WO1996013625A1

WO1996013625A1 - Surface coated aluminium material and a method for preparing it

Info

Publication number: WO1996013625A1
Application number: PCT/DK1995/000421
Authority: WO
Inventors: Jean Rasmussen; Per MØLLER
Original assignee: Danfoss A/S
Priority date: 1994-10-28
Filing date: 1995-10-25
Publication date: 1996-05-09

Abstract

The invention relates to a surface coated aluminium material comprising a base aluminium or aluminium alloy material having at least one surface part thereof and a first layer consisting of oxides of the base material and a second layer consisting essentially of indium, tin and/or gallium, and a method for preparing the material. The object of the invention is to provide an aluminium material having good wear resistance and low friction during the lifetime of the product made thereof.

Description

Surface coated aluminium material

and a method for preparing it

The present invention relates to a surface coated

aluminium material and a method for preparing it. The aluminium material is particularly useful for use in automobiles, aircraft appliances and buildings, which material has very good properties with respect to

corrosion resistance, wear resistance and low friction.

Recently aluminium materials being made of either pure aluminium or an aluminium alloy have been used

increasingly in various applications due to their light weight, high mechanical strength and good corrosion resistance. For example, aluminium sheets are used in automobile bodies for the purpose of saving weight and thereby reducing fuel consumption. In many situations the aluminium material is subjected to friction and wear, particularly when used in mechanical or movable units such as pistons, cylinders and hydraulic gears . Many attempts have been made to improve the properties of aluminium materials by surface treatment plating with nickel/PTFE (polytetrafluoroethylene or other fluoropolymers, aluminium oxide and cupper compounds. Attempts to improve the frictional property of the

aluminium material have been made by impregnating with PTFE, SIC or graphite.

US patent No. 5 277 788 discloses a method of protecting aluminium surfaces with a triple layer, i.a. a double oxide layer, an organo phosphorous layer.

This method is particularly useful for articles such as wings of aircrafts where water repel1ant properties are needed. However, there is still a need for aluminium materials having improved properties, particularly with respect to wear resistance, but also with respect to corrosion resistance and low friction.

US patent No. 3 891 519 discloses a method for treatment of surfaces of aluminium or aluminium alloys to improve their frictional properties and resistance to wear by forming on their surface a CU/In layer by diffusion.

In the described method the aluminium substrate is first subjected to an anodizing and thereafter plated first with indium, then with copper, and finally heated (temperature diffusion). Between each step a double rinse in water increases the necessary operational steps to 10, viz. 8 steps related to plating (2 steps) and rinsing (3 times a double rinse) and 2 steps related to first a heat

treatment and second a cooling period. However, this method is very time-consuming, expensive and includes many operational steps. Hence, it is not

economically profitable in most applications for

industrial large scale production. The first object of the present invention is to provide an aluminium material having good wear resistance and which can be produced in an economically beneficial way, also on a large scale basis. A second object of the present invention is to provide an aluminium material having improved wear resistance and low friction and which material keeps having good wear

resistance and low friction during the life time of the product made thereof. A third object of the present invention is to provide an aluminium material having improved wear and good corrosion resistance. These and other objects can be accomplished by a surface coated aluminium material as claimed in claim 1.

The aluminium alloy can be either "cast alloys" which are cast directly into their desired form by one of three methods (i.e. sandcasting, gravity die casting or pressure die casting) or the "wrought alloys" which are cast into ingots and mechanically hot or cold worked into extrusion, sheets, foils tubes or wires. The alloy preferably comprises at least 75 weight-% aluminium and most preferably at least 85 weight-%

aluminium depending on the solubility of the alloying constituent. The alloying constituents may be selected among copper

(Cu), magnesium (Mg), silicon (Si), manganese (Mn), nickel (Ni) and zinc (Zn), but the alloy may also comprise small amounts of other alloying elements such as iron,

molybdenum, chromium, zinc, gallium, vanadium, titanium, boron, lithium and zirconium. Most preferably the alloy belongs to one of the standardized aluminium alloy series such as the 2000 series (Al-Cu alloys), the 3000 series (Al-Mn alloys), the 5000 series (Al-Mn-Si alloys) and the 7000 series (Al-Zn-Mg alloys).

