Aluminum metal surface cleaning and activation by atmospheric-pressure remote plasma

Accepted Manuscript Title: Aluminum Metal Surface Cleaning and Activation by Atmospheric-Pressure Remote Plasma Authors: J. Mu˜noz, J.A. Bravo, M.D. Calzada PII: DOI: Reference:

S0169-4332(17)30454-3 http://dx.doi.org/doi:10.1016/j.apsusc.2017.02.092 APSUSC 35201

To appear in:

APSUSC

Received date: Revised date: Accepted date:

19-9-2016 18-1-2017 12-2-2017

Please cite this article as: J.Mu˜noz, J.A.Bravo, M.D.Calzada, Aluminum Metal Surface Cleaning and Activation by Atmospheric-Pressure Remote Plasma, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2017.02.092 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Aluminum Metal Surface Cleaning and Activation by Atmospheric-Pressure Remote Plasma

J. Muñoz*, J.A. Bravo and M.D. Calzada

Laboratorio de Innovación en Plasmas, Edificio Einstein (C2), Campus de Rabanales, Universidad de Córdoba, 14071 Córdoba, Spain

* E-mail: [email protected]

Graphical abstract

Highlights

    

Atmospheric-pressure postdischarges have been applied on aluminium surfaces The outer hydrocarbon layer is reduced by the action of the postdischarge The treatment promotes the appearance of hydrophilic OH radicals in the surface Effectivity for distances up to 5 cm allows for treating irregular surfaces Ageing in air due to the disappearance of OH radicals has been reported

Abstract

The use of the remote plasma (postdischarge) of argon and argon-nitrogen microwave plasmas for cleaning and activating the surface of metallic commercial aluminum samples has been studied. The influence of the nitrogen content and the distance between the treated samples and the end of the discharge on the hydrophilicity and the surface energy has been analyzed by means of the sessile drop technique and the Owens-Wendt method. A significant increase in the hydrophilicity has been noted in the treated samples, together with an increase in the surface energy from values around 37 mJ/m2 to 77 mJ/m2. Such increase weakly depends on the nitrogen content of the discharge, and the effectivity of the treatment extends to distances up to 5 cm from the end of the discharge, much longer than those reported in other plasma-based treatments. The analysis of the treated samples using X-ray photoelectron spectroscopy reveals that such increase in the surface energy takes place due to a reduction of the carbon content and an increase in the amount of OH radicals in the surface. These radicals tend to disappear within 24 - 48 hours after the treatment when the samples are stored in contact with ambient air,

resulting in the ageing of the treated surface and a partial retrieval of the hydrophobicity of the surface.

Keywords: aluminum, surface cleaning, surface activation, remote plasma, microwave discharge, atmospheric pressure

1. Introduction Over the last years, several techniques have been developed for modifying surfaces using nonthermal plasmas. These methods allow for a faster and environmentally friendlier treatment than those based on purely chemical treatments [1-2], especially when applied on an industrial scale. Initially, many of these techniques were developed using low-pressure plasmas [3-6] whose implementation requires the use of pumping systems, thus increasing both the cost and the maintenance of the required equipment. On the other hand, while there exists a large number of studies related to the modification of polymeric and textile surfaces [7-11], there are comparatively fewer studies devoted to the activation of metallic surfaces such as steel [12], chromium [13] or aluminum [14-16], and most of them typically make use of dielectric barrier discharges, given that their geometry is more suitable for treating flat surfaces in a continuous mode. There are, however, some studies that have explored the possibility of using other plasma sources, such as radiofrequency or microwave plasmas [17-19] sustained at atmospheric pressure for this purpose. Even though these plasma sources have lower effective treatment areas, they present the advantage of being better suited for their use on irregular surfaces. Riding on steady, sustained growth in worldwide consumption since the 1950s, aluminum is the leading non-ferrous metal in use due to its natural properties, such as its lightness, high electrical and thermal conductivity [20], also its ease of use in the customary processes of forming, bending, vessel making, etc. And, finally, aluminum can be recycled indefinitely without losing any of its intrinsic qualities. So, aluminum is ever more applied in sectors as varied as aeronautics, beverage containers, construction and energy transportation. Aluminum is usually supplied to the industry in the shape of rolled coils, and during the process of manufacturing, a series of lubricants and additives are used in order to avoid sticking of layers and prevent degradation or corrosion [21]. That treatment covers the surface of aluminum layers with a hydrocarbon film that is usually removed by heating the coils in large ovens [22] in a process that is very slow and energy demanding. Besides, the aluminum layers usually require of an additional chemical chromate process previous to the deposition of a hydrophilic layer [17]. In the present paper, an alternate method to remove the hydrocarbons deposited on the aluminum surface is proposed. This method consists in using the postdischarges of argon and argon-nitrogen plasmas to clean and activate aluminum surfaces in a single step. Since the

postdischarges show typically lower temperature values than those exhibited by the plasma, this process allows for avoiding surface damage, such as the appearance of cracks. The effects of both the gas composition of the discharge and the distance measured between the plasma end and the treated surface on both the wettability and the surface free energy has been examined. Besides, the modifications of the chemical composition of the treated surfaces have been studied using X-ray photoelectron spectroscopy (XPS) techniques. Finally, the transient behaviour of these surface properties (ageing) during 7 days after the treatment has been monitored and studied.

