Improving the wettability of 2024 aluminium alloy bymeans of cold plasma treatment

W. Polini*, L. SorrentinoDipartimento di Ingegneria Industriale, Universita degli Studi di Cassino, via G. di Biasio 43, 03043 Cassino, Italy

Received 10 October 2002; received in revised form 6 March 2003; accepted 6 March 2003

Abstract

Aluminium alloys are heavily used to manufacture structural parts in the aeronautic industry because of its lightness and its

corrosion resistance. These alloys are successfully used in other industrial fields too, such as railway, automotive and naval

industries. The need to contrast the severe use conditions and the heavy stresses developing in aeronautic field implies to protect

the surfaces of the structures in aluminium alloy by any deterioration. To preserve by deterioration, it is necessary to make

aluminium more suitable to be coated by protective paint. In the aeronautic industry, a complex and critical process is used in

order to enhance both wettability and adhesive properties of aluminium alloy surfaces. Cold plasma treatment represents an

efficient, clean and economic alternative to activate aluminium surfaces.

The present work deals with air cold plasma treatment of 2024 aluminium alloy surfaces. The influence of dc electrical

discharge cold plasma parameters on wettability of 2024 aluminium alloy surfaces has been studied. A set of process variables

(voltage, time and air flow rate) has been identified and used to conduct some experimental tests on the basis of design of

experiment (DOE) techniques. The experimental results show that the proposed plasma process may considerably increase

aluminium alloy wettability. These results represent the first step in trying to optimise the aluminium adhesion by means of this

non-conventional manufacturing process.

# 2003 Elsevier Science B.V. All rights reserved.

Keywords: Cold plasma; Glow-discharge plasma; Aluminium alloy; Wettability

1. Introduction

Metals undergo corrosive phenomena when one or

more of their constituting elements combine with

environmental elements, in particular with oxygen.

An idea of the importance and the spread of the

corrosive phenomena is obtained by estimating the

damage due to corrosion. A recent assessment shows

that the corrosion damages amount to 3.5% of the

gross home product of industrial countries. This

assessment takes into account both the direct and

the indirect damages of corrosion, such as stops in

production or pollution.

The aluminium and the aluminium alloy are

involved in corrosive phenomena when they are used

in critical applications, such as aeronautic ones. In

aeronautic applications, the corrosion of aluminium

involves safety problems. The solution commonly

adopted to solve the corrosive problems is the paint-

ing of the aluminium alloy surface and, therefore, of

2024 aluminium alloy too, which is the subject of

Applied Surface Science 214 (2003) 232–242

* Corresponding author. Tel.: þ39-0776-299679;

fax: þ39-0776-299886.

E-mail address: polini@unicas.it (W. Polini).

URL: http://www.unicas.it.

0169-4332/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0169-4332(03)00359-3

this study. The applied paint film protects the alumi-

nium alloy from corrosion. Therefore, the adhesion of

the paint film to the aluminium alloy and the stability

of the film over time are critical variables for corrosion

protection. In fact, after a temporary exposure to

atmospheric conditions the paint film often shows, a

widespread or localised adhesion loss or air bubbles

that form between the paint film and the aluminium

alloy. These entities represent the centres that cause a

quick spread of corrosion below the paint film. The

paint film has to resist humidity penetration, to contain

an anti-corrosive pigment, to have a good adhesion

both to metal and to the further films of paint in order

to protect effectively the aluminium alloy from

corrosion. No protective film with the requirements

described above exists at the moment. Therefore, it is

necessary to apply a primer and, then, a protective film.

Primer assures that the protective film adheres well to

the aluminium below and, eventually, inhibits the

corrosive process. The protective film should prevent

humidity penetration and corrosion. In the aeronautic

industry, a complex and critical process is used in order

to enhance both wettability and adhesive properties of

aluminium alloy surfaces. It consists of three steps:

solvent cleaning to remove grease and dust from

surfaces; an anodising process to protect surfaces from

corrosion; and a primer application to promote adhe-

sion of the paint to the aluminium alloy surfaces.

