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Laser densification of sol–gel silica coatings on aluminium matrix composites for corrosion and hardness improvement
Surface & Coatings Technology 203 (2009) 1474–1480
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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Laser densification of sol–gel silica coatings on aluminium matrix composites for corrosion and hardness improvement A.J. López a, A. Ureña b, J. Rams b,⁎ a b
Presently at CEIT, Manuel de Lardizábal 15, 20018 San Sebastián, Spain Dept. de Ciencia e Ingeniería de Materiales, ESCET, Universidad Rey Juan Carlos, C/ Tulipán s/n, Móstoles 28933 Madrid, Spain
a r t i c l e
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Article history: Received 9 July 2008 Accepted in revised form 24 November 2008 Available online 3 December 2008 Keywords: Sol–gel High power diode laser Composites Coatings Densification Corrosion
a b s t r a c t Sol–gel coatings are usually densified using a thermal treatment stage in oven at temperatures above 500 °C that can cause in low melting temperature substrates a decrease in the substrate mechanical properties. The use of a high power diode laser to densify sol–gel silica coatings previously settled on aluminium matrix composite substrates resulted in consolidated coatings that provide effective corrosion protection and unmodified or even improved substrate properties. After 168 h of immersion in an aggressive electrolyte (3.5 wt.% NaCl aqueous solution), the laser treated sol–gel silica coatings showed better corrosion rates and polarization resistances than the in oven heat treated coatings. Moreover, the hardness of the substrates after laser treatment of the coatings was 45% higher than that of the heat treated ones. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Ceramic coatings are an effective way to improve the corrosion and wear resistance of most materials. These coatings can be obtained by traditional PVD and CVD process; however, they demand cost intensive equipment. The sol–gel organic route allows fabricating ceramic coatings from dilutions of metal alkoxides. It is a costeffective alternative to generate amorphous or crystalline oxide coatings to provide corrosion, erosion and wear protection [1]. During sol–gel coating processing, heat treatments are used to get full evaporation of solvent, the gelification of the sol layer deposited, and the consolidation of the coatings. Temperature selection of this sintering treatment is usually done taking into account the melting point of the metalic substrate, so that in aluminium alloys and composites this temperature must be kept under 500 °C [2–4], while in other systems temperatures above 750 °C are feasible [5,6]. Aluminium matrix composites have been used in different fields where low weight and thermal stability are key requirements. Typical applications include aerospace components, electronic packaging, high precision instrumentation, and automobile engine components [7]. Particulate reinforced composites consist of a uniform distribution of strengthening particles within a matrix. In general, these materials exhibit good wear and erosion resistance, as well as higher stiffness, hardness and strength than unreinforced alloys [8,9].
However, the corrosion resistance of aluminium matrix composites is usually lower than that of unreinforced alloys. To improve it, composites have been coated using a number of methods, and among them we can find the sol–gel ones [10]. The main trouble found during the application of this route is the necessity of applying high temperature densification treatments, which usually soften the substrates. Sol–gel densification treatments with laser [1,11–17] or ultraviolet [18] radiation have been studied to avoid substrate deterioration. High Power Diode Laser (HPDL) seems to be the most feasible laser source for surface processing because of its lower cost per Watt, as compared with other laser sources, and to its emission in the near infrared. In the present work, we analyze the corrosion protection of sol–gel silica coatings densified by HPDL deposited on the surface of aluminium matrix composites reinforced with SiC particles. We have consolidated the coatings while keeping their integrity all over the substrate surface and, simultaneously, increasing the substrate hardness. Corrosion characterization of the different laser treated coatings was carried out in 3.5 wt.% NaCl aerated solution at room temperature using electrochemical continuous current (DC) and gravimetric tests. Low angle X-Ray diffraction (LXRD) was used to determine the nature of the corrosion products formed on the coated and uncoated systems after the corrosion tests. 2. Experimental 2.1. Substrates
⁎ Corresponding author. E-mail address: [email protected] (J. Rams). 0257-8972/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.11.024
Coatings were made on 35 × 20 × 2 mm3 A380.0/SiC/20p samples, i.e. a cast aluminium matrix composite reinforced with 20 vol.% SiC in
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form of particles, with a nominal composition of the aluminium matrix alloy (in wt.%) of: 10.07 Si, 0.97 Fe, 3.13 Cu, 0.59 Mn, 0.39 Mg, 0.01 Zn, 0.08 Ti, traces of Cr and Sr, and balance of Al. All samples were ground to P800 finish with SiC grit papers before being coated. 2.2. Sample coating and thermal treating Silica sol–gel layers were fabricated from a pre-hydrolysed solution of the silica precursor using the organic sol–gel route, in which an alkoxide precursor is hydrolized and condensed to obtain a ceramic material. A homogeneous, clear transparent solution was prepared by dissolving TEOS (tetraethyl orthosilicate), as silica precursor, in ethanol. The molar ratio was TEOS/C2H5OH = 1:11. This solution was partially hydrolyzed by adding 0.1 M HCl acidulated water (molar ratio TEOS/H2O = 1). This mixture was stirred for 2 h and aged at room temperature for 30 min more. Coatings were obtained by dipping the composites in the aged solution and extracting them at 35 cm/min. Then, the coatings were dried in an oven at 100 °C for 1 h. After this, some samples were treated at 500 °C for 1 h in a pre-heated oven and cooled inside it to room temperature; this procedure provided continuous coatings on the surface of the samples that were free of cracking [19]. Other set of samples were treated with a HPDL using a DL13S from Rofin-Dilas with a maximum power of 1300 W and with an emission wavelength of 940 nm. The laser head was placed on an anthropomorphic robot from ABB which allowed controlling the laser power, scan speed and the distance between consecutive laser lines. The area of the rectangular laser spot was kept at 5 mm2 and the incidence angle used was 5° to normal. The conditions used for the coating densification are indicated in Table 1. Fig. 1. Microstructure of the as-received composite substrate: a) general view and b) detail of SiC reinforcement (dark) and intermetalic precipitates (bright).