2. Materials and Methods 2.1 Experimental setup Figure 1 outlines the experiment setup used to treat metal surfaces in the present study. A surfatron [23] able to generate surface-wave discharges was used as energy coupling device using microwave (2.45 GHz) powers of 150 W supplied in continuous mode by a Sairem GMP 03 K/SM generator. The discharges were contained in capillary quartz tubes of 2 and 3 mm of inner and outer radii, respectively, opened to the atmosphere at one of its ends. Argon-nitrogen mixtures with nitrogen contents up to 2% were obtained from high-purity (>99.999%, Abelló-Linde S.A.) Ar and N2 gases using HI-TEC (Bronkhorst) mass flow controllers. The total gas flow was kept in all the experiences at 1.0 slm (standard litre per minute). These experimental conditions were chosen accordingly with previous experimental results allowing for obtaining a postdischarge long enough while ensuring the integrity of the quartz tube (figure 1) [24]. Square aluminum samples 2.5 2.5 cm were prepared from a commercial aluminum coil of 400 m thickness and gently flattened using a manual press. The pieces where cleaned in an ultrasonic bath with acetone for 5 minutes and dried in contact with air. Before the plasma treatment, the samples were rinsed with ethanol and dried. This procedure is very similar to that employed in other studies [13, 18-19] and its intention is to remove any solid residue before the postdischarge treatment. As it will be seen later, this process alone is unable to remove the hydrocarbon layer covering the samples. The aluminum samples were placed in an automated two-dimensional displacement system built from two Zaber T-LSM50B linear actuators, thus allowing for treating square surfaces with speeds up to 7500 m/s. The speed and limits of the linear actuators were controlled using a script executed in the Zaber Console system. Preliminary measurements showed that the maximum speed of this system was enough to achieve cleaning and activation of the surface. Consequently, the possibility of using lower speeds was dismissed. The aluminum samples were exposed to the postdischarge for different plasma compositions and distances measured from the end of the discharge tube (𝑑) (figure 1). Having into account the dependence of the length of the discharge with the nitrogen concentration, the relative position of the microwave launcher was modified to match the end of the tube and the

end of the discharge, thus limiting the contact of the plasma with the surrounding air to the postdischarge. To know the effect of the postdischarge in the surface cleaning and activation, two groups of experiments were carried out. In the first group, for examining the effect of the postdischarge gas composition, the nitrogen concentrations in the plasma gas Ar-N2 were varied from 0.0 to 2.0%, while the distance between the sample and the end of the discharge tube was kept constant at 2 cm. A second group, for examining the effect of the distance between the sample and the discharge end, the d distance was varied from 2 to 6 cm, while the nitrogen concentration in the plasma was kept constant at 0.5%. In order to control the temperature of the samples, thermographies of the surfaces were collected during the treatment using a Fluke Ti400 thermographic camera calibrated in the -20 to 1200 ºC range. The temperature of the samples was obtained correcting the surface emissivity to that of aluminum (0.12), leading to uncertainties lower than 5%. This emissivity value was obtained letting an aluminum sample reach thermal equilibrium with a heating plate in a similar range of temperatures to that resulting from the exposure of the samples to the postdischarge. 2.2 Analysis methods of the surfaces In order to measure the effect of the postdischarge on the surfaces of the treated samples under the different operational conditions utilized in this study, the wettability and the energy of the aluminum surfaces were measured. Besides, variations of the chemical composition surfaces were also experimentally determined. In the current research, we measured the contact angle using the sessile drop method [25] considering the spherical drop model with water drops of 5 l deposited on aluminum surfaces using a micropipette. A digital camera Casio EXFH20 with a focal distance of 5 mm and exposure time of 1/6 s was used to take the pictures right after the deposition on surfaces. Figure 2 shows an example of the photographs of two water drops taken on aluminum before (a) and after (b) of the postdicharge treatment.

The surface free energy of the aluminum plates was determined utilizing the OwensWendt method which appears detailed in [25]. Experimentally, drops of 5l of water, ethylene 𝑝 glycol and glycerol were used to calculate this energy. If the polar (𝛾𝑙 ) and dispersive (𝛾𝑙𝑑 ) 𝑝

𝑝

components of the free energy of the liquids are known, the polar (𝛾𝑠 ) and dispersive (𝛾𝑑 ) components of the surface free energy of the solid can be calculated knowing the contact angle of these liquids with the surface of the solid (𝜃𝑙𝑠 ) using equation (1). 𝑝

(𝛾𝑙 + 𝛾𝑙𝑑 )(1 + cos 𝜃𝑙𝑠 ) 2√𝛾𝑙𝑑

𝑝 = √𝛾𝑠 √

𝑝

𝛾𝑙

𝛾𝑙𝑑

+ √𝛾𝑠𝑑

(1)

