Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Forming nonmetal coating using specified waveform other than...
Reexamination Certificate
2002-03-27
2004-10-26
King, Roy (Department: 1742)
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Forming nonmetal coating using specified waveform other than...
C204S229500, C204S230600, C205S107000, C205S108000, C205S321000, C205S322000, C205S324000, C205S333000
Reexamination Certificate
active
06808613
ABSTRACT:
TECHNICAL FIELD
The subject of the present invention is an electrical oxidation process using a microarc plasma for the purpose of obtaining a ceramic coating on the surface of a metal having semiconducting properties.
Aluminium, titanium, their alloys and all metals which exhibit valve (diode) properties have a beneficial strength/weight ratio and are suitable for a wide range of applications such as in aeronautics, automobiles and mechanical engineering (especially for moving parts, with high mechanical loads and strains), etc.
BACKGROUND OF THE INVENTION
However, as these materials do not naturally exhibit suitable tribological and mechanical properties (hardness, friction coefficient, abrasion resistance, etc.), coatings are often used to improve the limited characteristics of the coatings on these materials.
They often allow complementary requirements to be met, such as the corrosion resistance in acid medium and/or alkaline medium, the possibility of momentarily withstanding high temperatures, or the obtaining of dielectric properties.
Several electrolytic coating processes are currently employed. The process most used, for wear and/or corrosion protection, is hard anodizing. However, this quite rapidly reaches its operating limits.
This anodizing process is used to form protective oxide layers on aluminium workpieces. However, the coatings produced by this method are limited in terms of thickness and have only a moderate hardness (a maximum of about 500 Hv).
A number of other techniques have been developed to produce coatings of higher performance, especially ceramic coatings, so as to meet severer operating requirements, namely arch-discharge plasma spraying, flame spraying or vacuum deposition techniques.
However, the drawback is that, to obtain good adhesion of the coating, these techniques require a high substrate temperature and prior processes for preparing the surface.
Moreover, these processes cannot compete with conventional anodizing in terms of coating uniformity and/or production costs.
A relatively old (1932) anodic oxidation process using anodic spark discharges or microarc discharges has been developed so as to obtain ceramic coatings for workpieces made of aluminium, titanium, magnesium and their alloys, as a means of protection against severe abrasion and corrosion.
Owing to the valve effect, microarc oxidation forms an insulating barrier film on the metals such as aluminium and titanium. By increasing the anodic potential to more than 200 V, the barrier film is broken and microarcs appear. If a high voltage is maintained, many microarcs are initiated and they move rapidly over the entire immersed surface of the specimen.
These dielectric breakdowns create tracks which pass through the oxide (barrier) layer formed. Complex compounds are synthesized within these tracks. They are composed of substrate material, surface oxides and addition elements from the electrolyte. Chemical interactions in the plasma phase occur in the multiple surface discharges and result in the formation of a coating which grows in two directions from the surface of the substrate. This causes a gradual change in the composition of the properties of the coating from the metal alloy within the substrate to a complex ceramic compound in the coating.
According to the historical description of this process, Gunterschulze and Betz were the first to mention, in 1932, the anodic spark deposition (ASD) process. They observed that the material underwent deposition of the electrolyte during the dielectric breakdown of an insulating film growing on the anode.
This dielectric breakdown causes sparks which appear and disappear while being distributed over the entire surface of the anode, giving the effect of movement.
The first practical applications of ASD were their use as anticorrosion coatings on magnesium alloys, dating from 1936, and these were included in a military specification in 1963.
Since then, the main research efforts have been pursued by Gruss, McNeill and coworkers at the Frankford Arsenal in Philadelphia, and by Brown, Wirtz, Kriven and coworkers at the University of Illinois in Urbana-Champaign.
At the same time, research was carried out in East Germany, mainly by Krysmann, Kurze, Dittrich and coworkers. The German process is called “anodic oxidation by spark discharges” (the German acronym for which is ANOF [Anodische Oxidation an Funkenanladung]). The reports of this work in the international literature make reference to patents in the German language.
It is clear that this research has made significant progress, yet it remains, despite everything, superficial and the compounds of the coating formed have not been clearly identified (only the &agr;-Al
2
O
3
and &ggr;-Al
2
O
3
(OH) (bohemite) have been identified by X-ray diffraction).
One process, patented in 1974, was developed in order to compete with coatings on aluminium for architectural purposes. The method allows the aluminium substrate to act as an anode in a potassium silicate solution so that an aluminosilicate coating of olive-grey colour is applied by using a 400 V half-wave rectified DC current. The process takes place by dielectric breakdown of the barrier layer, causing sparks or scintillations visible on the anodic substrate, while Bakovets, Dolgoveseva and Nikoforova postulate three parallel mechanisms during formation of the film, namely electrochemical, plasma oxidation and chemical oxidation mechanisms.
Several modifications have been made to this process, called “silicodisant”, comprising the addition of carboxylic acids and of vanadium components in the bath. Ceramic or tetrafluoroethylene resins have also been added to the bath so as to provide the coating with hardness or lubrication properties.
The drawback with such processes is the use, in terms of signal waveform, of a DC current of a few mA at voltages of less than 500 V. This results in the sparking being stopped after a few minutes (most of the deposit is formed in the first few minutes). Such operating conditions make it possible to produce only very small coating thicknesses and thus limit its physical properties.
Other processes describe the use, in electrolytic baths of variable composition, of AC voltages which may exceed 1000 V combined with a DC or AC current.
It should also be noted that the use in certain cases of high voltages with high current densities means that these processes cannot easily be exploited on an industrial scale.
On the other hand, the excellent adhesion to the substrate of this type of coating, the physical and tribological characteristics (high hardness, thermal resistance, electrical resistance, abrasion resistance, corrosion resistance, etc.), the wide variety of aluminosilicate mixtures for coating purposes and the fact that the coating can be performed within narrow surfaces of complex geometry are among the many advantages of this process.
We describe below a different type of microarc process capable of monitoring, imposing and controlling the change in a ceramic coating process in its various phases. A suitable device is used to achieve optimum programming according to various parameters (nature of the alloy or of the metal of the workpieces to be treated, characteristics of the ceramic that it is desired to obtain, etc.).
Three main process phases may be identified, according to the descriptions that may be found in the numerous scientific works and other publications on the subject generally called microarc oxidation and described above.
The workpieces to be treated and the electrodes immersed in the electrolyte constitute a dipole, to which the electrical energy delivered by a generator is applied.
The electrolyte is an aqueous solution, preferably based on demineralized water, and includes at least one oxyacid salt of an alkali metal and a hydroxide of an alkali metal. A wide variety of solutions are described in the numerous publications on the subject.
In the first phase, which lasts from a few seconds to a few minutes depending on the alloys, an insulating layer consisting of a hydroxide i
Cantor & Colburn LLP
King Roy
Leader William T.
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