Sol-gel based protective coatings for copper products.

Abstract Corrosion, fouling, and wearing of metal surfaces are the most common problems faced in industry as well as in environmental use. In the recent study, sol--gel based protective coatings were developed and deposited on different copper substrates. Different chemical pretreatments were used to enhance the adhesion and spreadability of coatings on copper surfaces. Substantial improvement of the corrosion resistance of sol--gel coated copper surfaces was obtained by a salt spray test. In addition, the solgel coatings increased hydrophobic and easy-to-clean properties of the copper surface. Variation of the curing temperature of the coatings caused changes in the morphology, adhesion, and topography of the sol-gel coatings. On the basis of the important information obtained in this study, the protective properties of sol-gel coatings can be tailored for copper and copper alloy substrates.

Keywords Sol-gel, Protective coatings, Copper

Introduction

Fouling of metal surfaces is a remarkable problem in industry as well as in environmental use. Impurities and dirt particles can generate wear and corrosion of metal surfaces and therefore decrease the value of the metal products as well as cause financial losses in the metal industry. Copper is widely used in electric, thermal, electronic, construction, architectural, and other everyday life applications. Different kinds of profiles of copper can be used, for instance, tubes, wires, coils, plates, rods, and anodes. Generally this is possible because cold and also warm working of copper is easy to process. Even though copper provides good corrosion properties, its surface can be vulnerable in certain conditions. For example, it has been noticed that copper is not sufficiently passive to resist wearing and strong acid or base chemicals as well as fingerprints.

During the last 10 years, there has been a general interest to study the potential of nanoscale inorganic-organic hybrid sol--gel coatings to improve the properties of metal surfaces. (1), (2) The sol-gel process involves the evolution of inorganic networks in a continuous liquid phase through the formation of colloidal suspension and the following gelation of the sol. Controlled hydrolysis and condensation reactions are the keys for the formation of the sol-gel matrix. Due to these reactions, the sol-gel technique offers a great opportunity to produce functional and premium transparent thin films. Hydrolysis acts as a rapid initial reaction of sol-gel process, where reactive alkoxide groups react with water molecules to form hydroxyl groups. Reaction 1 represents the hydrolysis, where M is usually silicon, zirconium, or titanium and n generally equals four. (3)

M[(OR).sub.n] + [H.sub.2]O[Rightwards arrow over leftwards arrow][(OR).sub.n - 1]M(OH) + ROH (1)

After the initiation of the reaction, the hydrolyzed alkoxides react easily with each other and produce the backbone of the sol--gel network. Water or alcohol is obtained as byproducts depending on the reaction mechanism. The condensation reactions are also explained by reactions 2 and 3. (3)

M(OH) [(OR).sub.n - 1] + M[(OR).sub.n][rightwards arrow over leftwards arrow][(OR).sub.n - 1]MOM[(OR).sub.n - 1] + ROH (2)

2M(OH) [(OR).sub.n - 1][rightwards arrow over leftwards arrow][(OR).sub.n - 1]MOM[(OR).sub.n - 1] + [H.sub.2]O (3)

Recently, different metal alkoxides, (4), (5) functional organosilanes, (6), (7) and corrosion inhibitors (8) have been used to prevent wearing, fouling, and corrosion of metal surfaces. In comparison to stainless steel or aluminum, only few studies concentrated on sol--gel coated copper substrates have been published. Boysen et al. (9) studied sol-gel based protective coatings on copper. They managed to prepare purely ceramic Si[O.sub.2]-Zr[O.sub.2]-based coatings to prevent corrosion of kitchen utensils. Detachment and crack formation, of the coatings became a problem when organic components were added. In addition. Garcia-Cerda et al. (10) developed silica-based sol-gel coatings to prevent the corrosion of copper. Slight increase in corrosion protection was observed after diluting samples in sodium chloride solution. Li et al. (11) applied silaneamine based hybrid sol--gel coating on copper and brass surfaces. According to abrasion tests with the Taber apparatus, the wear resistance of both soft metal surfaces was improved. Furthermore, spectroscopic characterization of dip-coated sol--gels on copper has been carried out by Li et al. (12) In order to study the possible covalent bonding between a copper substrate and silane-based sol-gel film, they used reflection and absorption infrared spectroscopy. A relationship between the coating adhesion and corrosion protection will be further studied by them.

