The CZ - Ematal - Process

a new surface treatment for aluminium alloys. Further development of the Ematal process - a special anodizing technique with applications previously limited to decorative effects - has led to major improvements. As a result, this technique can now be used in a wide range of technical applications to obtain remarkable surface characteristics.

chemical durability Ematal(german)

Ematal - CZ

Carried out in a special electrolyte bath, the process is based on the anodic conversion of the basic metal surface into a hard, wear-resistant oxide (and simultaneous desposition of titanium, zirconium and other compounds). The use of additives to increase and stabilize the characteristics of the Ematal coating is a new feature of particular significance for technical applications.

In addition to increasing corrosion and chemical resistance, the Ematal process offers the following advantages compared with conventional anodizin

Maintaining Dimensional Accuracy

The Ematal method can - if necessary - be controlled so that the dimensions of precision parts after coating are kept within extremely close tolerances (approximately +/- 3 µm).

Other coating methods (such as hard chromizing) always produce dimensional changes of the component treated, usually unevenly distributed, thus increasing the component's thickness. However, the precise control inherent in the Ematal method avoids this negative feature of the coating process. The Ematal coating has almost the same volume as the original base metal which was converted to from the layer.

To ensure that Ematal treated components retain the same dimensions, more severe criteria are to be applied to the alloy used and the thickness of the coating should not exceed approximately 20 µm, a value which - due to the high wear resistance of the Coating - is suffivient for practically all applications.

If lesser dimensional accuracy is required, the coating can be correspondingily increased by augmenting the thickness of the Ematal layer.

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High Resistance to Wear

The Ematal coating exhibits high wear resistance when exposed to friction or abrasion.

Resistance to friction results from a number of factors. Therefore tests must only be conducted to demonstrate whether this resistance withstands specific conditions of use. Extensive trials and experience gained from numerous applications have shown that the Ematal coating has a very high resistance to wear.

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Antifriction Properties

Thanks to the smooth, poreless surface the Ematal coating exhibits an extremly low coefficient of friction on suitably pretreated components.

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Increased Surface Hardness

The Ematal coating produces a good surface hardness on the parts treated with this new technique developed by Contraves.

The hardness of the Ematal coating itself is - in the case of suitable alloys - very high and exceeds that of quartz or topaz (Mohs scale: 7.8).

Applied rather thinly on the softer base material, the coating tends to break when high specific surface pressures are exerted. As a result, the penetration resistance of the alloy is of considerable importance in the case of wear due to fissures, cracks, scratches, and impacts. The customary hardness tests (e.g., Vickers, Brinell) are, therefore, not applicable.

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High Covering Capability

The Ematal coating is formed with the same thickness and characteristics even on very awkward spots such as very narrow holes or screw threads.

Coating begins at thepoint with the least resistance to the flow of current. Since the electrical resistance increases proportionally as the thickness of the coating increases, the coation effect spreads over the component. In other words, the Ematal process is carried out with high constant voltage so that other exposed areas of the surface are treated until the entire components is uniformly coated.

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Other Characteristics

Depending on the alloy, the color of the Ematal coating varies from bright gray to brownish gray. In the case of heterogeneous alloya the coating reproduces the texture of the alloy. If the alloy contains Si, the coating is black. The deposition of various substances gives the finished coating an opaque enamelled base material is thus lost.

Depending on the alloy and the thickness of the coating, the final state of the Ematal coating represents a good thermic and electric insulator (breakdown voltage, ca. 35-50 V/µm).

The thermal stability of the Ematal coating exceeds the melting point of aluminium. But excessive heating produces microcracks due to the varying thermal expansion of the coating and the basic meaterial.

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Choice of Alloys

The selection ot the right alloy is of utmost importance in order to attain the optimal characteristics of the Ematal coating.

Just as with all other methods for electrolytically anodizing aluminium, the base material forns the skeleton of the coating. Therefore the composition, the structure, and the texture of the aluminium alloy decisively affect the caracteristics of the coating. The desosition of titanium compounds and other active substances reduces this dependency to the extent that the main characteristics of the coating vary only slightly over a wide range of aluminium alloys. The optimal coating characteristics are obtained with an homogeneous aluminium alloy which contains very few nonoxidizable foreign substances. Trials of the Ematal process are recommended for any specific application because, in addition to the chemical analysis, the alloy structure resulting from the various types of treatment (e.g., heat treatment, aging) and machining (e.g., metal cutting and metal forming) of the semifinished product are important.

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Applications of the Ematal Process

Workpieces requuiring highly abraison-resistant surfaces: computer capacitors, rotors, washers or disks, bearing flanges, fuse bodies, gears, fuse castings, couplings, valve housings, toothed wheels, masks, fixtures, front panels, rods, screws, etc.

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Summary

The recently developed Ematal method offers a possibility for coating aluminium and aluminium alloys which provides very advantageous properties for technical applications. This process provides highly wear-resistant, smooth, poreless surface, presenting excellent antifriction properties, without essentially altering the dimensions of the parts treated. Uniform coatings having the same basic properties can also be formed on complex shapes.

Suitable use of this method - together with trials for optimizing and adapting the coating for particular applications - represents a technilogical advance with respect to other surface finishing methods.