The invention relates to an electrode comprising a substrate having at least at one of its sides a coating made of elec- troconductive diamond.

The DE 199 11 746 Al describes a method for manufacturing of a diamond coated electrode. On a electroconductive substrate there is deposited a layer made of a diamond powder having an average grain size in the range of 5 nm to 100 nm. This dia¬ mond layer acts as a seed layer on which by chemical vapor deposition (CVD) a diamond layer having a grain size usually in the range of 1 to 50 μm is deposited.

From DE 694 10 576 T2 it is known the use such a diamond coated electrode for the treatment of waste water. However, in practise it has turned out that the corrosion resistance of such an electrode is not very high. This may end up in a separation of the diamond layer from the substrate.

It is an object of the present invention to avoid the disad¬ vantages in the art. It is an aim of the invention to provide a diamond coated electrode having an improved resistance against corrosion.

This object is solved by the features of claim 1. Embodiments of the invention are described by the features of claims 2 to 21.

According to the present invention there is provided an elec¬ trode comprising a substrate having at least at one of its sides a coating made of an electroconductive diamond, the coating comprising at least one first diamond layer having a first average grain diameter and at least one second diamond layer having a second average grain diameter, the first aver- age grain diameter being bigger than the second average grain diameter, and the second layer overlying the first layer.

The proposed electrode shows an excellent resistance against corrosion. By depositing upon the first diamond layer a sec¬ ond diamond layer having a smaller average grain size it is possible to effectively prevent a liquid from seeping into the coating. The second diamond layer forms an effective seal which can advantageously be produced simply by varying one or more parameters during chemical vapor deposition. The first layer may have a fine grained base layer which is followed by- coarser grains. The coarse grains may have a columnar struc¬ ture.

According to an embodiment of the invention the first average grain diameter is in the range of 0.5 μm to 25 μm. The second average grain diameter is advantageously less than 1.0 μm, preferably in the range of 50 to 200 nm. A second diamond layer having the aforementioned second average grain diameter effectively protects an underlying first diamond layer against the penetration of liquids.

According to a further embodiment a thickness of the second diamond layer is smaller than a thickness of the first dia- mond layer. It has turned out to be advantageous that a ratio of the thickness of the second diamond layer to the first diamond layer is in the range of 0.05 to 0.99. For an effec¬ tive sealing against corrosion it is sufficient to deposit the second diamond layer with a relatively small thickness. As a result of this the cost for providing an effective pro¬ tection against corrosion can be kept low.

According to a further embodiment the first and second dia¬ mond layers form an alternating sequence. By this feature the resistance against corrosion can be enhanced further. It can be provided in particular an excellent resistance against corrosion caused by electrochemical attack. The overall thickness of the alternating sequence may be in the range of 1 to 200 μm. Preferably the thickness of the alternating se- quence is in the range of 2 to 25 μm.

An uppermost diamond layer forming an outer surface of the electrode is advantageously the second diamond layer. Thereby the number of the layers can be minimised and at the same time a good protection against corrosion can be achieved.

According to a further embodiment the diamond contains a dop¬ ing for increasing its electrical conductivity. The doping may comprise at least one of the following substances: boron, nitrogen. An amount of the doping contained in the diamond may be in the range of 10 ppm to 3000 ppm, preferably in the range of 100 ppm to 1000 ppm. The proposed doping is suitable to provide an excellent electrical conductivity of the dia¬ mond coating.

A first average amount of the doping contained in the first diamond layer may differ from a second average amount of the doping contained in the second diamond layer. In particular the first average amount is lower than the second average amount. Furthermore an third average amount of the doping contained in the uppermost diamond layer can be higher than the average amount of the diamond layers being provided be¬ tween the uppermost diamond layer and the substrate. By the aforementioned features the electrical properties of the electrode, in particular the electrical conductivity of the diamond coating can be improved. The diamond and/or the sub¬ strate may have an electrical resistance of less than 100 Ωcm, preferably of less than 0.1 Ωcm. According to a further embodiment at least 30 % by volume, preferably 50 % per area unit on the surface, of the diamond crystals of the uppermost diamond layer are twins. By this feature the electromechanical resistance of the diamond coat- ing can be enhanced. The growth of twins can be simply- achieved during chemical vapor deposition by choosing suit¬ able parameters, e. g. an increase of the temperature. Fur¬ thermore the uppermost diamond layer may have a hydrophobic or a hydrophilic surface. A hydrophilic surface can be made by an annealing the deposited diamond coating in an oxygen atmosphere. A hydrophobic surface of the uppermost diamond layer can be made by an annealing the diamond coating in an atmosphere containing hydrogen and/or methane.

It has been turned out to be advantageous if the substrate is made of a metal, preferably a self passivating metal. Under the term "self passivating metal" there is understood a metal which is passivated on its surface by the formation of an isolating layer by chemical or electrochemical oxidation. The metal may be selected from the following metals: titanium, niobium, tantalum, aluminium, zirconium, steel, steel being coated with a layer which separates the iron of the steel from the atmosphere used in the CVD-process and forms cova- lent bonds to the diamond layer. Suitable layers are for ex- ample made of titanium boron nitride or chromium carbide. The substrate has advantageously a thickness in the range of 0.1 to 20.0 mm.