2.3. Sample characterization The coatings were observed using an Environmental Scanning Electronic Microscope (ESEM) from Philips XL-30 in high-vacuum conditions without metalizing them, and by means of a light microscope (LM). The microstructure of the substrates was evaluated after metallographic etching with Keller's reactive (i.e. 0.5 mL HF 48%, 5 mL HCl, 2.5 mL HNO3 and 95 mL water). Vickers hardness tests were made using 0.1 N load to study the influence of the different treatments in the final densification of the coatings. Electrochemical measurements were made in a 3.5 wt.% NaCl solution using an Autolab PGStat 30 and exposing an area of 0.78 cm2 of the sample to the test medium. All electrochemical tests were done at room temperature without stirring. A three-electrode cell was used for the electrochemical measurements; the working electrode was the own tested material and the counter and reference electrodes were graphite and silver/silver chloride (Ag/AgCl), respectively. A potential of ±10 mV around the corrosion potential (Ecorr) with 1 mV/s scanning rate was applied for different immersion times in the electrolyte to calculate the polarization resistance (Rp), according to ASTM standard G59 [20]. Anodic–cathodic polarization measurements were carried out at 1 mV/s scan rate from −400 to 300 mV with respect to Ecorr and with a 5 mA/cm2 current density threshold limit. Gravimetric tests were made after 168 h of immersion time on other set of samples with the same aggressive solution. Before the
Table 1 Laser parameters used for silica sol–gel densification Sample
Power (W)
Speed (mm/s)
Interlinear distance (mm)
DL1 DL2 DL3 DL4
950 950 950 800
80 60 50 50
1.2 1.2 1.2 0.6
experiment, the samples were weighted with a 0.0001 g precision on a Sartorius BP 211S balance and their area was measured. After immersion tests, specimens were washed with distilled water in a ultrasonic bath, dried with hot air and weighted again. Low angle X-ray diffraction (LXRD) at 1° was used to determine the nature of the corrosion products formed on the coated and uncoated systems after the corrosion tests. 3. Results and discussion 3.1. Sol–gel coatings structure The A380.0/SiC/20p casting composite used as substrate had an α-Al dendritical microstructure in which the SiC reinforced particles were placed at interdendritical positions together with polyhedral AlFeMn precipitates (Fig.1a). Al2Cu and AlCuNi intermetallic precipitates were also identified in the substrates (Fig. 1b). Sol–gel silica coatings deposited had thicknesses of about 5 µm after the heat treatment in oven and were transparent to light microscopy (Fig. 2a), so they were not visible because they were continuous and free from cracking (Fig. 2b), as it has been reported previously [19]. The coatings sintered with DL1 conditions (Table 1 shows the main processing parameters used) were homogeneous and transparent to the light microscope, without any evidence of cracks in the coating (Fig. 3a). The appearance of the laser treated substrate revealed that it did not suffer any microstructural modification (Fig. 3b). The second laser treatment condition, DL2, consisted in increasing the DL1 input heat by 33%, to achieve a higher condensation degree of the sol–gel coating. Under these conditions, the coating still kept unbroken (Fig. 4a), and the substrate microstructure was not affected in terms of dendritical size and presence of intermetalic precipitates (Fig. 4b).
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Fig. 2. Images of the sol–gel silica coated specimens after thermal treatment in oven: a) light microscopy and b) SEM micrograph.
Fig. 4. Silica sol–gel coating densified using DL2 condition; a) light microscopy micrograph, b) SEM micrograph.
Fig. 3. Silica sol–gel coating densified using DL1 condition; a) light microscopy micrograph and b) SEM micrograph.
The heat applied by the DL1 and DL2 laser conditions seemed enough to obtain the densification of the silica sol–gel coating without causing any distortion of the substrate, or even without causing any modification of its microstructure. In the DL3 condition the input heat was increased by 20% respect to DL2 condition by reducing the laser speed from 60 mm/s to 50 mm/s. In this case, the input heat applied was excessive as the substrates were partially molten (Fig. 5a) and the coating cracked (Fig. 5b). Finally, in the DL4 condition we reduced the laser power, to avoid high local temperatures, and we also reduced the distance between scanning lines to compensate this effect and increase the input heat. Fig. 6 shows the formation of aluminium drops on the specimen surface, indicating that it was massively molten. The sol–gel silica layer acted as a physical barrier, maintaining the shape of the substrate, but the local breaking of the coating allowed the escaping of some molten aluminium from the substrate. Using input heats higher than those used in DL4, by increasing the laser power or by reducing the laser speed, gave rise to molten substrates, while reducing the input heat left the coatings untreated. It is important to take into consideration that the laser treatment values used in this work differ by large from those used by same authors in other works where a HPDL was used for the rapid melting and solidification of the substrate [21,22]. The differences are due to the presence of a silica layer on the substrate that behaves as a thermal isolator reducing the cooling rate of the samples. In other studies [23,24], excimer lasers directly heated the coating because of the ultraviolet absorption of the deposited gel, but in our case, as the coating was transparent for HPDL radiation, the first microns of the substrate absorbed the laser radiation and got heated, then they transmitted the heat to the silica layer and consolidated it, i.e. the coatings consolidated from the inside. The direct heating of the substrates was especially relevant for the highest input heat conditions, i.e. DL3 and DL4. In these cases, the specimens could not evacuate enough heat, giving rise to massive
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Fig. 6. Light microscopy image of an aluminium bubble on the surface of a sample treated with the DL4 condition.
