Abstract:
A method of producing oxide ceramic layers on Al, Mg, Ti, Ta, Zr, Nb, Hf, Sb, W, Mo, V, Bi or their alloys by a plasma-chemical anodical oxidation in a chloride-free electrolytic bath having a pH value of 2 to 8 and a constant bath temperature of -30° to +15° C. A current density of at least 1 A/dm 2  is maintained constant in the electrolytic bath until the voltage reaches a predetermined end value.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of producing oxide ceramic layers on barrier layer-forming metals or their alloys by plasma-chemical anodic oxidation in aqueous organic electrolytes, wherein the oxide ceramic layer may be further modified for specific applications. The present invention further relates to articles produced by the method. 
     2. Description of the Related Art 
     In aqueous electrolytes, the anodic oxidation described above is a gas/solid reaction under plasma conditions in which the high energy input at the base point of the discharge column produces liquid metal on the anode which forms with the activated oxygen a temporarily molten oxide. The layer formation is effected by anodes. The spark discharge is preceded by a forming process (P. Kurze; Dechema-Monographien Volume 121-VCH Verlagsgesellschaft 1990, pages 167-180 with additional literature references). The electrolytes are selected in such a way that their positive properties are combined and high-quality anodically produced oxide ceramic layers are formed on aluminum. By combining different salts, higher salt concentrations can be achieved in the electrolytic bath and, thus, higher viscosities can be achieved. Such high viscosity electrolytes have a high thermal capacity, they stabilize the oxygen film formed on the anode and, thus, they ensure a uniform oxide layer formation (DD-WP 142 360). 
     Because of the pattern of the current density/potential curves for the anodic spark discharge, three distinct portions can be distinguished. i.e. the Faraday portion, the spark discharge portion and the arc discharge portion, see P. Kurze mentioned above. 
     A barrier layer is naturally found on the metal or the metal alloy. By increasing the voltage of the anodically poled metal, the barrier layer increases. Consequently, a partial oxygen plasma which forms the oxide ceramic layer is created at the phase boundary metal/gas/electrolyte. The metal ion in the oxide ceramic layer is derived from the metal and the oxygen from the anodic reaction in the aqueous electrolyte. The oxide ceramic is liquid at the determined plasma temperatures of approximately 7,000° Kelvin. Toward the side of the metal, the time is sufficient for allowing the melted oxide ceramic to properly contract and, thus, form a sintered oxide ceramic layer which has few pores. Toward the side of the electrolyte, the melted oxide ceramic is quickly cooled by the electrolyte and the gases which are still flowing away, particularly oxygen and water vapor, leave an oxide ceramic layer having a wide-mesh linked capillary system. Pore diameters of 0.1 μm to 30 μm were determined by examinations using electron scan microscopes (Wirtz, G. P., et al., Materials and Manufacturing Processes, 1991, &#34;Ceramic Coatings by Anodic Spark Deposition,&#34;  6(1):87-115, particularly FIG. 12). 
     DE-A-2 902 162 describes a method in which spark discharge during the anodizing process is utilized for manufacturing porous layers on aluminum intended for use in chromatography. 
     EP-A-280 886 describes the use of the anodic oxidation with spark discharge on Al, Ti, Ta, Nb, Zr and their alloys for manufacturing decorative layers on these metals. 
     The above-described methods make it possible only to manufacture ceramic layers having relatively small thicknesses of up to a maximum of 30 μm which are insufficient for use as wear and corrosion protection layers. 
     SUMMARY OF THE INVENTION 
     Therefore, it is the object of the present invention to produce oxide ceramic layers on the above-mentioned metals which have a substantially greater layer thickness of up to 150 μm, are resistant to abrasion and corrosion and have a high alternating bending strength. 
