Method of Producing an Anode and Anode Thus Obtained

The invention relates to a method of producing an anode for X-ray tubes, wherein a target layer on the basis of tungsten is deposited by means of chemical vapour deposition (CVD) on a substrate of molybdenum or a molybdenum alloy. The invention also relates to an anode thus obtained.

Anodes are used in X-ray tubes, particularly as rotary anodes for X-ray tubes for medical examination.

French Patent Specification No. 2,153,765 discloses a method of producing an anode of the type described above. According to this prior art, a tungsten target layer for the electrons is provided on a molybdenum substrate. The tungsten layer is deposited by means of chemical vapour deposition (CVD). A barrier layer is provided between the target layer and the substrate, also by means of CVD.

The invention has for its object to improve the prior art method, whereby an improved bond is obtained between the target layer and the substrate.

The method according to the invention is characterized in that the following layers are applied, one after the other, on the substrate by CVD.

a. a layer (1) of molybdenum or a molybdenum alloy containing more than 95% by weight of molybdenum.

b. a layer (2) of a tungsten-molybdenum alloy the composition of which varies in thickness direction so that the molybdenum content at the side contiguous to layer (1) is 95-100% by weight and at the other side 0-5% by weight whereas the tungsten content varies from 0-5% by weight to 95-100% by weight.

c. a layer (3) consisting of tungsten or a tungsten alloy, whereafter the substrate with the layers deposited thereon is annealed in a non-oxidizing atmosphere for from 10 minutes to 6 hours at 1200.degree.-1700.degree. C. The use of layer (1) and layer (2) results in a gradual transition in the coefficient of expansion between the substrate and the layer (3). This results in an improved bond between the substrate and the layer (3). A further improvement of the bond is obtained by forming the layer (3) from two layers: an exterior layer (3b) and an intermediate layer (3a) between layer 2 and the exterior layer (3b). A suitable choice of the material of which layers 3a and 3b are made results in a more gradual variation of the coefficient of expansion.

Consideration has already been given to the provision between the substrate and the target layer of an intermediate layer having a gradually changing composition. German Patent Application No. 2,400,717 describes a method wherein by fusing a tungsten-rhenium alloy on a molybdenum substrate an intermediate layer having a molybdenum concentration which varies in the thickness direction would be obtained. The proposed method is, however, difficult to implement, at any rate it is not easily reproduceable. For mass production the method used must be reproduceable.

The method in accordance with the invention can be performed in a reproduceable manner in a very simple way. A suitable method of depositing the above-mentioned layer (2) is, for example, described in Electrodeposition and Surface Treatment, 2 (1973/74) pages 435-446, "Vapour deposition of Molybdenum-Tungsten" by J. G. Donaldson et al.

The invention will now be further described by way of example with reference to the accompanying drawing in which

FIG. 1 is a cross-sectional view through an anode in accordance with a preferred embodiment of the invention and

FIG. 2 shows a detail of the encircled portion in FIG. 1.

FIG. 1 shows an anode A formed by a substrate S and a target layer T deposited thereupon. The substrate S consists of molybdenum or a molybdenum alloy such as, for example, TZM (a molybdenum alloy containing 0.5% by weight of Ti; 0.07% by weight of Zr and 0.03% by weight of C). The target layer T may alternatively cover a smaller or a larger portion of the substrate S. The target T may alternatively be provided on a recessed portion in the substrate S.

As shown in FIG. 2, the target layer T comprises the layers 1, 2, 3a and 3b. Layer 1 consists of molybdenum or a molybdenum alloy with more than 95% by weight of molybdenum. Layer 2 consists of a tungsten-molybdenum alloy which has a gradually varying composition. At the side contiguous to layer 1, layer 2 contains 95-100% by weight of molybdenum and 0-5% by weight of tungsten; at the side contiguous to layer 3a it contains 95-100% by weight of tungsten and 0-5% by weight of molybdenum. Layer 3a consists of a layer containing 95-100% of tungsten, while layer 3b consists of tungsten or a tungsten alloy. The composition of layer 3b corresponds to the composition of the prior art target layers for X-ray anodes, such as, for example, tungsten, tungsten alloys having one or more of the elements rhenium, tantalum, osmium, iridium, platinum and similar elements.

The layers 1, 2, 3a and 3b are all deposited by means of CVD processes which are known per se. After deposition of the layers, an annealing operation is performed for 10 minutes to 6 hours at 1200.degree.-1600.degree. C. During said annealing operation some diffusion between the different layers occurs, which also results in an improved bond. In some cases it may be possible to perform the annealing operation after only a part of the layers has been deposited.

Preferably, the layers 1, 2, 3a and 3b are deposited with the following thicknesses: layer 1 has a thickness of 1-200 microns preferably 10-50 .mu.m, layer 2 has a thickness of 1-300 microns, preferably 50-100 .mu.m, layer 3a has a thickness of 10-500 .mu.m, preferably 200-300 .mu.m and layer layer 3b has a thickness of 50-1000 microns, preferably 200-300 .mu.m.

The invention will now be further described with reference to the following example.

A layer of molybdenum is first deposited with a thickness of 20 .mu.m (layer 1) by means of CVD on a suitable substrate made of TZM (a molybdenum alloy containing 0.5% by weight of Ti, 0.07% by weight of Zr, 0.03% by weight of C). The substrate is preheated at 1000.degree. C. The molybdenum is supplied as MoF.sub.6. The MoF.sub.6 and also the fluorides to be specified below are reduced by H.sub.2. The conditions during the process are as follows: gas pressure 15 mbar, temperature 1000.degree. C., flow rate of the H.sub.2 0.5 l per minute, flow rate of the MoF.sub.6 0.04 l per minute. The liters of gas have been converted for all cases into atmospheric pressure and room temperature. As soon as the desired layer thickness has been obtained, the flow rate of MoF.sub.6 is gradually reduced to zero and a gradually increasing quantity of WF.sub.6 is supplied (increasing from 0 to 0.05 l per minute), all this in such a way that a layer (2) is obtained having a thickness of 50 .mu.m, in which the molybdenum concentration decreases from 100 to 0% and the tungsten concentration increases from 0 to 100%. The feed forward of WF.sub.6 is continued until a layer (3a) of pure tungsten has been obtained having a thickness of 250 .mu.m. Then the feed of the WF6 is slightly reduced and ReF.sub.6 is simultaneously supplied so that a layer (3b) containing 4% of Re is deposited. This is continued until layer (3b) has a thickness of 250 .mu.m.

The substrate with the layers 1, 2, 3a and 3b deposited thereupon is finally heated for 3 hours at 1600.degree. C. in a non-oxidizing atmosphere. During this annealing operation some diffusion occurs between the substrate and the layers and between the respective layers. Such diffusion ensures a proper bond between the different layers and the substrate.