Patent Publication Number: US-2021179906-A1

Title: Abrasive coating including metal matrix and ceramic particles

Description:
TECHNICAL FIELD 
     The disclosure relates to abrasive coatings. 
     BACKGROUND 
     Systems that use compression of fluids, such as pumps, compressors, turbines, and the like use seals between rotating and static components to improve their functioning, such as efficiency. In low temperature applications, the seals may be formed from materials such as rubber or polymers. In high temperature systems, such as gas turbine engines, rotating components may include an abrasive coating on tips or end walls of the rotating components and adjacent static components may include abradable coatings. During use, the abrasive coating may wear a groove into the abradable coating to form a seal while reducing or substantially preventing damage to the rotating component. 
     SUMMARY 
     In some examples, the disclosure describes a method that includes controlling, by a computing device, a powder source to deliver metal powder to a powder delivery device; controlling, by the computing device, the powder delivery device to deliver the metal powder to a surface of an abrasive coating; and controlling, by the computing device, an energy delivery device to deliver energy to at least one of the abrasive coating or the metal powder to cause the metal powder to be joined to the abrasive coating. 
     In some examples, the disclosure describes a system that includes a powder source; a powder delivery device; an energy delivery device; and a computing device. The computing device is configured to: control the powder source to deliver metal powder to the powder delivery device; control the powder delivery device to deliver the metal powder to a surface of an abrasive coating; and control the energy delivery device to deliver energy to at least one of the abrasive coating or the metal powder to cause the metal powder to be joined to the abrasive coating. 
     In some examples, the disclosure describes a computer-readable storage device comprising instructions that, when executed, configure one or more processors of a computing device to: control a powder source to deliver metal powder to a powder delivery device; control the powder delivery device to deliver the metal powder to a surface of an abrasive coating; and control an energy delivery device to deliver energy to at least one of the abrasive coating or the metal powder to cause the metal powder to be joined to the abrasive coating. 
     In some examples, the disclosure describes a method including controlling, by a computing device, an energy delivery device to deliver energy to an abrasive coating, wherein the abrasive coating comprises a metal matrix and abrasive particles at least partially encapsulated by the metal matrix; and controlling, by the computing device, the energy delivery device to scan the energy across a surface of the abrasive coating and form a series of softened or melted portions of the metal matrix. 
     In some examples, the disclosure is directed to a system that includes an energy delivery device; and a computing device, wherein the computing device is configured to: control the energy delivery device to deliver energy to an abrasive coating, wherein the abrasive coating comprises a metal matrix and abrasive particles at least partially encapsulated by the metal matrix; and control the energy delivery device to scan the energy across a surface of the abrasive coating and form a series of softened or melted portions of the metal matrix. 
     In some examples, the disclosure describes a computer-readable storage device comprising instructions that, when executed, configure one or more processors of a computing device to: control an energy delivery device to deliver energy to an abrasive coating, wherein the abrasive coating comprises a metal matrix and abrasive particles at least partially encapsulated by the metal matrix; and control the energy delivery device to scan the energy across a surface of the abrasive coating and form a series of softened or melted portions of the metal matrix. 
     The details of one or more examples are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a conceptual block diagram illustrating an example system for processing an abrasive coating after deposition of the abrasive coating to improve properties of the abrasive coating. 
         FIG. 2  is a flow diagram illustrating an example technique for processing an abrasive coating after deposition of the abrasive coating to improve properties of the abrasive coating. 
         FIG. 3  is a conceptual diagram illustrating an example article including an abrasive coating. 
         FIG. 4  is a flow diagram illustrating an example technique for processing an abrasive coating after deposition of the abrasive coating to improve properties of the abrasive coating. 
         FIG. 5  is a flow diagram illustrating an example technique for processing forming an abrasive coating using directed energy deposition material addition an abrasive coating after deposition of the abrasive coating to improve properties of the abrasive coating. 
         FIG. 6  is a conceptual diagram illustrating a system that includes a rotating component, an abrasive coating on the rotating component, and a stationary component adjacent the rotating component. 
     
    
    
     DETAILED DESCRIPTION 
     The disclosure generally describes techniques for forming abrasive coatings that include a metal matrix and ceramic abrasive particles. Some such coatings have been formed using electroplating. However, electroplating limits the alloys that can be used to alloys compatible with electroplating processes. Many of these alloys are relatively low temperature alloys, which limits the systems in which the abrasive coatings can be used. 
     Other abrasive coatings that include a metal matrix and ceramic abrasive particles have been deposited using additive manufacturing techniques, such as blown powder directed energy deposition. While blown powder directed energy deposition may allow use of alloys that are not compatible with electroplating, abrasive coatings deposited using blown powder directed energy deposition may result in abrasive coatings that are more uneven (e.g., have a higher surface roughness) than abrasive coatings deposited using electroplating. The higher surface roughness may reduce effectiveness as an abrasive coating, as the seal formed by the abrasive coating wearing into an adjacent abradable coating may be less complete. Additionally, or alternatively, the abrasive particles may be less embedded in the metal matrix, leading to a greater chance of the abrasive particles becoming dislodged from the metal matrix and a shorter lifetime of the abrasive coating. 
     In accordance with some examples of this disclosure, an abrasive coating may be subjected to additional processing after initial deposition to improve properties of the abrasive coating. In some examples, after the abrasive coating is formed on a substrate, directed energy deposition may be used to deposit additional metal matrix material, which further encapsulates the abrasive particles in the abrasive coating, reduces surface roughness, or both. In other examples, after the abrasive coating is formed on a substrate, an energy source, such as a laser or a plasma may be used to soften or melt the metal matrix. The softening or melting of the metal matrix may allow surface tension/surface energy to cause the metal matrix to flow around the abrasive particles and make the height and surface roughness of the abrasive coating more uniform. 
