Abstract:
A nozzle element for applying powder material to a substrate is provided. The powdered material is applied from the nozzle element onto the substrate generating a coating of the powder material defined by a first film thickness and a first particle size of the powder material. A deformation nozzle element is provided for spraying shot toward the coating of powder material disposed upon the substrate deforming particles of the powder material disposed in the coating forming a second particle size being smaller than the first particle size and deforming the coating to define a second film thickness being less than the first film thickness.

Description:
PRIOR APPLICATIONS 
       [0001]    The present application claims priority to U.S. Provisional Patent Application No. 62/250,548 filed on Nov. 4, 2015, the contents of which are incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present application relates toward a cold spraying coating system and method used to apply a protective coating to a substrate. More specifically, the present application relates toward an improved method of cold spraying a coating onto a substrate using spray shot to enhance performance of the coating. 
       BACKGROUND 
       [0003]    Cold spraying particles onto a substrate surface to protect the substrate has been gaining increased acceptance as a viable method of coating a substrate. To obtain high-performance coatings the cold spraying is conducted at a high pressure with the assistance of a high-pressure gas, such as, for example, helium, nitrogen, and air having a coating material infused therein, which includes, for example, powder metals, refractory metals, alloys and composite materials. Powder particles having a size range of between about 20 to 50 micrometers are introduced at a high pressure into a supersonic gas stream generated by a spray gun and emitted from a nozzle. One such nozzle is disclosed in U.S. Pat. No. 8,132,740, the contents of which are incorporated herein by reference. The powder particles are accelerated to a supersonic velocity and directed to impact the substrate onto which the coating is to be formed. 
         [0004]    Kinetic energy generated from impact of the particles on the substrate causes the particles to deform to a slightly flat configuration and diffuse into the substrate. The deformation promotes adhesion to the substrate, interlocking between adjacent particles and the substrate, and metallurgical bonding with the substrate resulting in a protective coating on the substrate. Because the particles are cold sprayed at near ambient temperatures, oxidation while airborne and forming the coating is prevented or significantly reduced. 
         [0005]    However, because the distribution of the particles is not uniform and random, the structures of the coating and performance properties are not believed to be optimized. An effort to enhance the performance properties of the coating applied through conventional cold spraying includes a step of heat treatment or annealing of a cold spray coating in a furnace or by way of laser heating. However, heat treating or annealing the cold spray coatings is known to decrease the mechanical properties while resulting in more complexity and cost associated with cold spraying a substrate. Further, a laser heating process located adjacent the cold spraying operation is not viable due to airborne particles proximate the area of deposition and the inability to control necessary laser strength and other parameters to provide the desired annealing of the cold spray coating. 
         [0006]    Coatings applied by high pressure cold spraying processes are believed provide desirable durability properties. However, it is difficult to perform high pressure cold spraying in a conventional industrial environment without enclosing the high pressure cold spray system within a spray booth, cabinet, and helium and/or nitrogen shrouds to achieve the high particle velocity and prevent oxidation of the particles, which increases manufacturing complexity and cost. High pressure cold spray processes generate particle velocity in the range of 550 m/s to 900 m/s requiring environmental containment. 
         [0007]    One solution to some of these drawbacks of high pressure cold spraying technology is to reduce pressure of the cold spray nozzle to a speed of about 300 m/s to 500 m/s or a low pressure cold spray. However, low pressure cold spraying coatings provide an undesirable structure that does not perform well when compared with high pressure cold spray coatings. This is believed to be a result of insufficient particle velocity not providing desired particle deformation and resulting in weaker particle bonds and undesirable porosity of the resulting coating. 
         [0008]    Therefore, it would be desirable to provide a low pressure cold spray process that provides desired particle deformation, particle bonding, and coating porosity. 