Fig. 5. Laser treated sol–gel silica coating with DL3 condition: a) light microscopy; b and c) SEM images.
metallic precipitates, and by the existence of a massive anodic zone, i.e. aluminium matrix. Anodic–cathodic polarization curves of tested materials after 1 h in contact (Fig. 8a) and after 168 h (Fig. 8b) in the aggressive environment showed that the substrate had a reasonably protection capability, presumable because of the natural alumina passive layer that is usually formed on its surface. In the coated systems, the presence of silica on the substrate surfaces moved their polarization curves to less active corrosion potentials, being a clear indication of the cathodic behaviour of the coating, and improving the corrosion resistance of the specimens. This action was especially relevant for the sol–gel systems densified with the diode laser under conditions DL1 and DL2. The corrosion densities (icorr) have been determined from the anodic–cathodic polarization curves using the Stern–Geary equation [25] (Table 2). These corrosion density values measured represent the mean values measured after 168 h of immersion, calculated at the times described in Table 3. This is not the case of the corrosion rate, which correspond to punctual measurements and do not describe the full evolution of the system. The results obtained after 1 h of immersion (Fig. 8a) evidence that the corrosion behaviour of all the tested specimens was similar; all them showed a good protection behaviour. Among them, the coatings treated in an oven showed the best protective behaviour, and the DL1 treated sample showed the worst, although in all cases with minor differences. After 168 h in contact with the electrolyte, the tested materials presented a clear increase in their corrosion densities and a reduction
melting of the specimen. Therefore, the strategy used in the DL4 test, consisting in reducing the local input heat while increasing the number of laser lines applied, led to different results from those obtained in uncoated samples. The main goal of the HPDL treatment is that, as it was previously shown, the structure of the substrate was not modified. This has allowed keeping the hardness of the substrate (Fig. 7). The as-received samples had a hardness of 1.18 GPa; it reduced to 0.85 GPa after a heat treatment of 1 h at 500 °C in an oven. In the case of applying the HPDL working in the DL1 and DL2 conditions, the hardness did not reduced but increased because of the thermal stress caused by the laser treatment. DL3 and DL4 were not included in the hardness measurements because the melting of the substrates and the cracking of the coatings indicates that they are not appropriated treatments for any application. 3.2. Electrochemical tests The corrosion activity of the uncoated composites is caused by the presence of cathodic zones, i.e. reinforcement particles and inter-
Fig. 7. Vickers hardness of the substrates before and after the different consolidation treatments of the sol–gel silica coatings.
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A.J. López et al. / Surface & Coatings Technology 203 (2009) 1474–1480 Table 3 Results of linear polarization resistance (in kΩ cm2) studies of the different tested materials at different immersion times in NaCl 3.5 wt.% NaCl
Fig. 8. Anodic–cathodic polarization curves of the materials tested at: a) 1 h of immersion; b) 168 h of immersion.
in their corrosion potential (Fig. 8b) because of the progression of the corrosion process. The uncoated specimen curve was the most affected one, and its corrosion density after 168 h in contact with the electrolyte (Table 2) was nearly 40 times higher than that measured after one hour, indicating that the corrosion products formed on its surface had only a moderate protective behaviour. The curves obtained for the sol–gel coated specimens after 168 h of contact with the aggressive electrolyte showed that their protection behaviour was maintained, as the corrosion densities measured were similar to those obtained at the initial test time (Table 2). Corrosion densities of coated substrates were one or two orders of magnitude smaller than that of the uncoated substrate. In particular, the sol–gel densified with the DL1 laser condition was the most protective system, with even better behaviour than that of the coating treated in the oven. The DL2 treatment worked well for the first instants of the corrosion process but degraded faster than DL1 one. It is also remarkable that after 168 h of test the corrosion speeds of all coated systems were one order of magnitude smaller than in the
Material
1h
4h
24 h
48 h
72 h
96 h
168 h
Substrate Sol–gel TT Sol–gel DL1 Sol–gel DL2
6.34 9.98 7.84 3.74
8.43 13.00 11.35 6.00
3.28 13.57 17.63 6.12
1.77 8.38 27.86 9.75
1.40 5.94 26.11 5.43
2.14 13.14 26.33 8.16
2.28 16.86 44.57 14.35
case of the uncoated substrate. Sol–gel DL1 coated material presented the lowest corrosion rate, confirming that the grade of protection of this coating was even better than conventional densified sol–gel coating. This is probably due to a better consolidation process in the inner zones of the coating, which allowed the starting of the corrosion process but avoided that the corrosion products formed detached the coating from the composite. However, all the sol–gel silica coatings proportioned an effective physical barrier against corrosion, retarding the beginning and the progression of the corrosion process. Polarization resistance (Rp) tests were also carried out at different immersion intervals described in Table 3 to ascertain the behaviours observed in anodic–cathodic tests and to analyze the influence of the different densification treatments, as well as, to determine the role played by the corrosion products formed in the surface of the specimens during the corrosion process. The uncoated substrate showed a marked decrease in the Rp with time at the first 72 h of testing indicating that pitting corrosion of the substrate was taking place. Only after 92 h of contact with the electrolyte did the Rp slightly increase, presumably because the corrosion products were blocking the access of the electrolyte to the previously attacked zones of the substrate and, hence, retarding the progression of corrosion. The specimen coated using the traditional sol–gel treatment retained high values of Rp during all the testing time, only showing a small drop down from 48 to 72 h, which proves the effectiveness of the coating. At the end of the test, the material recovered its high resistance to corrosion (16.86 kΩ/cm2), most probably due to the inhibition effect of the corrosion products formed on the surface. The sample treated with the DL1 condition started from a relatively high polarization resistance (7.84 kΩ cm2) that monotonically grew to 44.6 kΩ cm2. The Rp values measured after the first 24 h were higher than those reached by any of the other specimens throughout the test. This evolution seems to be due to the combined effect of a protecting isolating coating and to the blocking of the existing pores in the coating by the corrosion products formed during the test. The DL2 condition treated samples started from the lowest Rp value among the samples tested, indicating that it may have a lower protective behaviour. However, the polarization resistance increased
Table 2 Corrosion current densities and corrosion rates of studied materials at initial and at the end of the test time Material Substrate Sol–gel TT Sol–gel DL1 Sol–gel DL2
icorr (1 h)
icorr (168 h)
Corrosion rate (168 h)
μ A/cm2
μ A/cm2
(mm/year)
0.4 0.34 1.08 0.62
15.15 0.69 0.42 1.18
0.18 0.014 0.04 0.012
Fig. 9. Mass loss after 168 h contact time in 3.5 wt.% NaCl solution.
