System and method for laser structuring of x-ray tube component surfaces

Abstract

A system and method of texturing a surface of an X-ray tube component, which comprises applying laser radiation to the surface of the X-ray component to remove material from the surface of the X-ray tube component by producing a pattern of varying structures on the surface of the X-ray tube component to improve the emissivity of the X-ray tube component.

Description

Docket No.701348-WO-2 SYSTEM AND METHOD FOR LASER STRUCTURING OF X-RAY TUBE COMPONENT SURFACES CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of and priority to U.S. Provisional Patent Application Nos.63 / 689,307 and 63 / 689,344, both filed on August 30, 2024, the disclosures of which are incorporated herein by reference in their entirety. BACKGROUND

[0002] Embodiments of the subject matter disclosed herein relate to implementation of a system and method for laser structuring or texturing of X-ray tube component surfaces, and in particular, surface texture modification of an X-ray tube cathode emitter or filament for improved electron emission and an X-ray tube anode target for emissivity enhancement.

[0003] Any high voltage space that requires materials with high emissivity requires a coating or some other way of changing the emissivity of the materials. Currently coatings are utilized in X-ray tubes to change the emissivity of the materials. Laser structuring or texturing of material surfaces would be a substantial benefit in high voltage spaces as there is no coating to flake off and render an X-ray tube unusable.

[0004] Emissivity of a surface of a material is its effectiveness in emitting energy as infrared energy or thermal radiation (radiated heat) into its surroundings. It's the ratio of energy radiated from a material's surface to the energy radiated from a perfect emitter, called a blackbody, at the same temperature, wavelength, and viewing conditions.

[0005] Early life failures and high voltage instability are problems of X-ray tubes used in medical imaging systems. This disclosure provides a system and method of reducing early life failures and high voltage instability by removing coatings from surfaces of X-ray tube components in the high voltage space, eliminating the potential for flaking of coatings on surfaces of X-ray tube components.

[0006] A typical solution for increasing the electron emission from an X-ray tube cathode emitter is increasing the temperature of the emitter or increasing the surface area of the emitter.Docket No.701348-WO-2 Eventually, performance and / or reliability limits are reached due to evaporation, material strength, or creep resistance of the material. Creep resistance of a material refers to a material’s ability to resist “creep,” which refers to the tendency of a material to slowly deform over a long period of exposure to high levels of stress. SUMMARY

[0007] This summary introduces concepts that are described in more detail in the detailed description. It should not be used to identify essential features of the claimed subject matter, nor to limit the scope of the claimed subject matter.

[0008] In an aspect, a system for texturing a surface of a component, comprising a laser source for applying pulsed laser radiation to the surface of the component; and a laser controller configured to control the laser source to apply the laser radiation as a series of laser pulses to produce an arrangement of varying structures on the surface of the component to improve emissivity of the component.

[0009] In an aspect, an X-ray tube component comprising a modified surface that is in a raster pattern of material removal in a series of linear lines, grooves, trenches, circles, or divots.

[0010] In an aspect, a method of texturing a surface of a component, comprising applying laser radiation to the surface of the component to produce an arrangement of varying structures on the surface of the component to improve the emissivity of the component. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Various aspects of this disclosure may be better understood upon reference to the accompanying drawings and reading the detailed description.

[0012] FIG.1 is a schematic diagram of a system that may be used for laser treatment of a material surface.

[0013] FIG.2 is a flow diagram of a process that may be used for laser treatment of a material surface.

[0014] FIG.3A is an embodiment of a cross-section of a standard emitter of an X-ray tube.Docket No.701348-WO-2

[0015] FIG.3B is an embodiment of a cross-section of a surface textured emitter of an X- ray tube.

[0016] FIG.4A is an embodiment of a cross-section of a standard filament of an X-ray tube.

[0017] FIG.4B is an embodiment of a cross-section of a surface textured filament of an X- ray tube.

[0018] FIG.5A is an embodiment of a portion of an anode target with a brazed graphite coating on a bottom surface of the anode target.

