CH709962A2 - An electrochemical process for producing adherent metal coatings on aluminum surfaces. - Google Patents

Abstract

A method for depositing metals on a fabricated alumina layer on a surface of a workpiece while the oxide layer comprises a nanoporous oxide layer having a plurality of nanopores that are perpendicular or nearly perpendicular to the surface plane and a barrier layer at the bottom of the nanoporous oxide layer should be improved to achieve more stable growth conditions during electrochemical deposition and achieving a high fill factor. This is accomplished by defining an optimum fill potential before conducting electrochemical deposition in the potentiostatic mode of a desired metal in a bath comprising a potentiostatic reverse pulse deposition metal component comprising cycles of successive pulses of short cathodic pulse at the determined optimum cathodic fill voltage Pulse times (Δt1) of at least 8 ms and a subsequent short anodic pulse at an anodic pulse voltage with anodic pulse times (Δt2) higher or equal to 0 ms.

Description

TECHNICAL AREA

The present invention describes a method of depositing metals on an aluminum oxide layer formed on a surface of a workpiece while the oxide layer comprises a nanoporous oxide layer having a plurality of nanopores that are perpendicular or nearly perpendicular to the surface plane and a barrier layer at the bottom includes nanoporous oxide layer.

STATE OF THE ART

Since the nineteen-fifties there has been a well-known way of coating surfaces of aluminum articles and workpieces with metals from the previous deposition of a thin Zn layer; This is the principle of the so-called zincate process.

The main objective is the deposition of a well-adhering layer of nickel on an aluminum oxide surface as a universal basis for further electrochemical deposition steps. Aluminum has an oxide layer of a few nanometers, due to the unique corrosion properties of this material. The oxide layer prevents reliable plating on aluminum and in all current aluminum plating processes it is attempted to remove this layer prior to plating.

As in Zelly, W.G., FORMATION OF IMMERSION ZINC COATINGS ON ALUMINUM (Formation of dip zinc coatings on aluminum). Journal of the Electrochemical Society 1953, 100 (7), 328-333, this thin Zn layer provides protection against oxidation and thus prevents the formation of a native dielectric oxide layer which would otherwise preclude generation of intermetallic bonds between the electrochemically deposited and the substrate would. However, the reliability and reproducibility of this process are not optimal and repeated stresses on the resulting Zn layer coating and a later metal layer could result in its delamination or delamination failure. Therefore, alternative methods are currently being sought.

In this application, the term "aluminum" refers to aluminum and aluminum alloys. An improved method for depositing metals such as nickel on surfaces of workpieces having a nanoporous oxide layer is sought.

During the last decades, the anodization of aluminum has attracted much commercial and scientific interest because, under suitable conditions, it leads to the growth of a porous oxide layer with nanopores arranged.

As disclosed in US 4,431,489, it is known to form a porous alumina coating by starting from anodizing an aluminum workpiece, particularly the surfaces of the aluminum workpiece, in an aqueous sulfuric acid. In a second step, electrolysis is carried out in an acidic aqueous bath containing various soluble metal salts, for example, nickel sulfamate, resulting in coating of the produced porous alumina surface. The nickel deposition achieved with this method is not reproducible due to an unclear disclosure of the optimal deposition parameter.

Masuda and Fukuda, Masuda, H .; Fukuda, K., ORDERED METAL NANOHOLE ARRAYS MADE BY A 2-STEP REPLICATION OF HONEYCOMB STRUCTURES OF ANODIC ALUMINA (Ordered Metal Nanochonochrome Assemblies Prepared by a 2-Step Modeling of Anodic Alumina Honeycombs), Science 1995, 268 (5216) have developed a two-step anodization process that allows to obtain a high order of nanopores whose axis is perpendicular to the substrate. The pores are organized in a hexagonal tightly packed cell, resulting in a very high pore density. Further, the pore and cell dimensions can be adjusted by the anodization conditions and it has been shown that nanopores with a diameter of less than 10 nm can be obtained in sulfuric acid. This oxide layer consists of two different parts: the porous oxide and the barrier layer. The latter makes electrochemical deposition within the nanoporous oxide layer difficult.