The treatment of the aluminium to enhance adhe-

sion is the subject of a very large amount of research,

as discussed in detail in [1] where 41 mechanical,

chemical, electrochemical or other treatments are

identified. Cold plasma surface treatment is known

as a powerful process used to modify surface char-

acteristics of aluminium. The effects of oxygen

plasma treatments on the wettability and surface

composition of cold-rolled aluminium foil were inves-

tigated in [2] from a chemical point of view. Low

temperature plasma treatment was shown to be an

efficient approach in order to prepare the surface of

7075-T6 aluminium for Parylene C coatings [3]. Cold

plasma treatment has been applied to adhesive bond-

ing in [4,5]: cold plasma treatment with oxygen on Al

6061 alloy improves shear strength and, therefore, the

quality of the joint. An important characteristic of cold

plasma deposition is the possibility of obtaining coat-

ings with different compositions along the thickness

by changing the process conditions to minimise the

interface between metal and coating, as reported in

[6,7]. Cold plasma represents an efficient, clean and

economic alternative to the conventional techniques in

order to activate the aluminium surfaces.

The present work deals with air cold plasma treat-

ment of aluminium alloy surfaces. In particular, the

possibility that the cold plasma treatment can replace

one or more steps of the traditional process has been

evaluated with a view of eliminating either the priming

or the anodising steps or both. In this way, it will be

possible to decrease the time and, consequently, the

cost of machining and to reduce the weight of the

aeroplane structure since some materials, that are

traditionally used to activate the surface, will not be

applied any more. The first step towards this aim is to

increase surface wettability in order to improve adhe-

sion, as shown for polypropylene in [8,9]. The study of

wettability is the subject of the present work.

The present work aims to study the relationship

between the cold plasma variables and aluminium

alloy wettability. Owing to the complexity of

plasma–workpiece interaction, currently no validated

theory exists to predict the changes in these properties

with process variables which require experimental

investigations. A set of process variables has been

identified and used to conduct some experimental tests

on the basis of design of experiment (DOE) techni-

ques. In Sections 2 and 3, the plasma equipment and

the experimental plan are presented. In Sections 4 and

5, the methodologies, instruments and results related

to wettability are reported.

2. Cold plasma apparatus

A plasma is an ionised gas containing both charged

and neutral particles, such as electrons, ions, atoms,

molecules and radicals. There are two kinds of plasma:

cold and hot. The difference is the temperature of the

heavy species: it is similar to the electrons (104–

105 K) in a hot plasma, while it is lower than 773 K

in a cold plasma (or non-equilibrium plasma). More-

over, the working pressure of a cold plasma is gen-

erally lower than that of a hot plasma: this allows

the production of reactive chemical species at low

temperatures.

A plasma is generally obtained by an electrical

discharge passing through a gas. It is generated by

W. Polini, L. Sorrentino / Applied Surface Science 214 (2003) 232–242 233

either direct current (dc with 50–60 Hz), or radio

frequency (rf with 1.5–50 MHz), or microwave

(MW with 150–104 MHz) power applied to a gas.

In our case, the plasma is produced using an ac

electrical discharge with a 50 Hz frequency. The

transformer clamps are connected with two electrodes,

which are inside a reactor. The gas is confined in it

and it is submitted to pressures around 1000 Pa. The

electric field generated is able to accelerate the

particles and to transfer kinetic energy to them. Their

consequent motion causes a chain reaction, which

increases the concentration of charged particles (ioni-

sation of neutral species). In this way, a cold plasma is

generated. The electric apparatus supplies a voltage at

the electrodes, varying from 0 to 30 kV, with 30 mA as

current maximum. The gas distributor operates in the

pressure range of 300–9000 Pa and the flow rate

interval is 10–500 dm3/h. The details of this equip-

ment are reported in [10].

3. Experimental plan

A large number of process variables affects the cold

plasma results: flow rates of the different gases, total

pressure in the reactor chamber, substrate temperature

and bias, reactor geometry and material, electrode

material and distance between electrodes, tensioned

electric power applied to the plasma [11]. Because of

the complexity of the surface treatment and of the fairly

recent development of cold plasma technology, the

selection of process variables is still largely empirical.

