Patent Publication Number: US-8542799-B1

Title: Anti-fretting coating for attachment joint and method of making same

Description:
BACKGROUND OF THE INVENTION 
     Embodiments of the invention relate generally to x-ray tubes and, more particularly, to an anti-fretting coating for an attachment joint and a method of making same. 
     Computed tomography X-ray imaging systems typically include an x-ray tube, a detector, and a gantry assembly to support the x-ray tube and the detector. In operation, an imaging table, on which an object is positioned, is located between the x-ray tube and the detector. The x-ray tube typically emits radiation, such as x-rays, toward the object. The radiation typically passes through the object on the imaging table and impinges on the detector. As radiation passes through the object, internal structures of the object cause spatial variances in the radiation received at the detector. The detector converts the received radiation to electrical signals and then transmits data received, and the system translates the radiation variances into an image, which may be used to evaluate the internal structure of the object. One skilled in the art will recognize that the object may include, but is not limited to, a patient in a medical imaging procedure and an inanimate object as in, for instance, a package in an x-ray scanner or computed tomography (CT) package scanner. 
     A typical x-ray tube includes a cathode that provides a focused high energy electron beam that is accelerated across a cathode-to-anode vacuum gap and produces x-rays upon impact with an active material or target provided. Because of the high temperatures generated when the electron beam strikes the target, typically the target assembly is rotated at high rotational speed for purposes of spreading the heat flux over a larger extended area. 
     As such, the x-ray tube also includes a rotating system that rotates the target for the purpose of distributing the heat generated at a focal spot on the target. The rotating subsystem is typically rotated by an induction motor having a cylindrical rotor built into an axle that supports a disc-shaped target and an iron stator structure with copper windings that surrounds an elongated neck of the x-ray tube. The rotor of the rotating subsystem assembly is driven by the stator. 
     The target is attached to a support shaft, which is in turn supported by roller bearings that are typically hard mounted to a base plate. Thus, the target provides a thermal path to the roller bearings that can cause the roller bearings to operate at elevated temperature, compromising the life thereof. In order to minimize or reduce the operating temperature of the bearings, often a thermally resistive material is placed between the target and the bearings. The thermally resistive material, referred to sometimes as a thermal barrier, can be designed having a high thermal resistance to include using a material having a relatively low thermal conductivity, a very thin wall and additional length—all resulting in an increased thermal resistance between the target and the bearing. Thermal resistance can be further increased by introducing a bolted joint between the shaft and the roller bearings, as it is well known that contact resistance in, for instance, a bolted joint can cause a large thermal resistance and temperature drop thereacross in conduction heat transfer. As known in the art, bolted joint strength may be enhanced by designing components such that they have an interference fit, and in some instances bolts may be foregone entirely, leaving joint strength entirely to the interference fit at an interface therebetween. Not only may such designs be intended to increase thermal conductivity, bolted and/or interference joints may be introduced into a design to facilitate assembly of components (such as an anode or target assembly) during fabrication of an x-ray tube. 
     However, because the target is typically rotated about its axis at a high rate of speed, typically 100 Hz or more, and because the x-ray tube itself is rotated at a high rate of speed on a gantry, typically 2 Hz or more, enormous periodic loads can be generated at interfaces that join the target and other rotating components. So, high-frequency periodic loads are applied to the joint due to the target rotation and some unavoidable residual unbalance of the rotating components and low-frequency periodic loads due to the tube rotation on the CT gantry. Such loads in a bolted joint can cause bending of the joints components causing small relative motion to occur, which can cause fretting, leading to particulate generation within the x-ray tube. Fretting and particulate generation can occur in bolted joints and at interfaces that include, for instance, interference joints. In fact, particles can be generated at any interface where materials are such as in a bolted joint or an interference fit pressed together (but not fused or otherwise bonded together, such as in a welded or brazed joint, as examples). And, the effect can increase significantly with increased gantry and/or increased target rotating speed, leading to increased fretting and particulate generation as x-ray tubes are rotated faster on gantries and as targets are rotated faster within x-ray tubes. 
     As known in the art, particulate in an x-ray tube can degrade performance and life in a number of ways that include, for instance, accelerated bearing wear if the wear particles fall into the bearing and electrical discharge activity in the high voltage environment of the x-ray tube. Both of these issues reduce the useful life of the x-ray tube. 
     Accordingly, it would be advantageous to have an x-ray tube that could be rotated at a high speed on a gantry and at a high target rotational speed without a reduction in life due to particulate generation at connection joints in the x-ray tube. 