Table 1 shows the free energy parameters of the used liquids (water, ethylene glycol and glycerol) for the evaluation of the surface free energy of the solid [19]. Measurement of the contact angles of the different liquids was carried out the photographies obtained in the same way than those utilized to measure the wettability of material. The chemical composition of the treated surfaces was analyzed by X-ray photon spectroscopy (XPS) in order to identify variations in the surface composition, as well as the

modifications in the oxidation state of its constituents. This technique can only analyze the superficial composition because the maximum scanning depth is of about 10 nm. The utilized instrumentation was a Phoibos 150 MXD XPS with an Al K (1486.7 eV) X-ray source operated at 300 W. Due to the physical limitations of the XPS sample holder, samples with 1  1 cm2 area prepared in a similar fashion to that described in section 2.1 were prepared for XPS analysis. Overview spectra for a 100 eV pass energy and 1 eV resolution were recorded in order to identify the main features of each surface. High resolution (0.1 eV) spectra of the main constituents of the surface samples were performed at a pass energy of 30 eV. The C 1s line, appearing at 285.0 eV was used to calibrate the energy of the spectra, and the analysis were performed using the CasaXPS software, substracting a Shirley type baseline. 3. Results and Discussion 3.1 Effect of the plasma composition Aluminum samples, placed at 2 cm from the discharge tube end, were treated with Ar-N2 postdischarges with nitrogen concentrations ranging from 0.0 to 2.0%. Changes in the contact angle of water drops placed on the surfaces give information about wettability modifications suffered by the material. Figure 3 shows the results obtained for the treated surfaces. The grey strip in the top represents the value of the contact angle on an untreated sample. These values were obtained from the average of three different samples treated in different days. The corresponding uncertainties were calculated as the root mean square deviation. As it can be seen, the surfaces treated with the plasma postdischarge show a drastic decrease of the contact angle from 72.0  1.7° to values ranging from 27.0  1.3° to 14.0  1.5° for samples treated with the postdischarge of Ar and Ar-N2 (2.0%), respectively. Similar treatments in which the surface was exposed to the direct action of a 250 W He microwave plasmas resulted in water contact angles ranging from 21º to 7º [17], while the use of microwave plasmas sustained in Ar, Ar-O2 (0.01%) and Ar-H2 (0.01%) [19] resulted in water contact angles of 20°, 15°, and 3°, respectively. Comparing our experimental conditions with those of other microwave systems based on the direct action of the discharge shows that, while treatment times are comparable [17, 19], the distance is typically limited to 1 cm. In [19], the treatment distance was increased to 2 cm, but at the costs of using much larger powers (600 W) and total gas flows (10 slm) to sustain the discharge. Moreover, these wettability results are comparable to those obtained using dielectric barrier discharges, where contact angles between 10º and 5º have been reported for aluminum surfaces exposed to an Air-O2 discharge [14. Though in general these devices allow for treating large surfaces at much higher speeds [14, 16, 22], the sample distance is limited to up to 0.5 cm, thus limiting their use to flat surfaces. On the other hand, the surface free energy was studied using the Owens-Wendt method introduced in section 2.2. The surface free energy values appearing in figure 4 were calculated from the slope and the intercept of the linear fits which, according to equation (1), correspond with the dispersive and polar components of the surface free energy, respectively. As it can be seen, there is a significant increase in the free energy of aluminum surfaces, growing from 37.0  1.8 mJ/m2 to values ranging from 69.0  7.3 mJ/m2 (Ar postdischarge) to 77.0  8.9 mJ/m2 (2% N2 postdischarge). While the results show a certain trend in the surface energy to increase

