Patent Publication Number: US-2019177828-A1

Title: Thermal spray coating

Description:
This application claims the benefit of U.S. Provisional Application No. 62/598,087, filed Dec. 13, 2017, which is incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The present disclosure generally relates to thermal spray coating. 
     BACKGROUND 
     Thermal spray systems are used in a wide variety of industrial applications to coat substrates with coating material to modify or improve the properties of the target surface. Coatings may include thermal barrier coatings, wear coatings, ablative coatings, or the like. Thermal spray systems use heat generated electrically, by plasma, or by combustion to heat material injected in a plume, so that softened or molten material propelled by the plume contacts the surface of the target. Upon impact, the material adheres to the target surface, resulting in a coating. The properties of coatings applied on substrates by thermal spraying may depend on the parameters used for controlling the thermal spraying. 
     SUMMARY 
     In some examples, the disclosure describes an example technique for thermal spraying. The example technique includes thermally spraying a substrate in a thermal spray cycle. The thermal spray cycle includes a plurality of passes of a coating material to form a coating. The example technique includes determining, by a computing device, a change in curvature of the substrate Δκ during a central pass of the plurality of passes. The example technique includes determining, by the computing device, residual stress σ of the coating based on the change in the curvature ΔK. 
     In some examples, the disclosure describes an example system including a thermal spray gun and a computing device. The computing device is configured to control the thermal spray gun to thermally spray a substrate in a thermal spray cycle. The thermal spray cycle includes a plurality of passes of a coating material to form a coating. The computing device is configured to determine a change in curvature of the substrate Δκ during a central pass of the plurality of passes. The computing device is configured to determine residual stress σ of the coating based on the change in the curvature Δκ. 
     In some examples, the disclosure describes a computer readable storage medium comprising instructions. The instructions, when executed, cause at least one processor to control a thermal spray gun to thermally spray a substrate in a thermal spray cycle including a plurality of passes of a coating material to form a coating. The instructions, when executed, cause at least one processor to determine a change in curvature of the substrate Δκ during a central pass of the plurality of passes. The instructions, when executed, cause at least one processor to determine residual stress σ of the coating based on the change in the curvature Δκ. 
     In some examples, the disclosure describes an example technique for thermal spraying. The example technique includes thermally spraying a substrate in a thermal cycle. The thermal cycle includes a plurality of passes of a coating material to form a coating. The example technique includes receiving, by a computing device, a first signal indicative of changes in bending deflection of the substrate at a predetermined location along the substrate over a first coating cycle. The example technique includes receiving, by the computing device, a second signal indicative of changes in bending deflection of the substrate at the predetermined location along the substrate over a second coating cycle. The example technique includes determining, by the computing device, a first frequency spectrum of the bending deflection over the first coating cycle. The example technique includes determining, by the computing device, a first plurality of peaks of the first frequency spectrum. The example technique includes determining, by the computing device, a second frequency spectrum of the bending deflection over the second coating cycle. The example technique includes determining, by the computing device, and a second plurality of peaks of the second frequency spectrum. The example technique includes determining, by the computing device, a plurality of frequency shifts between respective peaks of the first plurality of peaks and corresponding peaks of the second plurality of peaks. The example technique includes determining, by the computing device, a modulus of the coating based on the plurality of shifts. 
     In some examples, the disclosure describes an example system including a thermal spray gun and a computing device. The computing device is configured to control the thermal spray gun to thermally spray a substrate in a thermal spray cycle. The thermal spray cycle includes a plurality of passes of a coating material to form a coating. The computing device is configured to control the thermal spray gun to thermally spray a substrate in a thermal cycle. The thermal cycle includes a plurality of passes of a coating material to form a coating. The computing device is configured to receive a first signal indicative of changes in bending deflection of the substrate at a predetermined location along the substrate over a first coating cycle. The computing device is configured to receive a second signal indicative of changes in bending deflection of the substrate at the predetermined location along the substrate over a second coating cycle. The computing device is configured to determine a first frequency spectrum of the bending deflection over the first coating cycle. The computing device is configured to determine a first plurality of peaks of the first frequency spectrum. The computing device is configured to determine a second frequency spectrum of the bending deflection over the second coating cycle. The computing device is configured to determine a second plurality of peaks of the second frequency spectrum. The computing device is configured to determine a plurality of frequency shifts between respective peaks of the first plurality of peaks and corresponding peaks of the second plurality of peaks. The computing device is configured to determine a modulus of the coating based on the plurality of shifts. 
     In some examples, the disclosure describes a computer readable storage medium comprising instructions. The instructions, when executed, cause at least one processor to control a thermal spray gun to thermally spray a substrate in a thermal spray cycle. The instructions, when executed, cause at least one processor to receive a first signal indicative of changes in bending deflection of the substrate at a predetermined location along the substrate over a first coating cycle. The instructions, when executed, cause at least one processor to receive a second signal indicative of changes in bending deflection of the substrate at the predetermined location along the substrate over a second coating cycle. The instructions, when executed, cause at least one processor to determine a first frequency spectrum of the bending deflection over the first coating cycle. The instructions, when executed, cause at least one processor to determine a first plurality of peaks of the first frequency spectrum. 
     The instructions, when executed, cause at least one processor to determine a second frequency spectrum of the bending deflection over the second coating cycle. The instructions, when executed, cause at least one processor to determine a second plurality of peaks of the second frequency spectrum. The instructions, when executed, cause at least one processor to determine a plurality of frequency shifts between respective peaks of the first plurality of peaks and corresponding peaks of the second plurality of peaks. The instructions, when executed, cause at least one processor to determine a modulus of the coating based on the plurality of shifts. 
     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. 1A  is a conceptual and schematic block diagram illustrating an example system for thermal spraying. 
         FIG. 1B  is a conceptual and schematic block diagram illustrating an example thermal spray cycle of a coating applied to a substrate by the thermal spray system of  FIG. 1A . 
         FIG. 2  is a schematic chart illustrating temperature of substrate  14  as a function of time for multiple thermal spray cycles during an example thermal spray process. 
         FIG. 3  is a schematic chart illustrating thermal spray passes and cycles during an example thermal spray process. 
         FIG. 4A  is a schematic chart illustrating temporal changes in bending deflection of a substrate during a thermal spray process. 
         FIG. 4B  is a schematic chart illustrating peak shifts in frequency spectra of bending deflections during a thermal spray process. 
         FIG. 5  is a flow diagram illustrating an example technique for thermal spraying. 
         FIG. 6  is a flow diagram illustrating an example technique for thermal spraying. 
     
