This technical information has been contributed by
Microplasmic Corporation

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MICROPLASMIC ALUMINUM COATING

By: Jerry L. Patel and Nannaji Saka, Ph.D.

The practice of anodizing, or controlled oxidation, of aluminum and aluminum alloys is more than seven decades old. The primary intent of anodizing aluminum and aluminum alloy parts is to protect the highly reactive surface against corrosion in aqueous environments, such as humid air and sea water. Because the anodic coating can be produced in a variety of colors, painted anodized parts are used in architectural applications. Furthermore, because the anodization process produces a hard ceramic coating, many times harder than that of the substrate from which it is formed, anodic coatings are also used to protect aluminum parts from abrasion, especially sand abrasion.

Traditional Anodizing

Traditional anodizing is an electrochemical oxidation process. The part to be anodized is connected to the positive terminal of a Direct Current (DC) power source and a nonreactive metal, such as stainless steel, is connected to the negative terminal. The aluminum part, or the anode, and the stainless steel cathode are immersed in an electrolytic bath and a DC voltage is applied across them. The potential difference is of the order of 20 -100 V and the current densities are 1-10 A/dm2.

The electrolytic baths comprise aqueous solutions of chromic acid, orthophosphoric acid, sulfuric acid, oxalic acid, or combinations thereof. Because the electrolytic baths have appreciable resistivity and because the anodization process itself is exothermic the temperature of the electrolytic bath increases greatly during anodizing.

Since the anodizing process is quite sensitive to temperature, the bath temperature is controlled rather closely by heat exchanger or refrigeration equipment. Today's advanced anodizing technologies include several proprietary hard anodizing processes that employ a wide range of electrolyte compositions, operating conditions and a limited aluminum alloy compositions.

The type and thickness of coating obtained greatly depends on the composition of the electrolytic bath, operating conditions and alloy compositions. The military specification MIL-A-8625F, for example, lists at least six types and two classes of electrolytically formed anodic coatings on aluminum and aluminum alloys for non-architectural applications.

Despite the many decades of experience and the expensive equipment employed by the traditional anodizing plants, the acid bath based DC anodizing process has severe limitations:

This state of affairs is primarily due to the porosity of the coating produced by the traditional acid based electrolytic processes at low power levels, and to certain extent the poor bonding between the aluminum alloy substrate and the anodic layer.

The Microplasmic Process

In recent years, the Microplasmic Corporation, a start up R&D company of Peabody, MA, U.S.A. has developed a unique anodizing technology, called the Microplasmic Process for all types of aluminum alloys. It is an electrochemical micro arc oxidation process for which a US patent is pending. A controlled high voltage AC power is applied to the aluminum part submerged in an electrolytic bath of proprietary composition. Due to the high voltage and high current, intense plasma is created by micro arcing at the specimen surface and this plasma in turn oxidizes the surface of the aluminum specimen. Thus the process is called Microplasmic Process. The oxide film is produced by subsurface oxidation and considerably thicker coatings can be produced.

Much as the traditional process, the Microplasmic process is an electrochemical process, but there ends the similarity. The Microplasmic process is radically different from the traditional anodizing processes in many respects. The distinguishing features of the process may be summarized as follows:

Applications

Because the microplasmic process produces a thick, well-bonded ceramic coating on a variety of reactive light metal alloys, it can be used for a broad range of applications. The primary application could be the replacement of heavier metallic alloys or the more expensive composite materials required by the aerospace and automotive industries by light metals (e.g., Al, Ti, Mg, and their alloys) coated by the Microplasmic Process. Other applications can be divided into the following categories: Chemical, Mechanical, Thermal, Electrical and Electronics, and combinations of these:

Additionally, the Microplasmic Process is also well suited for hard coating interior surfaces (such as those of hollow cylindrical and conical parts), recesses, blind holes, threaded sections, and so on.

Many coating processes in the market, such as Chemical Vapor Deposition (CVD), Physical Vapor Deposition (PVD), Plasma Enhanced Physical Vapor Deposition (PEPVD), Sputtering, Thermal Spraying, etc. are unable to coat the inside surface of a long part. Thus, where appropriate these expensive coating processes can be readily replaced by the Microplasmic Process.

This technical information has been contributed by
Microplasmic Corporation

Click here to find suppliers

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