Patent Application
20080081122
The present invention is directed to an improved process for manufacturing a rotary anode for an x-ray tube, said rotary anode comprising a molybdenum support member on which a target layer consisting essentially of tungsten or a tungsten-rhenium alloy is provided by plasma spraying, the improvement comprising:
The invention is also directed to the anode produced by the improved process.
The general method of manufacturing rotary anodes using a plasma spray coating technique is known in the art and is described in U.S. Pat. Nos. 4,090,103, 4,390,368, 4,534,993 and 4,641,333, the disclosures of which are herein incorporated by reference.
The use of hydrogen and a slightly reduced pressure will improve the density, adherence, and purity of the tungsten or tungsten-rhenium track. Hydrogen will serve to remove oxygen from the surfaces of tungsten, rhenium, and molybdenum. The oxygen removal will have two effects—first is to “activate” the surface of both the coating and the substrate improving adherence and secondly to remove oxygen prior to deposition resulting in an improved density and purity of the coating. The slightly reduced pressure will serve to remove the reactant oxygen bearing species. When referring to “slightly reduced pressure”, the closer the process is too atmospheric the more suppressed is the volatilization of tungsten oxide bearing species.
The determination of the amount of oxygen present in the tungsten or tungsten-rhenium alloy as tungsten dioxide is readily determined. Techniques for measuring the oxygen content of metal powders are known and include, for example, total x-ray fluorescence, secondary ion mass spectroscopy, x-ray photoelectron spectroscopy and Auger spectroscopy. Suitable analyzers are also available form LECO Corporation (TC400, TC500 and RO500C series). Based on the amount of oxygen measured, the actual amount of oxygen present in the form of the oxide present in the form of tungsten dioxide can then readily calculated The devices noted report the oxygen content as a % by weight per one gram sample. The moles of tungsten dioxide can then be calculated according to the following formula:
Mole of tungsten dioxide=[OC times WGT]÷32
where OC is the % by weight of oxygen per 1 gram sample, WGT is the total weight of the powder to be sprayed and 32 is the molecular weight of oxygen.
The molybdenum support member is preheated to a temperature of from 1150° C. to 1600° C. (preferably from 1300° C. to 1500° C.) and placed in a gaseous atmosphere containing hydrogen and having a pressure of from 0.5 to 0.9 bars (preferably from 0.7 to 0.9 bars) and wherein hydrogen is present in a molar ratio of hydrogen to tungsten dioxide of from 5:1 to 50:1 (preferably in a molar ratio of from 10:1 to 30:1). The tungsten or tungsten-rhenium alloy is the plasma sprayed onto the support layer.
Suitable devices for use in plasma spray coating known in the art and are commercially available. Such devices are included DC, arc and inductively coupled plasma devises. Such devices are commercially available from Progressive Technologies, Inc. (Michigan), Plasma Processes, Inc. (Alabama) and Tekna Plasma Systems Inc. (Canada).
As noted above, a hydrogen excess is required. This improves the reduction of WO2. As the hydrogen to WO2 molar ratio increases, the temperature at which complete reduction is possible decreases, and the temperature at which the volatilization becomes detectable increases. At a hydrogen to WO2 molar ratio of 50:1, the temperature at which volatility of tungsten oxide species becomes detectable is almost 3000° C.; however, further temperature increase leads to increased volatility of elemental tungsten. At a hydrogen to WO2 ratio of 75, no further increase in temperature for onset of volatility is noted, but tungsten metal evaporation increases.
In the presently claimed process, hydrogen is intentionally required to remove oxide species and activate the molybdenum substrate. The activation of molybdenum substrate and powder surfaces will also lead to an increase in the adherence of the tungsten or tungsten-rhenium alloy coating to the molybdenum substrate. The process effectively removes oxygen from all the metals and would increase the density of the coating. The process also will prevent any future degassing that would occur if oxide species were present in the coating.
Although illustrated and described herein with reference to certain specific embodiments, the present invention is not intended to be limited to the details described. Various modifications may be made within the scope and range of equivalents of the claims that follow without departing from the spirit of the invention.