    
    
     DETAILED DESCRIPTION 
     The disclosure describes example systems and techniques for thermal spraying coatings on substrates. In some examples, properties of coatings may be determined based on bending deflections exhibited by substrate in response to the thermal spraying. For example, example techniques and systems according to the disclosure may be used to determine coating residual stress or coating modulus, for example, Young&#39;s modulus, and to control the thermal spraying based on the properties of the coatings. Such properties of coatings may relate to durability and other functional performance of thermally sprayed coatings. 
     In some examples, substrate deformation, for example, bending deflection or changes in curvature may be measured at different time intervals during the thermal spraying. Based on relationships between curvature and residual stress, or by evaluating finite element models, properties of coatings can be ultimately determined based on the substrate deformations. For example, change in curvature after the coating and substrate has reached a steady state may be used to determine residual stress in coatings, so that the curvature is substantially a result of quench stress, rather than of thermal gradients between a coating and a substrate. In some examples, such as when the coating is thin or when the substrate is stiff, the coating and the substrate may not reach a thermal steady state, or the change in curvature may be relatively small. The small change in curvature may reduce the accuracy of determining coating properties based on curvature changes. Further, if multiple layers of coating material are sprayed, prior layers may not cool down completely before subsequent layers are sprayed, leading to errors in determining coating properties. 
     In some examples in accordance with this disclosure, curvature determined during a mid-point of a thermal cycle may be used to increase the measured change in curvature, and increase the accuracy of determining coating properties. In other examples, finite element models may be iterated through different intervals of the thermal cycle, and shifts in frequency spectra of bending deflections or vibrations may be used to determine coating properties with increased accuracy. 
     Further, example techniques and systems according to the disclosure may be used to control thermal spraying based on the determined properties of the coating, for example, to prepare coatings meeting predetermined specifications. 
       FIG. 1A  is a conceptual and schematic block diagram illustrating an example system  10  for thermal spraying. In some examples, thermal spray system  10  includes components such as an enclosure  11 , a thermal spray gun  12 , a substrate  14 , and a computing device  30 . 
     Enclosure  11  encloses some components of thermal spray system  10 , including, for example, thermal spray gun  12  and substrate  14 . In some examples, enclosure  11  substantially completely surrounds thermal spray gun  12  and substrate  14  and encloses an atmosphere. The atmosphere may include, for example, air, an inert atmosphere, a vacuum, or the like. In some examples, the atmosphere may be selected based on the type (e.g., composition) of coating being applied using thermal spray system  10 , the composition of substrate  14 , or both. 
     Substrate  14  is coated with a coating  16  using thermal spray system  10 . In some examples, substrate  14  may include, for example, a substrate on which a bond coat, a primer coat, a hard coat, a wear-resistant coating, a thermal barrier coating, an environmental barrier coating, an abrasive coating, an abradable coating, or the like is to be deposited. Substrate  14  may include a body of any regular or irregular shape, geometry or configuration. In some examples, substrate  14  includes a substantially rectangular parallelepiped component, for example, a sheet, a block, or a rod with rectangular cross-section. In some examples, substrate  14  may include metal, plastic, glass, or the like. Substrate  14  may be a component used in any one or more mechanical systems, including, for example, a high temperature mechanical system such as a gas turbine engine. In some examples, substrate  14  may include a test coupon or test sample used to test performance of thermal spray system  10 . 
     Thermal spray gun  12  is coupled to a material reservoir  18  via material inlet port (not shown) and to a fluid supply  20  via a fluid inlet port (not shown). Thermal spray gun  12  is also coupled to, or includes, an energy source. Fluid supply  20  provides a flow of an energizable fluid, for example, gas, to the fluid inlet port of thermal spray gun  12 . Depending upon the type of thermal spray process being performed, the fluid flow may be a carrier gas for the coating material, may be a fuel that is ignited to at least partially melt the coating material, or both. While fluid supply  20  may be enclosed in enclosure  11  as shown in  FIG. 1A , in other examples, fluid supply  20  may be external to enclosure  11 . 