EP 0 368 470 discloses a method for depositing a metal layer on an anodized aluminum surface. After anodizing the aluminum substrate (ie, forming a porous anodized layer having pores therein), an AC or modified AC (AC and superimposed DC) deposition of a pore filling metal into the pores to fill the pores with the metal up to Surface of the anodized layer, whereby a continuous support layer on the surface of the anodized layer of a thickness in the micrometer range before depositing at least one coating of a final metal on the support layer performed. By superimposing a DC deposition current during AC voltage deposition, breakage of the barrier layer is avoided. This method results in insufficient pore filling and a slow rate of deposition.

The known anodization methods are widely used in the industry, for example, for aluminum dyeing, but have limitations in reproducibility and are highly dependent on the alloy composition.

It has been proven that some metals (e.g., nickel) can be deposited directly on the barrier oxide layer present at the bottom of the nanopores on a surface of aluminum and aluminum alloy articles. However, it requires a high potential for the electrons to tunnel through the barrier oxide layer, and it is therefore recommended to thinner this dielectric layer beforehand.

The electrochemical deposition of various metals within the nanopores is by various methods, for example, reverse pulse galvanostatic settling, Nielsch, K., Muller, F., Li, AP, Gosele, U., Uniform nickel deposition into ordered alumina pores by pulsed electrodeposition ( Uniform nickel deposition into ordered alumina pores by pulsed electrochemical deposition). Advanced Materials 2000, 12 (8). So far, the cathodic pulses have always been applied in galvanostatic mode. Thus, this is very sensitive to modifications of the active surface area that occur during the various stages of nanopore filling.

Because the active surface area is formed during the filling of nanopores and the growth of the cover metal layer, the current-regulated deposition is not suitable when formation of a nickel capping layer is required. What is needed is a process that results in the metal filling of nanopores with a high filling ratio and builds up a covering metal layer of adjustable thickness.

DESCRIPTION OF THE INVENTION

The object of the present invention is to provide a reliable electrochemical deposition process for growing metals on an anodized aluminum surface of a workpiece by achieving more stable growth conditions during electrochemical deposition, achieving a high fill factor, avoiding the removal of an oxide layer, and / or previous deposition to create a previous Zn layer before further metal deposition.

In order to solve the problem of reliable deposition before an electrochemical deposition step, the optimum filling potential is determined in a determining step. After the fill potential of the nanoporous aluminum surface has been determined with respect to the barrier layer thickness, the electrochemical deposition is performed in the form of potentiostatic reverse pulse deposition comprising at least a plurality of short cathodic pulse cycles followed by a short anodic pulse at the defined optimum fill potential , It has been seen that pulse parameters of potentiostatic reverse pulse deposition have an impact on fill ratio and metal coating morphology. The optimal filling potential should be determined by analytical methods (e.g., voltammetry) since it can vary widely depending on the alloy composition, barrier layer thickness, and electrolytic conditions.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred exemplary embodiment of the subject invention is described below in conjunction with the accompanying drawings. <Tb> FIG. FIG. 1 shows a schematic sectional view of an aluminum workpiece during metal filling of the porous oxide layer, while FIG <Tb> FIG. Figure 2 shows <SEP> current passage curves recorded during nickel filling of a porous anodic aluminum oxide layer by potentiostatic reverse pulse deposition. <Tb> FIG. 3 <SEP> show cyclic voltammograms of a thinned out Watts bath recorded at 1 V / s for the AAO patrix <tb> <SEP> a) a sample with an applied thinning potential of 10 V and b) various samples prepared with thinning potentials between 2.5V and 10V showing different cathodic peak potentials, and c) for samples made with the same thinning potential measured in sulfamate bath and Watts baths, resulting in different cathodic peak potentials. <Tb> FIG. FIG. 4 shows a schematic sectional view of an aluminum workpiece during a manufacturing process including multi-step anodization. FIG <Tb> FIG. 5 shows the potential and current passage curve recorded during the second anodization step, a subsequent barrier layer thinning step, and a barrier layer homogenization step. <Tb> FIG. 6a) to 6c) show SEM micrographs of the manufacturing steps during which the anodized aluminum surface of a pure aluminum sample has been formed <Tb> FIG. Figure 6d shows a SEM micrograph of a nickel plating grown by potentiostatic reverse pulse deposition on a prepared anodized aluminum surface of a pure aluminum sample.