     BRIEF DESCRIPTION OF THE INVENTION 
     Embodiments of the invention provide an apparatus and method of attaching a target to a bearing having a reduced amount of particulate generation at interfaces of attachment locations thereof. 
     According to one aspect of the invention, an x-ray tube includes a cathode adapted to emit electrons, a bearing assembly comprising a bearing hub, a target assembly positioned to receive the emitted electrons, the assembly having a target hub coupled to the bearing hub at an attachment face, wherein the attachment face comprises a first material compressed against a second material, and a first anti-wear coating attached to one of the first material and the second material and positioned between the first material and the second material. 
     In accordance with another aspect of the invention, a method of fabricating an anode assembly for an x-ray tube includes applying a first anti-wear coating to one of a first material and a second material, and coupling an x-ray target to a bearing at an interface that is comprised of the first material and the second material. 
     Yet another aspect of the invention includes an x-ray imaging system that includes a gantry, a detector attached to the gantry, and an x-ray tube attached to the gantry. The x-ray tube includes a bearing having a bearing hub, a target having a target hub coupled to the bearing hub at a contact location, and a first anti-fretting coating. The contact location includes a first material attached to a second material, and the first anti-fretting coating is attached to one of the first material and the second material at the contact location and is positioned between the first material and the second material. 
     Various other features and advantages of the invention will be made apparent from the following detailed description and the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawings illustrate one preferred embodiment presently contemplated for carrying out the invention. 
       In the drawings: 
         FIG. 1  is a block diagram of an imaging system that can benefit from incorporation of an embodiment of the invention. 
         FIG. 2  is a cutaway view of an x-ray tube or source incorporating embodiments of the invention. 
         FIG. 3  is an illustration of an interference fit joint, according to an embodiment of the invention. 
         FIG. 4  is an illustration of a bolted joint, according to an embodiment of the invention. 
         FIG. 5  is a joint including a thermal barrier, according to an embodiment of the invention. 
         FIG. 6  is a pictorial view of a CT system for use with a non-invasive package inspection system. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a block diagram of an embodiment of an imaging system  10  designed both to acquire original image data and to process the image data for display and/or analysis in accordance with the invention. It will be appreciated by those skilled in the art that the invention is applicable to numerous medical imaging systems implementing an x-ray tube, such as x-ray or mammography systems. Other imaging systems such as computed tomography (CT) systems and digital radiography (RAD) systems also benefit from the invention. In a CT system, for instance, x-ray source  12  and detector  18  may be mounted on a gantry (not shown) and rotated about object  16  at a high rate of speed or, for instance, 2 Hz or greater. The following discussion of x-ray system  10  is merely an example of one such implementation and is not intended to be limiting in terms of modality. 
     As shown in  FIG. 1 , x-ray system  10  includes an x-ray source  12  configured to project a beam of x-rays  14  through an object  16 . Object  16  may include a human subject, pieces of baggage, or other objects desired to be scanned. X-ray source  12  may be a conventional x-ray tube producing x-rays having a spectrum of energies that range, typically, from 30 keV to 200 keV. The x-rays  14  pass through object  16  and, after being attenuated by the object, impinge upon a detector  18 . Each detector in detector  18  produces an analog electrical signal that represents the intensity of an impinging x-ray beam, and hence the attenuated beam, as it passes through the object  16 . In one embodiment, detector  18  is a scintillation based detector, however, it is also envisioned that direct-conversion type detectors (e.g., CZT detectors, etc.) may also be implemented. 
     A processor  20  receives the signals from the detector  18  and generates an image corresponding to the object  16  being scanned. A computer  22  communicates with processor  20  to enable an operator, using operator console  24 , to control the scanning parameters and to view the generated image. That is, operator console  24  includes some form of operator interface, such as a keyboard, mouse, voice activated controller, or any other suitable input apparatus that allows an operator to control the x-ray system  10  and view the reconstructed image or other data from computer  22  on a display unit  26 . Additionally, console  24  allows an operator to store the generated image in a storage device  28  which may include hard drives, flash memory, compact discs, etc. The operator may also use console  24  to provide commands and instructions to computer  22  for controlling a source controller  30  that provides power and timing signals to x-ray source  12 . 
       FIG. 2  illustrates a cutaway portion of an x-ray source or tube  50  constructed in accordance with the invention. X-ray source or tube  50  may be used in any system using x-rays for imaging, and in one example is x-ray source  12  of  FIG. 1 . X-ray tube  50  includes a frame or casing  52  that encloses a vacuum  54  and houses an anode assembly  56 , a bearing assembly  58 , a cathode  60 , and a rotor  62 . X-rays  14  are produced when high-speed electrons are suddenly decelerated when directed from cathode  60  to anode assembly  56 , and particularly to a focal spot  64  via a potential difference therebetween of, for example, 60 thousand volts or more. The electrons impact focal spot  64  and x-rays  14  emit therefrom toward a detector, such as detector  18  illustrated in  FIG. 1 . To avoid overheating anode  56  from the electrons, anode  56  is rotated  65  at a high rate of speed about a centerline  66  at, for example, 90-250 Hz. 