In the present work, the potential of sol--gel based protective hybrid coatings to improve antifouling, wear, corrosion, and chemical resistance of copper products is studied. Prior to the depositions of the nanocoatings the coating process parameters were investigated. The hybrid thin films were coated on three different copper substrates. The study was carried out as a part of the Clean and Environmentally Friendly Metal Products--PUHTEET--project in collaboration with Finnish metal industry, Finnish Funding Agency for Technology and Innovation (TEKES), and VTT Technical Research Centre of Finland.

Experimental

Materials

In the current study, protective surface treatments for three copper products including two different phosphorus deoxidized copper metals (DHP) and one aluminum-copper alloy (aluminum bronze) were investigated. The two copper metals were plain and dark oxidized DHP-coppers. The details of the substrates are listed in Table 1. Prior to the coating, the copper substrates were pretreated in acetone and ethanol baths, immersing and rinsing 1 min in each solution. In addition, commercial alkaline pretreatment agent METEX PE E5 (Mac Dermid Inc.) was used to enhance the spreadability and adhesion of the coatings. METEX PE E5 powder was blended in concentration of 40 g/L with ionized water. The pretreatment procedure is described in Table 2.

Table 1: Chemical composition of substrates

Copper substrates                      Chemical compositin (%)

DHP-copper                           Cu 99.90, P 0.015-0.040

DHP-copper with copper oxide layer   Cu 99.90, P 0.015-0.040, oxide
                                     layer consist of C[u.sub.2]O
                                     and CuO

Aluminum bronze rest                 Zn 4-6, Sn 0.3-1.5, Al 4-6, Cu

Table 2: The pretreatment procedure of the copper substrates

Treatment  Temperature [[degrees]C]  Time [min]

Acetone             RT                  1

Ethanol             RT                  1

METEX               65                  8

Water               RT                  1

Water               RT                  1

After the pretreatment the copper substrates were coated with sol-gel coatings (Fig. 1). The coatings developed and studied by VTT were silane-based and acrylate-modified sol-gel coatings, which were named as PSG11 and PSG21. PSG1J is organically modified ceramic coating and PSG21 is low surface energy hybrid coating. The main composition of the experimental sol-gel coatings from VTT varied in relation to the ratio between the polymeric and ceramic components. Ethanol was used as a solvent in the sol and water was added for the hydrolysis reaction. The desired properties to achieve were antifouling, abrasion, corrosion, chemical, and oxidation resistance and also environmental friendliness. In addition, semicommercial sol-gel coatings from Millidyne Ltd. (AS02, MD110, MD115, MD117, and MD118) were studied. The coating thickness for VTT's experimental coatings was 3-5 [mu]m. The thicknesses were measured with Elcometer 456 apparatus and optical profilometer. Millidyne's semicommercial ASO2 coating was reported to be 0.2 [mu]m thick.

The coatings were deposited by different wet coating methods, e.g., spraying and spilling. The preliminary lab scale samples of 50 x 100 mm were spill-coated in order to see the spreadability and wettability of the coatings. After pretreatment and curing process optimization, the larger A4 size samples were spray-coated (Fig. 1). The coatings were heat-treated in a conventional heating chamber to evaporate the solvents and to cure the coatings. The heat treatment temperature varied from 120 [degrees]C to 150 [degrees]C depending on the substrate material and the preliminary tests. The curing time was 1 h.