     In some examples, thermal spray gun  12  may include a material inlet port coupled to material reservoir  18 . Material reservoir  18  may be enclosed in enclosure  11 , or may be located external to enclosure  11 . Coating material may be fed from material reservoir  18  to thermal spray gun  12  in powder form, and may mix with fluid from fluid supply  20  within thermal spray gun  12 . In other examples, thermal spray gun  12  may omit material inlet port, and a material feed line may provide coating material from material reservoir  18  at a region outside thermal spray gun  12 , for example, near a nozzle or outlet of thermal spray gun  12 . The composition of the coating material may be based upon the composition of the coating to be deposited on substrate  12 , and may include, for example, a metal, an alloy, a ceramic, or the like. 
     Thermal spray system  10  also includes an energy source, which may be included in thermal spray gun  12  or may be separate from thermal spray gun  12 . The energy source provides energy to at least partially melt (e.g., partially melt or substantially fully melt) the coating material provided through the material inlet port. In some examples, the energy source includes a plasma electrode, which may energize fluid provided through a fluid supply line to form a plasma. In other examples, the energy source includes an electrode that ignites gas provided through the fluid supply line  20 . 
     As shown in  FIG. 1A , thermal spray  17  exits the outlet of thermal spray gun  12 . In some examples, the outlet includes a spray gun nozzle. Thermal spray  17  may include at least partially melted coating material carried by a carrier fluid. Thermal spray gun  12  may be configured and positioned to direct the at least partially melted coating material at substrate  14  to eventually form coating  16 . 
     While enclosure  11  completely surrounds thermal spray gun  12  and substrate  14  in example system  10  shown in  FIG. 1A , in other examples, enclosure  11  may only partially surround one or both of thermal spray gun  12  or substrate  14 , or may not be included in system  10 . 
     In some examples, system  10  may include a spray controller  22 . Spray controller  22  may include circuitry for controlling the operation, orientation, or location of one or more of thermal spray gun  12 , substrate  14 , material reservoir  18 , or fluid supply  20 . For example, spray controller  22  may send control signals to one or more of thermal spray gun  12 , substrate  14 , or material reservoir  18 , or to an industrial robot, platform, a movable multi-axis stage, or one or more suitable mechanisms for holding one or more of thermal spray gun  12 , substrate  14 , material reservoir  18 , or fluid supply  20  in respective locations and orientations. In some examples, computing device  30  may send control signals to spray controller  22  for ultimately controlling the operation of system  10 . In other, examples, system  10  may not include spray controller  22 , and computing device  30  may act as a spray controller by sending respective control signals to other components of system  10 . 
     Computing device  30  may thus ultimately control the operation of thermal spray gun  12  to apply coating  16  to substrate  14 . In some examples, computing device  30  may control thermal spray gun  12  to thermally spray substrate  14  in a thermal spray cycle. An example of a thermal spray cycle is shown in  FIG. 1B . 
       FIG. 1B  is a conceptual and schematic diagram illustrating an example thermal spray cycle of coating  16  applied to substrate  14  by thermal spray system  10  of  FIG. 1A . As shown in  FIG. 1B , the thermal spray cycle may include a plurality of passes  19  of a spraying cycle of the coating material sprayed from thermal spray gun  12  to form coating  16 . For example, computing device  30  may control thermal spray gun  12  to apply plurality of passes  19  on substrate  14 . In some examples, plurality of passes  19  may include an alternating plurality of passes. In some examples, a single thermal spray cycle may include forming one layer of coating  16 , for example, including plurality of passes  19  that substantially cover an underlying layer, for example, a major surface of substrate  14 , or a previous layer of coating  16 . A single pass of the plurality of passes may include a single traversal or substantially linear sweep along substrate  14  from a beginning of a pass path to an end of a pass path (e.g., from one edge of substrate  14  to another edge of substrate  14 ). While alternating passes are shown in  FIG. 1B , thermal spray gun  12  apply coating  16  using any suitable combination of passes, for example, staggered, undulating, crisscross, zigzag, spiral, curved, parallel, or unidirectional passes. In some examples, at least some passes of plurality of passes  19  overlap. 
     A single thermal spray cycle may thus be initiated at time t 1 , for example, near one corner of substrate  14 , and terminated at time t 2 , for example, at the diagonally opposite corner of substrate  14 . In other examples, thermal spray cycle may be initiated and terminated at any other suitable locations along substrate  14 . The mid-point of the thermal spray cycle may be determined by the approximate location of the plume of thermal spray  17  along substrate  14  at time (t 1 +t 2 )/2. In some examples, the location of the plume at the mid-point may be substantially at or near a geometric mid-point of coating  16  on substrate  14 . 
     Computing device  30  may thus control thermal spray gun  12  to execute at least one thermal spray cycle, thus applying at least one layer of coating material that eventually forms completed coating  16  on substrate  14 . 
