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Attributes

Cold Spray is an entirely distinct family of processes for applying surfacing materials. Not only are coatings applied in solid state with Cold Spray, but the processes also have different application, installation, operation, and maintenance requirements than other types of processes such as Thermal Spray (THSP). In fact, the unique process attributes of Cold Spray can compliment THSP processes and enable users to enjoy a broad range of new surfacing opportunities.
Attributes of Cold Spray technology include:

  • Suitability to wide varieties of substrate and coating combinations
    • Cold Spray coatings can be applied to almost any solid substrate including metals and non-metals. Common examples include: aluminum, bronze, copper, cast iron, magnesium, nickel, steel,  zinc, ceramics, concrete, glass and stone.
    • Minimal heat input allows coatings to be deposited on materials that cannot accommodate other coating methods. For example, cast iron and die cast zinc or pot metal.
    • Process remains consistent over a wide range of substrate temperatures.
  • Substrate thickness is not considered an essential process variable since:
    • Low spray temperatures rarely cause thermal shock to the substrates
    • There is no requirement to raise the substrate temperature to promote fusion.
    • Process does not induce significant substrate heating so a change in heat sinking capacity of the material is not generally of consequence.
    • The main limitation to thinness of the substrate is its resilience to particle impact.
  • The process eliminates the detrimental effects of high temperatures on coatings and substrates such as:
    • Oxidization.
    • Decomposition.
    • Loss of constituents.
    • Crystallization.
    • Grain growth.
    • Residual tensile stress that can lead to delamination.
  • Unique surfacing deposit (coating) properties include:
    • Porosity of the deposit is typically less than 0.5% for metals.
    • High bond strength.
    • Ultra-thick coatings are possible because there is no accumulation of residual thermal stresses to decrease the bond strength.
    • Coating has compressive residual stress that contributes to improved fatigue performance without the need for post coating shot peening.
    • Highly-wrought microstructure yields a deposit with higher hardness than traditional coating methods.
    • High degree of intergranular bonding, few oxide impurities, and low porosity results in cold-sprayed coatings that typically have high thermal and electrical conductivity.
  • Production Capability
    • Minimal set-up and warm-up time result in increased productivity.
    • Because the process occurs essentially at room temperature, copper, aluminum, and many other reactive metals can be cold sprayed in an open-air environment with little or no oxidation.
    • Inert gas may be used to propel the feedstock powder and shield the deposited coating to form ultra-pure coatings.
    • Solid state surfacing with no bulk particle melting means that material retains composition and phases of initial particles.
    • Spray distances (nozzle to substrate distance) of 10 mm or less are possible ensuring maximum control over the particle flight path.
    • Lower heat input to workpiece reduces cooling requirement
    • Deposition rate and travel speed easily adjusted to deposit coatings from a few grains thick to millimeters per pass.
    • Thickness can be controlled accurately (can be varied across the deposit to match the required fill profile or to feather the coating edges).
    • Follows substrate contours very closely.
    • Excellent surface finishes are achievable.
    • Roughened coating surface accepts additional coatings such as paint and plating very well.
    • Variable composition is possible both across and through the coating thickness.
  • Operational safety improved by:
    • The absence of high temperature jets and radiation.
    • The simplicity of technical implementation and operation.
    • No fuel gases employed.
  • Environmentally friendly
  • Grit blast, spray coating, and shot peening performed in a single operation so that finished coatings can be applied quickly and consistently.
  • Spray powder materials include:
    • Ductile metals. Both amorphous and nanostructured metals. Common metals include: aluminum, copper, nickel, and zinc.
    • Alloys. The absence of melting means that the alloy you start with will be the alloy in the coating. Not only can alloys be used that match the substrate but you can also use alloys (e.g. an Aluminum alloy different than the substrate Aluminum alloy) to achieve:
      • Localized improvement of chemical, electrical, mechanical, and thermal properties.
      • Color match or contrast.
    • Composites. Cold Spray deposits can contain entrained materials to increase hardness or wear resistance, promote shear, increase lubricity, etc.
    • Blends. Cold Spray feedstock powder can be a mixture of different constituents such as:
      • Chemically dissimilar materials.
      • A mixture of flux and brazing or soldering filler metal.
      • A metal and foaming agent to facilitate placement of metal foam.
  • Substrate preparation
    • Minimal surface cleaning is required since feedstock powder will not bond until there is intimate, contaminant free, contact with the substrate.
    • Supplemental substrate surface preparation (such as roughening) is not generally required. In some cases it is undesirable if it introduces stress sites in the substrate.
    • Minimal need for masking because the spray profile has sharply defined edges.
  • Un-deposited powders can be easily collected for reuse or reclamation.
Attributes specific to the Low Pressure Cold Spray Process:
  • Low noise level (70-85dB).
  • Portability enables repairs to be done on-site to avoid expensive and time consuming disassembly.
  • Since compressed air is used the process is economically scalable to very large sizes.
  • Powder does not go through the throat of the nozzle so there is less wear and lower chance of feedstock agglomeration.
Attributes specific to the High Pressure Cold Spray Process:
  • Deposition efficiencies may be as high as 95%.
  • Higher velocity and temperature permit harder feedstock and substrate materials to be employed and larger particles may be sprayed.
Limitations of the Cold Spray Process:
  • Hard brittle materials like ceramics cannot be sprayed without using ductile binders.
  • Substrates must be resilient or well supported to accept coating.
  • Low ductility substrates typically have low bond strengths.
  • Line-of-sight process making internal surfaces and complex shapes difficult to spray.
  • Deposition rate substantially lower than thermal spray.
  • Coating displays near-zero ductility in the as-sprayed condition.
  • If large quantities of Helium or Nitrogen carrier gas are required, recycling may be necessary to manage the expense.
  • Many technical standards have yet to be revised or prepared.

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