[FIGURE 1 OMITTED]

Methods

The properties of the deposited sol-gel coatings were characterized with different methods. Prior to depositions, optimal processing parameters for thermal curing of the coating, and for the pretreatment of the copper surface were studied. In order to optimize the processing parameters the thermal curing of the sol-gel coating was examined with TA Instruments MDSC 2920 differential scanning calorimeter (DSC). Before the measurement, the sol sample was allowed to gelate in a heating chamber at 50[degrees]C. The test method was dynamic heating with heating rate of 10[degrees]C/min and heating range from the room temperature to +400[degrees]C in nitrogen atmosphere. In addition, Fourier transform infrared spectroscopy (FTIR) was used to investigate the curing process in elevated temperatures. Firstly, the sol samples were thermally cured in a small melting pot and after that milled and mixed with potassium bromide to prepare KBr pellets. Curing temperatures were used from 250[degrees]C to 350[degrees]C. In addition, 3D-laser profilometry and coating thickness measurements were used to investigate the influence of the curing process on the surface topography of the sol-gel coatings. The influence of the curing conditions on the adhesion of the coatings was examined by the tape test ASTM D 3359-02.

Hydrophobic/hydrophilic properties of the deposited sol-gel coatings were studied with water contact angle measurements (CAM 200 Optical Contact Angle Meter, KSV Instruments Ltd., CAM 200 software). The abrasion resistance of sol-gel coatings was tested with a standard paint washing tester apparatus (modified standard DIN 53778) where scotch brite was used as an abrasive material. The abrasion resistance of the coatings was evaluated by comparing the contact angle measurements before and after abrasion. Also, the visual examination of the abraded surface was performed. Each coating type was subjected to 700 abrasion cycles.

A salt spray test according to the standard SFS-ISO 92227-1:2001 was used to test the corrosion resistance of the sol-gel coatings. An X-shaped groove was made on the border of the test panel before beginning of the test. Uncoated test panels were also exposed to salt spray as a reference. The corrosion resistance was evaluated visually immediately after the experiment.

The chemical resistance of the sol-gel coatings was tested with three different chemicals: 100% MEK (pH = 5.9), 10% acetic acid (pH = 2.4), and 2% sodium carbonate aqueous solution (pH = 11). In the test, a filter paper was soaked in the test chemical and set on the surface. To prevent the evaporation of the test chemical, a convex glass was placed on the top of the filter paper. Exposure times of test chemical were 30 min, 1, 3, 5, 10, and 16 h. After the exposure, the soaked filter papers were removed and the test area was washed with water and dried with lint-free cloth. Chemical resistance of the coatings was visually evaluated immediately after cleaning the test area.

[FIGURE 2 OMITTED]

Results and discussion

Processing parameters of thermal curing and pretreatments

The processing parameters, e.g., thermal curing and pretreatments, were studied before coating depositions. The commercial alkaline pretreatment solution METEX PE E5 was used to pretreat copper substrates prior to depositions to enhance the spreadability and adhesion of the coatings. Before and after the pretreatment, water contact angle measurements were used to estimate the difference in surface chemistry. The pretreatment was carried out for DHP-copper and aluminum bronze substrates. The water contact angle value for untreated copper and aluminum bronze was measured as approximately 90 [degrees], which obviously should be lower to ensure a sufficient wetting of sol-gel coatings. After the pretreatment of copper surfaces, the measured water contact angle value was decreased to 60 [degrees] for DHP-copper and to 20-40[degrees] for aluminum bronze. Unfortunately, alkaline pretreatment solution dyed the golden color of aluminum bronze to violet. Therefore, only acetone and ethanol solution was used for aluminum bronze to prevent the unwanted color changes. The acetone and ethanol treatment did slightly reduce the hydrophobicity of aluminum bronze surface. However, spreadability and wetting of the sol-gel coatings were sufficient without alkaline treatment.