A set of experimental tests has been designed by

means of a factorial experimental plan. The designed

plan is given in Table 1. The available reactor has

imposed some process variables: reactor geometry and

material, electrode material and distance between

electrodes. Among the many other variables that

influence the treatment results, only three have been

selected and considered as factors in the experimental

phase. The selection has been made considering their

influence on the surface treatment like results from

preliminary tests. The levels of the single process

variables have been chosen in order to cover ade-

quately all the possible experimental conditions, which

are consistent with aluminium alloy thermal stability.

The voltage levels (10, 20 and 30 kV) have been

uniformly spread along the reactor capability range,

its maximum value is related to the highest plasma gas

temperature. The flow rate has been chosen as 15, 35

and 60 dm3/h, and the corresponding pressure inside

the reactor has been 1000, 1500 and 2000 Pa. The use

of a flow rate over 60 dm3/h would involve a very high

pressure and temperature inside the reactor which

would not be consistent with aluminium alloy thermal

stability. The time levels (30, 75 and 120 s) have been

selected in order to study aluminium alloy reactivity to

quite long treatments. The gas used in the experiments

has been air, which is the cheapest gas used in cold

plasma treatment. The small dimensions of our reactor

prototype imposed the dimensions of our rectangular

test piece (120 mm � 30 mm area). The surfaces to be

treated have been cleaned by MEC1 and, then, placed

at a distance of 40 mm from the glow discharge to

avoid possible surface overheating. A steel support has

been used for the test piece positioning. As shown in

Table 1, each test has been replicated five times,

yielding a total of 135 measured surfaces. The experi-

ments have been performed in a random sequence, to

reduce the effect of any possible systematic error [12].

Further experiments have been carried out to eval-

uate the increase of wettability due to the most

economic treatment conditions, i.e. those conditions

characterised by a minimum value of voltage supply

(5 kV). This low voltage clearly reduces the achiev-

able wettability. Therefore, the aim is to solve the

trade-off: to find the best value of wettability achiev-

able at minimum cost. We have looked for the best

value of wettability by changing the treatment time for

a fixed value of air flow rate at 60 dm3/h, since the cost

of cold plasma treatment depends on time and on

electric power used to pump the air inside the reactor.

This electric power does not depend on the adopted

flow rate. The number and the value of levels of time

are the same as those of the first experimental plan.

Each test has been replicated three times.

Table 1

Experimental plan

Process variables Number of levels Values

Tension (kV) 3 10, 20, 30

Air flow rate (dm3/h) 3 15, 35, 60

Treatment time (s) 3 30, 75, 120

Replications 5

Measured surfaces 135

234 W. Polini, L. Sorrentino / Applied Surface Science 214 (2003) 232–242

4. Measurement of wettability

As often happens when the same topic is shared by

many fields, every field has developed its own termi-

nology, so that different definitions of what is meant

for ‘wetting’ exist [13]. In general, it is connected with

the area covered by a liquid drop put on a solid surface.

It depends on the liquid and surface and it is measured

by the contact angle, defined as the angle between

the drop and the surface, as shown in Fig. 1. When the

contact angle has a value near zero, it is said that the

liquid is wetting the surface perfectly, otherwise it is

said that it is not wetting the surface [13]. In ASTM

D724 standard, the procedure to measure the contact

angle is described.

From an energetic point of view, surface wettability

follows Young’s equation concerning the equilibrium

between the three interface surface energies: solid–

liquid (gsl), liquid–gas (glg) and solid–gas (gsg):

gsg ¼ gsl þ glg cos y: (1)

The procedure used to measure the contact angle is

based on the ASTM D724 standard. Five distilled

water drops (7.5 mm3 volume) have been uniformly

dispersed on each surface by a micrometer pipette,

as shown in Fig. 2. The image of each single

drop has been acquired by an optical microscope

(Leica1).

Thanks to the surface energies equilibrium, the

drop takes the shape of an exact semi-sphere whose

contact angle is lower than 908. The drop will take

the shape of a spherical cap whose diameter is lower

than the inscribed sphere diameter, as shown in

Fig. 3. The volume of the drop is known and equal

to 7.5 mm3, since its value is connected to the

accuracy of the micrometer pipette. The base (b)

of the drop profile has been measured by means of

the optic microscope, while its height has been

calculated by means of the equation related to the

volume of a spherical cap:

V ¼ phb2

8þ h2

6

� �¼ 7:5 mm3: (2)

The contact angle’s value has been calculated by

means of the following equation:

W ¼ arcsin4bh

4h2 þ b2

� �: (3)

Fig. 1. Contact angle and gas–liquid–solid related tensions.