with the nitrogen content in the discharge, especially if we take into account the wettability results shown in figure 3, the uncertainties prevent us from establishing such relationship. These results are similar to those obtained using other methods based on plasma technology, where values in the 72.0 – 77.5 mJ/m2 and the 78.0 – 82.0 mJ/m2 ranges have been obtained using dielectric barrier discharges [14-15, 22] and microwave plasmas [19], respectively. In our case, one of the advantages as compared with other microwave plasmas is that the surface is exposed to significantly lower temperatures. On the other hand, for industry processing of aluminum, it is usual to apply the EN-5454 standard [26] to determine whether the surface is clean enough or not prior to further process. That happens when the surface energy is larger than that of water (72.8 mJ/m2) [22]. According to our experiments, the surface free energies obtained are very close or even larger than those required by the EN-545-4 standard. 3.2 Effect of the distance One of the advantages of the use of postdischarges is that they extend beyond the most active zone of the plasma, thus allowing for treating materials at longer distances [27]. As it was previously explained, if the surface is directly treated by the discharge of a microwave plasma sustained at atmospheric pressure, this distance will be limited by the plasma dimensions, whose species will be quickly deactivated in lengths of about 1 cm, depending on the flow and the applied power [17-19]. In the case of dielectric barrier discharges [12-16], the geometry of the discharge limits the treatment to distances below 0.5 cm. In order to determine the limitations of the postdischarge treatment as compared to the direct action of the plasma, a set of experiences was carried out modifying the distance between the aluminum samples and the end of the discharge between 2 and 6 cm. During these experiments, a 0.5 % nitrogen discharge was sustained using a constant absorbed power of 150 W. From figure 5 it can be seen that there is a significant difference between the samples treated at distances bellow 5 cm and those treated at longer distances. While samples treated at distances lower than 5 cm show characteristic surface free energies of 71  5 mJ/m2, this parameter is reduced to 47  4 mJ/m2 for the samples placed at 6 cm away from the end of the discharge. These results have also interesting implications from the point of view of the implementation of this technology since, for the shown experimental conditions, there is a relatively wide zone between 2 and 5 cm where the treatment is efficient, resulting in similar surface free energy values of the sample surface. This clearly indicates that, for our experimental conditions, it is possible to treat not only flat surfaces, but also relatively irregular ones [28] without excessively increasing the complexity of the experiment setup, obtaining similar results at positions placed at different distances from the end of the discharge. Besides, these results also show that there is still room for improving the efficiency of the process by increasing the speed of the treatment, thus reducing its costs. Similar experiments were performed for the case of a pure argon plasma, resulting in a shorter distance for an effective treatment. Thus, the aluminum samples treated with a pure argon postdischarge at a distance of 5 cm showed surface free energy values of 50  7 mJ/m2,

while samples treated at a distance of 4 cm showed a surface free energy value of 72  2 mJ/m2. As it can be seen, these values are similar to those shown in figure 5 for the aluminum samples treated at distances of 5 and 6 cm from the end of an argon-nitrogen (0.5% nitrogen) discharge, respectively. As it has been shown in [29], one of the characteristics of an Ar-N2 plasma is that it shows higher gas temperature values than an Ar plasma. So, it seems reasonable to think that the temperature of the postdischarge for these two discharges will also be different. Figure 6 shows the gas temperature measured in the postdischarge of both an Ar and Ar-N2 discharges, sustained with 150 W and a total gas flow of 1 slm. This temperature was measured using a K thermocouple of 6 mm radium, which is able to measure temperature between -200 and 1250 °C with an error of 1 °C. This temperature was similar to that obtained from the thermographic images of the aluminum samples at the treatment spot taking into account the indeterminacies. The average temperature of the samples was, however, 100 ºC lower than the temperature at the treatment spot. As it can be seen from Figure 6, there is a significant difference in the temperature of both postdischarges. Moreover, the temperature values at positions 4 and 5 cm away from the end of an Ar discharge are very similar to those at 5 and 6 cm away from the end of the Ar-N2 discharge. Comparing figures 5 and 6, it seems to show a correlation between the temperature in the postdischarge and the effectiveness of the surface treatment. Besides, another conclusion derived from this comparison is that the sample can be placed at a greater distance from the end of the Ar-N2 discharge than in the case of the plasma created with pure Ar and obtain similar effects on the surface of the material. In order to discard the thermal effect as the sole responsible for the changes in the aluminum surface, further tests were carried out heating aluminum samples prepared in the same way and exposed to the flow of a heating gun for 20 seconds. The selected temperature was 200 °C, significantly higher than that measured in the limit for the effectiveness of the Ar and Ar-N2 postdischarge treatment. As a result, a water contact angle of 63.0  1.8° was obtained and a surface energy of 39  2 mJ/m2, slightly higher values than those obtained in untreated samples, but significantly lower than those obtained for the case of aluminum samples treated with the postdischarge (figure 5). These results showed that heat is not the only mechanism by which the postdicharge treatments clean the aluminum surfaces. 3.3 Action of the postdischarge on the aluminum surface With the purpose of optimizing and understanding the processes involved in the modification of the physicochemical properties of aluminum surfaces by the plasma postdicharge, XPS analysis of samples were performed. A batch of 11 cm square samples was prepared using the same process described in the previous sections. Tests were also performed on aluminum samples exposed to the outflow of a heating gun at 200 ºC for 20 seconds. All samples were introduced in the vacuum chamber of the XPS spectrograph within one hour after treatment in order to avoid contamination or ageing effects (see next section). Figure 7 shows the XPS spectrum characteristic of an untreated aluminum surface. Signals for Al(2s) and (2p), O(1s), C(1s) and F(1s) were detected. Oxygen usually appears in the surface of metallic aluminum due to the fast oxidation of the surface when exposed to ambient air, while the presence of fluor can be due to the hydrogen fluoride used in the extraction of aluminum from bauxite. On the other hand, carbon typically appears in the form of a mix of hydrocarbon impurities due to the use of machining lubricants in the processing of