       SUMMARY 
       [0009]    A method of applying a coating to a substrate includes a nozzle element for applying powder material to the substrate. The powder material is sprayed from a nozzle element onto the substrate generating a coating of powder material defined by a first film thickness and a first particle size and shape of the powder material. A deformation nozzle element is provided for spraying shot onto the coating applied to the substrate. The deformation nozzle sprays shot toward the coating of powder material disposed on the substrate to deform particles of the powder material disposed in the coating resulting in a second particle size is smaller than the first particle size and includes a second particle shape being flatter than the first particle shape. The coating is further deformed to a second film thickness that is less than the first film thickness by the spray shot directed toward the coating. 
         [0010]    The method of the present invention enables a low pressure cold spray process be performed upon a substrate to overcome some of the manufacturing difficulties of using a high pressure coating process, while achieving performance qualities of the high pressure coating process. For the first time, a desired particle deformation and reconfiguration of crystalline structure and film build are achieved using a low pressure cold spray process. Further, the use of a deformation spray nozzle to spray shot onto the low pressure cold spray coating enhances performance characteristics beyond that of a high pressure cold spray process by the significantly improved coating structure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    Other advantages of the present invention will be readily appreciated as the same becomes better understood by reference to the following detailed description, when considered in connection with the accompanying drawing, wherein: 
           [0012]      FIG. 1  shows a schematic view of the cold spray coating deposition apparatus of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    Referring to  FIG. 1 , a schematic of a low pressure, cold spray coating assembly is generally shown at  10 . The assembly  10  includes a nozzle element  12  for applying powder material  14  to a substrate  16 . For the purpose of this application, a low pressure cold spray assembly is defined as a nozzle element  12  operating at a particle velocity of between about 300 m/s to about 500 m/s, which is distinguished from a high pressure, cold spray nozzle that operates at a supersonic velocity. 
         [0014]    The nozzle element  12  sprays the powder material  14  onto the substrate  16  forming a first coating  18  having a first film thickness and a first particle  13  size of the powder material  14 . While the first coating thickness of the first coating  18  is tailored for desirable for performance characteristics of a particular application, the average first particle  13  size of the first coating  18  is believed to range between about 20 microns to 50 microns. A characteristic of the low pressure cold spray process, the average particle size is believed to decrease by less than 0.1 microns upon contact with the substrate  16 . However, the particles disposed in the first coating  18  become slightly deformed from a substantially spherical shape to an egg shape or oval disposition. 
         [0015]    The nozzle element  12  includes a particulate nozzle  20  that delivers a supersonic flow of delivery gas  22  into which the powder material  14  is infused. The delivery gas  22  increases the speed of the particles defining the powder material to about 300 m/s to 500 m/s with a target speed above 342 m/s or above the speed of sound. 
         [0016]    A temperature control nozzle  24  circumscribes or substantially circumscribes the particulate nozzle  20  and provides a stream of temperature control gas  26  toward the location on the substrate  16  onto which the powder material  14  is deposited. It should be understood by those of ordinary skill in the art that the temperature control gas  26 , in one embodiment is used to cool both the powder material  14  and the first coating  18 . However, for other embodiments, it may be desirable to heat both the powder material  14  and the first coating  18  to achieve a desired deposition temperature. In addition, the temperature control gas  26  also helps shape a spray pattern of the powder material  14  as it is delivered from the particulate nozzle  20  toward the substrate  16 . 
         [0017]    A deformation nozzle element  28  is positioned proximate the powder nozzle element  12 . The deformation nozzle element  28  emits a stream of shot gas identified by arrows  30  infused with shot  32 . The shot  32  is directed toward the first coating  18  shortly after deposition onto the substrate  16 . The shot  32  reshapes the first coating  18  into a second coating  34 . The shot  32  reduces the size of the particles disposed in the first coating  18  from a range of 20 microns to 50 microns to less than about 0.1 micron average particle size defining a second particle  35 . In addition, the film build of the first coating  18  is significantly reduced to a desired film thickness by the shot  32  in the second coating  34  the thickness of which depends upon the needs of a given application. 