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throughout the test, indicating that there was a good stability of the corrosion products in the sample surface. 3.3. Gravimetric tests Gravimetric tests were carried out to measure the accumulated corrosion degradation of the different coated systems immersed in 3.5 wt.% NaCl aqueous aerated solution after 168 h and to support the results obtained in electrochemical tests. All coatings system, with the exception of sol–gel treated with the DL2 condition, proportioned protection to the corrosion of the substrate, showing a smaller mass loss after 168 h than the uncoated substrate (Fig. 9). The mass loss results confirmed the tendency of the electrochemical results. The sol–gel coatings densified with DL1 and inside the oven showed the same mass loss, indicating that, at least, the same corrosion protection was obtained with this laser treatment condition. Among coated systems, sol–gel coating DL2 showed the worst corrosion protection of the fully tested coatings, and it showed a clear signature of generalized corrosion processes at the end of the immersion time. This was also previously confirmed by linear polarization resistance tests, being it the coating with the lowest polarization resistance at any time of immersion. However, it is important to take into consideration that the mass loss due to the dilution of the matrix may be partially hidden by the mass gain caused by the formation of insoluble corrosion products. Moreover, gravimetric test can only be used for evaluation of corrosion when generalized corrosion takes place, and in our case some pitting corrosion seems to happen mainly at the beginning of immersion time. 3.4. Morphology and characterization of corrosion attack The uncoated samples appeared clearly damaged after 7 days of immersion in the aggressive environment (Fig. 10a and detail in
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Fig. 10b). An insoluble white substance appeared on all the surface of the tested material, as result of the corrosion reactions that took place once the chloride anions reached the metallic surface of the composite, after breaking its natural alumina coating. In the case of these composites, this alumina layer is always disrupted by the presence of the SiC reinforcement particles and intermetallic precipitates, causing a lower corrosion resistance than that of the matrix itself because of the despolarization of the anodic reaction of the RedOx system. The sol–gel silica coating densified in the oven reduced by far the formation of corrosion products (Fig. 10c); even when examining the corroded zone at higher magnifications (Fig. 10d) we observed that nearly no corrosion products were formed. A similar scenario was obtained for the coating treated with the DL1 condition (Fig. 11a and b). There were nearly no corrosion products on the material surface, even studying it at higher magnifications; evidencing less damage than in the previous case. The specimen with the silica coating densified using the DL2 laser condition showed more damage than any of the other coated samples tested (Fig. 11c and d). There was an extensive presence of corrosion products on the coating surface. However, the amount of products was clearly lower than those formed in the uncoated substrate. In all the specimens, we have determined by applying low angle XRD that the corrosion product formed in the surface of the samples was bayerite (β-Al2O3∙3H2O), as it has been observed in other systems [26]. It is important to take into consideration that the DL2 treatment inputs more heat to the coatings, so that its porosity should be lower than after DL1 treatment. Therefore, other type of factors may reduce the corrosion protection of the coating. The most feasible one is the formation of stress in the coating by means of the higher consolidation speed. This stress may generate microcraks in the coating that speed up the chloride penetration into the coating and so reaching the surface of the composite earlier. Visual comparison of the different samples ascertains that the mass loss measurements made (Fig. 9) do not correlate with the actual
Fig. 10. Specimens after 168 h in contact with the aggressive environment: a) and b) uncoated substrate; c) and d) heat treated sol–gel.
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Fig. 11. Specimens after 168 h in contact with the aggressive environment: a) and b) DL1 conditions; c) and d) DL2 conditions.
corrosion of the samples because of the formation of corrosion products on the samples surface. In particular, in the case of the uncoated substrate the mass gain due to the formation of corrosion products hid a big dissolution of the aluminium matrix.
Acknowledgement
4. Conclusions
References
A summary of the most remarkably conclusions of this study are: • Sol–gel silica coatings have been successfully consolidated on the surface of aluminium matrix composites containing 20% SiC particles using a HPDL. Crack-free and continuous ceramic layers that provide corrosion protection have been obtained. • The substrate microstructure was not affected by means of the HPDL consolidation treatment giving rise, when optimum laser parameters were used, to samples that were 45% harder than those heat treated. • Sol–gel coating consolidated applying the DL1 condition showed better corrosion resistances than those consolidated in an oven. After 7 days of contact with 3.5 wt.% NaCl, this coating system protected the substrate from the aggressive electrolyte attack. • Using heat inputs above the optimum (DL2 condition) produced coatings with less corrosion protection from the beginning of the contact with the electrolyte. This indicates that stress may appear in the coating favouring the evolution of the corrosion mechanisms. • In all tested cases, the corrosion product was mainly bayerite β-Al2O3∙3H2O and it had a strong influence in the evolution of the corrosion process by blocking the access of aggressive solution to the substrate corroded zones. • HPDL results a feasible technique to consolidate sol–gel ceramic coatings to obtain corrosion protective silica coatings, and at the same time to retain the mechanical properties of the substrates, at a difference from in-oven heat treatments.