     In accordance with the present invention, oxide ceramic layers are produced on aluminum, magnesium, titanium, tantalum, zirconium, niobium, hafnium, antimony, tungsten, molybdenum, vanadium, bismuth or their alloys by plasma-chemical anodic oxidation while maintaining the following parameters: 
     1. The electrolytic bath should be substantially free of chloride, which means that it contains less than 5×10 -3  mol/l chloride ions; 
     2. The electrolytic bath is adjusted to a pH value of 2 to 8; 
     3. The temperature of the bath is in the range of -30° to +15° C. and preferably between -10° and +15° C.; 
     4. The temperature of the bath is maintained constant within the limits of ±2° C.; and 
     5. The current density of at least 1 A/dm 2  is maintained constant until the voltage reaches an end value. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Within the scope of the present invention, aluminum and its alloys are very pure aluminum and, inter alia, the alloys AlMn; AlMnCu; AlMg1; AlMg1,5; E--AlMgSi; AlMgSi0,5; AlZnMgCu0,5; AlZnMgCu1,5: G--AlSi--12; G--AlSi5Mg; G--AlSi8Cu3; G--AlCu4Ti; G--AlCu4TiMg. 
     For the purposes of the invention, also suitable in addition to pure magnesium are the magnesium casting alloys with the ASTM designations AS41, AM60, AZ61, AZ63, AZ81, AZ91, AZ92, HK31, QE22, ZE41, ZH62, ZK51, ZK61 EZ33, HZ32 as well as the wrought alloys AZ31, AZ61, AZ 80, M1, ZK60, ZK40. 
     Moreover, pure titanium or also titanium alloys, such as, TiAl6V4; TiAl5Fe2,5, etc. can be used. 
     The chloride-free electrolytic bath may contain inorganic anions which are conventional in methods for the plasma-chemical anodic oxidation, namely, phosphate, borate, silicate, aluminate, fluoride or anions of organic acids, such as, citrate, oxalate and acetate. 
     The electrolytic bath preferably contains phosphate ions, borate ions and fluoride ions in combination and in an amount of at least 0.1 mol/l of each individual of these anions up to a total of 2 mol/l. 
     The cations of the electrolytic bath are selected in such a way that they form together with the respective anions salts which are as soluble as possible in order to facilitate high salt concentrations and viscosities. This is usually the case in alkali-ions, ammonium, alkaline earth ions and aluminum ions up to 1 mol/l. 
     In addition, the electrolytic bath contains urea, hexamethylenediamine, hexamethylenetetramine, glycol or glycerin in an amount of up to a total of 1.5 mol/l as stabilizer. 
     For producing particularly wear-resistent oxide ceramic layers on aluminum or its alloys by plasma-chemical anodic oxidation at a current density of at least 5 A/dm 2  which is maintained constant until the voltage reaches an end value, it is possible to utilize even very significantly diluted electrolytic baths of the above-described composition in which the concentration of the anions is only 0.01 to 0.1 mol/l. In these significantly diluted baths, the pH value is between 10 and 12, preferably 11. Because of the low conductivity of this electrolytic bath, the voltage end value may reach up to 2000 V. The energy input caused by the plasma-chemical reaction is accordingly very high. The oxide ceramic layer formed on the aluminum materials consists of corundum, as was shown by X-ray diffraction examinations. A hardness of the oxide ceramic layer of up to 2000 HV is obtained. These oxide ceramic layers can be particularly used where an extremely high abrasive wear protection is required. 
     The selection of the type of voltage and current, such as, direct current, alternating current, 3-phase current, impulse current and/or interlinked multiple-phase current with frequencies of up to 500 Hz has surprisingly no influence on the process of forming ceramic layers on the metals. 
     The current supply to the plasma-chemical anodizing process for forming the ceramic layer is carried out in such a way that the required current density of at least 1 A/dm 2  is maintained constant and that the voltage is applied until an end value is reached. The voltage end value is between 50 and 400 volts and is determined by the metal used, i.e. by the alloy components of the metal, by the composition of the electrolytic bath and by the control of the bath. 
     As mentioned above, the invention also relates to articles produced by the above-described method, wherein the articles are of barrier layer-forming metals or their alloys with plasma-chemically produced oxide ceramic layers having a thickness of 40 to 150 μm, preferably 50 to 120 μm. 
     The following examples describe the present invention in more detail without limiting the scope of the invention. 
     EXAMPLE 1 
     A test plate of AlMgSi1 having a surface area of 2 dm 2  is degreased and subsequently washed with distilled water. 