     In some examples, the additional powder deposition technique and the softening or melting technique may be used together, in either order. For example, after the abrasive coating is formed on a substrate, directed energy deposition may be used to deposit additional metal matrix material. The abrasive coating including the additional metal matrix material then may be softened or melted using a laser or a plasma energy source. As another example, after the abrasive coating is formed on a substrate, the abrasive coating may be softened or melted using a laser or a plasma energy source, then directed energy deposition may be used to deposit additional metal matrix material. In either case, the combination of the two technique may be used to provide lower surface roughness and more uniform height to the abrasive coating, while allowing use of high temperature metal matrices, such as nickel- or cobalt-based superalloys. 
       FIG. 1  is a conceptual block diagram illustrating an example system  10  for processing an abrasive coating  22  after deposition to improve properties of abrasive coating  22 . In the example illustrated in  FIG. 1 , system  10  includes a computing device  12 , a powder delivery device  14 , an energy delivery device  16 , a stage  18 , a first material source (FMS)  24 , and an optional second material source (SMS)  24 . Computing device  12  is communicatively connected to powder delivery device  14 , energy delivery device  16 , and stage  18 . 
     Abrasive coating  22  is on a surface or surfaces of component  20 . Component  20  may be any component of a mechanical system in which a seal is to be made between component  20  and an adjacent component. For example, component  20  may be a rotating component in a mechanical system, which is adjacent to a stationary component or a counter-rotating component. For instance, component  20  may be a knife seal, a blade (such as a gas turbine engine blade), a wall of a scroll compressor or other compressor or pump, or the like. The adjacent component may include a stator, a runner, a blade track or blade shroud, or the like. In some examples, component  20  is formed from a metal or an alloy, such as a steel (e.g., stainless steel), a nickel-based alloy, a cobalt-based alloy, a titanium-based alloy, or the like. 
     Abrasive coating  22  may include a metal matrix and abrasive particles. The metal matrix may include any suitable metal or alloy. In some examples, the metal matrix includes a composition that is the same as the composition of component  20 . In other examples, the metal matrix includes a composition that is different from the composition of component  20 . 
     In some examples, the metal matrix may include a high-performance metal or alloy, such as a steel (e.g., stainless steel), a nickel-based alloy, a cobalt-based alloy, a titanium-based alloy, or the like. In some examples, the metal matrix may include a nickel-based, iron-based, or titanium-based alloy that includes one or more alloying additions such as one or more of Mn, Mg, Cr, Si, Co, W, Ta, Al, Ti, Hf, Re, Mo, Ni, Fe, B, Nb, V, C, and Y. In some examples, the metal matrix may include a polycrystalline nickel-based superalloy or a polycrystalline cobalt-based superalloy, such as an alloy including NiCrAlY or CoNiCrAlY. For example, the metal matrix may include an alloy that includes 9 to 10.0 wt. % W, 9 to 10.0 wt. % Co, 8 to 8.5 wt. % Cr, 5.4 to 5.7 wt. % Al, about 3.0 wt. % Ta, about 1.0 wt. % Ti, about 0.7 wt. % Mo, about 0.5 wt. % Fe, about 0.015 wt. % B, and balance Ni, available under the trade designation MAR-M 247™, from various suppliers. In some examples, the metal matrix may include an alloy that includes 22.5 to 24.35 wt. % Cr, 9 to 11 wt. % Ni, 6.5 to 7.5 wt. % W, less than about 0.55 to 0.65 wt. % of C, 3 to 4 wt. % Ta, and balance Co, available under the trade designation MAR-M 509™, from various suppliers. In some examples, the metal matrix may include an alloy that includes 19 to 21 wt. % Cr, 9 to 11 wt. Ni, 14 to 16 wt. % W, about 3 wt. % Fe, 1 to 2 wt. % Mn, and balance Co, available under the trade designation HAYNES® 25 alloy from Haynes International, Kokomo, Ind. In some examples, a metal matrix may include a chemically modified version of MAR-M 247™ that includes less than 0.3 wt. % C, between 0.05 and 4 wt. % Hf, less than 8 wt. % Re, less than 8 wt. % Ru, between 0.5 and 25 wt. % Co, between 0.0001 and 0.3 wt. % B, between 1 and 20 wt. % Al, between 0.5 and 30 wt. % Cr, less than 1 wt. % Mn, between 0.01 and 10 wt. % Mo, between 0.1 and 20. % Ta, and between 0.01 and 10 wt. % Ti. In some examples, the metal matrix may include a nickel based alloy available under the trade designation IN-738 or INCONEL® 738, or a version of that alloy, IN-738 LC, available from Special Metals Corporation, New Hartford, N.Y., or a chemically modified version of IN-738 that includes less than 0.3 wt. % C, between 0.05 and 7 wt. % Nb, less than 8 wt. % Re, less than 8 wt. % Ru, between 0.5 and 25 wt. % Co, between 0.0001 and 0.3 wt. % B, between 1 and 20 wt. % Al, between 0.5 and 30 wt. % Cr, less than 1 wt. % Mn, between 0.01 and 10 wt. % Mo, between 0.1 and 20 wt. % Ta, between 0.01 and 10 wt. % Ti, and a balance Ni. In some examples, the metal matrix may include an alloy that includes 5.5 to 6.5 wt. % Al, 13 to 15 wt. % Cr, less than 0.2 wt. % C, 2.5 to 5.5 wt. % Mo, Ti, Nb, Zr, Ta, B, and balance Ni, available under the trade designation IN-713 or INCONEL® 713 from Special Metals Corporation, New Hartford, N.Y. 
     In some examples, the metal matrix may include a refractory metal or a refractory metal alloy, such as molybdenum or a molybdenum alloy (such as a titanium-zirconium-molybdenum or a molybdenum-tungsten alloy), tungsten or a tungsten alloy (such as a tungsten-rhenium alloy or an alloy of tungsten and nickel and iron or nickel and copper), niobium or a niobium alloy (such as a niobium-hafnium-titanium alloy), tantalum or a tantalum alloy, rhenium or a rhenium alloy, or combinations thereof. 
     The abrasive particles may include a ceramic, such as a metal nitride, a metal carbide, a metal oxide, or the like. For example, the abrasive particles may include cubic boron nitride, aluminum oxide, zirconium oxide, silicon carbide, silicon nitride, titanium nitride, zirconium nitride, tantalum nitride, hafnium nitride, or the like. 