DESCRIPTION

In the following, a coating method for metal is presented on the surface of a workpiece 0 comprising aluminum or aluminum alloys. On the surface of the workpiece 0, an aluminum oxide layer 1 comprising a nanoporous oxide layer 10 having a multiplicity of nanopores and a barrier layer 11 are formed on the nanopore bottom in a preceding manufacturing process. The electrochemical deposition of nickel is carried out on the oxide layer 1, also called anodic alumina (AAO) patrix.

Electrochemical deposition

Electrochemical deposition of a metal, preferably nickel, is carried out in the potentiostatic mode in an electrolytic cell comprising electrodes in a Watts bath or a Sulfamatbad. Potentials can be applied by the electrodes and measuring determinations could be carried out.

In addition to nickel, other metals such as Cu, Fe, Co, Ag, Au may also be deposited while adjusting the use of electrolytes and taking account of the chemical and physical stability of alumina in the choice of electrochemical conditions.

For the experiments described here nickel electrolytes were prepared using boric acid and nickel sulfate and nickel chloride for the Watts- or nickel sulfamate for the sulfamate. Sodium dodecyl sulfate (SDS) and saccharin were added as additives to the nickel sulfamate electrolyte, if mentioned. This electrochemical deposition is shown schematically in FIG.

The nickel concentration used has been divided by a factor of 10 as compared with conventional watt and sulfamate baths to reduce the growth rate of nickel nanowires; This restricts joule heating and the ohmic decrease of the barrier layer. The best results were achieved with Cl-free nickel sulfamate baths with additives.

The potentiostatic mode was preferred for surface area development because of its high selectivity and stability. The optimum deposition potential can vary widely depending on the alloy composition, the bath conditions and the barrier layer thickness; therefore, it must be determined prior to performing electrochemical deposition by electroanalytical techniques such as voltammetry.

[0024] Potentiostatic reverse pulse deposition

During electrochemical deposition in an electrolytic cell, the electrolytes used were gently stirred and thereby maintained at temperatures below or equal to 45 ° C to completely eliminate alumina hydration.

After immersing the manufactured workpiece 0 in the Watts bath or sulfamate bath, the electrochemical deposition is then carried out by potentiostatic reverse pulse deposition, which consists of cycles of successive pulses: a short cathodic pulse at the determined optimum filling voltage for the cathodic pulse time Δt1 of at least 8 ms, and preferably less than 100 ms and a subsequent short anodic pulse at an anodic pulse voltage having an anodic pulse time Δt2 of less than Δt1 and preferably less than 20 ms.

An optimal pause with zero current on the previous two pulses in the open cell, which allows the relaxation of the diffusion layer, can be performed for a pause time Δt3 of more than zero seconds to 10 seconds. The addition of the pause is not mandatory, but results in a higher filling ratio of the porous volume through the electrochemically deposited material.

The electrochemical deposition must be performed in cycles until three transitions in the current transfer curve are observed. In Fig. 2, these three transitions are shown, namely the nucleation and growth of nanowires within nanopores a), filling the nanopores b) and the formation of a smooth homogeneous cover layer c).

The entire electrochemical deposition in the form of cycles of potentiostatic reverse pulse deposition can be performed in a few minutes to several hours with respect to the selected pause time .DELTA.t3.

The duration of a cycle is the sum of the cathodic pulse time .DELTA.t1, anodic pulse time .DELTA.t2 and each of the pause time .DELTA.t3. The duty cycle is the cathodic pulse time Δt1 divided by the duration of a cycle. Measurement determinations have shown that the filling ratio increases with low duty cycle values. The highest filling ratios could be achieved with cathodic pulse times Δt1 of 20 ms and low duty cycles of 2%, as can be seen in Table 1. <tb> 7.5 <SEP> -11 <SEP> 8 <SEP> 80 <SEP> 40 ± 10 <tb> 7.5 <SEP> -11 <SEP> 20 <SEP> 20 <SEP> 55 ± 20 <tb> 7.5 <SEP> -11 <SEP> 20 <SEP> 8 <SEP> 76 ± 17 <tb> 7.5 <SEP> -11 <SEP> 20 <SEP> 4 <SEP> 89 ± 20 <tb> 7.5 <SEP> -11 <SEP> 20 <SEP> 2 <SEP> 95 ± 17 <tb> 7.5 <SEP> -11 <SEP> 40 <SEP> 4 <SEP> 83 ± 16 <tb> 7.5 <SEP> -11 <SEP> 8 <SEP> 4 <SEP> 70 + 14

Table 1: Influence of pulse parameters on the filling ratio estimated by GD-OES

The potentiostatic reversed pulse deposition was carried out in an aqueous electrolyte containing at least one metal salt, preferably nickel salt, in a concentration in the range of 10 <-> <3> to 2 mol / 1, preferably 10 <-2> 1 mol / 1 contained.