Authors wish to thank to MEC under projects MAT2003-04931-C02-02 and CAM (S-0505/MAT/0077).
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Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Laser densification of sol–gel silica coatings on aluminium matrix composites for corrosion and hardness improvement A.J. López a, A. Ureña b, J. Rams b,⁎ a b
Presently at CEIT, Manuel de Lardizábal 15, 20018 San Sebastián, Spain Dept. de Ciencia e Ingeniería de Materiales, ESCET, Universidad Rey Juan Carlos, C/ Tulipán s/n, Móstoles 28933 Madrid, Spain
a r t i c l e
i n f o
Article history: Received 9 July 2008 Accepted in revised form 24 November 2008 Available online 3 December 2008 Keywords: Sol–gel High power diode laser Composites Coatings Densification Corrosion
a b s t r a c t Sol–gel coatings are usually densified using a thermal treatment stage in oven at temperatures above 500 °C that can cause in low melting temperature substrates a decrease in the substrate mechanical properties. The use of a high power diode laser to densify sol–gel silica coatings previously settled on aluminium matrix composite substrates resulted in consolidated coatings that provide effective corrosion protection and unmodified or even improved substrate properties. After 168 h of immersion in an aggressive electrolyte (3.5 wt.% NaCl aqueous solution), the laser treated sol–gel silica coatings showed better corrosion rates and polarization resistances than the in oven heat treated coatings. Moreover, the hardness of the substrates after laser treatment of the coatings was 45% higher than that of the heat treated ones. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Ceramic coatings are an effective way to improve the corrosion and wear resistance of most materials. These coatings can be obtained by traditional PVD and CVD process; however, they demand cost intensive equipment. The sol–gel organic route allows fabricating ceramic coatings from dilutions of metal alkoxides. It is a costeffective alternative to generate amorphous or crystalline oxide coatings to provide corrosion, erosion and wear protection [1]. During sol–gel coating processing, heat treatments are used to get full evaporation of solvent, the gelification of the sol layer deposited, and the consolidation of the coatings. Temperature selection of this sintering treatment is usually done taking into account the melting point of the metalic substrate, so that in aluminium alloys and composites this temperature must be kept under 500 °C [2–4], while in other systems temperatures above 750 °C are feasible [5,6]. Aluminium matrix composites have been used in different fields where low weight and thermal stability are key requirements. Typical applications include aerospace components, electronic packaging, high precision instrumentation, and automobile engine components [7]. Particulate reinforced composites consist of a uniform distribution of strengthening particles within a matrix. In general, these materials exhibit good wear and erosion resistance, as well as higher stiffness, hardness and strength than unreinforced alloys [8,9].
However, the corrosion resistance of aluminium matrix composites is usually lower than that of unreinforced alloys. To improve it, composites have been coated using a number of methods, and among them we can find the sol–gel ones [10]. The main trouble found during the application of this route is the necessity of applying high temperature densification treatments, which usually soften the substrates. Sol–gel densification treatments with laser [1,11–17] or ultraviolet [18] radiation have been studied to avoid substrate deterioration. High Power Diode Laser (HPDL) seems to be the most feasible laser source for surface processing because of its lower cost per Watt, as compared with other laser sources, and to its emission in the near infrared. In the present work, we analyze the corrosion protection of sol–gel silica coatings densified by HPDL deposited on the surface of aluminium matrix composites reinforced with SiC particles. We have consolidated the coatings while keeping their integrity all over the substrate surface and, simultaneously, increasing the substrate hardness. Corrosion characterization of the different laser treated coatings was carried out in 3.5 wt.% NaCl aerated solution at room temperature using electrochemical continuous current (DC) and gravimetric tests. Low angle X-Ray diffraction (LXRD) was used to determine the nature of the corrosion products formed on the coated and uncoated systems after the corrosion tests. 2. Experimental 2.1. Substrates
⁎ Corresponding author. E-mail address: [email protected] (J. Rams). 0257-8972/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.11.024
Coatings were made on 35 × 20 × 2 mm3 A380.0/SiC/20p samples, i.e. a cast aluminium matrix composite reinforced with 20 vol.% SiC in
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form of particles, with a nominal composition of the aluminium matrix alloy (in wt.%) of: 10.07 Si, 0.97 Fe, 3.13 Cu, 0.59 Mn, 0.39 Mg, 0.01 Zn, 0.08 Ti, traces of Cr and Sr, and balance of Al. All samples were ground to P800 finish with SiC grit papers before being coated. 2.2. Sample coating and thermal treating Silica sol–gel layers were fabricated from a pre-hydrolysed solution of the silica precursor using the organic sol–gel route, in which an alkoxide precursor is hydrolized and condensed to obtain a ceramic material. A homogeneous, clear transparent solution was prepared by dissolving TEOS (tetraethyl orthosilicate), as silica precursor, in ethanol. The molar ratio was TEOS/C2H5OH = 1:11. This solution was partially hydrolyzed by adding 0.1 M HCl acidulated water (molar ratio TEOS/H2O = 1). This mixture was stirred for 2 h and aged at room temperature for 30 min more. Coatings were obtained by dipping the composites in the aged solution and extracting them at 35 cm/min. Then, the coatings were dried in an oven at 100 °C for 1 h. After this, some samples were treated at 500 °C for 1 h in a pre-heated oven and cooled inside it to room temperature; this procedure provided continuous coatings on the surface of the samples that were free of cracking [19]. Other set of samples were treated with a HPDL using a DL13S from Rofin-Dilas with a maximum power of 1300 W and with an emission wavelength of 940 nm. The laser head was placed on an anthropomorphic robot from ABB which allowed controlling the laser power, scan speed and the distance between consecutive laser lines. The area of the rectangular laser spot was kept at 5 mm2 and the incidence angle used was 5° to normal. The conditions used for the coating densification are indicated in Table 1. Fig. 1. Microstructure of the as-received composite substrate: a) general view and b) detail of SiC reinforcement (dark) and intermetalic precipitates (bright).