     In some examples, stage  18  is movable relative to energy delivery device  16  and/or energy delivery device  16  is movable relative to stage  18 . Similarly, stage  18  may be movable relative to powder delivery device  14  and/or powder delivery device  14  may be movable relative to stage  18 . For example, stage  18  may be translatable along at least one axis (e.g., the z-axis shown for purposes of illustration in  FIG. 1 ) to position component  20  relative to energy delivery device  16  and/or powder delivery device  14 . Similarly, energy delivery device  16  and/or powder delivery device  14  may be translatable and/or rotatable along at least one axis (e.g., translatable along the x- and y-axes shown in  FIG. 1 ) to position energy delivery device  16  and/or powder delivery device  14 , respectively, relative to component  20 . Stage  18  may be configured to selectively position and restrain component  20  in place relative to stage  18  during manufacturing and/or processing of abrasive coating  22 . 
     Powder delivery device  14  may be configured to deliver material to the location of abrasive coating  22 . In some examples, the material may be supplied by powder delivery device  14  in powder form, e.g., as a metal powder, an abrasive powder, or a mixture of metal powder and abrasive powder. 
     In some examples, system  10  may be a blown powder directed energy deposition system. In some such systems, powder delivery device  14  may deliver the powder adjacent to the surface of component  20  and/or abrasive coating  22  by blowing the powder in a powder stream adjacent to the surface, e.g., as a mixture of the powder with a gas carrier. In some examples, powder delivery device  14  thus may be fluidically coupled to a material source (such as first material source  24 ) that provides a fluidized powder (e.g., a powder carried in a gas), and powder delivery device  14  may include at least one nozzle or other mechanism for directing the powder stream to a particular location adjacent component  20 . In some examples, powder delivery device  14  may be mechanically coupled or attached to energy delivery device  16  to facilitate delivery of powder and energy for heating the powder or a location of component  20  and/or abrasive coating  22  at or near where the powder is delivered. 
     In other examples, system  10  may be a powder bed directed energy deposition system. In some such examples, powder delivery device  14  may deliver the powder adjacent to the surface of component  20  and/or abrasive coating  22  by spreading the powder on the surface of component  20  and/or abrasive coating  22 , such that the powder rests on the surface prior to the powder, component  20  and/or abrasive coating  22  being heated. In some examples of a powder bed additive manufacturing system, powder delivery device  14  may include a device that spreads the powder or can otherwise manipulate the powder to move the powder within system  10 . 
     In some examples, powder delivery device  14  may be coupled to a single material source, e.g., first material source  24 . First material source  24  may include a source of a mixture of the metal powder and the abrasive powder. Thus, to deposit abrasive coating  22 , computing device  12  may be configured to control first material source  24  to deliver the mixture of the metal powder and the abrasive powder to powder delivery device  14 , which computing device  12  controls to deliver the mixture of the metal powder and the abrasive powder to component  20  to deposit abrasive coating  22 . 
     In some examples, powder delivery device  14  may couple to two material sources. In some implementations, first material source  24  may include a source of metal powder and second material source  26  may include a source of abrasive powder. Thus, to deposit abrasive coating  22 , computing device  12  may be configured to control first material source  24  to deliver the metal powder and control second material source  26  to deliver the abrasive powder to powder delivery device  14 , which computing device  12  controls to deliver the mixture of the metal powder and the abrasive powder to component  20  to deposit abrasive coating  22 . 
     In other implementations, first material source  24  may include a source of metal powder mixed with abrasive powder and second material source  26  may include a source of metal powder. Thus, to deposit abrasive coating  22 , computing device  12  may be configured to control first material source  24  to deliver the mixture of the metal powder and the abrasive powder to powder delivery device  14 , which computing device  12  controls to deliver the mixture of the metal powder and the abrasive powder to component  20  to deposit abrasive coating  22 . Computing device may be configured to control second material source  26  to deliver the metal powder to powder delivery device  14  during further processing of abrasive coating  22  to deposit additional metal matrix. 
     The metal powder may include any suitable metal or alloy configured to form a metal matrix in which abrasive particles are at least partially encapsulated. For example, the metal powder may include any of the metals or alloys described above with respect to the metal matrix. 
     The abrasive powder may include ceramic particles, such as a metal nitride, a metal carbide, a metal oxide, or the like. For example, the abrasive powder may include boron nitride, aluminum oxide, zirconium oxide, silicon carbide, silicon nitride, titanium nitride, zirconium nitride, tantalum nitride, hafnium nitride, or the like. 
     Energy delivery device  16  may include an energy source, such as a laser source, an electron beam source, plasma source, or another source of energy that may be absorbed by component  20 , the metal powder, and/or the metal matrix of abrasive coating  22 . Example laser sources include a CO laser, a CO 2  laser, a Nd:YAG laser, or the like. In some examples, the energy source may be selected to provide energy with a predetermined wavelength or wavelength spectrum that may be absorbed by component  20 , the metal powder, and/or the metal matrix of abrasive coating  22 . Similarly, the energy source may be selected to provide a powder and energy density (e.g., energy per unit focal volume) sufficient to soften and/or melt component  20 , the metal powder and/or the metal matrix of abrasive coating  22 . 
     In some examples, energy delivery device  16  also includes an energy delivery head, which is operatively connected to the energy source. The energy delivery head may aim or direct the energy toward predetermined positions adjacent to and/or within a volume of component  20  and/or abrasive coating  22  during the processing technique. As described above, in some examples, the energy delivery head may be movable in at least one dimension (e.g., translatable and/or rotatable) under control of computing device  12  to direct the energy toward a selected location adjacent to d/or within a volume of component  20  and/or abrasive coating  22 . 