Determination of the optimum filling voltage by voltammetry study

Before performing the electrochemical deposition described above, the optimum fill voltage may be determined by an electroanalytical method, preferably a voltammetry or cyclic voltammetry study. Regardless of the origin of the workpiece 0, whether it is made by itself or not, the optimum filling voltage can be determined after positioning the workpiece 0 in the electrolytic cell, for example, by determining cyclic voltammograms. In the measurement determinations, the electrode potential was increased linearly against time at a scan rate of 1 V / sec between two fixed potentials of +/- 20 volts.

An example of a resulting cyclic voltammogram of a diluted wetting bath for an alumina layer 1 recorded at a scan rate of 1 V / sec is shown in Fig. 3a). The cathodic peak is attributed to nickel electrochemical deposition and the cathodic peak potential remains at about -12.5V for each cycle.

This broad peak has a shoulder at about -10V, which occurs only during the first scan. It can be explained by the modification of the electrode surface, since the nickel, which has been electrically deposited on the barrier layer, is not completely dissolved during the reverse scan. This shoulder can thus be associated with a cathodic side reaction that occurs only at the alumina / electrolyte interface. A voltammetric scan was then performed in 0.73M H3B03, but no significant faradic response was observed at the corresponding potential, which implies that nickel salt is involved in the reaction.

Depending on the thinning potential with which the aluminum oxide layer 1 has been modified in the barrier layer thinning step II, the cathodic peak potential varies. This is shown in Fig. 3a) for samples with thinning potentials between 2.5V and 10V.

Figure 3c) shows the results of cyclic voltammetry studies of oxide layers 1 after thinning with a 5V thinning potential after immersion in a sulfamate bath and a Watts bath. Although the barrier layer thinning step II was identical, the cathodic peak potential of the sulfamate bath is slightly less negative than that of the Watts bath. In either case, the particular cathodic peak potential will be used as the optimal fill potential in potentiostatic reverse pulse deposition.

Table 2 shows the cathodic peak potentials of samples in the Watts bath and sulfamate bath as compared to various thinning potentials used in preparing the samples. <tb> 10 <SEP> -12, 5 <SEP> -10, 5 <tb> 7.5 <SEP> -10, 8 <SEP> -9.5 <Tb> 5 <September> -8.5 <September> -7.8 <tb> 2, 5 <SEP> -10 <SEP> -8,3

Table 2: List of peak potentials determined for the watt and sulfamate baths at various barrier layer thicknesses.

The cathodic peak potential measured before the actual potentiostatic reverse pulse deposition corresponds to the reduction of Ni <2 +> / Ni <0>, and therefore, this cathodic peak potential is later selected as the optimum filling potential for the electrochemical deposition.

Determining optimum fill voltage during the barrier layer thinning step

The optimum filling may be determined during the barrier layer thinning step as described later. The thicker the barrier layer, the more energy is required to tune electrons, resulting in a more negative potential value. However, it also depends on the Al composition and the electrolyte conditions. The most recently used thinning potential resulted in the optimum fill potential to be used in the later electrochemical deposition step.

It is possible to use samples made by third parties. For each sample, it is necessary to determine the approximate potential for the electrochemical deposition prior to the deposition step.

Another way of determining the optimal fill potential prior to potentiostatic reverse pulse deposition may be achieved by knowing the last potential applied during the barrier layer thinning step after formation of the oxide layer 1.

When the manufacture, and more specifically the barrier layer thinning step, has been performed by the user, the last applied thinning potential is known.

If the sample was purchased, the thinning potential can be obtained from the manufacturer.

Possible production

In a manufacturing process, formation of a porous anodic alumina layer 1 comprising ordered nanopores is performed by multi-step anodization of the surface of the workpiece 0.