     Bearing assembly  58  includes a center shaft  68  attached to rotor  62  at a first end  70  and attached to anode assembly  56  at a second end  72 . A front inner race  74  and a rear inner race  76  rollingly engage a plurality of front balls  78  and a plurality of rear balls  80 , respectively. Bearing assembly  58  also includes a front outer race  82  and a rear outer race  84  configured to rollingly engage and position, respectively, the plurality of front balls  78  and the plurality of rear balls  80 . Bearing assembly  58  includes a stem  86  which is supported by a backplate  88  of x-ray tube  50 . A stator (not shown) is positioned radially external to and drives rotor  62 , which rotationally drives anode assembly  56 . 
     Anode assembly  56  includes a target  90  having a heat sink material  92  such as graphite attached thereto. Target  90  is attached to a bearing hub  94  at an attachment location or contact region  96  via a number of means that are illustrated in subsequent embodiments of  FIGS. 3-5 . As known in the art, x-ray tube  50  may be positioned on a gantry (not shown) and caused to rotate  97  about a gantry rotational axis  98 . Thus in operation, still referring to  FIG. 2 , at least two factors can combine to cause relative part motion and fretting in an x-ray tube. First, as anode  56  is caused to rotate about centerline  66  at a high rate of speed, such as 100 Hz or greater, a high frequency input is thus imparted on components at, for instance, contact region  96 . Second, by rotating  97  x-ray tube  50  about gantry rotational axis  98  at typically 2 Hz or greater, a bending moment  99  is imposed on components of anode  56  and specifically on contact region  96 . As such, relative motion occurs at attachment location or contact region  96  due to the high frequency input of 100 Hz or more, which is exacerbated when compounded with the low frequency component of 2 Hz or greater that is caused by moment  99 . As such, as gantry rotational speeds increase above 2 Hz, the effect of wear and fretting of components is compounded, leading to early life failure. 
     Referring now to  FIG. 3 , an enlarged view of attachment location  96  of x-ray source or tube  50  of  FIG. 2  is illustrated. Attachment location  96  includes center shaft  68  having bearing hub  94  inserted into an interference-fit region  98  of anode assembly  56  and target  90 . Interference-fit region  98  includes an inner surface  100  of attachment location  96  having an interference-fit diameter  102  that corresponds to a hub diameter  104 . As known in the art, an interference fit between mating components may be formed by designing components such that they interfere at operating temperature. That is, through appropriate analysis, knowledge of material properties such as material expansion coefficients, and knowledge of for instance temperatures of components during operation, parts fabricated at or near room temperature may be sized appropriately such that an interference fit is formed between components at elevated temperature and during operation. 
     Referring still to  FIG. 3 , bearing hub  94  is inserted into interference-fit region  98  such that bearing hub  94  and target  90  are essentially locked together and rotate together during operation. As known in the art, the interference fit may be formed by, for instance, inserting bearing hub  94  into interference-fit region  98  using a lever to force the components together (i.e., a press-fit). In another example, the interference fit may be formed by heating interference-fit region  98  of target  90  to excess temperature such that interference-fit diameter  102  expands to be greater than hub diameter  104  of bearing hub  94 . That is, target  90  may be heated to excess temperature above, for instance, 300° C. or more, such that bearing hub  94  may fit therein without interference. As target  90  cools, interference-fit region  98  contracts and forms an interference fit with bearing hub  94 . In one example an expanded diameter  106  of target  90  may be included such that an axial interference contact length  108  is formed that is sufficient to maintain component integrity, facilitating insertion of bearing hub  94  into interference-fit region  98 . Thus, one skilled in the art will recognize that using appropriate and known techniques, axial interference contact length  108  may be formed such that sufficient interference is maintained during operation when both bearing hub  94  and interference-fit region  98  are at operating temperatures. 