[0019] FIG.5B is an embodiment of a portion of an anode target with a coating on a bottom surface of the anode target.

[0020] FIG.6 is an embodiment of a material surface enhancement by removing material “pixel” by “pixel” from the material surface. DETAILED DESCRIPTION

[0021] Embodiments of the present disclosure will now be described, by way of example, with reference to the Figures.

[0022] Coatings serve as a highly volatile high voltage (HV) instability mechanism that has been required of X-ray tube components in need of different emissive values to ensure adequate radiative heat transfer to meet X-ray tube performance requirements. Coatings that go through multiple heat cycles have tendencies to flake off, therefore causing unstable X-ray tubes and early or late life failures. Additionally, some X-ray components undergo cyclical thermal loads that no currently available coatings can handle without flaking or losing adherence, making the design space and operating ranges much narrower. Changing emissive properties of material surfaces by laser ablation of structuring or texturing the material surface that leads to a more stable emissive layer, which does not flake. The X-ray tube design space also increases vastly as the areas that can be laser structured textured can vary in size and location without concern of coating flaking.Docket No.701348-WO-2

[0023] Additionally, there are substantial process simplifications utilizing laser ablation over any thermal spray or other coating method. Reducing the critical variables required to meet the process requirements makes laser ablation a preferred method that is easier to control and maintain. Laser ablation is a process that uses a focused laser beam to remove material from a solid material’s surface.

[0024] Laser structuring or texturing technology through laser ablation modifies surface conditions of materials at the nano structure level as well into the micro scale. Laser radiation is applied to a surface to modify the material’s surface properties by modifying its texture and / or roughness. A pulsed laser beam creates micropatterns on the surface through laser ablation, removing layers with micrometer precision and perfect repeatability. Typical patterns include dimples, grooves, and other free forms. The structure consists of differing shapes at varying depths which modify the way light waves react with the surface of the component, sometimes appearing black or other colors in the visible spectrum. The added surface area and diffraction of different wavelengths changes the emissivity of the surface of the material.

[0025] Laser surface texturing may be used to improve properties like adherence, wettability, electrical and thermal conductivity, and friction. For example, it may increase surface adherence before applying common coatings like adhesives, paints, or ceramics. Laser surface texturing may also be used to prepare surfaces for thermal spray coatings and laser cladding as well as to improve the performance of mechanical seals.

[0026] For example, materials such as copper that have a very low emissivity in the raw material state, that would have traditionally needed a coating applied for high temperature vacuum applications, can now be laser structured to a comparable emissivity of a coating. In high temperature vacuum applications this is critical as coatings have tendency to flake over numerous thermal cycles, while laser structuring changes the surface of the material from its original state. As shown in the scanning electron microscope (SEM) images the nano ripple effect is causing visible light entrapment, while the composition varies insignificantly from the original material.

[0027] Alternatives to laser ablation include physical vapor deposition (PVD) and thermal sprays and coatings, including thin film deposition techniques where a solid material is vaporizedDocket No.701348-WO-2 in a vacuum environment and deposited on substrates as a pure material or alloy composition coating. Physical vapor deposition, sometimes called physical vapor transport, includes a variety of vacuum deposition techniques which may be used to produce thin films and coatings on substrates including metals, ceramics, glass, and polymers.

[0028] FIG.1 is a schematic diagram of a system 10 that may be used for laser treatment of a material surface. The system 10 may include a laser 20 coupled to a laser controller 16 which is used to control operation of the laser 20 to emit a pulsed laser radiation beam of desired characteristics. The laser controller 16 is coupled to a power supply 14, which supplies power to the laser controller 16 and laser 20. The laser 20 is aligned with a component 18 having a surface 12 in a chamber 24. Operation of the laser 20 under control of the laser controller 16 forms periodic structures, texturing, or profiling on the surface 12 of the component 18.