The oxide layer normally consists essentially of Al₂O₃, and preferably at least 85% by weight of the surface oxide layer is aluminium oxide. The oxide layer is made by an anodization process which will be described in detail below. The total oxide layer preferably has a thickness of between 1 μm to 500 μm. Most preferably the thickness is between 5-100 μm. The oxide layer consists of unit cells, each having a pore extending from the surface of the oxide layer down towards the aluminium substrate. At the bottom of the pores a thin part of the oxide layer - the barrier layer - covers the aluminium substrate, shielding it from exposure to e.g. aggressive environments. The corrosion resistance of anodized aluminium is highly correlated with the thickness of the barrier layer, which, depending on the processing conditions, are in the magnitude of 100 to 300 Angstrøm (10-30 nm).

The oxide layer provides the aluminium material with a hard surface. The thickness of the oxide layer has

influence on the hardness of the surface. The second layer of metallic indium, tin and/or gallium is deposited into the open pores of the oxide layer, and functions as a metallic plug which physically blocks the corrosion mechanism of the aluminium. The aluminium is protected by the barrier layer and the metallic layer, the last typically being in the order from 30-52 μm (300000 to 500000 Angstrøm).

In order to obtain optimum wear and corrosion resistance and low friction, at least 80% and preferably all the pores of the oxides should be filled with the second metallic layer. Therefore, the weight of the second layer is correlated with the porosity and the pore volume, which depends on the anodizing conditions. The second coating layer may consist of pure indium, pure tin or pure gallium, but it may also consist of a combination of these metals. The second coating layer may also comprise small impurities of other metallic

compounds. Preferably, the impurities constitute less than 1% by weight, more preferably less than 0.1% by weight.

Indium is a very soft, silvery-white metal. It has found application in the making of low melting alloys and is also used in the making of bearing alloys. It wets glass and aluminium. It can be plated onto metals and evaporated onto glass, forming a mirror as good as that made with silver.

Gallium is also a soft metal which also wets on glass. It has found application in the making of low melting alloys.

Ordinary tin is a soft silvery-white metal which is malleable, somewhat ductile, and has a highly crystalline structure. It is particularly preferred that the second layer is pure or essentially pure indium.

The present invention also comprises a method of preparing the surface coated aluminium material.

The base aluminium or aluminium alloy material is first subjected to an anodizing treatment at least of a surface thereof, viz. the surface that is to be coated. In the following it should be understood that the surface of the base material means the surface that is subjected to be coated. The anodizing pretreatment can be carried out by any known method, e.g. by anodizing in phosphorous acid, chromic acid, sulphuric acid, oxalic acid or mixtures thereof . Prior to anodizing the surface of the base material is preferably decreased e.g. by vapour decreasing and/or by being subjected to an alkaline cleaner. This must be followed by a pretreatment, typically using alkaline etching and acid desmutting, ensuring a surface clean of impurities and activated for the following electrochemically conversion of aluminium into aluminium oxide. The surface of the aluminium article is then subjected to an anodizing treatment in an electrolyte containing an acid as mentioned above. The acid is

preferably sulphuric acid.

The properties and structure of the oxide layer are very dependent on the anodizing process.

The wear resistance and the hardness are reduced when the processing time is increased in order to obtain thicker oxide coatings in the conventional anodizing processes using direct current. When the formation of aluminium oxide is made using pulse-modulated current, this relation between properties of the oxide coating and the processing time cannot be detected. In order to obtain thick, hard and dense oxide layers, the industrial conversion of aluminium into aluminium oxide is normally carried out at low process temperature (-5 °C to +5 °C) and at high current densities (2-3 A/dm²). Some methods of anodizing, however, require other temperatures and current densities.