     As stated, due to enormous loads during operation from high frequency-induced relative motion that is compounded by low frequency input from rotation about the gantry, fretting and relative motion of components may cause particulate to generate at a first interference location  110  such as where outer diameter of bearing hub  94  contacts target  90 , and/or at a second location  112  such as along an axial surface where bearing hub  94  contacts target  90 . Thus, according to the invention an anti-wear or anti-fretting coating may be applied to bearing hub  94  at a first hub location as a first hub coating  114 , or a second hub location as a second hub coating  116 . Similarly, an anti-wear or anti-fretting coating may be applied to target  90  at a first target location as a first target anti-wear or anti-fretting coating  118  or a second target location as a second anti-wear or anti-fretting target coating  120 . According to the invention, coatings  114 - 120  may be chromium nitride, titanium nitride, diamond-like carbon, tungsten carbide, tungsten carbon-carbon (WC/C), TiCN, TiAlN, AlTiN, and ZrN, as examples. Further, although a number of examples are provided, it is contemplated that the invention is not to be so limited. According to the invention, coatings  114 - 120  may include any material for a coating that reduces fretting, wear of components, and ultimately particulate generation for rotating components in a vacuum, such as in an x-ray tube, that have counterfaces pressed or otherwise maintained against each other. In one example coatings  114 - 120  include materials having a hardness of 1750 measured on the Vickers HV scale. 
     Coatings  114 - 120  reduce wear and fretting via one or more processes. Firstly, the coating is harder than the base material to which it is adhered, so its wear rate (adhesive and abrasive wear rate) is lower than the base material. Secondly, in a vacuum its coefficient of friction can be lower than the base material system thereby lower friction wear action. Also, the metallurgical affinity between the counterface materials is much less by using dissimilar materials. These factors all combine to reduce the rate of particulate production in high temperature and high vacuum environments, such as experienced in an x-ray tube, of up to approximately 600° C. in a vacuum of 1E-6 torr. Thus, particulate generation can be reduced by using preferably different coatings on each mating surface (e.g., CrN-WC). In another example coatings  114 - 120  are applied having a thickness of approximately 2-5 microns (although coatings such as coatings  114 - 120  for this and other embodiments are shown having thicknesses greater than 2-5 microns for illustrative purposes). Further, it is contemplated that any coating thickness may be applied for coatings  114 - 120  and other coatings described herein, and that the invention is not limited to coating thicknesses of 2-5 microns, but may have greater or lesser thicknesses than 2-5 microns. 
     According to the invention, coatings  114 - 120  may be applied using physical vapor deposition (PVD) (such as but not limited to sputtering and ion plating, as examples) and other known techniques for applying a smooth and uniform application of material. Further, embodiments of the invention include having coatings applied to each part such that a first coating is pressed against a second coating. For instance, in one embodiment coating  114  may be applied to bearing hub  94  and coating  118  may be applied to target  90  at attachment location  96  such that coating  114  is pressed against coating  118  when the interference fit is formed. In this embodiment, coatings  114  and  118  are preferably of different materials. That is, as one example coating  114  may be chromium nitride and coating  118  may be titanium nitride. In another example, coating  118  is diamond-like carbon and bearing hub  94  is uncoated (i.e., coating  114  is not present). As such, embodiments of the invention include a first material pressed against a second material, and the opposing materials are preferably of different materials. Thus, because of the different materials, friction therebetween the two is minimized and there is a reduced amount of adhesive wear because an amount of diffusion bonding between the materials is reduced, as compared to an interface of two of the same materials pressed against each other. 
     As stated,  FIG. 3  illustrates an interference fit between a bearing hub and a target that may be assembled using known techniques such as a press fit or an interference fit that is formed by heating the target to cause expansion of the target such that the bearing hub may be positioned therein. However, according to the invention the target may be attached to the bearing hub using other known techniques. For instance,  FIG. 4  illustrates a bolted joint that may also include an interference fit, for additional joint stability, similar to that illustrated in  FIG. 3 . In yet another embodiment of the invention, illustrated in  FIG. 5 , a thermal barrier may be provided that includes at least two bolted joint regions and may include interference fits of components, as well. 
     Referring now to  FIG. 4 , a bolted joint  122  may be used to directly attach target  90  to bearing hub  94 . In this embodiment bearing hub  94  includes a flange  124  having flange holes  126 , and target  90  having target holes  128  that match with locations of flange holes  126  such that target  90  may be bolted to bearing hub  94 . According to the invention a flange face coating  130  may be applied to flange  124 , or a target wear coating  132  may be applied to target  90 . In such fashion, when target  90  is attached to bearing hub  94  via bolts  134 , coatings  130  or  132  applied as illustrated at one or the other location reduces an amount of fretting and particulate generation by having a low coefficient of friction therebetween, and materials that are not chemically compatible so as to avoid diffusion bonding. 