Fig. 2. Drop positions.

Fig. 3. Drop spherical cap.

W. Polini, L. Sorrentino / Applied Surface Science 214 (2003) 232–242 235

The drop contact angle of 10 non-processed 2024

aluminium samples has been measured. Five drops

have been placed on each single sample. The contact

angle average is equal to 488. This value has been

taken as the reference for contact angle analysis.

5. Wettability results

To evaluate the influence of plasma variables on

wettability, a multifactor analysis of variance on the

contact angle measurements has been used [14]. The

fundamental hypothesis of the ANOVA applicability

to the experimental data (residual normality and

homogeneity of residuals variance) have been tested

for each step. The main effect plots have been used

in order to have a graphical image of the response

trend.

Firstly, the influence of plasma variables and drop

position have been investigated. The supply voltage,

the gas pressure and the treatment time have shown to

be the most significant factors (see Fig. 4). In Table 2,

the analysis output is shown.

An increase of voltage or treatment time and a

decrease of gas pressure cause a decrease of the contact

angle and thus an improvement in wettability. This is

probably due to the increase of plasma ionisation

degree that involves an enhancement of the charged

species. The drop position is not a significant factor

and, therefore, denotes that plasma effects are homo-

geneously distributed on the surface. Furthermore,

there are some significant interactions: voltage–flow

rate, time–flow rate and voltage–time. When the flow

rate is at the minimum level (15 dm3/h), an increase of

the time causes a decrease of the contact angle, i.e. an

improved wettability. At 35 dm3/h, an increase of the

time up to 75 s causes an increase of the contact angle,

while an increase of time up to 120 s causes a decrease

of the contact angle. At 60 dm3/h, there is an opposite

behaviour. This is probably due to the increase of

pressure inside the discharge tube, connected with

the increase of the flow rate. It produces an increase

in gas impedance and, therefore, a decrease in ionisa-

tion degree that may carry out a low value of contact

angle by increasing the treatment time.

To identify the plasma variables that provide the

best wettability, it has been determined which condi-

tion, among the 27 possibilities, has a distribution of

the contact angle characterised by minimum values of

both mean and standard deviation. The data follow a

Gaussian distribution, as previously verified during

the validation of ANOVA approach. The contact angle

average values and the boxplot of the best experi-

mental conditions are shown in Table 3 and Fig. 5 [15].

Comparing all the results only on the basis of the

voltage supply, the 30 kV condition gives the best

average increase of the surface wettability (�W10 kV ¼21:958; �W20 kV ¼ 21:148; �W30 kV ¼ 20:398). All the

Fig. 4. Main effects plot with regards to voltage supply, air flow rate, time and drop position.

236 W. Polini, L. Sorrentino / Applied Surface Science 214 (2003) 232–242

obtained results have been demonstrated to be lower

than the value of the contact angle of non-processed

2024 aluminium surfaces (48.318).To establish the process conditions that lead to impro-

ved wettability and minimise the cost of treatment, the

ANOVA has been carried out for the data involving

5 kV voltage supply. An increase of treatment time

causes a decrease of contact angle and thus an

improvement in wettability (see Fig. 6). Comparing

all the results on the basis of the time, the 120 s con-

dition gives the best average increase of the surface

wettability (�W30 s ¼ 26:038; �W75 s ¼ 24:808; �W120 s ¼22:828). However, all the obtained average values

have been resulted to be significantly inferior to the

value of the contact angle of non-processed 2024

aluminium surfaces (48.318).