The influence of thermal curing on coating adhesion was measured with DSC, FTIR, and a tape test. The adhesion of sol-gel coating was improved when thermal curing temperature was increased from 100[degrees]C to 150[degrees]C. In Fig. 2b, it can be seen that the coating was sufficiently attached on the copper surface after the curing was done at 150[degrees]C. On the basis of the DSC study, the improvement of adhesion can be due to polymerization of organic components. Figure 3 represents a DSC spectrum which shows one endothermic peak and two exothermic peaks. The DSC experiment was carried out with heating rate of 10[degrees]C/min and range from room temperature to +400[degrees]C. Firstly, the endothermic peak showed that most of the solvents and water had evaporated at ~100[degrees]C. Secondly, thermal polymerization of the organic parts of the sol-gel was observed by the first exothermic peak. Based on the first exothermic peak, the thermal polymerization of the organic component began at nearby 120[degrees]C and most of the reactions occurred approximately at 170[degree]C. Based on DSC study, the decomposition of the organic components started at 250[degrees]C. DSC and FTIR studies of PSG11 and 21 coatings indicated that the absolutely highest thermal curing temperature for this application could be 300[degrees]C. The FTIR results in Fig. 4 represent the chemical composition of the sol-gel after thermal curing in 250, 300, and 350[degree]C. The stretching vibration peaks of carbon-hydrogen bond (C-H) near 3000 c[m.sub.-1] and also carbony1 peaks C=O and C=C near 1600 c[m.sub.-1] indicates the presence of organic component in sol-gel coating. After the heat treatment in 350[degrees]C the peaks of organic component is absent. Therefore, it can be concluded that if the curing temperature exceeded 300[degrees]C, the organic part and hydrocarbon bonds start to decompose. Consequently, on the basis of these measurements, the optimized curing temperatures were determined to be in the range 120-150[degrees]C.

The effect of curing temperature on the coating topography and the thickness was measured. Heat treatment seemed to have minor effects on surface roughness and coating thicknesses of the sol-gel deposited copper products. The coating thickness varied from 3 [mu]m to 5[mu]m when the curing temperature was 100[degrees]C or 150[degrees]C. However, after the increase of curing temperature over 200[degrees]C the coating thickness decreased to 2 [mu]m. The sol-gel coatings seemed to smooth the topography of copper surface which was seen in the arithmetical mean deviation of the surface profile (Ra) of deposited sol-gel coatings, which was about 0.10 [mu]m. However, the average surface profile did not absolutely describe the surface roughness of the sol-gel thin film. For instance, it is quite difficult to obtain information of individual high peaks or deep valleys with the Ra parameter. Nevertheless, it was useful to detect the microstructured roughness of the coating. In order to obtain a more reliable view of the topography, the peak-to-valley roughness parameters like [R.sub.z] and [R.sub.max] were also measured. After all, the surface roughness decreased slightly when curing temperature was increased.

[FIGURE 3 OMITTED]

Water repellence properties of sol-gel coated copper products

The water repellence properties of sol-gel coated DHP-copper and aluminum bronze substrates were studied with water contact angle measurements. It is a well-known phenomenon that hydro- and oleophobic surface reduces the fouling and makes the surface easier to clean. The contaminants do not stick so easily on hydrophobic surface and therefore the dirt particles can be washed away.

[FIGURE 4 OMITTED]

In everyday life, dirt particles and contaminants are usually wiped away with abrasive cleaning cloth. Therefore, effect of abrasion on water repellence properties was examined also. The studied coatings PSG21 and ASO2 showed a hydrophobic nature as was expected. The water contact angle value increased from the initial value of copper, which can be seen in Figs. 5 and 6. However, after 100 abrasion cycles the hydrophobic behavior was slightly decreased. Furthermore, after 700 abrasion cycles, the measured water contact angle value was 60[degrees] for both the coatings. The organically modified ceramic coating PSG11 resisted abrasion relatively well (Fig. 7). After the abrasion test there were only few scratches on the surface and the water contact angle remained almost the same. Visual examination of abraded surfaces showed that the abrasion resistance of PSG11 and PSG21 coatings was slightly better than ASO2, because only few scratches were detected on PSG coatings.

However, different coating techniques and therefore a different coating thickness of the coatings can have an effect on the wear resistance behaviour of sol-gel coatings. In this study, the coating thickness for the PSG coatings was 3 [mu]m and for the ASO2 coating only about 0.2 [mu]m. The absolute wear resistance of sol-gel coatings cannot be compared, but qualitatively it can be said that the coatings improved the water repellence properties and resisted relatively well the abrasive wearing. Similar behavior was also noticed with coated aluminum bronze substrate. In addition, it can be concluded that the increase of curing temperature might improve the wear resistance with increasing coating adhesion. The influence of coating thickness and deposition techniques as well as curing temperature on wear resistance will be further studied.