[0029] In the embodiment of FIG.1, the component 18 is a metal component comprising, for example, aluminum, copper, stainless steel, or other appropriate material used for X-ray tube components. In operation, pulsed laser radiation of desired characteristics is scanned across the surface 12 of the component 18 by the laser 20 under control of the laser controller 16 to produce a periodic arrangement of structures, texturing, or profiling on the surface 12 of the component 18. For example, in order to form peaks and troughs arranged in parallel rows or in a circular pattern, the laser beam may be scanned along parallel or circular, spaced-apart paths across the surface 12 to form parallel or circular troughs separated by peaks. Any other suitable arrangements of structures or textures can be formed by suitable scanning of the laser beam over the surface 12 of the component 18. The laser 20 may be controlled by a laser controller 16 and powered by a power supply 14 to form structures, texturing, or profiling over a desired area.

[0030] In experiments of metal samples, such as copper and stainless steel, following laser treatment of the surfaces to form structures, texturing, or profiling on the surfaces of the metal samples. For copper and stainless steel samples emissivity values of 0.8 or greater were achieved utilizing pulse rates exceeding 1 MHz, with variable pulse widths of 0.3 PS to 20 PS at travel speeds from 50mm / s to 1000 mm / s. The available power for the tests was 20 watts, however the relationship between pulse rate and power are relatively linear (for given pulse energy). Providing similar ablation rates at a linear rate, making it possible to utilize more than 20 wattsDocket No.701348-WO-2 up to near 1000 watts where pulse rates can reach the same relationship (for example, 2 MHz, at 20 watts, and 50 mm / s would yield the same result as 20 MHz, at 200 watts, and 500 mm / s).

[0031] The laser treatment of the surfaces to form structures, texturing, or profiling on the surfaces of the metal samples formed a geometry of a raster pattern in a series of linear lines or circles yielding the best results. This process is key to creating the right micro-geometry to visibly trap light and yield high emissive values.

[0032] There are currently three known and unique X-ray tube components for leveraging this technology and process. These include X-ray tube emitters, X-ray tube emitter filaments, X- ray tube targets, and X-ray tube frames.

[0033] X-ray tube emitters or emitter filaments are challenged with coating process limitations where adherence of a coating to the emitter or filament surface is a major issue. Where coating adhesion is not robust, this results in multiple significant quality issues.

[0034] X-ray tube frames are challenged with a complex thermal chemical conversion coating, an oxide coating process that is known to cause supply chain limitations, variable emissive results based on material composition, and limited HV standoffs. Design arounds of coating processes all have a high risk coating spallation with different processing steps that may result in excursions. The other type of design around could be a media blast to roughen the surface, which has particulate risks resulting in HV instability.

[0035] X-ray tube targets are challenged by applying any coatings as operating temperatures of X-ray tube targets are extremely high and create limitations. Where coatings break down potentially at operating temperatures and produce particulate or gas load. Current X-ray tubes exceeding temperature limits are required to sustain stable coating chemistry.

[0036] The process of using lasers for material conditioning, structuring, or texturing is material surface structuring by means of laser profiling, for X-ray tube components to increase emissivity. Utilizing femtosecond laser technology with laser power exceeding 20 watts, or pulse rates exceeding 1 MHz, and pulse widths from 0.3 ps to 20 ps. Utilizing infrared (1000-1060 nm) wavelength or visible (500-550 nm) wavelength laser sources. Where rastering of the material surface may contain linear or nonlinear shapes to achieve the desired emissivity.Docket No.701348-WO-2

[0037] This process has not been used for changing emissivity values of different materials. Generally, materials are coated to change emissivity values in X-ray tube component applications. There has been laboratory testing at smaller scales that are not manufacturable due to cycle times that would be required. However, with the current changes in technology this can be scaled for production components.