     Thermal spray  17  may exert a thrust force and heat on substrate  12 . Thus, as thermal spray gun  12  sprays the plurality of passes on substrate  12 , one or both of substrate  12  and coating  16  may be subjected to bending deflections and temperature fluctuations or changes. System  10  may include components for detecting such bending deflections and temperature changes. For example, system  10  may include one or more of a respective strain gauge  24 , or a respective laser sensor  26   a  ( 26   b ,  26   c ) adjacent or at each respective predetermined location of the at least one predetermined location to detect the deflection of substrate  14 . For example, respective strain gauge  24  may be adjacent to or in contact with substrate  14  at or adjacent each respective predetermined location of the at least one predetermined location. Strain gauge  24  may be configured to generate a signal indicative of a respective deflection of substrate  14  at the respective at least one predetermined location. While system  10  includes a single strain gauge  24  in the example shown in  FIG. 1A , in other examples, system  10  may include two, three, or more strain gauges. 
     Laser sensor  26   a  ( 26   b ,  26   c ) may be at or adjacent each respective predetermined location of the at least one predetermined location along substrate  14 . In some examples, system  10  includes at least three respective laser sensors  26   a ,  26   b , and  26   c , respectively adjacent at least three respective predetermined locations along substrate  14 . While system  10  includes three laser sensors  26   a ,  26   b , and  26   c  in the example shown in  FIG. 1A , in other examples, system  10  may include one, two, four, or more laser sensors. Laser sensor  26   a  may be configured to generate a signal indicative of a respective deflection of substrate  14  at the respective at least one predetermined location. Computing device  30  may be configured to determine the bending deflection of substrate  14  after receiving, from laser sensor  26   a  ( 26   b ,  26   c ), the signal indicative of the respective deflection. 
     The at least one location of respective strain gauge  24  or respective laser sensor  26   a  ( 26   b ,  26   c ) may include any suitable location along substrate  14 . In some examples, the at least one location may include a location at or near substantially a center of substrate  14 , or at or adjacent ends of substrate  14 . In some examples, respective strain gauge  24  may be adjacent or at one or more predetermined location of the at least one predetermined locations, while respective laser sensor  26   a  (or other laser sensors) may be adjacent or at other of the predetermined locations of the at least one predetermined location. 
     System  10  may include at least one thermocouple  27  to detect a temperature of substrate  14 , at least one pyrometer  28  to detect a temperature of coating  16 , or both. In some examples, system  10  may alternatively or additionally include infrared temperature sensors to detect the respective temperature of substrate  14 , coating  16 , or both. Computing device  30  may receive signals from one or more of respective strain gauge  24 , respective laser sensor  26   a  (or  26   b , or  26   c ), at least one thermocouple  27 , or at least one pyrometer  28  indicative, respectively, of deflection or temperature of substrate  14  or coating  16 , and may analyze the signal to determine the respective temperature or deflection. As described elsewhere in the disclosure, computing device  30  may determine a curvature of substrate  14  from the deflection. Computing device  30  may thus monitor the temperature and curvature of one or both of substrate  14  or coating  16  at predetermined intervals during thermal spray pass  19 , or during a thermal spray cycle including a plurality of passes  19 , or during a thermal spray process including a series of thermal spray cycles. 
       FIG. 2  is a schematic chart illustrating temperature of substrate  14  as a function of time for multiple thermal spray cycles during an example thermal spray process. As shown in  FIG. 2 , substrate  14  may be preheated to a first temperature before commencing spraying substrate  14  with a series of thermal spray cycles (one thermal cycle of which is labelled C). During one thermal spray cycle C, the temperature (upper curve) and the curvature (lower curve) of substrate  14  may change. For example, both the temperature and curvature of substrate  14  may rise to peak at approximately the mid-point of cycle C, and then reduce towards the end of cycle C, before rising again during a subsequent cycle. As shown in  FIG. 2 , after the thermal spraying is completed to apply a series of layers of coating material forming coating  16 , substrate  12  eventually cools down, and relaxes to a substantially undeflected (or uncurved) configuration from the deflected or curved configurations assumed during thermal spraying. 
     While  FIG. 2  illustrates a single peak for each thermal spray cycle, substrate  14  and coating  16  may be exhibit further sub-changes in temperature and deflection or curvature during thermal spray pass  19  of the plurality of passes within a single thermal cycle C, as shown in  FIG. 3 . 