aluminum coils. Since some of the samples were exposed to Ar-N2 postdischarge, which are known to contain long lived excited nitrogen species, the N(1s) signal was monitored too. Figure 8 shows the high-resolution spectra obtained for the most relevant XPS signals (carbon, aluminum and oxygen), while table 2 contains their characteristic parameters (binding energy and full width at half the maximum). These parameters were obtained as an average of all the XPS spectra analyzed and are in good agreement with those found in the literature [3033]. Also in Table 2 the quantification results obtained upon analyzing the aluminum samples considered in the present study are shown. To that end, the calibration factors of the CasaXPS software were used. Given that the carbon signal is actually the envelope of several contributions originating from many bond types (C–C, C–O, C=O,…), the spectrum shown in figure 8(a) has generally a very complex deconvolution, whose correct interpretation requires a certain degree of knowledge about the exact chemical nature of the organic substances present in the surface [34]. Since in our case the analysis of this signal is only intended to quantitatively estimate the amount of contaminating hydrocarbons at the aluminum surface, no additional components were assigned and only the total intensity (area) of the signal was considered. On the other hand, since the carbon signal is well known, it is frequently used to calibrate the energy of the rest of the signals in the XPS spectrum [22, 30] correcting its maximum to a position of 285.0 eV and applying the same correction to the rest of signals in the sample. As it can be seen from figure 8(b), the aluminum 2p signal is characterized by having two well defined components, placed around 71.8 and 74.5 eV, respectively. These signals are originated by two different oxidation states, namely metallic aluminum (Al1 or Al0) and oxidized aluminum (Al2 or Al+3) [14, 22, 31]. In a similar way, it is possible to assign three components to the oxygen 1s signal placed around 531.7 eV, O1, O2 and O3, corresponding to oxygen atoms in different chemical environments. According to [31], the first of these components, O1, with a characteristic energy of 533.0 eV, was assigned to oxygen atoms bonded with hydrogen in water molecules adsorbed in the surface (H2O(surface)). The second component, O2, with a characteristic energy of 531.9 eV, was assigned to oxygen atoms bonded with hydrogen forming hydroxide groups (O-H). Finally, the third component, O3, with a characteristic energy of 530.8 eV, was assigned to oxygen atoms forming oxide groups, as in aluminum oxide (Al=O). A deconvolution of the Al (Figure 9) and O (Figure 10) signals obtained from the XPS analysis of the different samples considered in this work allowed for the determination of the contribution of each of these components to the total signal. For ease of comparison, these contributions also appear in Table 2 under each of the considered elements. Finally, nitrogen can appear in several forms in the aluminum surface [14, 22]. In our case, the analysis of our samples revealed nitrogen signals appearing at 400.2, 403.8 and 407.3 eV binding energies, and labeled N1, N2 and N3, respectively. These signals are usually related to NO–, NO2– and NO3– groups, respectively. In our case, even though nitrogen was detected (figure 7), the corresponding signals were relatively low. The analysis of the XPS spectra allows for obtaining the percent composition of the first 10 nm of the surface, approximately, which are shown in Table 2 (center) for the cases of untreated and heated samples, as well as for those treated with Ar and Ar-N2 postdischarges. As it can be seen, one of the most relevant effects of the action of the postdischarge over the aluminum surface is that it significantly reduces the carbon content from values near 50% to 10%, both when argon or argon-nitrogen postdischarges are used. This reduction of the carbon

content due to the action of the postdischarge is similar to that obtained while treating aluminum [14, 22] and chromium [13] surfaces with dielectric barrier discharges. However, simply heating the sample reduces the carbon content to values near 20%. The presence of oxygen even in the case of relatively clean surfaces indicates that, as it has been shown in other studies, the surface of our samples is covered by an oxidized aluminum layer which typically contains a mixture of aluminum oxide, Al2O3, aluminum oxhidroxide, AlO(OH), and aluminum hydroxide Al(OH)3 [22, 30, 33]. Over this layer, another one consisting of a mixture of contaminating hydrocarbons exists, whose exact nature is normally undetermined (Figure 11). While the evaluation of the thickness of the hydrocarbon layer would require additional information concerning the distribution of -OH an =O radicals in both the hydrocarbon and oxidized aluminum layer the surface of the material, the decrease in the C(1s) shown in Table 2 clearly points to a reduction of 𝑑𝐶 . On the other hand, according to [35] it is possible to estimate the thickness of the oxidized layer 𝑑𝑂 , using equation (2), being 𝐼𝑜 and 𝐼𝑚 the intensities of the Al2 and Al1 component presented in Table 2. 𝑑𝑜 (Å)=28 ln (1 + 1.4

𝐼𝑜 ) 𝐼𝑚

(2)