6. Physicochemical analysis of aluminiumsurfaces

A SEM analysis has been carried out in order to

study the surface changes of 2024 aluminium alloy

connected with wettability increase. A physicochem-

ical analysis of both non-treated surfaces and treated

ones by cold plasma of the samples in 2024 aluminium

Table 2

ANOVA output

Factor Levels Values

Voltage (kV) 3 10, 20, 30

Time (s) 3 30, 75, 120

Air flow rate (dm3/h) 3 15, 35, 60

Drop position 2 1, 2

Source d.f. Seq SS Adj SS Adj MS F P

Voltage 2 109.850 109.850 54.925 35.65 0.000�

Time 2 155.782 155.782 77.891 50.55 0.000�

Flow rate 2 147.065 147.065 73.533 47.72 0.000�

Position 1 2.256 2.256 2.256 1.46 0.228

Voltage � time 4 75.487 75.487 18.872 12.25 0.000�

Voltage � flow rate 4 37.857 37.857 9.464 6.14 0.000�

Voltage � position 2 3.931 3.931 1.966 1.28 0.281

Time � flow rate 4 884.929 884.929 221.232 143.59 0.000�

Time � position 2 2.114 2.114 1.057 0.69 0.505

Flow rate � position 2 4.452 4.452 2.226 1.44 0.238

Voltage � time � flow rate 8 64.707 64.707 8.088 5.25 0.000�

Voltage � time � position 4 6.604 6.604 1.651 1.07 0.371

Voltage � flow rate � position 4 4.235 4.235 1.059 0.69 0.602

Time � flow rate � position 4 9.832 9.832 2.458 1.60 0.177

Voltage � time � flow rate � position 8 2.862 2.862 0.358 0.23 0.985

Error 216 332.807 332.807 1.541

Total 269 1844.771

The asterisk represents significant variable.

Fig. 5. Boxplot of the best conditions in terms of wettability.

W. Polini, L. Sorrentino / Applied Surface Science 214 (2003) 232–242 237

alloy has been developed. In particular, the surfaces

treated by cold plasma according to wettability

optimal condition (30 kV, 120 s, 35 dm3/h) has been

further investigated and compared with non-treated

one.

The chemical X-ray analysis has underlined the

elements that are present on the surface of the samples,

as weight (wt.%) and atomic (at.%) percentages, as

shown in Table 4. Both the elements constituting 2024

aluminium alloy (aluminium, copper and so on) and

the organic contaminants deriving from the manufac-

turing process of the aluminium alloy (carbon and

Table 3

Contact angle averages (�W) for the 27 experimental conditions

Experiment

ID

Parameter

values

�W (8) sW (8)

1 10 kV, 30 s, 15 dm3/h 23.58 1.34

2 10 kV, 30 s, 35 dm3/h 21.20 1.38

3 10 kV, 30 s, 60 dm3/h 19.49 1.11

4 10 kV, 75 s, 15 dm3/h 21.40 1.10

5 10 kV, 75 s, 35 dm3/h 22.70 1.13

6 10 kV, 75 s, 60 dm3/h 19.44 1.06

7 10 kV, 120 s, 15 dm3/h 24.87 1.29

8 10 kV, 120 s, 35 dm3/h 20.87 1.23

9 10 kV, 120 s, 60 dm3/h 24.97 1.77

10 20 kV, 30 s, 15 dm3/h 22.75 1.23

11 20 kV, 30 s, 35 dm3/h 20.22 1.10

12 20 kV, 30 s, 60 dm3/h 18.65 1.40

13 20 kV, 75 s, 15 dm3/h 19.78 0.98

14 20 kV, 75 s, 35 dm3/h 24.22 1.21

15 20 kV, 75 s, 60 dm3/h 17.94 1.62

16 20 kV, 120 s, 15 dm3/h 24.76 1.29

17 20 kV, 120 s, 35 dm3/h 18.57 1.22

18 20 kV, 120 s, 60 dm3/h 23.35 1.11

19 30 kV, 30 s, 15 dm3/h 19.51 1.16

20 30 kV, 30 s, 35 dm3/h 18.73 0.97

21 30 kV, 30 s, 60 dm3/h 18.45 1.17

22 30 kV, 75 s, 15 dm3/h 20.21 1.09

23 30 kV, 75 s, 35 dm3/h 24.51 1.27

24 30 kV, 75 s, 60 dm3/h 19.21 1.03

25 30 kV, 120 s, 15 dm3/h 22.62 1.14

26 30 kV, 120 s, 35 dm3/h 16.97 1.18

27 30 kV, 120 s, 60 dm3/h 23.26 1.42

Fig. 6. Main effects plot with regards to time (5 kV).