       FIG. 3  is a schematic chart illustrating thermal spray passes and cycles during an example thermal spray process. As shown in  FIG. 3 , each thermal spray cycle (cycle  1 , cycle  2 , and cycle  3 ), includes sub-fluctuations in temperature (diamonds), and curvature, as detected using single laser sensor  26   a  (triangles) and using three laser sensors  26   a ,  26   b , and  26   c  (x&#39;s). Thus, both temperature and curvature may rise within a single pass to a respective peak, and then lower to a respective minimum, before rising again in a subsequent pass. 
     Computing device  30  may determine these fluctuations of temperature and curvature, and based on such fluctuations, eventually determine properties of coating  16 , for example, residual stress or modulus of coating  16 . Based on the properties of coating  16 , for example, one layer of coating  16  or of coating  16  as a whole, computing device  30  may determine thermal spray parameters to be used for controlling thermal spray gun  12  to generate coating  16  having properties meeting predetermined specifications. 
     Returning to  FIG. 1A , computing device  30  may include non-volatile storage for storing instructions and data and may include a processor  31  for executing the instructions. In some examples, the non-volatile storage of computing device  30  may include one or more modules including one or both of instructions and data. For example, computing device  30  may include one or more of curvature detection module  32 , coating analysis module  34 , finite element analysis module  36 , fast Fourier transform (FFT) module  38 , or spray control module  39 . Curvature detection module  32  may receive signals from respective strain gauge  24  or respective laser sensor  26   a  ( 26   b ,  26   c ) and may determine a bending deflection of substrate  14  at at least one predetermined location based on the signal. Curvature detection module  32  may further calculate a curvature of substrate  14  based on the deflection. For example, by comparing the deflection at at least one predetermined location during a particular time interval during spraying with an initial deflection at the same at least one predetermined location before the spraying, curvature detection module  32  may determine the curvature of substrate  14 . Coating analysis module  34  may determine properties of the coating, for example, residual stress, modulus, or other properties, based on the curvature. For example, coating analysis module  34  may use a thin film equation or a thick film equation described elsewhere in the disclosure to determine residual stress based on the curvature. 
     In some examples, coating analysis module  34  may analyze peak shifts in frequency spectra of bending deflections of substrate  14 . In some such examples, FFT module  38  may analyze variations in bending deflections of substrate  14  over predetermined windows of time to determine a frequency spectrum of bending deflection over that window, as described with reference to  FIGS. 4A and 4B . 
       FIG. 4A  is a schematic chart illustrating temporal changes in bending deflection of substrate  14  during a thermal spray process.  FIG. 4B  is a schematic chart illustrating peak shifts in frequency spectra of bending deflections during the thermal spray process. Finite element analysis module  36  may store a digital representation of a finite element model (FEM) of substrate  14  and coating  16 . Finite element analysis module  36  may simulate the response of substrate  14  to thermal spraying by iterating the FEM of substrate  14  based on predetermined constraints or boundary conditions and subjected to a model simulating the thermal spraying of the substrate. Finite element analysis module  36  may determine shifts in one or more frequency peaks for the FEM for conditions simulating those of the thermal spraying process, based on a test modulus of the coating. Coating analysis module  34  may change the test modulus and compare the shifts in frequency peaks for the FEM for different test moduli with shifts in frequency peaks for substrate  14 , until the shifts in the frequency peaks substantially match. For example, finite element analysis module  36  may determine a value of an objective function based on differences between predicted plurality of shifts and corresponding observed plurality of shifts. Finite element analysis module  36  may determine the test modulus associated with the predicted plurality of shifts of the FEM such that the objective function satisfied a predetermined condition. 
     Based on the properties of coating  16  determined by computing device  30 , for example, one or both of residual stress or modulus of coating  16 , computing device  30  may control a spray process to produce a subsequent coating having properties within predetermined specifications. The predetermined specifications may include a coating thickness, a coating modulus (for example, a Young&#39;s modulus), a residual stress in coating  16 , or a coating hardness. The subsequent coating may be a subsequent layer of coating  16 , or a coating similar to coating  16  applied to a second substrate, or a coating similar to coating  16  re-applied to substrate  14  after scrubbing or cleaning a previous coating from substrate  14 . For example, spray control module  39  may generate one or more control signals based on one or both of modulus or residual stress determined for coating and based on acceptable ranges of modulus, residual strength, or other predetermined properties of coating  16 . The control signals may be sent to and received by spray controller  22  to control the operation of thermal spray gun. While different modules of computing device  30  have been described, example techniques according to the disclosure are described with reference to computing device  30 . An appropriate module of computing device  30  may perform one or more steps of example techniques according to the disclosure. In some examples, computing device  30  may not include one or more of such modules, and may instead executing instructions corresponding to operations performed by one or more modules. 