Substituting the data from table 2 results in 𝑑𝑂 values ca. 71 Å for untreated, treated and heated samples, with differences lower than 5% between them, owing to the small changes in the Al1 and Al2 components. However, it must be kept in mind that, while equation (2) relies only on the intensities of the Al1 and Al2 components, it assumes the existence of a single uniform oxidized layer, neglecting the influence of the hydrocarbon layer, which can be expected to have little influence according to [35]. Some studies [22] have pointed to the removal of the hydrocarbon surface layer as the only reason for the increase in the surface energy and the corresponding surface energy. However, in our case, the heated samples show a noteworthy decrease of the carbon content similar to that reported for dielectric barrier discharges but this decrease does not come with a significant increase in the surface energy, as it has been explained in previous sections. This increase in the surface free energy and hydrophilicity in the surfaces treated with the postdischarge can be explained, besides the reduction of the hydrocarbon layer, by a chemical modification of the surface. As it can be seen from figure 10 and Table 2, those samples treated with the postdischarge of an argon or argon-nitrogen discharge are characterized by a lower O1 component, while the O2 component shows a significant increase with respect to the untreated and heated samples. This shows a relative reduction of the amount of adsorbed water in the surface, accompanied by an increase in the proportion of OH functional groups at the surface. These groups are considered by a number of studies [14, 17, 19, 34, 36] as responsible for the increase of the hydrophilicity of the surfaces. Recent studies have reported the existence of significant amounts of OH radicals in the postdischarges of RF Ar and Ar-H2O plasmas sustained at atmospheric-pressure [38], which could react at the surface, either implanting in the hydrocarbon layer or to give place to hydroxylated aluminum species such as aluminum hydroxide, Al(OH)3, or aluminum oxyhydroxide AlO(OH). There are also important differences in the nitrogen XPS signals before and after the treatment of the samples, as it can be seen in table 2. In the untreated sample, only the N1 signal, assigned to NO- groups, can be detected, while the samples treated with an argon postdischarge

show a slight increase in the N2 component and a more significant increase in the N3 component, respectively assigned to NO2- and NO3- groups. According to [13], the apparition of the later groups can be interesting from the point of view of biological applications. Moreover, the N3 component is the most intense nitrogen signal in those samples treated with an argonnitrogen postdischarge, which is known to contain a large amount of nitrogen active species, while the existence of these species in an argon discharge can be attributed to the impurities contained in the plasmogen gas. On the other hand, it has been pointed in [39] to the possibility of nitrogen taking part in the apparition of OH radicals at the surface of some metallic oxides though the formation of peroxynitrose acid, ONOOH via reaction (3), giving place to an XPS signal around 408.1 eV which would match that of component N3 Al-OH+NO2 →Al-ONOOH

(3)

In fact, some studies on the effect of dielectric barrier discharges on aluminum [14] and chromium [13] surfaces have shown that, after the plasma treatment, there is an increase in the component assigned to nitrate groups which, according to both studies, tends to disappear some time after the treatment. 3.4 Ageing of the postdischarge surface treatment Figure 12 shows the variation along time of the surface energy for aluminum samples treated with an argon-nitrogen (0.5% N2) postdischarge sustained with 150 W of absorbed power. These measurements were carried out following the same procedure of sections 3.1 and 3.2. In order to avoid the measurement process to contaminate and modify the chemical composition of the surface, each measurement was performed on a fresh sample allowed to age in contact with air. It is possible to observe that the effectiveness of the treatment partially disappears during the first 24 hours after the treatment. During this time, the surface energy decreases to 50 mJ/m2. For aging times longer than 24 h, the ageing process becomes slower, with the surface free energy decreasing to 44 mJ/m2 after 7 days. These results are similar to those found for aluminum samples treated with a dielectric barrier discharge [14], while in that case it was also found that the ageing time required to recover similar values of contact angle and surface energy (recovery time) was substantially increased if samples were treated with an oxygen-containing plasma. This ageing process could take place due to the partial recovery of the hydrocarbon layer in contact with the air ambience. In order to determine the chemical modifications of the surface after ageing, an XPS analysis of samples aged 48 hours, similar to that of section 3.3, was performed. Table 2 (right) shows the results of the atomic composition of the aluminum surfaces. Comparing these results with those obtained for freshly treated surfaces, it can be seen that there is an increase in the carbon concentration, similar to that found in other studies [1314]. This partial recovery of the carbon at the surface in contact with air could take place due to incorporation of volatile hydrocarbons or other carbon-containing molecules existing in the ambience.

In order to determine the influence of the existence of OH radicals, high resolution spectra of the oxygen 1s signal of the aged samples was also performed. As it can be seen in Table 2, comparing these results with those obtained for freshly treated surfaces, all cases exhibit little changes in the O1 component, representing water adsorbed in the surface. It is important to notice that, in the case of the untreated sample, there is a noticeable difference in the O1 component that increases about 3%. Since there is no reason to expect a substantial change in untreated samples after ageing, such variation must owe to the indeterminacy of the deconvolution process, which in the case of the O(1s) signal is very difficult since the components are very close to one another, giving rise to a smooth envelope. Consequently, differences in the components of the O(1s) signal below 3% must be carefully considered. On the other hand, the O2 component is significantly reduced in samples treated with the postdischarge, signifying a lower content in OH radicals, thus reducing the hydrophilic character of the surface and explaining the decrease in the surface energy. The fact that this decrease takes place simultaneously with an increase of the O3 component, related to aluminum oxide, together with the effectiveness of the treatment not disappearing completely, suggests that besides the implantation of OH radicals that could disappear over time in contact with air, the treatment produces the hydroxylation of the aluminum surface, as pointed by other authors [14-19]. The oxidation of the surface could take place again with time, as it happens in the case of chromium [13], while this possibility does not seem equally feasible for aluminum since its hydroxide is its most stable chemical form under normal conditions Finally, the differences in the high resolution spectra of the nitrogen 1s signal, shown in Table 2, were also analyzed. As it can be seen by comparing these results with those found for freshly treated surfaces, component N3, representing nitrate groups, tends to increase with the ageing process for the case of samples treated with the postdischarge of an argon plasma, probably due to the interaction of the surface with the ambience air after removal of the hydrocarbon layer. This component remains stable after 48 hours ageing for the case of aluminum samples treated with argon-nitrogen postdischarges. These results differ from those found in dielectric barrier discharges, both in the case of aluminum [14] and chromium [13] samples, where it was found that the presence of nitrate groups disappeared with the ageing process.