2.3. Sample characterization The coatings were observed using an Environmental Scanning Electronic Microscope (ESEM) from Philips XL-30 in high-vacuum conditions without metalizing them, and by means of a light microscope (LM). The microstructure of the substrates was evaluated after metallographic etching with Keller's reactive (i.e. 0.5 mL HF 48%, 5 mL HCl, 2.5 mL HNO3 and 95 mL water). Vickers hardness tests were made using 0.1 N load to study the influence of the different treatments in the final densification of the coatings. Electrochemical measurements were made in a 3.5 wt.% NaCl solution using an Autolab PGStat 30 and exposing an area of 0.78 cm2 of the sample to the test medium. All electrochemical tests were done at room temperature without stirring. A three-electrode cell was used for the electrochemical measurements; the working electrode was the own tested material and the counter and reference electrodes were graphite and silver/silver chloride (Ag/AgCl), respectively. A potential of ±10 mV around the corrosion potential (Ecorr) with 1 mV/s scanning rate was applied for different immersion times in the electrolyte to calculate the polarization resistance (Rp), according to ASTM standard G59 [20]. Anodic–cathodic polarization measurements were carried out at 1 mV/s scan rate from −400 to 300 mV with respect to Ecorr and with a 5 mA/cm2 current density threshold limit. Gravimetric tests were made after 168 h of immersion time on other set of samples with the same aggressive solution. Before the
Table 1 Laser parameters used for silica sol–gel densification Sample
Power (W)
Speed (mm/s)
Interlinear distance (mm)
DL1 DL2 DL3 DL4
950 950 950 800
80 60 50 50
1.2 1.2 1.2 0.6
experiment, the samples were weighted with a 0.0001 g precision on a Sartorius BP 211S balance and their area was measured. After immersion tests, specimens were washed with distilled water in a ultrasonic bath, dried with hot air and weighted again. Low angle X-ray diffraction (LXRD) at 1° was used to determine the nature of the corrosion products formed on the coated and uncoated systems after the corrosion tests. 3. Results and discussion 3.1. Sol–gel coatings structure The A380.0/SiC/20p casting composite used as substrate had an α-Al dendritical microstructure in which the SiC reinforced particles were placed at interdendritical positions together with polyhedral AlFeMn precipitates (Fig.1a). Al2Cu and AlCuNi intermetallic precipitates were also identified in the substrates (Fig. 1b). Sol–gel silica coatings deposited had thicknesses of about 5 µm after the heat treatment in oven and were transparent to light microscopy (Fig. 2a), so they were not visible because they were continuous and free from cracking (Fig. 2b), as it has been reported previously [19]. The coatings sintered with DL1 conditions (Table 1 shows the main processing parameters used) were homogeneous and transparent to the light microscope, without any evidence of cracks in the coating (Fig. 3a). The appearance of the laser treated substrate revealed that it did not suffer any microstructural modification (Fig. 3b). The second laser treatment condition, DL2, consisted in increasing the DL1 input heat by 33%, to achieve a higher condensation degree of the sol–gel coating. Under these conditions, the coating still kept unbroken (Fig. 4a), and the substrate microstructure was not affected in terms of dendritical size and presence of intermetalic precipitates (Fig. 4b).
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Fig. 2. Images of the sol–gel silica coated specimens after thermal treatment in oven: a) light microscopy and b) SEM micrograph.
Fig. 4. Silica sol–gel coating densified using DL2 condition; a) light microscopy micrograph, b) SEM micrograph.
Fig. 3. Silica sol–gel coating densified using DL1 condition; a) light microscopy micrograph and b) SEM micrograph.
The heat applied by the DL1 and DL2 laser conditions seemed enough to obtain the densification of the silica sol–gel coating without causing any distortion of the substrate, or even without causing any modification of its microstructure. In the DL3 condition the input heat was increased by 20% respect to DL2 condition by reducing the laser speed from 60 mm/s to 50 mm/s. In this case, the input heat applied was excessive as the substrates were partially molten (Fig. 5a) and the coating cracked (Fig. 5b). Finally, in the DL4 condition we reduced the laser power, to avoid high local temperatures, and we also reduced the distance between scanning lines to compensate this effect and increase the input heat. Fig. 6 shows the formation of aluminium drops on the specimen surface, indicating that it was massively molten. The sol–gel silica layer acted as a physical barrier, maintaining the shape of the substrate, but the local breaking of the coating allowed the escaping of some molten aluminium from the substrate. Using input heats higher than those used in DL4, by increasing the laser power or by reducing the laser speed, gave rise to molten substrates, while reducing the input heat left the coatings untreated. It is important to take into consideration that the laser treatment values used in this work differ by large from those used by same authors in other works where a HPDL was used for the rapid melting and solidification of the substrate [21,22]. The differences are due to the presence of a silica layer on the substrate that behaves as a thermal isolator reducing the cooling rate of the samples. In other studies [23,24], excimer lasers directly heated the coating because of the ultraviolet absorption of the deposited gel, but in our case, as the coating was transparent for HPDL radiation, the first microns of the substrate absorbed the laser radiation and got heated, then they transmitted the heat to the silica layer and consolidated it, i.e. the coatings consolidated from the inside. The direct heating of the substrates was especially relevant for the highest input heat conditions, i.e. DL3 and DL4. In these cases, the specimens could not evacuate enough heat, giving rise to massive
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Fig. 6. Light microscopy image of an aluminium bubble on the surface of a sample treated with the DL4 condition.