[0038] FIG.2 is a flow diagram of a process 30 that may be used for laser treatment of a material surface of a component for an X-ray tube. The process 30 includes component preparation 32. The component preparation may include machining, grinding, sanding, electrical discharge machining (EDM), mechanical cutting, etc. in order to prepare the component for further processing. That further processing may include a laser structuring or laser ablation process 34 where one or more surfaces of the component are subjected to pulsed laser radiation of desired characteristics that are scanned across the one or more surfaces of the component by a system such as the one shown in FIG.1 to produce a periodic arrangement of structures, texturing, or profiling on the one or more surfaces of the component. A post-processing step 36 of cleaning the component by using one or more of a solvent, aqueous solution, plasma, vacuum firing, hydrogen firing, etc. may be conducted after the laser structuring process step 34. The component may then be visually inspected for defects and / or inspected to verify dimensional metrics, emissivity metrics, etc. in step 38. The component may then be assembled or installed into an X-ray tube assembly 40. In an embodiment, the above process 30 may be an automated manufacturing process where the component is loaded into a manufacturing system, the manufacturing system executes a predefined program to complete the structuring process through laser ablation (automated movement, rastering, etc.).

[0039] As mentioned previously, another application of the laser ablation process is X-ray tube emitter filament surface profiling for improved electron emission. Surface profiling of flat emitters or coil emitter filaments for increasing electron emission density and / or operating at a lower temperature in X-ray tubes. Electrons are generated by heating a tungsten flat emitter or coil emitter filament to high temperatures. The number of electrons produced is dependent on the temperature of the filament material and the surface area available.Docket No.701348-WO-2

[0040] Increasing the filament temperature becomes impractical at some point due to: 1) evaporation rates increasing with temperature resulting in insufficient filament life; and 2) material properties degrading with increasing temperature, including yield strength and creep resistance, resulting in insufficient filament life.

[0041] Increasing the surface area with conventional means of increasing filament length and width becomes impractical at some point due to: 1) increasing gantry loading with increasing size / mass, resulting in insufficient emitter / filament life; 2) increasing thermal loading with increasing size, resulting in insufficient emitter / filament life; and 3) ability to focus the electron beam from a large emitter / filament area to the desired focal spot size, resulting in decreased imaging resolution.

[0042] An example of an X-ray tube emitter or filament with the same footprint and the same electron emission density, and with a lower temperature. Using surface profiling of the emitter or filament to maintain electron emission at a lower temperature by increasing the surface area of the electron emission surface within the same footprint. This reduces evaporation rate by potentially multiple orders of magnitude given the exponential dependency on temperature. For example, a ten times surface area multiplier nominally dropping temperature by 270 degrees Celsius, reducing evaporation rate by forty times. It also increases material robustness by increasing yield strength and creep resistance by a significant amount. For example, a ten times surface area multiplier drops temperature by 270 degrees Celsius, increasing yield strength by 50% and reducing creep rate by ten times.

[0043] Another example of an X-ray tube emitter or filament with the same footprint operating at the same temperature with a higher electron emission density. Using emitter surface profiling to increase electron emission by maintaining footprint, maintaining temperature, and increasing surface area. This increases emitter performance by increasing electron emission density. For example, a ten times surface area multiplier at the same temperature increases electron emission by ten times.

[0044] In yet another example of an X-ray tube emitter or filament operating at the same temperature, with a higher electron emission density, and with a smaller footprint. Using surface profiling to reduce the footprint by maintaining the emitter filament temperature and maintainingDocket No.701348-WO-2 the emitter filament surface area. This increases design space possibilities for emitter or filament sizes by increasing electron emission density. For example, a five times emitter filament surface area multiplier with the same emitter filament footprint and operating at the same temperature while nominally maintaining the same total electron emission.

[0045] Surface profiled emitters and emitter filaments have not been used in X-ray tube products before for increasing available surface area for electron emission. Standard emitters and emitter filaments have a smooth surface resulting from their forming processes and may even be polished or grinded for achieving an ultra-smooth surface. See FIGS.3A and 4A.

[0046] FIG.3A is an embodiment of a cross-section of a standard flat emitter 52 of an X-ray tube. The flat emitter 52 having a surface 50. FIG.3B is an embodiment of a cross-section of a surface profiled or textured flat emitter 52 of an X-ray tube. The surface profile having a structured or textured pattern that may have been applied in a non-uniform manner including “peaks” 54 representing unremoved material and “trenches” 56 representing removed material from the surface 50 of the flat emitter 52. This provides for a non-uniform emissivity enhancement control.