4. Conclusions The capability of argon and argon-nitrogen postdischarges produced by surface-wave plasmas at atmospheric pressure to clean and activate aluminum surfaces has been demonstrated. The action of the postdischarge on the surface provokes a significant increase of the hydrophilicity and surface free energy from 37 to 77 mJ/m2, values comparable and even larger than those required by the industrial processing standards. On the other hand, it has been tested that the effectiveness of the postdischarge treatment under the studied conditions reaches distances up to 5 cm, larger than those typically allowed by dielectric barrier discharges by an order of magnitude, which on one hand allows for treating non-planar geometries and leaves room for improvement in terms of treatment speed. The cleaning and activation mechanism of the postdischarge has also been studied. While it is not possible to completely discard the role of thermal effects in the removal of the

hydrocarbon layer which is typically present in industrial aluminum, the comparison of the aluminum samples with those treated only with a thermal process has shown that, besides a larger reduction of the hydrocarbon layer, the species in the postdischarge play a significant role in the activation of the surface via OH radical implantation and/or hydroxylation of aluminum, giving place to a significant increase in the hydrophilicity of the surface. Finally, the ageing effect of the surfaces treated with postdicharges has been studied. While the surface remains clean, the activation process reverts 24 hours after the treatment and the surface energy of the samples is reduced to 44 mJ/m2 after one week. The analysis of the surfaces suggests that OH radicals appearing during the treatment disappear in contact with air, indicating that any further treatment after this cleaning and activation process should take place within the following hours. Acknowledgements This work was supported by the Andalusia Regional Council from Spain (Consejería de Economía e Innovación) under project no. P11-FQM-7489 (FEDER co-funded).

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Figure 1. Experiment setup (left) and detail of an Ar-N2 plasma and postdischarge (right) sustained at atmospheric pressure.

Figure 2. Photographs of water drops deposited in an aluminum surface a) untreated and b) treated with an Ar-N2 (0.5 % N2) postdischarge.

90

Water Contact Angle (Degrees)

80 Untreated

70 60 50 40 30 20 10 0 0.0

0.5

1.0

1.5

2.0

[N2] (%)

Figure 3. Water contact angle as a function of the nitrogen content of the discharge (150 W, d = 2 cm).

90

Polar Dispersive

2

Surface Free Energy (mJ/m )

80 70 60 50 40 30 20 10

]= 2. 0% 2

[N

]= 1. 5% 2

[N

.0 % ]= 1 2

[N

]= 0. 5% 2

[N

]= 0. 0% 2

[N

U

nt re

at ed

0

Figure 4. Surface free energy of the aluminum samples as a function of the nitrogen content in the discharge (150 W, d = 2 cm).

90

Polar Disp.

2

Surface Free Energy (mJ/m )

80 70 60 50 40 30 20 10

cm 6 d

=

5 = d

= d

= d

cm

cm 4

cm 3

cm 2 = d

U

nt re

at ed

0

Figure 5. Surface free energy of the aluminum samples as a function of the distance between the aluminum samples and the plasma (150 W, 0.5% nitrogen).

300 Ar Ar+N2 (0.5%)

280 260 240

200

o

T ( C)

220

180 160 140 120 100 2

3

4

5

6

d (cm)

Figure 6. Gas temperature measured at different positions from the end of the discharge.

100

O(1s)

Intensity (a.u.)

75

50

F(1s)

C(1s) Al(2p) N(1s)

Al(2s)

25

0 1200 1000

800 600 Be (eV)

400

200

0

Figure 7. Survey XPS spectrum of an untreated aluminum sample.

40

Untreated

a) Carbon C(1s)

Ar Ar-N2

Intensity (a.u.)

30

Heated

20

10

0 292

290

288

286

284

282

280

Be (eV) Untreated

16 14

Ar-N2

12

Intensity (a.u.)

b) Aluminium Al(2p)

Ar Heated

10 8 6 4 2 0 80

78

76

74

72

70

Be (eV)

Intensity (a.u.)