Fig. 5. Laser treated sol–gel silica coating with DL3 condition: a) light microscopy; b and c) SEM images.
metallic precipitates, and by the existence of a massive anodic zone, i.e. aluminium matrix. Anodic–cathodic polarization curves of tested materials after 1 h in contact (Fig. 8a) and after 168 h (Fig. 8b) in the aggressive environment showed that the substrate had a reasonably protection capability, presumable because of the natural alumina passive layer that is usually formed on its surface. In the coated systems, the presence of silica on the substrate surfaces moved their polarization curves to less active corrosion potentials, being a clear indication of the cathodic behaviour of the coating, and improving the corrosion resistance of the specimens. This action was especially relevant for the sol–gel systems densified with the diode laser under conditions DL1 and DL2. The corrosion densities (icorr) have been determined from the anodic–cathodic polarization curves using the Stern–Geary equation [25] (Table 2). These corrosion density values measured represent the mean values measured after 168 h of immersion, calculated at the times described in Table 3. This is not the case of the corrosion rate, which correspond to punctual measurements and do not describe the full evolution of the system. The results obtained after 1 h of immersion (Fig. 8a) evidence that the corrosion behaviour of all the tested specimens was similar; all them showed a good protection behaviour. Among them, the coatings treated in an oven showed the best protective behaviour, and the DL1 treated sample showed the worst, although in all cases with minor differences. After 168 h in contact with the electrolyte, the tested materials presented a clear increase in their corrosion densities and a reduction
melting of the specimen. Therefore, the strategy used in the DL4 test, consisting in reducing the local input heat while increasing the number of laser lines applied, led to different results from those obtained in uncoated samples. The main goal of the HPDL treatment is that, as it was previously shown, the structure of the substrate was not modified. This has allowed keeping the hardness of the substrate (Fig. 7). The as-received samples had a hardness of 1.18 GPa; it reduced to 0.85 GPa after a heat treatment of 1 h at 500 °C in an oven. In the case of applying the HPDL working in the DL1 and DL2 conditions, the hardness did not reduced but increased because of the thermal stress caused by the laser treatment. DL3 and DL4 were not included in the hardness measurements because the melting of the substrates and the cracking of the coatings indicates that they are not appropriated treatments for any application. 3.2. Electrochemical tests The corrosion activity of the uncoated composites is caused by the presence of cathodic zones, i.e. reinforcement particles and inter-
Fig. 7. Vickers hardness of the substrates before and after the different consolidation treatments of the sol–gel silica coatings.
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Fig. 8. Anodic–cathodic polarization curves of the materials tested at: a) 1 h of immersion; b) 168 h of immersion.
in their corrosion potential (Fig. 8b) because of the progression of the corrosion process. The uncoated specimen curve was the most affected one, and its corrosion density after 168 h in contact with the electrolyte (Table 2) was nearly 40 times higher than that measured after one hour, indicating that the corrosion products formed on its surface had only a moderate protective behaviour. The curves obtained for the sol–gel coated specimens after 168 h of contact with the aggressive electrolyte showed that their protection behaviour was maintained, as the corrosion densities measured were similar to those obtained at the initial test time (Table 2). Corrosion densities of coated substrates were one or two orders of magnitude smaller than that of the uncoated substrate. In particular, the sol–gel densified with the DL1 laser condition was the most protective system, with even better behaviour than that of the coating treated in the oven. The DL2 treatment worked well for the first instants of the corrosion process but degraded faster than DL1 one. It is also remarkable that after 168 h of test the corrosion speeds of all coated systems were one order of magnitude smaller than in the
Material
1h
4h
24 h
48 h
72 h
96 h
168 h
Substrate Sol–gel TT Sol–gel DL1 Sol–gel DL2
6.34 9.98 7.84 3.74
8.43 13.00 11.35 6.00
3.28 13.57 17.63 6.12
1.77 8.38 27.86 9.75
1.40 5.94 26.11 5.43
2.14 13.14 26.33 8.16
2.28 16.86 44.57 14.35
case of the uncoated substrate. Sol–gel DL1 coated material presented the lowest corrosion rate, confirming that the grade of protection of this coating was even better than conventional densified sol–gel coating. This is probably due to a better consolidation process in the inner zones of the coating, which allowed the starting of the corrosion process but avoided that the corrosion products formed detached the coating from the composite. However, all the sol–gel silica coatings proportioned an effective physical barrier against corrosion, retarding the beginning and the progression of the corrosion process. Polarization resistance (Rp) tests were also carried out at different immersion intervals described in Table 3 to ascertain the behaviours observed in anodic–cathodic tests and to analyze the influence of the different densification treatments, as well as, to determine the role played by the corrosion products formed in the surface of the specimens during the corrosion process. The uncoated substrate showed a marked decrease in the Rp with time at the first 72 h of testing indicating that pitting corrosion of the substrate was taking place. Only after 92 h of contact with the electrolyte did the Rp slightly increase, presumably because the corrosion products were blocking the access of the electrolyte to the previously attacked zones of the substrate and, hence, retarding the progression of corrosion. The specimen coated using the traditional sol–gel treatment retained high values of Rp during all the testing time, only showing a small drop down from 48 to 72 h, which proves the effectiveness of the coating. At the end of the test, the material recovered its high resistance to corrosion (16.86 kΩ/cm2), most probably due to the inhibition effect of the corrosion products formed on the surface. The sample treated with the DL1 condition started from a relatively high polarization resistance (7.84 kΩ cm2) that monotonically grew to 44.6 kΩ cm2. The Rp values measured after the first 24 h were higher than those reached by any of the other specimens throughout the test. This evolution seems to be due to the combined effect of a protecting isolating coating and to the blocking of the existing pores in the coating by the corrosion products formed during the test. The DL2 condition treated samples started from the lowest Rp value among the samples tested, indicating that it may have a lower protective behaviour. However, the polarization resistance increased
Table 2 Corrosion current densities and corrosion rates of studied materials at initial and at the end of the test time Material Substrate Sol–gel TT Sol–gel DL1 Sol–gel DL2
icorr (1 h)
icorr (168 h)
Corrosion rate (168 h)
μ A/cm2
μ A/cm2
(mm/year)
0.4 0.34 1.08 0.62
15.15 0.69 0.42 1.18
0.18 0.014 0.04 0.012
Fig. 9. Mass loss after 168 h contact time in 3.5 wt.% NaCl solution.