     Computing device  12  may include, for example, a desktop computer, a laptop computer, a workstation, a server, a mainframe, a cloud computing system, or the like. Computing device  12  is configured to control operation of system  10 , including, for example, powder delivery device  14 , energy delivery device  16 , stage  18 , first material source  24 , and/or second material source  26 . Computing device  12  may be communicatively coupled to powder delivery device  14 , energy delivery device  16 , stage  18 , first material source  24 , and/or second material source  26  using respective communication connections. In some examples, the communication connections may include network links, such as Ethernet, ATM, or other network connections. Such connections may be wireless and/or wired connections. In other examples, the communication connections may include other types of device connections, such as USB, IEEE 1394, or the like. In some examples, computing device  12  may include control circuitry, such as one or more processors, including one or more microprocessors, digital signal processors (DSPs), application specific integrated circuits (ASICs), field programmable gate arrays (FPGAs), or any other equivalent integrated or discrete logic circuitry, as well as any combinations of such components. The term “processor” or “processing circuitry” may generally refer to any of the foregoing logic circuitry, alone or in combination with other logic circuitry, or any other equivalent circuitry. A control unit including hardware may also perform one or more of the techniques of this disclosure. 
     Computing device  12  may be configured to control operation of powder delivery device  14 , energy delivery device  16 , and/or stage  18  to position component  20  and/or abrasive coating  22  relative to powder delivery device  14  and/or energy delivery device  16 . For example, as described above, computing device  12  may control stage  18 , powder delivery device  14 , and/or energy delivery device  16  to translate and/or rotate along at least one axis to position component  20  and/or abrasive coating  22  relative to powder delivery device  14  and/or energy delivery device  16 . Positioning component  20  and/or abrasive coating  22  relative to powder delivery device  14  and/or energy delivery device  16  may include positioning a predetermined surface (e.g., a surface to which material is to be added, a surface which is to be re-melted, or the like) of component  20  and/or abrasive coating  22  in a predetermined orientation relative to powder delivery device  14  and/or energy delivery device  16 . 
     In accordance with some examples of this disclosure, computing device  12  may be configured to control system  10  to subject abrasive coating  22  to additional processing after initial deposition of abrasive coating  22  to improve properties of abrasive coating  22 . In some examples, after abrasive coating  22  is formed on component  20  (e.g., a glade tip, knife tip, or the like), computing device  12  may be configured to control powder delivery device  14  and energy delivery device  16  to perform directed energy deposition to deposit additional metal matrix material, which further encapsulates the abrasive particles in the abrasive coating, reduces surface roughness, or both. During the deposition of additional metal matrix material, computing device  12  may control a material source (e.g., first material source  24  or second material source  26 , depending on the powder that the material source provides) to provide metal powder to powder delivery device  14 . Computing device  12  also may control powder delivery device  14  to provide the metal powder to a surface of abrasive coating  22  and control energy delivery device  16  to delivery energy at or near a surface of abrasive coating  22  to soften or melt the metal powder and/or the metal matrix near the location at which powder delivery device  14  is delivering the metal powder to cause the metal powder to be added to the metal matrix. This may build up metal matrix around the abrasive particles, increasing encapsulation of the abrasive particles in the metal matrix and reducing surface roughness of abrasive coating  22 . This may lead to greater durability for abrasive coating  22  (due to reduced loss of abrasive particles during abrasion of an adjacent surface), reduced surface roughness (which may result in a better seal between component  20  and an adjacent component), or the like. 
     In other examples of the disclosure, after abrasive coating  22  is formed on component  20 , computing device  12  may control energy delivery device  16  to soften or melt the metal matrix of abrasive coating  22 , without delivery of additional metal matrix. The softening or melting of the metal matrix of abrasive coating  22  may allow surface tension/surface energy to cause the metal matrix to flow around the abrasive particles and make the height and surface roughness of abrasive coating  22  more uniform. 
     In some examples, computing device  12  may be configured to perform both the additional powder deposition technique and the softening or melting technique, in either order. In either order, the combination of the two technique may be used to provide lower surface roughness and more uniform height to abrasive coating  22 , while allowing use of high temperature metal matrices, such as nickel- or cobalt-based superalloys. 
     An example technique that may be implemented by system  10  will be described with concurrent reference to  FIG. 2 .  FIG. 2  is a flow diagram illustrating an example technique for processing an abrasive coating after deposition of the abrasive coating to improve properties of the abrasive coating. Although the technique of  FIG. 2  is described with respect to system  10  of  FIG. 1  and component  20  of  FIG. 3 , in other examples, the technique of  FIG. 2  may be performed by other systems, such as systems including fewer or more components than those illustrated in  FIG. 1 . Similarly, system  10  may be used to performed other techniques (e.g., the technique illustrated in  FIGS. 4 and 5 ). 
     The technique of  FIG. 2  includes controlling, by computing device  12 , a material source (e.g., second material source  26 ) to deliver metal powder to powder delivery device  14  ( 30 ). For example, computing device  12  may be configured to control at least one of a valve, a pump, or the like to enable flow of fluidized powder with a selected flow rate of metal powder from second material source  26  to powder delivery device  14 . As described above, the metal powder may include any suitable metal for forming a metal matrix of an abrasive coating. In the example of  FIG. 2 , abrasive particles may be omitted, as an abrasive coating is already present on the surface of the article being processed. 
     The technique of  FIG. 2  also includes controlling, by computing device  12 , powder delivery device  14  to deliver the metal powder to a surface  44  of abrasive coating  22  ( 32 ; see  FIG. 3 ). Computing device  12  may control powder delivery device  14  to move relative to component  20  so that metal powder is delivered to selected location(s) of the surface  44  of abrasive coating  22 . Additionally, computing device  12  may control stage  18  to position component  20  relative to powder delivery device  14 . 