140

Untreated

120

Ar

100

Ar-N2

c) Oxygen O(1s)

Heated

80 60 40 20 0 540 538 536 534 532 530 528 526 524

Be (eV) Figure 8. High resolution XPS spectra of a) carbon C(1s), b) aluminum Al(2p) and c) oxygen O(1s) signals of the different analysed samples.

16 a)

Experimental

14

Intensity (a.u.)

8 6 88.3%

11.7%

74 Be (eV)

16 c)

72

70

80

Ar Plasma

Al2

8

89.5% 10.5%

6

76

74 Be (eV)

72

4 2

70

Ar-N2 Plasma

Experimental Fitting

12

Al1

10

78

14

Fitting

12

10.8%

16 d)

Experimental

14

89.2%

4

0 76

Al2

6

2 78

Al1

8

0 80

Fitting

10

2

Intensity (a.u.)

Intensity (a.u.)

Al2

4

Experimental

12

Al1

10

Heated

14

Fitting

12

Intensity (a.u.)

16 b)

Untreated

Al1

10

Al2

90.3%

8

9.7%

6 4 2

0

0

80

78

76

74 Be (eV)

72

70

80

78

Figure 9. Fitting of the high resolution XPS spectra of the Al(2p) signal.

76

74 Be (eV)

72

70

120

Intensity (a.u.)

100

120

Comp. O3

60 29.7%

40 35.9%

20

34.4%

Comp. O1 Comp. O2

80

Comp. O3

60 40

34.9%

33.4%

31.7%

0

538

536

534 532 530 Be (eV)

528

Ar Plasma

120 100 80 60

526

140

c)

120

Experimental Fitting Comp. O1 Comp. O2

53.8%

Comp. O3 33.3%

20

538

0

534 532 530 Be (eV)

528

80 60

526

d)

Experimental Fitting Comp. O1

54.6%

Comp. O2 Comp. O3

40

25.1% 20.3%

20

12.9%

536

Ar-N2 Plasma

100

40

540

540

Intensity (a.u.)

540

Intensity (a.u.)

Fitting

20

0

140

b)

Heated Experimental

100

Comp. O2

80

140

a)

Untreated Experimental Fitting Comp. O1

Intensity (a.u.)

140

0 538

536

534 532 530 Be (eV)

528

526

540

538

536

534 532 530 Be (eV)

Figure 10. Fitting of the high resolution XPS spectra of the O(1s) signal.

528

526

Hydrocarbon layer Hidrocarburos

𝑑𝐶

Oxidized Aluminum (Al+3) layer

𝑑𝑂

Metallic Aluminum (Al0) core

Figure 11. Schematic draw of a transversal section of an aluminum sample.

80 2

Surface Free Energy (mJ/m )

75 70 65 60 55 50 45 40

Untreated

35 0

24

48

72

96

120

144

168

t (h)

Figure 12. Ageing effect on surface energy of aluminum samples treated with an argon-nitrogen (0.5% N2) postdischarge.

Table 1. Surface free energy of the liquids used in the determination of the surface free energy

of aluminum. 𝒑

Liquid

𝜸𝒍 (mJ/mm2)

𝜸𝒅𝒍 (mJ/mm2)

𝜸𝒍 (mJ/mm2)

Ethylene glycol

48.3

29.3

19.6

Glycerol

64.0

34.0

30.0

Water

72.8

21.8

51.0

Table 2. Characteristic parameters of the XPS signals analyzed in this study (left), and XPS quantification results of the elements and components existing in the

surface after the treatment (center) and after 48 hour ageing (right).

Element

Signal

C Al

C(1s) Al(2p)

Component

Al1 (Al0) Al2 (Al+3) O

O(1s) O1 (H2O) O2 (–OH) O3 (=O)

F N

F(1s) N(1s) N1 (NO–) N2 (NO2–) N3 (NO3–)

Be (eV)

FWHM (eV)

After Treatment Untreated

Ar

Ar + N2

Heated

Untreated

Ar

Ar + N2

Heated

285.0 -71.8 74.5 531.7 533.0 531.9 530.8 685.5 -400.2 403.8 407.3

--1.4 2.1 3.0 2.2 2.0 2.1 2.5 -2.4 1.8 1.8

47 19

11 36

10 32

19 30

45 20

15 36

16 32

20 28

11.7 88.3

48 hour ageing

10.5 89.5

9.7 90.3

10.8 89.2

11.2 88.8

10.8 89.2

9.3 93.7

10.7 89.3

32

51

56

51

32

46

50

50

35.9 29.7 34.4 <2 <1 100 0 0

12.9 53.8 33.3 <2 <1 81.8 8.0 10.2

20.3 54.6 25.1 <2 <1 29.6 9.9 60.5

33.4 34.9 31.7 <2 <1 100 0 0

39.1 28.7 32.2 <2 <1 100 0 0

14.2 40.9 44.9 <2 <1 69.8 4.4 25.8

9.3 42.4 48.3 <2 <1 32.4 8.5 59.1

32.3 34.6 33.1 <2 <1 100 0 0