         [0018]    The shot  32  results in nano-crystallization of the particles forming the coating  18 / 34 . Nano-crystallization is more pronounced at an upper surface  36  than it is at the subsurface  38  of the second coating  34  proximate the substrate. Therefore, the second particle  35  size is believed to gradually decrease in the coating  34  approaching proximity to the substrate  16 . Reduction in the second particle  35  size of the second coating  36  is also defined by impact milling, or plastic deformation, during bombardment of the first coating  18  by the shot  32 . The deformation achieved in the second coating  34  by the shot  32  enhances the performance of the second coating  34  over that achievable by the first coating  18  as will be explained further herein below. The shot  32  propelled by the gas  30  travels at a velocity of between about 60 m/s to about 80 m/s. This velocity is achieved by pressure ranges of the gas of between about 5 bar to about 6 bar. 
         [0019]    The deformation of the second coating  34  also provides an increase in density of the second coating over that of the first coating  18 . In addition, the egg-shaped particles disposed in the first coating  18  are further flattened by the shot  32  increasing particle contact. The increased density and particle contact reduces the propensity of oxygen and moisture from penetrating the second coating  36  over that of the first coating  18 , which is known to cause oxidation of metallic substrates. Therefore, the second coating  36  substantially seals the substrate  16  relative to the first coating  18  or a mere low pressure cold spray coating. 
         [0020]    The shot  32  is selected from materials useful to deform the first coating  18  while not removing substantive amounts of the first coating  18  during bombardment. Therefore, the shot  32  is tailored to the material composition of the first coating  18 . As such, as hardness of a particular coating is increased, a durometer of the shot  32  may also be increased to achieve the desired deformation of the first coating  18 . Alternatively, softer coatings likely may make use of a softer or lower durometer shot. The shot grades included S100, S130, S170, and S280 with shot diameter including 0.03 mm, 0.04 mm, 0.5 mm and 0.8 mm. It is further contemplated that hardness of the shot is selected based upon a desired amount of nano-crystallization and deformation of the particles forming the first coating. The shot  32  is contemplated to be harder than the first coating  18  and includes a hardness value of about 50 HRC. 
         [0021]    The shot  32  is selected from a variety of ceramic granules, or other materials including, but not limited to, SiO 2 , SiC, Al 2 O 3  or equivalents. In one embodiment, the shot  32  includes a size range of between 150-200 microns, which is substantially larger than the particle size of the powder material  14  disposed in the first coating  18 . In one embodiment, the shot is used only once to avoid contamination of the resultant second coating  34 . However, in alternative embodiments, the shot is re-used after cleaning, or when contamination of the second coating  34  is not critical. 
         [0022]    In one embodiment, the assembly  10  achieves a fixed orientation between the powder nozzle element  12  and the deformation nozzle element  28 . In this embodiment, the powder nozzle element  12  is oriented substantially perpendicular to the substrate  16 , while the deformation nozzle element  28  is oriented at a fixed angle to the substrate  16  to achieve desired deformation. The angle of the deformation nozzle element  28  to the substrate  16  includes a range between about 75° to about 90° to achieve desired nano-crystallization, particle deformation and coating thickness. Alternatively, the powder nozzle element  12  and the deformation nozzle element  28  are not fixed relative to the other so that various types of deformation may be achieved on such as, for example, three-dimensional objects. 
         [0023]    As set forth above, temperature of the first coating  18  upon deformation is controlled between a desired range. The deformation nozzle  28  also provides further control of the temperature deposition of the first coating  18  by way of temperature control of the shot gas  30 . Alternatively, the deformation nozzle  28  is oriented relative to the powder nozzle  12  so that the first coating  18  achieves a desired temperature prior to deformation by the shot  32 . 
         [0024]    The invention has been described in an illustrative manner, and it is to be understood that the terminology that has been used is intended to be in a nature of words of description rather than of a limitation. Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the specification the referenced numerals are merely for convenience, and are not to be in any way limiting, so that the invention may be practiced otherwise therein specifically described.