     Thus, system  10  may be used for controlling thermal spraying of substrate  14  with coating material from thermal spray gun  12  to form coating  16  having predetermined properties. In addition to example systems for thermal spraying, the disclosure also describes example techniques for thermal spraying, for example, as described with reference to  FIGS. 5 and 6 . 
       FIG. 5  is a flow diagram illustrating an example technique for thermal spraying. The example technique of  FIG. 5  is described with reference to example system  10  of  FIGS. 1A and 1B , and the charts of  FIGS. 2 and 3 , for convenience and conciseness. However, example techniques according to the disclosure may be implemented using any suitable system. 
     In some examples, the example technique of  FIG. 5  includes, thermally spraying substrate  14  in a thermal spray cycle including a plurality of passes of a coating material to form at least one layer of coating  16  ( 40 ). For example, computing device  30  (e.g., spray control module  39 ) may cause spray controller  22  to control material reservoir  18 , fluid supply  20 , and thermal spray gun  12  to coat substrate  14  using a plurality of passes of a coating material to form coating  16  ( 40 ). The plurality of passes may include plurality of passes  19 , as shown in  FIG. 1B , or any other suitable plurality of passes of a coating material to form at least one layer of coating  16 . 
     In some examples, the example technique of  FIG. 5  includes, determining, by computing device  30  (e.g., by curvature detection module  32 ), a change in curvature, Δκ, of substrate  14  during a central pass of the plurality of passes ( 42 ). In some examples, the central pass is a respective pass of the plurality of passes that includes the mid-point in time of the spraying cycle, for example, mid-point (t 1 +t 2 )/2, as shown in  FIG. 1B . Thus, the change in curvature may be the difference in minimum curvature and maximum curvature during the central pass, for example, Δκ 2 , as shown in  FIG. 3 . The change in curvature at the central pass may be larger than changes in curvature during other time intervals of the thermal spray cycle, and may thus result in one or more of better precision or accuracy in determination of properties, such as residual stress, of coating  16  based on the change in curvature. 
     In some examples, determining the change in curvature of substrate  14  ( 42 ) includes determining, by computing device  30  (e.g., by curvature detection module  32 ), a bending deflection of substrate  14  at at least one predetermined location along substrate  14 . In some examples, the changes in bending deflection of the substrate include vibrations of substrate  14  in response to one or both of forces exerted on substrate  14  by the thermal spraying, for example, by thermal spray  17 , or by cooling of coating  16 . In some examples, the at least one predetermined location includes at least three locations. In some examples, determining the bending deflection includes receiving, by computing device  30 , from respective laser sensor  26   a  ( 26   b ,  26   c ) adjacent each respective predetermined location of the at least one predetermined location, a signal indicative of a respective deflection of substrate  14  at the respective at least one predetermined location. 
     In some examples, determining the bending deflection includes receiving, by computing device  30  (e.g., by curvature detection module  32 ), from respective strain gauge  24  in contact with substrate  14  at each respective predetermined location of the at least one predetermined location, a signal indicative of a respective deflection of substrate  14  at the respective at least one predetermined location. Computing device  30  may determine change in curvature of substrate  14  based on the bending deflection. For example, computing device  30  may relate a known relation between deflections and curvature associated with a geometry of substrate  14 . 
     In some examples, the example technique of  FIG. 5  includes, determining, by computing device  30  (e.g., by coating analysis module  34 ), residual stress σ of the coating based on the change in the curvature Δκ ( 44 ). For example, computing device  30  may determine the residual stress σ ( 44 ) by determining a change in a thickness Δt D  of coating  16  during the central pass. Computing device  30  may determine the residual stress σ based on a relationship including Δt D  and Δκ. Computing device  30  may receive a signal, for example, from a laser sensor or optical sensor, or any suitable sensor, or from user input, indicative of the change in thickness Δt D . Computing device  30  may then evaluate properties of coating  16  based on Δt D  and other factors. For example, computing device  30  (e.g., coating analysis module  34 ) may determine the residual stress σ ( 44 ) by evaluating a thin film equation: 
     