It is particularly preferred that the anodizing is a pulse modulated current process carried out in a 5-15% by weight sulphuric acid at a temperature between 0 and 50 ºC and a processing time between 5 and 100 minutes and the pulse modulated currents being as stated in the following steps a ) and b): a) 40-100 seconds at 3-8 A/dm² followed by b) 10-50 seconds at 0.3-5 A/dm², wherein the current density of step a) is higher than the current density of step b). Optionally, the material is treated in the electrolyte with a current density of 0.3-5 A/dm² for 1-5 minutes prior to the pulse modulated current process.

Another very preferred method of anodizing is a pulse modulated process carried out in an electrolyte comprising U-acid (U-acid is a sulphonated dicarboxylic acid) and preferably also H₃SO₄.

The electrolyte comprises at least 1% v/v U-acid and preferably between 5 and 13% v/v U-acid. High

concentration of U-acid results in high microhardness of the formed aluminium oxide. When the electrolyte also comprises H₂SO₄ (preferably 0.1-3.0% v/v H₂SO₄), the microhardness is further increased.

The anodizing in this U-acid containing electrolyte is preferably carried out at a temperature above 5 °C, as lower temperature prevents the formation of thick and dense oxide coating. On the other hand, at high

temperature (above 30 ºC) the formation rate decreases. A preferred temperature is 14-20 °C.

The anodizing is also preferably carried out without any substantial agitation of the electrolyte, as such

agitation may result in local formation of amorphous aluminium hydroxide and local destruction of the surface. The base current (i₁) is preferably between 0.3 and 5 A/dm², most preferably between 1 and 3 A/dm².

The pulsed current (i₂) is preferably between 3 and 9 A/dm², most preferably between 5 and 7 A/dm².

The time (t₁) with the base current is 5-50 seconds, preferably 5-20 seconds, and the time with the pulsed current is 30-200 seconds, preferably 50-100 seconds.

It has been found that both the initial wear and the post-wear resistance are increased when using a U-acid

comprising electrolyte as described above in the anodizing process.

Further information about the anodizing process may be found in J.Cl. Puippe & F. Leaman (Ed.), Theory and practice of pulse plating, AESF, Orlando, 1986. The next step is the application of the second coating layer. In the following it is referred to as the indium layer, but this is only a preferred embodiment. Tin and gallium and combinations may be used in the same manner. The indium can be applied or coated in a number of ways, e.g. by coating methods such as electroplating, flame spraying, plasma spraying, rapid solidification method, CVD, PVD, ion plating, or ion splattering. Electrolytic plating is preferred and is conveniently made in an indium sulphamate electrolyte, established as a second surface treatment process following immediately after the

anodizing process in a plating shop.

In an electroplating process the indium ions present in the solution deposit as metallic indium at the cathode. Each time an ion converts into metallic indium three electrons are consumed. These electrons may be supplied externally by a rectifier. The anode is in this respect made of inert material such as stainless steel panels. The process of depositing metallic indium is not taking place with 100% current efficiency. A part of the total current consumed in the deposition process is used for gas

evolution at the cathode.

The electrodeposition of indium can also be made in other commercial available electrolytes.

The plating conditions can be made galvanostatic or potentiostatic using direct current (potential) or pulse modulated current (potential). Current control is

preferred because the deposition rate of indium is kept constant during the process.

The electrodeposition is preferably made at a current density of between 0.1 and 2.0 A/dm² and most preferably about 1 A/dm² (the dm² corresponds to the actual surface area at the bottom of the pores), taking into account that the apparent surface (all of the pore surface) is several times larger than the actual surface for deposition.