     Concurrently, the technique of  FIG. 2  further includes controlling, by computing device  12 , energy delivery device  16  to deliver energy to metal matrix  40  and/or the metal powder to cause the metal powder to be added to metal matrix  40  at surface  44  ( 34 ). Computing device  12  may control the power, spot size, scanning rate, frequency, pulse duration, and the like of the energy output by energy delivery device  16  to control the energy delivered to metal matrix  40  and/or the metal powder so that the metal powder is joined to metal matrix  40 . For example, computing device  12  may control the power, spot size, scanning rate, frequency, pulse duration, and the like of the energy output by energy delivery device  16  to cause the energy to melt portions of metal matrix  40  to form a melt pool, to which the metal power is added upon the metal powder impacting the melt pool. For instance, computing device  12  may be configured to control operation and movement of powder delivery device  14 , energy delivery device  16 , stage  18 , or both, based on a computer aided manufacturing or computer aided design (CAM/CAD) file. The CAM/CAD file may define one or more operating parameters of powder delivery device  14 , energy delivery device  16  and/or stage  18  such as, for example, the powder flow rate (e.g., mass flow rate, volumetric flow rate, or the like) of metal powder delivered by powder delivery device  14 ; the tool path (e.g., scanning pattern), scanning velocity, and other movement parameters of powder delivery device  14 ; power, spot size, frequency, pulse duration, and the like of the energy output by energy delivery device  16 ; the tool path (e.g., scanning pattern), scanning velocity, and other movement parameters of energy delivery device  16 ; the movement parameters of stage  18 ; or the like. 
     Computing device  12  may control energy delivery device  16  to move the energy across surface  40  of abrasive coating  22  in concert with controlling powder delivery device  14  such that metal powder is added at locations across surface  40 . 
     In this way, computing device  12  may be configured to control system  10  to add additional metal matrix  40  to an already-present abrasive coating  22 . As described above, during initial formation of abrasive coating  22 , abrasive particles  44  may tend to project from metal matrix  40  as abrasive particles  44  may be less dense than metal matrix  40  (and thus buoyant in the melt form of metal matrix  40 ). By further processing abrasive coating  22  by depositing addition metal matrix without additional abrasive particles, the additional metal matrix  40  may further encapsulate abrasive particles  44  in abrasive coating  22 . This may reduce surface roughness of abrasive coating  22 , improve incorporation of abrasive particles  44  in abrasive coating  22  such that abrasive particles  44  are less likely to be removed from abrasive coating  22  due to contact with an adjacent abradable coating, or both. Reduced surface roughness may improve a seal between a rotating component that includes multiple rotating elements and an adjacent stator or sealing component. 
     In some examples, further processing of abrasive coating  22  may omit any additional material delivery.  FIG. 4  is a flow diagram illustrating an example technique for processing an abrasive coating after deposition of the abrasive coating to improve properties of the abrasive coating. Although the technique of  FIG. 4  is described with respect to system  10  of  FIG. 1  and component  20  of  FIG. 3 , in other examples, the technique of  FIG. 4  may be performed by other systems, such as systems including fewer or more components than those illustrated in  FIG. 1 . Similarly, system  10  may be used to performed other techniques (e.g., the technique illustrated in  FIGS. 2 and 5 ). 
     During the technique of  FIG. 4 , system  10  may delivery no additional material to abrasive coating  22 . Instead, system  10  may use energy delivery device  16  to soften or re-melt one or more portions of metal matrix  40  to allow metal matrix  40  to flow and reduce surface roughness of abrasive coating  22 . 
     The technique of  FIG. 4  includes controlling, by computing device  12 , energy delivery device  16  to delivery energy to metal matrix  40  to soften or melt portions of metal matrix  40  ( 52 ). Computing device  12  may control the power, spot size, scanning rate, frequency, pulse duration, and the like of the energy output by energy delivery device  16  to control the energy delivered to metal matrix  40  such that portions of metal matrix  40  absorb sufficient energy to soften or melt. The energy delivered to any selected portion may be selected so that softening or melting is relatively localized, e.g., so that abrasive particles  42  do not have sufficient mobility to flow from abrasive coating  22  and/or metal matrix  40  does not flow from its position on component  20 . During this process, in systems that include powder delivery device  14 , computing device  12  may be configured to control powder delivery device  14  to not deliver powder to surface  44 , or computing device  12  may be configured to not cause powder delivery device  14  to deliver powder to surface  44 . 
     Computing device  12  also may control energy delivery device  16  and/or stage  18  to move energy delivery device  16  relative to abrasive coating  22  ( 54 ). More particularly, computing device  12  may control energy delivery device  16  and/or stage to move the energy (e.g., focal spot of an energy beam, central portion of a plasma plume, or the like) relative to surface  44  of abrasive coating  22 . Computing device  12  may control energy delivery device  16  and/or stage  18  to move in a selected pattern and rate to move the energy (e.g., focal spot of an energy beam, central portion of a plasma plume, or the like) relative to surface  44  of abrasive coating  22  to expose portions of abrasive coating  22  to a selected power and energy density to cause the selected softening and/or melting of metal matrix  40 . 
     For example, computing device  12  may be configured to control operation and movement of energy delivery device  16 , stage  18 , or both, based on a computer aided manufacturing or computer aided design (CAM/CAD) file. The CAM/CAD file may define one or more operating parameters of energy delivery device  16  and/or stage  18  such as, for example, power, spot size, frequency, pulse duration, and the like of the energy output by energy delivery device  16 ; the tool path (e.g., scanning pattern), scanning velocity, and other movement parameters of energy delivery device  16 ; the movement parameters of stage  18 ; or the like. 
     In this way, system  10  may be used to process an abrasive coating using energy delivery to soften or remelt metal matrix  40  after deposition of abrasive coating  22 . The softening or melting of metal matrix  40  may allow surface tension/surface energy to cause metal matrix  40  to flow around abrasive particles  42  and make the height and surface roughness of abrasive coating  22  more uniform. This may reduce surface roughness of abrasive coating  22 , improve incorporation of abrasive particles  44  in abrasive coating  22  such that abrasive particles  44  are less likely to be removed from abrasive coating  22  due to contact with an adjacent abradable coating, or both. Reduced surface roughness may improve a seal between a rotating component that includes multiple rotating elements and an adjacent stator or sealing component. 