       
         
           
             
               
                 
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     In some examples, computing device  30  (e.g., coating analysis module  34 ) may determine the residual stress σ ( 44 ) by evaluation a thick film equation, for example, when a thickness of coating  16  is greater than a predetermined threshold: 
     
       
         
           
             
               
                 
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     In EQUATION 2, t s  is a thickness of substrate  14 , 
     
       
         
           
             
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     E s  is the Young&#39;s modulus of substrate  14 , v s  is the Poisson&#39;s ratio of substrate  14 , 
     
       
         
           
             
               
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     is the Young&#39;s modulus of coating  16 , and v D  is the Poisson&#39;s ratio of coating  16 . 
     Computing device  30  (e.g., coating analysis module  34 ) may select between EQUATION 1 and EQUATION 2 based on an average thickness of coating  16 . For example, if the average thickness of coating  16  is less than a predetermined threshold, computing device may select and apply EQUATION 1 (thin film equation), while if the average thickness is greater than or equal to the predetermined threshold, computing device  30  may select and apply EQUATION 2 (thick film equation). 
     Thus, computing device  30  (e.g., coating analysis module  34 ) may determine residual stress σ of coating  16 . Computing device  30  (e.g., spray control module  39 ) may control a subsequent thermal spray process based on the residual stress of coating  16 . 
     For example, the example technique of  FIG. 5  may further include determining, by computing device  30  (e.g., by spray control module  39 ), a plurality of thermal spray parameters based on the residual stress σ ( 46 ). The thermal spray parameters are configured to produce a second coating with residual stress within a predetermined acceptable range. For example, the example technique may include forming a plurality of test coatings, and determining respective residual stresses for each respective test coating of the plurality of test coatings. Computing device  30  may associate the respective residual stresses with respective thermal spray parameters that produced the respective coating. Computing device  30  may create calibration curves or determined thresholds for one or more thermal spray parameters based on parameter values associated with acceptable residual stresses, and determine thermal spray parameters for a subsequent thermal spray cycle or process. Thus, the subsequent thermal spray cycle or process may form a coating having an acceptable residual stress, for example, residual stress lower than a predetermined threshold. The second coating may be a different coating on component. For example, the component may include substrate  14 , or a different substrate. In some examples, the second coating may be a subsequent layer of or a coating applied on coating  16 . 
     In some examples, the example technique of  FIG. 5  further includes thermally spraying substrate  14  with a plurality of passes based on the plurality of thermal spray parameters to produce the second coating ( 48 ). For example, computing device  30  (e.g., spray control module  39 ) may control the thermal spraying, for example, by sending control signals to spray controller  22  based on one or more of the residual stress or other properties of coating  16  (or an intermediate layer of coating  16 ), to eventually form coating  16  or a subsequent coating having properties substantially meeting predetermined coating specifications on a component, for example, a component including substrate  14  or a different substrate. 
     While computing device  30  may determine residual stress based on curvature using known relationships between curvature and residual stress, as described with reference to the example technique of  FIG. 5 , in other examples, computing device  30  may determine other properties of coating  16  without relying on such relationships between properties. 
       FIG. 6  is a flow diagram illustrating an example technique for thermal spraying. The example technique of  FIG. 6  is described with reference to example system  10  of  FIGS. 1A and 1B , and the curves of  FIGS. 2 and 3 , for convenience and conciseness. However, example techniques according to the disclosure may be implemented using any suitable system. 
     In some examples, the example technique of  FIG. 6  includes thermally spraying substrate  14  in a thermal cycle including a plurality of passes of a coating material to form coating  16  ( 50 ). For example, computing device  30  (e.g., spray control module  39 ) may cause spray controller  22  to control material reservoir  18 , fluid supply  20 , and thermal spray gun  12  to coat substrate  14  using a plurality of passes of a coating material to form coating  16  ( 50 ). 
     In some examples, the technique of  FIG. 6  includes receiving, by computing device  30  (e.g., by curvature detection module  32 ), a first signal indicative of changes in bending deflection of substrate  14  at a predetermined location along the substrate over a first coating cycle ( 52 ). In some examples, the at least one predetermined location comprises at least three locations. In some examples, the example technique includes receiving, by computing device  30 , a second signal indicative of changes in bending deflection of substrate  14  at the predetermined location along substrate  14  over a second coating cycle ( 54 ). In some examples, the changes in bending deflection of the substrate include vibrations of substrate  14  in response to force exerted on substrate  14  by the thermal spraying, for example, by thermal spray  17 . In some examples, receiving one or both of the first signal or the second signal ( 52  or  54 ) includes receiving, by computing device  30  (e.g., by curvature detection module  32 ), from respective gauge  24  in contact with substrate  14  at each respective predetermined location of the at least one predetermined location, one or both of the first signal or the second signal. In some examples, one or both of receiving the first signal or the second signal ( 52  or  54 ) includes receiving, by computing device  30 , from respective laser sensor  26   a  ( 26   b ,  26   c ) one or both of the first signal or the second signal. 