Depending on the anodizing conditions, the porosity of anodized aluminium in sulphuric acid is typically from 10 to 20%. Electrodeposition of indium may in this situation be made at a current of 0.01 - 0.2 amps per square

decimeter apparent surface. Electrodeposition at higher or lower current densities may result in an uneven distribution of indium, especially at corners and other places having defects in the oxide coating. This is so, because of gas evolution relating to a low current efficiency (at high current densities) and differences in the secondary current distribution because of differences in the resistance polarization at the interface electrolyte/substrate (low current density). An uneven distribution of indium may have a deteriorating effect over the wear resistance and low friction along with the corrosion resistance.

Electrodeposition of indium is preferably continued until the voltage drops because of changed deposition

conditions, indicating that the pores are filled with indium. Continued electrodeposition results in the

formation of an indium coating at the surface of the oxide layer.

An indium treatment of anodized aluminium involves 5 operational steps after the anodizing process, viz. 2 rinsing steps in water, electrodeposition in an indium electrolyte and finally two rinsing steps after plating.

The following examples are presented as a specific

illustration of the claimed invention. It should be understood, however, that the invention is not limited to the specific details set forth in the examples.

EXAMPLE 1 4 Samples of a plate of aluminium alloy A10, 5 MgSi (AA 6063 ) were surface treated by an anodizing treatment carried out in 15% w/w sulphuric acid at a temperature of 14 °C. All test pieces were processed using galvanostatic, square waved pulses. The adjustment of the pulse

parameters (if not otherwise specified) are shown in Table 1.

Prior to anodizing the aluminium was decreased and etched for 1 minute in a sodium hydroxide solution (40 g/1; 60 ºC), followed by 15 seconds immersion in a 50% v/v nitric acid solution.

The electron deposition of indium was carried out in a sulphamate electrolyte at room temperature and carried out at an apparent current density of 1 A/dm² with vigorous gas evolution. The actual surface area is determined by the porosity of the oxide coating. Assuming a porosity of 10 per cent, the actual current density is increased 10 fold.

Electrodeposition of indium was terminated at 100 Coulomb.

EXAMPLE 2

2 Samples of a plate of aluminium alloy as in Example 1 were surface treated as in Example 1 except that the processing time was 50 minutes instead of 20 minutes.

The oxide coating obtained had a thickness of 50 μm and a microhardness of 500 Vickers. Wear Test

The samples 1, 2, 3 and 4 of Example 1 and 5 and 6 of Example 2 were wear tested.

The wear test in this survey is an abrasive one. The method used is a reciprocating test in accordance with the international standard ISO 8251. This is a wear test specially designed for abrasive wear measurements on anodized aluminium.

The surface of the test piece reciprocates on a wheel which is covered by an abrasive strip. The abrasive wheel turns 1/400 full rotations for each reciprocal movement of the sample. It is pressed against the test surface by a spring load of 4.9 N (500 g). The test sample makes 40 reciprocating movements (cycles) per minute, giving a testing time of 10 minutes per 400 wear cycles.

Thereafter, the abrasive tape (silicon carbide, mesh 320) is changed. The wear is presented as the number of cycles needed to remove 1 μm of the surface. A high value

corresponds to a high wear resistance.

The thickness before and after the abrasive test is measured with an eddy current meter, which is cablibrated between 10 to 45 μm. The thickness is determined on the basis of three to four measurements (n=40). The precision of the wear resistance, calculated as +/-2σ, is better than 95 per cent (Table 2).

Until wear tested, the anodized samples are stored in a desiccator. All tests are carried out at 58 per cent relative humidity and a temperature of 20 ºC.

EXAMPLE 3

24 samples (Experiments 1-33) of AlSi7M_g were anodized in U-acid containing electrolytes having varying

concentrations of the U-acid and the sulphuric acid, the processing times, currents and temperatures.

The end voltage, the thickness of the formed oxide layer, the formation rate and the hardness were

measured/calculated.

The results are shown in Tables 9-12.

Claims

P a t e n t C l a i m s - - - - - - - - - - - - - - - - - - - - - - - - - -

1. A surface coated aluminium material comprising a base aluminium or aluminium alloy material having at least one surface part thereof and a first layer consisting of oxides of the base material and a second layer consisting essentially of indium, tin and/or gallium.