[0047] FIG.4A is an embodiment of a cross-section of a standard coil emitter filament 62 of an X-ray tube. The coil emitter filament 62 having a surface 60. FIG.4B is an embodiment of a cross-section of a surface profiled or textured coil emitter filament 62 of an X-ray tube. The surface profile having a structured or textured pattern that may have been applied in a non- uniform manner including “peaks” 64 representing unremoved material and “trenches” 66 representing removed material from the surface 60 of the coil emitter filament 62. This provides for a non-uniform emissivity enhancement control.

[0048] In an embodiment, the geometry of a structured or textured raster pattern of a series of linear lines, or circles yielded the best results. Spacing between the raster lines or circles vary from 30um to 500um. Where pulse overlap and ablation rates utilizing the above parameters were key to creating the right micro-geometry to visibly trap light and yield high emissive values.Docket No.701348-WO-2

[0049] Benefits include improved emitter or filament reliability reducing X-ray tube replacement costs and higher X-ray tube performance improving the value proposition. Other benefits include higher X-ray tube emission (higher emission performance), longer emitter or filament life, and a smaller emitter or filament footprint.

[0050] Alternatives may include using larger surface area emitters and emitter filaments with more sophisticated focusing of focal spots, and using coatings on emitters and emitter filaments to reduce the energy needed to release electrons from the surface of the emitter or filament.

[0051] This laser ablation process may be used on various X-ray tube component surfaces to change the emissivity of X-ray tube component materials to desired emissivity values.

[0052] X-ray tube performance is often limited by the X-ray tube target's ability to shed thermal energy it is environment. This can be accomplished through a variety of methods. Traditionally graphite or emissive coatings have been used to provide an effective means of radiating heat. However, each of the prior methods have their limitations.

[0053] FIGS.5A and 5B illustrate two examples of X-ray tube anode targets. FIG.5A is an embodiment of a portion of an anode target 70 with a brazed graphite coating 72 on a bottom surface of the anode target 70. FIG.5B is an embodiment of a portion of an anode target 76 with a coating 78 on a bottom surface of the anode target 76.

[0054] In an embodiment, a method of surface modification of an X-ray tube anode target enhances the emissivity of the X-ray tube target surface and subsequently the radiative heat transfer of the X-ray tube target to its surroundings. This surface modification is not a coating but rather a subtractive modification which may be achieved by use of an energy source such as a laser or through conventional machining equipment. This enhancement may be applied non- uniformly to control the radiative heat transfer of the X-ray tube target so as to control X-ray tube target temperatures in an advantageous manner.

[0055] FIG.6 shows another embodiment of radiation surface enhancement with control in a discrete “digital” way by removing material “pixel” by “pixel”. In this embodiment, each circle or dot 92 of a removal pattern 90 may be a “divot” or crater created by a laser pulse or otherDocket No.701348-WO-2 means. The area density of these discrete material removal features controls radiation heat transfer across the surface.

[0056] The subtractive process includes a laser source that textures the surface of the X-ray tube anode target in a pattern. The pattern is made such that emissivity of the surface of the X- ray tube target when tested at and above room temperature exceeds that of the base material. The result is an enhanced radiative heat transfer at elevated temperatures without the need for spray on coatings or the use of highly emissive materials.

[0057] In the embodiments shown in FIGS.3B, 4B and 6, the distribution of material removal may be established with results from thermal-structural analyses and used for control of a material’s thermal-structural response to loads.

[0058] This disclosure introduces a novel way of enhancing surface emissivity through a subtractive process. By adopting this methodology, one may apply varied levels of surface modification at areas where enhanced emissivity is necessary and less enhancement where it is not needed. By adopting this methodology emissivity may be modified without the introduction of subsequent thermal processing which may degrade mechanical properties of the base material.