[FIGURE 5 OMITTED]

Improvement of the corrosion resistance of copper products

The corrosion resistance of the coatings was studied with salt spray test according to the standard SFS-ISO 92227-1:2001. Sol--gel coated DHP-copper and dark oxidation copper as well as aluminum bronze were studied. Untreated copper was used as a reference. The uncoated copper products were extensively corroded after 2 h exposure, which can be seen in Fig. 8.

[FIGURE 6 OMITTED]

The sol-gel coatings developed by VTT gave corrosion resistance to the copper surfaces. After 24 h exposure, only minor defects were observed. The next observation was done after 96 h. Signs of corrosion products were seen on the surfaces, which were most likely CuC[l.sub.2] or patina. This indicates that PSG coatings were more or less porous. DHP-copper and dark oxidation copper seemed to be more corroded than aluminum bronze. It was also noticed that after the first damages had appeared the corrosion process went on gradually. After 764 h, the samples coated with PSG coatings were removed from the salt spray chamber. This was done because the samples seemed to be in same condition as uncoated samples (Fig. 8). Figure 9 represents the final state of the sol-gel coated samples after 764-h exposure.

Millidyne Ltd.'s semicommercial nanocoatings were studied as a reference to VTT's coatings. It was noticed that MD coatings provided excellent corrosion protection on copper substrates. Especially, the MD117 (Figs 10b and 10f) resisted corrosion efficiently. Figure 10 represents the MD coatings after a 438 h salt spray test.

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

[FIGURE 9 OMITTED]

[FIGURE 10 OMITTED]

[FIGURE 11 OMITTED]

Generally, the PSG sol-gel coatings developed by VTT did improve the corrosion and chemical resistance of copper substrates, even though a salt spray test is quite a hard test for thin sol-gel coatings. Also, the porosity of the coatings could be a problem for thin sol-gel coatings. In the future, the porosity of coatings and the effect of coating thickness on corrosion resistance will be studied.

Table 3: Results of the chemical resistance test

                           30 min                    1 h

                      MEK  Acetic  N[a.sub.2]  MEK   Acetic  N[a.sub.2]
                            acid   C[O.sub.3]         acid   C[O.sub.3]

DHP- Copper   PSG 11   0      0         0       0       0         0
              PSG 21   0      0         0       0       1         0

Aluminum      PSG 11   0      0         0       0       0         0
bronze        PSG 21   0      0         0       0       1         0

DHP-copper    PSG 11   0      0         0       0       0         0
  with oxide  PSG 21   0      0         0       0       0         0
  layer

                                3 h                  5 h

                      MEK   Acetic  N[a.sub.2]  MEK  Acetic  N[a.sub.2]
                            acid     C[O.sub.3]      acid    C[O.sub.3]

DHP-Copper    PSG 11   0     1          0        0     1+       0
              PSG 21   0     1          0        0     1++      1-

Aluminum      PSG 11   0     0          0        0     0        0
bronze        PSG 21   0     1+         1        0     1+       1-

DHP-copper    PSG 11   1     0          0        1     0        0
  with oxide  PSG 21   0     1          0        0     1        1-
  layer

                                    16 h

                          MEK   Acetic acid   N[a.sub.2]C[O.sub.3]

DHP-Copper        PSG 11   0        1++               1
                  PSG 21   0        1++               1+

Aluminum bronze   PSG 11   0        1                 1
                  PSG 21   0        1++               1

DHP-copper        PSG 11   1        1                 0
  with oxide      PSG 21   0        1+                1-
  layer

Note: pH MEK (100%): 5.9; pH Acetic acid (10%): 2.3; pH
N[s.sub.2]C[O.sub.3](2%): 11.0
(1) = Corrosion
(-) = Slight corrosion
(+) = Medium corrosion, oxidation
(++) = Large corrosion, coating has oxidated and corroded

Chemical resistance of sol-gel coated copper products

The chemical resistance of the PSG sol-gel coatings on all copper substrates was tested with three different chemicals: MEK, 10% acetic acid, and 2% sodium carbonate. Differences between the coatings subjected to different chemicals and exposure times were noticed. Sol-gel coated coppers resisted relatively well after 16-h exposure of sodium carbonate and methyl ethyl ketone. Bluish color was observed after 16 h exposure to acetic acid, which seemed to be typical for copper products. PSG11 coating resisted chemicals better than PSG21 coating. Figure 11 and Table 3 show the chemical resistance test results of PSG sol-gel coating on aluminum bronze and DHP-copper with and without oxide layer. The sol-gel coatings showed good chemical resistance for alkaline and MEK solutions.