     In some examples, the techniques of  FIG. 2  and  FIG. 4  may be used together, along with initial deposition of an abrasive coating, to form a finished abrasive coating.  FIG. 5  is a flow diagram illustrating a technique for forming an abrasive coating using directed energy deposition material addition and post-processing the abrasive coating to improve properties of the abrasive coating. Although the technique of  FIG. 5  is described with respect to system  10  of  FIG. 1  and component  20  of  FIG. 3 , in other examples, the technique of  FIG. 5  may be performed by other systems, such as systems including fewer or more components than those illustrated in  FIG. 1 . Similarly, system  10  may be used to performed other techniques (e.g., the technique illustrated in  FIGS. 2 and 4 ). 
     The technique of  FIG. 5  includes controlling, by computing device  12 , one or more material sources to deliver metal powder and abrasive particles to powder delivery device  14  ( 60 ). As described above with reference to  FIG. 1 , in some examples a single material source (e.g., first material source  24 ) is configured to provide a mixture of metal powder and abrasive particles. In other examples, a first material source  24  is configured to provide metal powder and second material source is configured to provide abrasive particles. Thus, computing device  12  may control one or more materials sources to cause the one or more material sources to delivery metal powder and abrasive particles to powder delivery device  14  ( 60 ). 
     Computing device  12  may control the one or more material sources to deliver metal powder and abrasive particles to powder delivery device  14  in a selected ratio to result in a selected concentration of abrasive powder  42  in the metal matrix  40  of abrasive coating  22 . For example, computing device  12  may control opening and closing of one or more valves or other flow control devices to control the flow rate of fluidized powder from the one or more material sources to powder delivery device  42 . 
     Computing device  12  also may control powder delivery device  14  to deliver the mixture of metal powder and abrasive particles to the surface of component  20  and/or abrasive coating  22  ( 62 ). Computing device  12  may control powder delivery device  14  to move relative to component  20  so that metal powder and abrasive particles are delivered to selected location(s) of the surface on which abrasive coating  22  is to be formed. Additionally, computing device  12  may control stage  18  to position component  20  relative to powder delivery device  14 . 
     Concurrently, the technique of  FIG. 5  further includes controlling, by computing device  12 , energy delivery device  16  to deliver energy to a surface of component  20 , metal matrix  40  and/or the metal powder to cause the metal powder and abrasive particles to be joined to component  20  to form abrasive coating  22  ( 64 ). Computing device  12  may control the power, spot size, scanning rate, frequency, pulse duration, and the like of the energy output by energy delivery device  16  to control the energy delivered to metal matrix  40  and/or the metal powder so that the metal powder is joined to metal matrix  40 , resulting in the abrasive particles being incorporated in abrasive coating  22 . For example, computing device  12  may control the power, spot size, scanning rate, frequency, pulse duration, and the like of the energy output by energy delivery device  16  to cause the energy to melt portions of component  20  and/or metal matrix  40  to form a melt pool, to which the metal power is added upon the metal powder impacting the melt pool and which abrasive particles impact and are incorporated. For instance, computing device  12  may be configured to control operation and movement of powder delivery device  14 , energy delivery device  16 , stage  18 , or both, based on a computer aided manufacturing or computer aided design (CAM/CAD) file. The CAM/CAD file may define one or more operating parameters of powder delivery device  14 , energy delivery device  16  and/or stage  18  such as, for example, the powder flow rate (e.g., mass flow rate, volumetric flow rate, or the like) of metal powder and abrasive particles delivered by powder delivery device  14 ; the tool path (e.g., scanning pattern), scanning velocity, and other movement parameters of powder delivery device  14 ; power, spot size, frequency, pulse duration, and the like of the energy output by energy delivery device  16 ; the tool path (e.g., scanning pattern), scanning velocity, and other movement parameters of energy delivery device  16 ; the movement parameters of stage  18 ; or the like. 
     Computing device  12  may control energy delivery device  16  to move the energy across surface  40  of abrasive coating  22  in concert with controlling powder delivery device  14  such that metal powder and abrasive particles are added at locations across a surface of component  20  to form abrasive coating  22 . 
     Computing device  12  may continue controlling powder source(s) ( 60 ), controlling powder delivery device  14  ( 62 ), and controlling energy delivery device  16  ( 64 ) until sufficient material (metal powder and abrasive particles) has been deposited to form abrasive coating  22 . As such, computing device may control powder delivery device  14  and energy delivery device  16  to cause the powder stream and energy to follow a tool path that defines a pattern (e.g., of adjacent rows) along the surface of component  20 , then control powder delivery device  14  and energy delivery device  16  to cause the powder stream and energy to follow a tool path that defines a pattern (e.g., of adjacent rows) along a surface of the previously deposited layer until a desired thickness of material (metal power and abrasive particles) has been deposited on component  20  to form abrasive coating  22 . 
     Once system  10  has formed abrasive coating  22 , system  10  may be used to post-process abrasive coating  22  to improve properties (e.g., surface roughness, incorporation of abrasive particles, or both) using the techniques of  FIGS. 2 and 4 . Although  FIG. 5  illustrates the technique of  FIG. 2  being performed before the technique of  FIG. 4 , in other examples, the technique of  FIG. 4  may be performed before the technique of  FIG. 2 . In the example of  FIG. 5 , computing device  12  may control one or more a powder source to deliver metal powder to powder delivery device  14  ( 30 ), control powder delivery device to deliver the metal powder to surface  44  of abrasive coating  22  ( 32 ), and, concurrently, control energy delivery device  16  to deliver energy to metal matrix  40  and/or the metal powder to cause the metal powder to be added to metal matrix  40  ( 34 ). Computing device  12  also may control energy delivery device  16  to deliver energy to metal matrix  40  to cause metal matrix  40  to soften or flow (without delivering metal powder to the surface of metal matrix) ( 52 ) and control energy delivery device  16  and/or stage  18  to move energy delivery device  16  relative to abrasive coating  22  such that energy is scanned across surface  40  of abrasive coating. 
