode potential drop. An optimum combination of mechanical properties (high hardness, low coefficient of friction, good wear-resistance, fracture toughness and good adhesion to the substrate) of Be coatings was achieved by operating at higher thermionic arc voltage values (1.5–2.0 kV). The dense structure, low lattice defect density and crystallographic orientation of grains seems to be the main responsible factors for the improvement of Be thin film properties. Acknowledgements This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the EURATOM research and training programme 2014–2018 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission. The activity was performed in the scope of the WP PFC programme. We would like to thank Professor Gheorghe Popa for his valuable guidance and useful discussion in the experiments and during manuscript drafting. D. Cristea acknowledges the structural funds project PRO-DD (POS-CCE, O.2.2.1., ID 123, SMIS 2637, ctr. no 11/2009) for providing the CSM Instruments infrastructure used in this work. Part of this work was completed in the ELTE Institutional Excellence Program (1783-3/2018/FEKUTSRAT) supported by the Hungarian Ministry of Human Capacities. References [1] Wallace W. Beaver, W. Dean Trautman, “Beryllium Alloys,” Access Science, McGraw-Hill Education, 2014, https://doi.org/10.1036/1097-8542.079800. [2] S.A. Bel'kov, G.V. Dolgoleva, G.G. Kochemasov, E.I. Mitrofanov, Application of beryllium deuteride as a material for laser X-ray target shells, Quantum Electron. 32 (2002) 27. [3] F. Scaffidi-Argentina, G.R. Longhurst, V. Shestakov, H. Kawamura, Beryllium R&D for fusion applications, Fusion Eng. 51–52 (2000) 23–41. [4] https://jwst.nasa.gov/faq.html. [5] C. Thomser, et al., Plasma facing materials for the jet ITER-like wall, Fusion Sci. Technol. 62 (2012) 1–8. [6] A.R. Raffray, M. Merola, Overview of the design and R&D of the ITER blanket system, Fusion Eng. Des. 87 (5–6) (2012) 769–776. [7] T. Hirai, et al., Characterization and heat flux testing of beryllium coatings on inconel for JET ITER-like wall project, Phys. Scr., T 128 (2007) 166.

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