Conclusion

In the present study, the potential of the sol-gel based protective hybrid coatings to improve the hydrophobic properties, wear, corrosion, and chemical resistance of copper products was studied. Prior to the coating, pretreatments were used to enhance the spreadability, wetting, and adhesion of the coatings. On the basis of this study, the optimized curing temperature of sol-gel coating for copper products was determined to be in the range 120-150[degrees]C. The studied sol-gel coatings PSG21 and AS02 improved hydrophobic properties and resisted the abrasive wear. PSG11 coating did not show hydrophobicity, but it resisted abrasive wearing relatively well. The uncoated copper products corroded extensively already after 2 h exposure. After 764 h, the PSG coated coppers were corroded similarly as uncoated. It was also noticed that sol-gel coatings developed by Millidyne Ltd. showed excellent corrosion protection on copper substrates. On the basis of the important information obtained in this study, the protective properties of sol-gel coatings can be tailored for different purposes.

Acknowledgment Finnish metal industry, TEKES, and VTT are acknowledged for funding the PUHTEET-project.

References

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(3.) Brinker, CJ, Scherer, GW, Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing. Academic Press, Inc., (1990)

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(5.) Castro, Y et al., "Silica-Zirconia Coatings Produced by Dipping and EPD from Colloidal Sol-Gel Suspensions."J. Sol-Gel Sci. Technol., 35 51-55 (2005)

(6.) Chou, TP et al., "Organic-Inorganic Hybrid Coatings for Corrosion Protection." J. Non-Cryst. Solids, 290 153-162 (2001)

(7.) Zandi-Zand, R et al., ''Organic-Inorganic Hybrid Coatings for Corrosion Protection of 1050 Aluminium Alloy." J. Non-Cryst. Solids, 351 1307-1311 (2005)

(8.) Khramov, AN et al., "Sol-Gel-Derived Corrosion-Protective Coatings with Controllable Release of Incorporated Organic Corrosion Inhibitors." Thin Solid Films, 483 191-196 (2005)

(9.) Boysen, W et al., "Protective Coatings on Copper Prepared by Sol-Gel for Industrial Applications." Surf. Coat. Technol., 122 14-17 (1999)

(10.) Garcia-Cerda, LA et al., '"Structural Characterization and Properties of Colloidal Silica Coatings on Copper Substrates." Matter. Lett., 56 450-453 (2002)

(11.) Li, C et al., "Abrasion-Resistant Coalings for Plastic and Soft Metallic Substrates by Sol-Gel Reactions of a Trieth-oxysilylated Diethylenetriamine and Tetramethoxysilane." Wear, 242 152-159 (2000)

(12.) Li, Y-S et al., "Spectroscopic Studies of Trimethoxypropylsilane and Bis(trimethoxysilyl)ethane Sol-Gel Coatings on Aluminium and Copper." Spectrochim. Acta part A. 65 779-786 (2006)

[C] FSCT and OCCA 2008

J. Nikkola ([??]), J. Mannila

VTT Technical Research Centre of Finland. Sinitaival 6,

P.O. Box 1300. Tampere 33101. Finland

e-mail: [e-mail]

R. Mahlberg

VTT Technical Research Centre of Finland,

Betonimiehenkuja 5, P.O. Box 1000, Espoo, 02044 VTT, Finland

J. Siivinen, A. Mahiout

VTT Technical Research Centre of Finland,

Metallimiehenkuja 8, P.O. Box 1000, Espoo, 02044 VTT, Finland

M. Kolari

Millidyne Ltd., Hermiankatu 6-8 G, Tampere 33720,

Finland