     In this way, system  10  may be used to process abrasive coating  22  by delivering additional metal matrix to abrasive coating  22  to further encapsulate previously deposited abrasive particles and by using energy delivery to soften or remelt metal matrix  40  after deposition of abrasive coating  22  (in either order). The softening or melting of metal matrix  40  may allow surface tension/surface energy to cause metal matrix  40  to flow around abrasive particles  42  and make the height and surface roughness of abrasive coating  22  more uniform. This may reduce surface roughness of abrasive coating  22 , improve incorporation of abrasive particles  44  in abrasive coating  22  such that abrasive particles  44  are less likely to be removed from abrasive coating  22  due to contact with an adjacent abradable coating, or both. Reduced surface roughness may improve a seal between a rotating component that includes multiple rotating elements and an adjacent stator or sealing component. 
     Abrasive coating  22  may be used as a coating on a substrate of a rotating component, e.g., in a mechanical system, to protect the substrate from damage due to intentional or unintentional contact with an adjacent component, such as a stator. As described above, the rotating component may include, for example, a pump rotor, a turbine blade, a compressor blade, a fan blade, a knife in a knife seal, or the like.  FIG. 6  is a conceptual diagram illustrating a system  70  that includes a rotating component  78 , an abrasive coating  80  on rotating component  78 , and a stationary component  76  adjacent rotating component  78 . In the example of  FIG. 6 , system  70  is a gas turbine engine and rotating component  78  is a rotating blade (e.g., a compressor blade or a turbine blade) of the gas turbine engine. 
     System  70  includes stationary component  76 , which may be a blade track or blade shroud. Abrasive coating  80  is on a tip or radially outer surface of rotating component  78 , and is configured to impact an abradable coating  74  on stationary component  76  during at least some operating conditions of system  70 . Although a single rotating component  78  is shown in system  70  for ease of description, in actual operation, system  70  may include a plurality of rotating components. 
     During operation of system  70 , rotating component  78  rotates relative to stationary component  76  in a direction indicated by arrow  82 . In general, the power and efficiency of system  70  can be increased by reducing the gap between stationary component  76  and rotating component  78 , e.g., to reduce or eliminate gas leakage around a tip of rotating component  78 . Thus, system  70 , in various examples, is configured to allow rotating component  78  to abrade into abradable coating  74  on stationary component  76  to define an abraded portion  84 , which creates a seal between abradable coating  74  and rotating component  78 . The abrading action may create high thermal and shear stress forces at abrasive coating  80 . 
     In some examples, the disclosure may be described by the following clauses. 
     Clause 1. A method comprising: controlling, by a computing device, a powder source to deliver metal powder to a powder delivery device; controlling, by the computing device, the powder delivery device to deliver the metal powder to a surface of an abrasive coating; and controlling, by the computing device, an energy delivery device to deliver energy to at least one of the abrasive coating or the metal powder to cause the metal powder to be joined to the abrasive coating. 
     Clause 2. The method of clause 1, wherein joining the metal powder to the abrasive coating reduces an average surface roughness of the abrasive coating. 
     Clause 3. The method of clause 1 or 2, wherein the metal powder joined to the abrasive coating further encapsulated at least some of the abrasive particles. 
     Clause 4. The method of any one of clauses 1 to 3, wherein the abrasive coating comprises a metal matrix and abrasive particles at least partially encapsulated in the metal matrix. 
     Clause 5. The method of clause 4, wherein the metal powder comprises a composition substantially similar to the metal matrix. 
     Clause 6. The method of clause 4 or 5, wherein the metal matrix comprises a Ni- or Co-based alloy. 
     Clause 7. The method of any one of clauses 1 to 6, wherein the abrasive particles comprise at least one of a metal nitride, a metal carbide, or a metal oxide. 
     Clause 8. The method of any one of clauses 1 to 7, wherein the energy delivery device comprises a laser or a plasma source. 
     Clause 9. The method of any one of clauses 1 to 8, wherein the powder delivery device comprises a blown powder deposition device. 
     Clause 10. A system comprising: a powder source; a powder delivery device; an energy delivery device; and a computing device, wherein the computing device is configured to: control the powder source to deliver metal powder to the powder delivery device; control the powder delivery device to deliver the metal powder to a surface of an abrasive coating; and control the energy delivery device to deliver energy to at least one of the abrasive coating or the metal powder to cause the metal powder to be joined to the abrasive coating. 
     Clause 11. The system of clause 10, wherein joining the metal powder to the abrasive coating reduces an average surface roughness of the abrasive coating. 
     Clause 12. The system of clause 10 or 11, wherein the metal powder joined to the abrasive coating further encapsulated at least some of the abrasive particles. 
     Clause 13. The system of any one of clauses 10 to 12, wherein the abrasive coating comprises a metal matrix and abrasive particles at least partially encapsulated in the metal matrix. 
     Clause 14. The system of clause 13, wherein the metal powder comprises a composition substantially similar to the metal matrix. 
     Clause 15. The system of clause 13 or 14, wherein the metal matrix comprises a Ni- or Co-based alloy. 
     Clause 16. The system of any one of clauses 10 to 15, wherein the abrasive particles comprise at least one of a metal nitride, a metal carbide, or a metal oxide. 
     Clause 17. The system of any one of clauses 10 to 16, wherein the energy delivery device comprises a laser. 
     Clause 18. The system of any one of clauses 10 to 17, wherein the powder delivery device comprises a blown powder deposition device. 
     Clause 19. A computer-readable storage device comprising instructions that, when executed, configure one or more processors of a computing device to: control a powder source to deliver metal powder to a powder delivery device; control the powder delivery device to deliver the metal powder to a surface of an abrasive coating; and control an energy delivery device to deliver energy to at least one of the abrasive coating or the metal powder to cause the metal powder to be joined to the abrasive coating. 
     Clause 1. A method comprising: controlling, by a computing device, an energy delivery device to deliver energy to an abrasive coating, wherein the abrasive coating comprises a metal matrix and abrasive particles at least partially encapsulated by the metal matrix; and controlling, by the computing device, the energy delivery device to scan the energy across a surface of the abrasive coating and form a series of softened or melted portions of the metal matrix. 
     Clause 2. The method of clause 1, wherein the softened or melted portions of the metal matrix allow the metal matrix to further encapsulate at least some of the abrasive particles. 