A first anodization step I is carried out in a first aqueous solution at temperatures between -5 ° C and 30 ° C for several hours, preferably for 4 to 8 hours, at voltages between 10 and 400 volts DC. The first aqueous solution contains an acid including, but not limited to, the following: sulfuric acid, oxalic acid, phosphoric acid, tartaric acid. This first anodization step I leads to an adhesive porous oxide layer 1.

After the first anodizing step I, the porous oxide layer 1 is then selectively exposed in a second aqueous solution, which may be a mixture of mineral acid such as phosphoric acid, sulfuric acid and chromic acid, at temperatures equal to or higher than room temperature for at least 5 minutes solved.

This exposure step I leads to the exposure of a prestructured aluminum surface, which consists of nanoholes, which are organized as a honeycomb structure. The first and second aqueous solutions may be different.

After the exposure step I and after the aluminum surface has been prestructured, the workpiece 0 is treated by a second anodizing step I. Therefore, the workpiece 0 is introduced into a third aqueous solution which, unlike or for practical reasons, may be identical to the first aqueous solution.

In the acid bath of the third aqueous solution, a second anodization step I is carried out for at least 1 minute at temperatures between -5 ° C and 30 ° C using a DC voltage between 10 and 400 volts.

Most preferably, for practical reasons, the third aqueous solution and the first aqueous solution are identical and the anodization temperature, time and / or voltage of the first anodization step I and the second anodization step I are also identical.

After the second anodization step I, an ordered porous oxide layer having a plurality of nanopores is achieved.

This porous oxide layer grows on a barrier oxide layer formed during the first few seconds of anodization and its thickness can be correlated with the electrochemical conditions and more specifically the anodization potential. Thus, by gradually reducing the applied cell voltage, this dielectric layer can be thinned once the porous layer has reached the desired thickness.

Upon the second anodization step I at a constant DC voltage, the barrier layer thinning step II and the barrier layer homogenization step III are performed as shown in FIG.

The barrier layer thinning step II is performed by reducing the applied voltage from the output anodization voltage V to the last applied thinning voltage. The thinning potential is lower than the anodization potential and preferably equal to or lower than 10 V. The potential reduction may be performed continuously or stepwise with a linear or gradual reduction, and preferably with a stepwise exponential reduction.

Thinning the barrier layer allows it to reduce its impedance and therefore facilitates the tunneling of the electrons involved in electrochemical metal deposition.

After the stepwise reduction of the anodization voltage, the barrier layer homogenization step III is performed by a constant DC voltage below the anodization voltage of the anodization steps I and I at the thinning potential.

Embodiment of a manufacturing method

In a first experimental set-up, the first anodization step I was carried out in 0.3 M sulfuric acid at 3 ° C. for 8 hours at 25 V. The result is shown in Fig. 6a).

After the first anodizing step I, the workpiece 0 was in the exposure step I, during which the anodic oxide layer is selectively dissolved in a mixture of 0.4 M H3PO4 and 0.2 M H2CrO4 at 60 ° C for one hour. Fig. 6b) shows the surface after the exposure step I.

The second anodization step I was then carried out by anodizing in an equilibrium state at 25 V under exactly the same conditions as in the first anodizing step I but for only 30 minutes, resulting in the ordered porous layer shown in Fig. 6c) is before the barrier layer thinning step II follows for 20 minutes. The applied voltage is exponentially reduced by 60 steps of 20 seconds until it reaches the selected potential value corresponding to the targeted barrier layer thickness. During this stage, the average current density decreases exponentially. At each potential step, the current density starts at a minimum value and increases logarithmically due to the formation of new cracks in the barrier layer, ultimately inducing the growth of new generations of smaller diameter nanopores.

Finally, the barrier layer homogenization step III was performed with the selected thinning potential applied for 600 seconds to homogenize the barrier layer thickness. Good results were achieved by choosing a thinning potential of 8 volts. Thinning potentials between 5 V and 10 V were investigated and led to sufficient results.

Embodiment of the electrochemical deposition method

Good results were achieved by the electrochemical deposition performed by potentiostatic reverse pulse deposition in a nickel sulfamate bath containing 0.7 mM sodium dodecyl sulfate (SDS) and 10.9 mM saccharin at 45 ° C.