     Still referring to  FIG. 4 , bolted joint  122  may include an interference fit between flange  124  and target  90  at flange outer diameter  136 , in order to enhance the strength of bolted joint  122 . Thus, similar to that described with respect to  FIG. 3 , in an embodiment that includes an interference fit, additional coatings may be applied as a flange outer diameter coating  138  and an interference fit inner diameter coating  140   
     Referring now to  FIG. 5 , a thermal barrier  142  is used to attach target  90  to bearing hub  94  via a first bolted joint  144  and a second bolted joint  146 . In one example thermal barrier  142  is Incoloy 909® (Incoloy is a registered trademark of Inco Alloys International, Inc. of Delaware), selected for its relatively low thermal conductivity (compared to, for instance, a bearing hub) and stability for machining and during operation, as examples. According to one embodiment, bolted joints  144 ,  146  are sufficient to provide attachment of bearing hub  94  to target  90 . However, in another embodiment additional joint strength may be provided between a bearing flange  148  and an inner diameter  150  of thermal barrier  142  by providing a first interference fit  152  as described above with respect to other embodiments. Similarly, additional joint strength may be provided between an outer diameter  154  of thermal barrier  142  and an inner diameter  156  of target  90  to form a second interference fit  158 . Thus, a material  160  may be applied to thermal barrier  142 , a material  162  may be applied to bearing hub  94 , and a material  164  may be applied to target  90 , as described above with respect to other embodiments of the invention, such that dissimilar materials are applied at contact locations formed by the two bolted joints  144 ,  146 . 
     Thus, according to the embodiments illustrated, a target may be attached to a bearing hub by using interference fits, bolted joints, or combinations thereof. Further, such attachment may also be accomplished using a thermal barrier and bolted joints, interference fits, or combinations thereof. In locations where contact points or surfaces are formed, anti-wear or anti-fretting coatings may be applied to one contact surface, the other contact surface, or both. As such, embodiments of the invention include a first material pressed against a second material, and the opposing materials are preferably of different materials. Thus, because of the different materials, friction therebetween the two is minimized and there is a reduced amount of adhesive wear because an amount of diffusion bonding between the materials is reduced, as compared to two of the same materials pressed against each other. 
     Further, although the embodiments described are for an x-ray tube application and for a joint attaching an x-ray tube target to a bearing hub, it is to be understood that the invention is not to be so limited, and it is contemplated that the invention may be applicable to any rotating components where fretting may occur, causing particulate generation. 
       FIG. 6  is a pictorial view of an x-ray system  500  for use with a non-invasive package inspection system. The x-ray system  500  includes a gantry  502  having an opening  504  therein through which packages or pieces of baggage may pass. The gantry  502  houses a high frequency electromagnetic energy source, such as an x-ray tube  506 , and a detector assembly  508 . A conveyor system  510  is also provided and includes a conveyor belt  512  supported by structure  514  to automatically and continuously pass packages or baggage pieces  516  through opening  504  to be scanned. Objects  516  are fed through opening  504  by conveyor belt  512 , imaging data is then acquired, and the conveyor belt  512  removes the packages  516  from opening  504  in a controlled and continuous manner. As a result, postal inspectors, baggage handlers, and other security personnel may non-invasively inspect the contents of packages  516  for explosives, knives, guns, contraband, etc. One skilled in the art will recognize that gantry  502  may be stationary or rotatable. In the case of a rotatable gantry  502 , system  500  may be configured to operate as a CT system for baggage scanning or other industrial or medical applications. 
     According to an embodiment of the invention, an x-ray tube includes a cathode adapted to emit electrons, a bearing assembly comprising a bearing hub, a target assembly positioned to receive the emitted electrons, the assembly having a target hub coupled to the bearing hub at an attachment face, wherein the attachment face comprises a first material compressed against a second material, and a first anti-wear coating attached to one of the first material and the second material and positioned between the first material and the second material. 
     According to another embodiment of the invention, a method of fabricating an anode assembly for an x-ray tube includes applying a first anti-wear coating to one of a first material and a second material, and coupling an x-ray target to a bearing at an interface that is comprised of the first material and the second material. 
     Yet another embodiment of the invention includes an x-ray imaging system that includes a gantry, a detector attached to the gantry, and an x-ray tube attached to the gantry. The x-ray tube includes a bearing having a bearing hub, a target having a target hub coupled to the bearing hub at a contact location, and a first anti-fretting coating. The contact location includes a first material attached to a second material, and the first anti-fretting coating is attached to one of the first material and the second material at the contact location and is positioned between the first material and the second material. 
     The invention has been described in terms of the preferred embodiment, and it is recognized that equivalents, alternatives, and modifications, aside from those expressly stated, are possible and within the scope of the appending claims.