[0059] The benefits of this system and method include that the reduced temperature may allow for reduced time between patients because the X-ray tube target may be used sooner without exceeding a temperature limitation. This disclosure also enhances reliability of the X-ray tube target by improving thermal margins which decreases structural load and reduces sources of particulation which can pose a risk to high voltage stability.

[0060] This disclosure may also eliminate a graphite burst failure mode on some X-ray tube target designs by replacing graphite with a surface modification to the base X-ray tube target material.

[0061] Successful implementation of the system and method may increase X-ray tube reliability which in turn helps reduce imaging system down time. The system and method provide quicker cooling and greater thermal / structural margin for rotating X-ray tube anode targets. This translates to higher system throughput and higher mechanical reliability of the anode target. This methodology would allow for enhanced heat exchange without the drawbackDocket No.701348-WO-2 of additional thermal processing. The surface modification enhances radiative heat transfer would violate the concept.

[0062] Embodiments of the present disclosure shown in the drawings and described above are example embodiments only and are not intended to limit the scope of the appended claims, including any equivalents as included within the scope of the claims. Various modifications are possible and will be readily apparent to the skilled person in the art. It is intended that any combination of non-mutually exclusive features described herein are within the scope of the present disclosure. That is, features of the described embodiments can be combined with any appropriate aspect described above and optional features of any one aspect can be combined with any other appropriate aspect. Similarly, features set forth in dependent claims can be combined with non-mutually exclusive features of other dependent claims, particularly where the dependent claims depend on the same independent claim. Single claim dependencies may have been used as practice in some jurisdictions require them, but this should not be taken to mean that the features in the dependent claims are mutually exclusive.

Claims

Docket No.701348-WO-2 CLAIMS What is claimed is:

1. A system for texturing a surface of a component, comprising: a laser source for applying pulsed laser radiation to the surface of the component; and a laser controller configured to control the laser source to apply the laser radiation as a series of laser pulses to produce an arrangement of varying structures on the surface of the component to improve emissivity of the component.

2. The system of claim 1, wherein the component is a flat emitter of an X-ray tube.

3. The system of claim 1, wherein the component is a coil emitter filament of an X- ray tube.

4. The system of claim 1, wherein the component is an anode target of an X-ray tube.

5. The system of claim 1, wherein the component is a frame of an X-ray tube.

6. The system of claim 1, wherein the varying structures are modifications of the surface of the component in a raster pattern of material removal in a series of linear lines, grooves, trenches, circles, or divots.

7. The system of claim 6, wherein the modified surface of the component has an emissivity that exceeds the emissivity of the base material of the component.

8. An X-ray tube component comprising a modified surface that is in a raster pattern of material removal in a series of linear lines, grooves, trenches, circles, or divots.

9. The X-ray tube component of claim 8, wherein the X-ray tube component is a flat emitter.Docket No.701348-WO-2 10. The X-ray tube component of claim 8, wherein the X-ray tube component is a coil emitter filament.

11. The X-ray tube component of claim 8, wherein the X-ray tube component is an anode target.

12. The X-ray tube component of claim 8, wherein the X-ray tube component is a frame.

13. The X-ray tube component of claim 8, wherein the modified surface of the X-ray tube component has an emissivity that exceeds the emissivity of the base material of the X-ray tube component.

14. A method of texturing a surface of a component, comprising: applying laser radiation to the surface of the component to produce an arrangement of varying structures on the surface of the component to improve the emissivity of the component.

15. The method of claim 14, wherein the applying laser radiation to the surface of the component is through laser ablation that modifies the surface of the component.

16. The method of claim 15, wherein the surface of the component is modified in a raster pattern of material removal in a series of linear lines, grooves, trenches, circles, or divots.

17. The method of claim 14, wherein the component is a flat emitter of an X-ray tube.

18. The method of claim 14, wherein the component is a coil emitter filament of an X-ray tube.

19. The method of claim 14, wherein the component is an anode target of an X-ray tube.Docket No.701348-WO-2 20. The method of claim 14, wherein the component is a frame of an X-ray tube.

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