As shown in Fig. 6d), it was demonstrated that a very flat and smooth opaque nickel coating could be achieved by performing electrochemical deposition in a thinned sulfamate bath containing SDS and saccharin as an additive.

Experiments:

All electrochemical experiments were performed in a dual electrode electrochemical cell using a platinum counter electrode and an Autolab 302N potentiostat / galvanostat equipped with a voltage multiplier. The electrolyte temperatures were accurately controlled by a Julibo / cooled Julipo circulator F12-ED and checked with a glass thermometer before each experiment.

High-resolution SEM images were obtained with a Hitachi S-4800 FE-SEM.

After electrochemical deposition, further metal finishing may be added, but is not necessary.

The appropriate stress for optimum filling of the pores and the high quality of the topcoat was determined by a cyclic voltammetry study for the first time for pure aluminum and the aluminum alloy EN AW-5005 H24. The filling ratio of the nanopores could be increased by optimizing the manufacturing process and the electrochemical deposition step.

Claims (13)

1. A method for depositing metals on an aluminum oxide layer (1) produced on a surface of a workpiece (0), while the oxide layer (1) comprises a nanoporous oxide layer (10) having a plurality of nanopores that are perpendicular or almost perpendicular to the surface plane. and a barrier layer (11) at the bottom of the nanoporous oxide layer (10), characterized in that an optimum filling potential of the aluminum oxide layer (1) with respect to the aluminum composition and the barrier layer (11) thickness at the bottom of the nanoporous oxide layer (10) is determined either by a previous manufacturing process in at least one performed barrier layer thinning step (11) or by a voltammetry study, before conducting electrochemical deposition in the potentiostatic mode of a desired metal in an electrolytic cell with an aqueous medium comprising at least one metal salt in the form of potentiostatic reverse pulse deposition, comprising cycles of successive pulses of a short cathodic pulse at the predetermined optimum fill voltage for a cathodic pulse time (Δt1) of at least 8 ms and a subsequent short anodic pulse at an anodic pulse voltage with anodic pulse time (Δt2) equal to or greater than 0 ms, carried out in repeated minute-long cycles resulting in a nucleation step a), the filling of nanopores b) and the formation of a smooth homogeneous cover layer c) of the metal component on the alumina layer (1) before the electrochemical deposition is terminated. A deposition method according to claim 1, wherein a zero-potential pause is applied to each anodic pulse for a pause time (Δt3) of up to 10 seconds before the next cycle of the next cathodic pulse is started. 3. A deposition method according to claim 1 or claim 2, wherein a duty cycle is a ratio between the cathodic pulse time Δt1 divided by the duration of a cycle, the sum of the cathodic pulse time (Δt1), the anodic pulse time (Δt2) with or without the pause time (Δt3). is set below 8%, preferably below 4%, and most preferably 2%. The deposition method of claim 1, wherein cyclic voltammetry has been performed to determine the optimum fill potential prior to electrochemical deposition with a sacrificial sample. A deposition method according to any one of the preceding claims, wherein the amount of voltage of the anodic pulse is equal to the amount of the optimum filling voltage of opposite polarity. A deposition process according to any one of the preceding claims, wherein the Watts bath or sulfamate bath in the electrolytic cell has temperatures below 45 ° C. A deposition method according to any one of the preceding claims, wherein the current density is measured at time intervals while the electrochemical deposition is performed, to evaluate the nucleation step a), fill nano-pores b), and form a smooth homogeneous cover layer c). The deposition method according to claim 1, wherein the alumina layer (1) has been previously prepared by a manufacturing method comprising the barrier layer thinning step (II) carried out by a voltage reduction from an initial anodization voltage to the most recently applied thinning voltage. The deposition method of claim 8, wherein the value of the last applied thinning voltage is between 2.5V and 10V. 10. Deposition method according to one of the preceding claims, wherein the potentiostatic reverse pulse deposition is carried out in an aqueous electrolyte containing at least one metal salt in a concentration in the range of 10 <-> <3> to 2 mol / 1, preferably 10 <->. 2> to 1 mol / 1 contains. The deposition method of claim 10, wherein admixtures have been added to the Watts bath or sulfamate bath. The deposition process of claim 11, wherein a chloride-free sulfamate bath is used. The deposition method according to claim 11 or 12, wherein sodium dodecyl sulfate (SDS) and / or saccharin have been the chosen additives.