2. A surface coated aluminium material according to claim 1, CHARACTERIZED in that the base material is an

aluminium alloy comprising at least 75% by weight

aluminium and up to 25% by weight of one or more of the constituents copper (Cu), magnesium (Mg), silicon (Si), manganese (Mn), nickel (Ni) and zinc (Zn).

3. A surface coated aluminium material according to claim 1 or 2, CHARACTERIZED in that the oxide layer is at least partly porous and has a total thickness between 1 and 500 μm, preferably between 5 and 100 μm.

4. A surface coated aluminium material according to any one of claims 1, 2 or 3, CHARACTERIZED in that the second layer comprises 95-100% by weight of indium and preferably 99-100% by weight.

5. A surface coated aluminium material according to any one of claims 1-4, CHARACTERIZED in that the second layer has an average thickness between 10 and 100 μm, preferably between 30 and 50 μm.

6. A method of preparing a surface coated aluminium material as claimed in claims 1-5, CHARACTERIZED in that it comprises the following steps: i) anodizing at least one surface part of a base

aluminium or aluminium alloy material, ii) application of indium, tin, gallium or a combination thereof.

7. A method according to claim 6, CHARACTERIZED in that the anodizing is a process using direct current at a current density between 0.5 and 10 A/dm² and preferably between 1.0 and 4.0 A/dm².

8. A method according to claim 7, CHARACTERIZED in that the anodizing is a pulse modulated current process carried out in a 5-15% by weight sulphuric acid at a temperature between 0 and 50 °C and a processing time between 5 and 100 minutes and the pulse modulated currents being as in the following steps a) and b): a) 40-100 seconds at 3-8 A/dm² followed by b) 10-50 seconds at 0.3-5 A/dm².

9. A method according to claim 8, CHARACTERIZED in that the aluminium material prior to the pulse modulated current is treated in the sulphuric acid with a current density between 0.3 and 5.0 A/dm² for 1-5 minutes.

10. A method according to claim 7, CHARACTERIZED in that the anodizing is a pulse modulated current process carried out in a U-acid comprising electrolyte, preferably having a U-acid concentration of at least 1% v/v.

11. A method according to claim 10, CHARACTERIZED in that the anodizing is carried out at a temperature between 5 and 30 ºC, preferably 14-20 °C, and that the electrolyte comprises H₂SO₄, preferably in a concentration of 0.1-3.0% v/v.

12. A method according to claim 10 or 11, CHARACTERIZED in that the pulse modulated step being as in the following step:

1) 5-50 seconds at a current density between 0.3 and 5 A/dm², 2) 30-200 seconds at a current density between 3 and 9 A/dm².

13. A method according to claims 6-12, CHARACTERIZED in that the application of indium, tin, gallium or a

combination thereof is made by mechanical application of indium, tin and/or gallium in the form of powder or melted metal.

14. A method according to claims 6-12, CHARACTERIZED in that the application of indium, tin, gallium or a

combination thereof is made by physical precipitation of indium, tin and/or gallium by laser induced plating or flame spraying.

15. A method according to claims 6-12, CHARACTERIZED in that the application of indium, tin, gallium or a

combination thereof is made by electrochemical

precipitation of indium, tin and/or gallium from aqueous or organic solutions.

16. A method according to claims 6-12, CHARACTERIZED in that the application of indium, tin, gallium or a

combination thereof is made by chemical precipitation of indium, tin and/or gallium from aqueous or organic

solutions or in gaseous form and by use of powdered, fluid and/or gaseous reduction compounds preferably selected between sodiumboronhydride, alkylamineborane and derivates thereof.

17. A method according to claim 15 or 16, CHARACTERIZED in that the anodizing and the chemical or electrochemical precipitation is carried out in the same aqueous or organic solution.