According to an advantageous embodiment of the invention the diamond coating is provided on the opposite sites of the sub¬ strate. Thereby the effectiveness of the electrode can be en¬ hanced remarkably. The substrate may have an angular, pref¬ erably a rectangular, form which makes it easy to manufacture for example a large-dimension flat electrode. However, it is also possible that the substrate has a curved surface. It may be a tube, slab, rod or plate. Furthermore, the substrate may be an expanded metal. It may have one or more apertures.

Embodiments of the invention are now described by way of non- limiting examples with references to the accompanying draw¬ ings in which:

Fig. 1 is a schematic sectional view of an electrode,

Fig. 2 is a SEM photo of a transverse section of an elec¬ trode,

Fig. 3 is a first 3-dimensional plot of the surface of a first diamond-layer and

Fig. 4 is a second 3-dimensional plot of a second diamond- layer.

Fig. 1 a seed layer A made of a nanocristalline diamond pow- der is deposited upon a substrate F. The substrate F is made preferably of titanium or steel coated with a layer made of titanium boron nitrite or chromium carbide. Such a layer separates the iron of the steel from the atmosphere used in the CVD-process and forms covalent bonds to the diamond layer.

The seed layer A is overlaid by a first diamond layer B, the thickness of which may be in the range 0.5 to 25 μm. A first average grain diameter in the direction of growth is prefera- bly bigger than 0.5 μm. The direction of growth is essen¬ tially perpendicular to the surface of the substrate F.

A second diamond layer C overlays the first diamond layer B. The thickness of the second diamond layer C is preferably smaller than the thickness of the first diamond layer B. A second average grain diameter is preferably smaller than 0.5 μm in the direction of growth.

As can be seen from Fig. 1 the second diamond layer C is overlaid by a further first diamond layer D. The further first diamond layer D is overlaid by further second diamond layer E which forms an uppermost diamond layer.

Compared with the second diamond layer C the further second diamond layer E may exhibit some special features. In order to enhance the electromechanical resistance of the uppermost diamond layer E it may contain a considerable amount of dia¬ mond twins. The amount may be 30 to 60 % or more per area unit on the surface. Furthermore the uppermost diamond layer E may contain a higher amount of a doping, preferably boron, than the first B, D and second diamond layers C. Finally, a surface S of the uppermost diamond layer E may have hydropho¬ bic or hydrophilic properties.

A change in the grain size of the first B, D and the second diamond layers C, E can be made by changing the content of methane in the atmosphere and/or by varying the temperature during CVD-process. A low content of methane in the atmos¬ phere and/or a high temperature leads to the deposition of grains with a large grain diameter whereas a high content of methane and/or a low temperature leads to the deposition of a small grain diameter. The high temperature may be a substrate temperature in the range of 8500C, the low temperature may be a substrate temperature in the range of 7500C. The first dia- mond layers B, D usually contain an amount of boron smaller than 1000 ppm. The second diamond layers C, E typically con¬ tain an amount of boron of more than 500 ppm.

Furthermore, by choosing suitable parameters during CVD- process it is possible to produce diamond grains having a texture with respect to the fastest growth direction. The first diamond layer can preferably contain diamond grains which have a texture in [100]- or [HO]- or [111] -direction. First diamond layers B, D exhibiting a texture have an im- proved mechanical strength and an improved resistance against corrosion.

The following tables show by way of example suitable deposi¬ tion parameters for the first B, D and the second diamond- layers C, E.

Table 1 Deposition parameter for the first diamond layer B, D (coarse grain size)

Table 2. Deposition parameter for the second diamond layer C, E (fine grain size)

Fig. 2 shows a SEM-photo of an electrode which is similar to the electrode shown schematically in Fig. 1. As can be seen from Fig. 2 the seed layer A has a thickness of about 0.4 μm. The thickness of the first diamond layer is in the range of 1.2 μm. The second diamond layer C and the further second diamond layer E have a thickness of about 1.8 μm. The further first diamond layer which is sandwiched between the second diamond layer E and the further second diamond layer E has also a thickness of about 1.8 μm.

Fig. 3 and 4 show 3-dimensional plots of the surface of the first diamond layer B and the second diamond layer C. The plots have been calculated on basis of data which have been obtained by the record of a picture via atomic force micros¬ copy (AFM) . As can be seen from Fig. 3 the first diamond layer has a surface roughness in the range of 200 nm.

As can be seen from Fig. 4 the second diamond layer C has a remarkably smoother surface with a roughness in the range of about 50 nm.

By providing a fine grained second diamond layer C or a fur¬ ther second diamond layer E an underlying first diamond layer B or a further first diamond layer D can be effectively pro¬ tected against a penetration of a liquid. The corrosion re¬ sistance of the proposed electrode is enhanced remarkably. As shown in Fig. 1 and 2 an alternating sequence of first B, D and second diamond layers C, E can be used. The provision of such an alternating sequence further enhances the resis- tance of the electrode against corrosion. Reference list

A seed layer B first diamond layer C second diamond layer D further first diamond layer E further second diamond layer or uppermost layer F substrate S surface