     Clause 3. The method of clause 1 or 2, wherein the softened or melted portions of the metal matrix allow surface tension or surface energy to cause the softened or melted portions of the metal matrix to flow between abrasive particles. 
     Clause 4. The method of any one of clauses 1 to 3, wherein the method reduces an average surface roughness of the abrasive coating. 
     Clause 5. The method of any one of clauses 1 to 4, wherein additional metal powder is not delivered to the metal matrix as the energy delivery device delivers energy to the abrasive coating. 
     Clause 6. The method of any one of clauses 1 to 5, wherein the metal matrix comprises a Ni- or Co-based alloy. 
     Clause 7. The method of any one of clauses 1 to 6, wherein the abrasive particles comprise at least one of a metal nitride, a metal carbide, or a metal oxide. 
     Clause 8. The method of any one of clauses 1 to 7, wherein the energy delivery device comprises at least one of a laser or a plasma. 
     Clause 9. The method of any one of clauses 1 to 8, further comprising: controlling, by the computing device, a powder source to deliver metal powder to a powder delivery device; controlling, by the computing device, the powder delivery device to deliver the metal powder to a surface of an abrasive coating; and controlling, by the computing device, the energy delivery device to deliver energy to at least one of the abrasive coating or the metal powder to cause the metal powder to be joined to the abrasive coating. 
     Clause 10. A system comprising: an energy delivery device; and a computing device, wherein the computing device is configured to: control the energy delivery device to deliver energy to an abrasive coating, wherein the abrasive coating comprises a metal matrix and abrasive particles at least partially encapsulated by the metal matrix; and control the energy delivery device to scan the energy across a surface of the abrasive coating and form a series of softened or melted portions of the metal matrix. 
     Clause 11. The system of clause 10, wherein the softened or melted portions of the metal matrix allow the metal matrix to further encapsulate at least some of the abrasive particles. 
     Clause 12. The system of clause 10 or 11, wherein the softened or melted portions of the metal matrix allow surface tension or surface energy to cause the softened or melted portions of the metal matrix to flow between abrasive particles. 
     Clause 13. The system of any one of clauses 10 to 12, wherein the method reduces an average surface roughness of the abrasive coating. 
     Clause 14. The system of any one of clauses 10 to 13, wherein additional metal powder is not delivered to the metal matrix as the energy delivery device delivers energy to the abrasive coating. 
     Clause 15. The system of any one of clauses 10 to 14, wherein the metal matrix comprises a Ni- or Co-based alloy. 
     Clause 16. The system of any one of clauses 10 to 15, wherein the abrasive particles comprise at least one of a metal nitride, a metal carbide, or a metal oxide. 
     Clause 17. The system of any one of clauses 10 to 16, wherein the energy delivery device comprises at least one of a laser or a plasma. 
     Clause 18. The system of any one of clauses 10 to 17, further comprising: a powder source; and a powder delivery device, wherein the computing device is further configured to: control the powder source to deliver metal powder to the powder delivery device; control the powder delivery device to deliver the metal powder to a surface of an abrasive coating; and control the energy delivery device to deliver energy to at least one of the abrasive coating or the metal powder to cause the metal powder to be joined to the abrasive coating. 
     Clause 19. A computer-readable storage device comprising instructions that, when executed, configure one or more processors of a computing device to: control an energy delivery device to deliver energy to an abrasive coating, wherein the abrasive coating comprises a metal matrix and abrasive particles at least partially encapsulated by the metal matrix; and control the energy delivery device to scan the energy across a surface of the abrasive coating and form a series of softened or melted portions of the metal matrix. 
     Clause 20. The computer-readable storage device of clause 19, further comprising instructions that configure one or more processors of the computing device to: control a powder source to deliver metal powder to a powder delivery device; control the powder delivery device to deliver the metal powder to a surface of an abrasive coating; and control an energy delivery device to deliver energy to at least one of the abrasive coating or the metal powder to cause the metal powder to be joined to the abrasive coating. 
     The techniques described in this disclosure may be implemented, at least in part, in hardware, software, firmware, or any combination thereof. For example, various aspects of the described techniques may be implemented within one or more processors, including one or more microprocessors, digital signal processors (DSPs), application specific integrated circuits (ASICs), field programmable gate arrays (FPGAs), or any other equivalent integrated or discrete logic circuitry, as well as any combinations of such components. The term “processor” or “processing circuitry” may generally refer to any of the foregoing logic circuitry, alone or in combination with other logic circuitry, or any other equivalent circuitry. A control unit including hardware may also perform one or more of the techniques of this disclosure. 
     Such hardware, software, and firmware may be implemented within the same device or within separate devices to support the various techniques described in this disclosure. In addition, any of the described units, modules or components may be implemented together or separately as discrete but interoperable logic devices. Depiction of different features as modules or units is intended to highlight different functional aspects and does not necessarily imply that such modules or units must be realized by separate hardware, firmware, or software components. Rather, functionality associated with one or more modules or units may be performed by separate hardware, firmware, or software components, or integrated within common or separate hardware, firmware, or software components. 
     The techniques described in this disclosure may also be embodied or encoded in an article of manufacture including a computer-readable storage medium encoded with instructions. Instructions embedded or encoded in an article of manufacture including a computer-readable storage medium encoded, may cause one or more programmable processors, or other processors, to implement one or more of the techniques described herein, such as when instructions included or encoded in the computer-readable storage medium are executed by the one or more processors. Computer readable storage media may include random access memory (RAM), read only memory (ROM), programmable read only memory (PROM), erasable programmable read only memory (EPROM), electronically erasable programmable read only memory (EEPROM), flash memory, a hard disk, a compact disc ROM (CD-ROM), a floppy disk, a cassette, magnetic media, optical media, or other computer readable media. In some examples, an article of manufacture may include one or more computer-readable storage media. 
     In some examples, a computer-readable storage medium may include a non-transitory medium. The term “non-transitory” may indicate that the storage medium is not embodied in a carrier wave or a propagated signal. In certain examples, a non-transitory storage medium may store data that can, over time, change (e.g., in RAM or cache). 
     Various examples have been described. These and other examples are within the scope of the following claims.