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throughout the test, indicating that there was a good stability of the corrosion products in the sample surface. 3.3. Gravimetric tests Gravimetric tests were carried out to measure the accumulated corrosion degradation of the different coated systems immersed in 3.5 wt.% NaCl aqueous aerated solution after 168 h and to support the results obtained in electrochemical tests. All coatings system, with the exception of sol–gel treated with the DL2 condition, proportioned protection to the corrosion of the substrate, showing a smaller mass loss after 168 h than the uncoated substrate (Fig. 9). The mass loss results confirmed the tendency of the electrochemical results. The sol–gel coatings densified with DL1 and inside the oven showed the same mass loss, indicating that, at least, the same corrosion protection was obtained with this laser treatment condition. Among coated systems, sol–gel coating DL2 showed the worst corrosion protection of the fully tested coatings, and it showed a clear signature of generalized corrosion processes at the end of the immersion time. This was also previously confirmed by linear polarization resistance tests, being it the coating with the lowest polarization resistance at any time of immersion. However, it is important to take into consideration that the mass loss due to the dilution of the matrix may be partially hidden by the mass gain caused by the formation of insoluble corrosion products. Moreover, gravimetric test can only be used for evaluation of corrosion when generalized corrosion takes place, and in our case some pitting corrosion seems to happen mainly at the beginning of immersion time. 3.4. Morphology and characterization of corrosion attack The uncoated samples appeared clearly damaged after 7 days of immersion in the aggressive environment (Fig. 10a and detail in
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Fig. 10b). An insoluble white substance appeared on all the surface of the tested material, as result of the corrosion reactions that took place once the chloride anions reached the metallic surface of the composite, after breaking its natural alumina coating. In the case of these composites, this alumina layer is always disrupted by the presence of the SiC reinforcement particles and intermetallic precipitates, causing a lower corrosion resistance than that of the matrix itself because of the despolarization of the anodic reaction of the RedOx system. The sol–gel silica coating densified in the oven reduced by far the formation of corrosion products (Fig. 10c); even when examining the corroded zone at higher magnifications (Fig. 10d) we observed that nearly no corrosion products were formed. A similar scenario was obtained for the coating treated with the DL1 condition (Fig. 11a and b). There were nearly no corrosion products on the material surface, even studying it at higher magnifications; evidencing less damage than in the previous case. The specimen with the silica coating densified using the DL2 laser condition showed more damage than any of the other coated samples tested (Fig. 11c and d). There was an extensive presence of corrosion products on the coating surface. However, the amount of products was clearly lower than those formed in the uncoated substrate. In all the specimens, we have determined by applying low angle XRD that the corrosion product formed in the surface of the samples was bayerite (β-Al2O3∙3H2O), as it has been observed in other systems [26]. It is important to take into consideration that the DL2 treatment inputs more heat to the coatings, so that its porosity should be lower than after DL1 treatment. Therefore, other type of factors may reduce the corrosion protection of the coating. The most feasible one is the formation of stress in the coating by means of the higher consolidation speed. This stress may generate microcraks in the coating that speed up the chloride penetration into the coating and so reaching the surface of the composite earlier. Visual comparison of the different samples ascertains that the mass loss measurements made (Fig. 9) do not correlate with the actual
Fig. 10. Specimens after 168 h in contact with the aggressive environment: a) and b) uncoated substrate; c) and d) heat treated sol–gel.
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Fig. 11. Specimens after 168 h in contact with the aggressive environment: a) and b) DL1 conditions; c) and d) DL2 conditions.
corrosion of the samples because of the formation of corrosion products on the samples surface. In particular, in the case of the uncoated substrate the mass gain due to the formation of corrosion products hid a big dissolution of the aluminium matrix.
Acknowledgement
4. Conclusions
References
A summary of the most remarkably conclusions of this study are: • Sol–gel silica coatings have been successfully consolidated on the surface of aluminium matrix composites containing 20% SiC particles using a HPDL. Crack-free and continuous ceramic layers that provide corrosion protection have been obtained. • The substrate microstructure was not affected by means of the HPDL consolidation treatment giving rise, when optimum laser parameters were used, to samples that were 45% harder than those heat treated. • Sol–gel coating consolidated applying the DL1 condition showed better corrosion resistances than those consolidated in an oven. After 7 days of contact with 3.5 wt.% NaCl, this coating system protected the substrate from the aggressive electrolyte attack. • Using heat inputs above the optimum (DL2 condition) produced coatings with less corrosion protection from the beginning of the contact with the electrolyte. This indicates that stress may appear in the coating favouring the evolution of the corrosion mechanisms. • In all tested cases, the corrosion product was mainly bayerite β-Al2O3∙3H2O and it had a strong influence in the evolution of the corrosion process by blocking the access of aggressive solution to the substrate corroded zones. • HPDL results a feasible technique to consolidate sol–gel ceramic coatings to obtain corrosion protective silica coatings, and at the same time to retain the mechanical properties of the substrates, at a difference from in-oven heat treatments.
Authors wish to thank to MEC under projects MAT2003-04931-C02-02 and CAM (S-0505/MAT/0077).
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