Table 4

Results of X-ray chemical analysis

Element Non-treated Treated with cold plasma

wt.% at.% wt.% at.%

CK 47.63 63.92 22.52 38.83

OK 13.15 13.26 6.21 8.05

MgK 2.36 1.57 3.60 3.07

AlK 34.72 20.75 63.58 48.84

AgL 0.52 0.08 1.10 0.21

MnK 0.21 0.06 0.44 0.17

CuL 1.41 0.36 2.55 0.83

Total 100.00 100.00 100.00 100.00

EDAX ZAF quantification (standardless)—element normalised.

V ¼ 25:00 kV.

238 W. Polini, L. Sorrentino / Applied Surface Science 214 (2003) 232–242

oxygen) exist on sample surfaces. Therefore, the

contaminants remain anchored to the aluminium sur-

face, even after the cleaning by silver nitrate; this

process is favoured by the heterogeneous structure of

the aluminium alloy.

Cold plasma treatment results in a large decrease in

the percentage of both carbon and oxygen, as shown in

Table 4. This cleaning effect is due to the mechanical

effect of the bombardment by the plasma ions on the

surface and to the chemical interaction of the charged

particles with the contaminants.

The same considerations may be carried out by

observing the X-ray emission spectrum of both the

main elements constituting the 2024 aluminium

alloy and the organic contaminants (Figs. 7 and 8).

A further confirmation of the presence of contami-

nants on sample surfaces has been given by the

microstructural analysis: the alloy surface after cold

Fig. 7. Emission spectrum of a surface of a non-treated sample.

Fig. 8. Emission spectrum of a surface of a sample treated by cold plasma.

W. Polini, L. Sorrentino / Applied Surface Science 214 (2003) 232–242 239

plasma treatment (Figs. 9 and 10) seems more

polished than that due to the industrial process

(Figs. 11 and 12).

The ageing time of cold plasma treatment applied to

aluminium alloy surface is a fundamental step to plan

the insertion of a cold plasma process inside an

industrial manufacturing plant. We have found out a

decay of about 4% in surface wettability after 1 h from

the cold plasma treatment by leaving the samples in

air, while it reaches 14% after 5 h. Moreover, the

decay is of about 65% after 1 day from treatment,

while it assumes an asymptotic trend from the 2nd day

up to the 7th day where the surface returns to an

untreated condition.

Fig. 9. Microstructural analysis (50�) of a surface of a non-treated sample.

Fig. 10. Microstructural analysis (2350�) of a surface of a non-treated sample.

240 W. Polini, L. Sorrentino / Applied Surface Science 214 (2003) 232–242

7. Conclusions

We have demonstrated that voltage, flow rate and

time significantly influence wettability. In particular,

an increasing voltage or time improves wettability.

This is probably due to an increase in ionisation

degree during the treatment time. In this case, a

stronger activation process is generated by the growth

of number of charged particles in the plasma. More-

over, an increase in air flow rate improves wettability.

This probably occurs because the voltage is so high

that it is not influenced by impedance and increase in

air rate. As a matter of fact an increasing air flow rate

raises the number of ionisable species.

The best results in terms of wettability are obta-

ined for condition 30 kV, 120 s, 35 dm3/h and the

Fig. 11. Microstructural analysis (50�) of a surface of a treated sample.

Fig. 12. Microstructural analysis (2350�) of a surface of a treated sample.

W. Polini, L. Sorrentino / Applied Surface Science 214 (2003) 232–242 241

corresponding contact angle is equal to about 178. The

most economic treatment condition is 5 kV, 120 s,

60 dm3/h that carries out a contact angle equal to

about 218. The contact angle of non-treated 2024

aluminium alloy is about 488. Therefore, the contact

angle reduction connected with plasma surface treat-

ment is higher than 50%. The characterisation of

adhesion properties of aluminium alloy 2024 is cur-

rent matter of further study.

Acknowledgements

This work was carried out with the funding of the

Italian M.U.R.S.T. (Ministry of University and Scien-

tific and Technological Research) and CNR (National

Research Council of Italy). The authors are grateful to

Agusta Frosinone and especially to Dr. Natalizia and

Mr. Colosimo for supporting this work. A special

thanks to engineer Annarita Viperino for participating

in the experimental work.

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