     The test plate treated in this manner is plasma-chemically anodically oxidized in an aqueous/organic chloride-free electrolytic bath having the following composition. 
     (a) Cations 
     0.13 mol/l sodium ions 
     0.28 mol/l ammonium ions 
     (b) Anions 
     0.214 mol/l phosphate 
     0.238 mol/l borate 
     0.314 mol/l fluoride 
     (c) Stabilizer and complex forms 
     0.6 mol/l hexamethylenetetramine 
     With a current density of 4 A/dm 2  and an electrolyte temperature of 12° C.±2° C. After a coating time of 60 minutes, the voltage end value of 250 volts is reached. 
     The test plate with ceramic layer is washed and dried. The thickness of the ceramic layer is 100 μm. The hardness of the ceramic layer is 750 (HV 0.015). 
     EXAMPLE 2 
     A dye cast housing of GD--AlSi12 having a surface area of 1 dm 2  is treated for one minute at room temperature in a pickle composed in equal halves of 40% HF and 65% HNO 3  and the housing is subsequently washed with distilled water. 
     The dye cast housing pickled in this manner is plasma-chemically anodically oxidized in the aqueous/organic chloride-free electrolytic bath of Example 1 at a current density of 8 A/dm 2  and an electrolyte temperature of 10° C.±2° C. After a coating time of 30 minutes, a voltage end value of 216 volts is registered. 
     The dye cast housing with ceramic layer is washed and dried. 
     The thickness of the ceramic layer is 40 μm. 
     EXAMPLE 3 
     A test plate of magnesium alloy of the type AZ 91 having a surface area of 1 dm 2  is pickled for 1 minute at room temperature in a 40% hydrofluoric acid. 
     The test plate treated in this manner is plasma-chemically anodically oxidized in an aqueous/organic chloride-free electrolytic bath of Example 1 at a current density of 4 A/dm 2  and an electrolyte temperature of 12° C.±2° C. 
     The voltage end value of 252 volts is reached after 17 minutes. 
     The ceramic layer has a thickness of 50 μm. 
     EXAMPLE 4 
     A rod of pure titanium having a length of 30 millimeters and a diameter of 5 millimeters is pickled in a pickle as in Example 2 and is subsequently washed with distilled water. 
     The rod treated in this manner is plasma-chemically anodically oxidized in an aqueous chloride-free electrolytic bath having the composition 
     a) Cations 0.2 mol/l calcium ions 
     b) Anions 0.4 mol/l phosphate 
     At a current density of 18 A/dm 2  and an electrolyte temperature of 10° C.±2° C. 
     After a coating time of 10 minutes the voltage end value of 210 volts is reached. 
     The rod with ceramic layer is washed with distilled water and is dried. 
     The thickness of the layer is 40 μm. 
     EXAMPLE 5 
     A gear wheel of AlMgSi1 having a surface area of 6 dm 2  is degreased and washed with distilled water. An electrolytic bath of Example 1 diluted 100 times with water is used as aqueous/organic chloride-free electrolytic bath which additionally contains 0.1 mol/l each of sodium aluminate and sodium silicate. 
     The gear wheel is plasma-chemically anodically oxidized at a current density of 10 A/dm 2 . After a coating time of 120 minutes, a voltage end value of 800 volts is reached. 
     The gear wheel with ceramic layer is washed and dried. The thickness of the oxide ceramic layer is 130 μm. The hardness of the ceramic layer is 1900 HV (0.1). The gear wheel coated in this manner has a service life which is 4 times that of a conventionally eloxated gear wheel having the same dimensions. 
     EXAMPLE 6 
     An ultrasonic sonotrode of AlZnMgCu1,5 having a surface area of 6.4 dm 2  is degreased and subsequently washed with distilled water. 
     The ultrasonic sonotrode treated in this manner is plasma-chemically anodically oxidized in an aqueous/organic chloride-free electrolytic bath, as described in Example 1, at a current density of 3.5 A/dm 2  and an electrolyte temperature of 15° C. After a coating time of 25 minutes, the voltage end value of 250 volts is reached.