     Computing device  30  (e.g., coating analysis module  34 ) may determine properties of coating  16  based on the bending deflections. For example, computing device  30  (e.g., fast Fourier transform module  38 ) may perform a fast Fourier transform (FFT) on a time-domain representation of bending deflections to obtain a frequency-domain representation, for example, a frequency spectrum. Computing device  30  (e.g., coating analysis module  34 ) may compare the frequency spectra at different time intervals to determine properties of coating  16 . For example, the example technique of  FIG. 6  may include, determining, by computing device  30 , a first frequency spectrum of the bending deflection over the first coating cycle based on the first signal ( 56 ). Computing device  30  may determine a second frequency spectrum of the bending deflection over the second coating cycle based on the second signal ( 60 ). In some examples, computing device  30  may determine the first frequency spectrum ( 56 ) the second frequency spectrum ( 60 ) by respectively performing fast Fourier transform (FFT) on the first signal and on the second signal. Computing device may compare the first frequency spectrum and the second frequency spectrum to determine properties of coating  16 . For example, computing device  30  (e.g., coating analysis module  34 ) may determine a first plurality of peaks of the first frequency spectrum ( 58 ), and determine a second plurality of peaks of the second frequency spectrum ( 62 ). In some examples, computing device  30  may determine a plurality of frequency shifts between respective peaks of the first plurality of peaks and corresponding peaks of the second plurality of peaks ( 64 ), for example, to compare the first and second spectra. 
     Based on the comparison, computing device  30  (e.g., coating analysis module  34 ) may determine a property of coating  16 , for example, a modulus of coating  16 . For example, computing device  30  may determine a modulus of coating  16  based on the plurality of shifts ( 66 ). In some examples, computing device  30  (e.g., finite element analysis module  36 ) may determine the modulus of coating  14  based on the plurality of shifts by iterating a finite element model (FEM) of substrate  14  subjected to a model simulating the thermal spraying of substrate  14 , assigning a test modulus to the FEM, determining a predicted plurality of shifts based on the FEM and on the test modulus, comparing the predicted plurality of shifts of the FEM to the observed plurality of shifts by varying the test modulus, determining a value of an objective function based on the differences between the predicted plurality of shifts and corresponding observed plurality of shifts, and determining the modulus to be the test modulus associated with the predicted plurality of shifts of the FEM such that the objective function satisfies a predetermined criterion. The objective function is a variable to be minimized, for example, a difference between actual and observed plurality of shifts. The predetermined criterion may include a threshold value, a threshold value of a derivative of the objective function, or a second derivative of the objective function, or any suitable criterion indicative of a local or a global minimum of the objective function. In some examples, the model incorporates a predetermined test force exerted on the FEM by the thermal spraying. For example, the test force may be substantially similar to or include one or both of a thrust force exerted by thermal spray  17  from thermal spray gun  12  on substrate  14 , or cooling force resulting from cooling of coating  16 . 
     In some examples, the example technique of  FIG. 6  further includes determining, by computing device  30  (e.g., by coating analysis module  34 ), a plurality of thermal spray parameters based on the modulus ( 68 ). The thermal spray parameters may be configured to produce a second coating with modulus within a predetermined acceptable range. For example, the example technique may include forming a plurality of test coatings, and determining respective moduli for each respective test coating of the plurality of test coatings. Computing device  30  may associate the respective moduli with respective thermal spray parameters that produced the respective coating. Computing device  30  may create calibration curves or determined thresholds for one or more thermal spray parameters based on parameter values associated with acceptable moduli, and determine thermal spray parameters for a subsequent thermal spray cycle or process. Thus, the subsequent thermal spray cycle or process may form a coating having an acceptable modulus, for example, modulus within a predetermined range. The second coating may be a different coating on a component. For example, the component may include substrate  14 , or a different substrate. In some examples, the second coating may be a subsequent layer of or a coating applied on coating  16 . 
     In some examples, the example technique of  FIG. 6  further includes thermally spraying substrate  14  with a plurality of passes based on the plurality of thermal spray parameters to produce the second coating ( 69 ). For example, spray control module  39  may send one or more control signals to spray controller  22  to control thermal spray gun to produce the second coating ( 69 ). For example, computing device  30  (e.g., spray control module  39 ) may control the thermal spraying, for example, by sending control signals to spray controller  22  based on one or more of the moduli or other properties of coating  16  (or an intermediate layer of coating  16 ), to eventually form coating  16  or a subsequent coating having properties substantially meeting predetermined coating specifications on a component, for example, a component including substrate  14  or a different substrate. 
     Thus, systems and techniques described above may be used to control thermal spray of substrates with coatings having properties satisfying predetermined specifications. 
     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 a computer system-readable medium, such as a computer system-readable storage medium, containing instructions. Instructions embedded or encoded in a computer system-readable medium, including a computer system-readable storage medium, 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 system-readable medium are executed by the one or more processors. Computer system 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 system readable media. In some examples, an article of manufacture may comprise one or more computer system-readable storage media. 
     Various examples have been described. These and other examples are within the scope of the following claims.