ATTRIBUTES

 

The Cold Gas-Dynamic Spray Advantage

Established Thermal Spray (THSP) processes such as arc spraying (ASP), flame spraying (FLSP), high velocity oxy fuel (HVOF) spraying, and plasma spraying (PSP) are well understood and widely used. These THSP processes are used to deposit finely divided metallic or nonmetallic surfacing materials in a molten or semimolten condition to form thermal spray deposits. Since the surfacing materials in most THSP processes are propelled as high velocity droplets it was natural for the pioneers of the Cold Spray industry to associate the processes.

Today, we know that Cold Gas-Dynamic Spray (Cold Spray) is an entirely distinct family of process for applying surfacing materials. Not only are surfacing materials applied in solid state with Cold Spray, but the processes also have different application, installation, operation, and maintenance requirements than required for THSP. Cold Spray processes have unique attributes that position them well to compliment THSP processes and to enable a broad range of new surfacing opportunities.

The unique attributes of Cold Spray technology include:

o  Performs the functions of grit blast, spray coating, and shot peening in a single operation so a finished surfacing deposit (coating) can be applied quickly and consistently.
 

o  A wide selection of substrate and coating combinations may be applied, many of which are difficult or impossible to produce by conventional surfacing processes.
 

o  The process eliminates the detrimental effects of high temperature on coatings and substrates such as:

·   Oxidization

·    Decomposition

·    Loss of constituents.

·    Crystallization.

·    Grain growth.

·    Residual tensile stress that can lead to delamination.
 

o  Range of substrates materials

·   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, and zinc; ceramics, concrete, glass, and stone.

·   Substrate thickness is not typically an essential variable since:

-      There is no requirement to raise the substrate to a temperature that promotes fusion.

-      The process remains consistent over a wide range of substrate temperatures.

-      The process does not induce significant substrate heating so a change in heat sinking capacity of the material is not generally of consequence.

·   The principal limitation to thinness of the substrate is its resilience when subjected to particle impacts.

·   Spray temperature is low enough that thermal shock in the substrate is rarely a problem.

·   Minimal heat input to substrate so coatings can be deposited on materials that cannot withstand the high temperatures of other coating methods. For example, cast iron and die cast zinc or pot metal.
 

o  Spray feedstock materials include:

·   Ductile metals. Both amorphous and nanostructured metals. Common metals include: aluminum, copper, nickel, and zinc.

·   Alloys. There are too many alloys to begin listing them. 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 now you can 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.
 

o  Substrate preparation

·   Minimal surface cleaning is required because the 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 and 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
 

o  Unique surfacing deposit (coating) properties

·   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.
 

o  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.

·   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.

·   Thickness can be varied across the deposit to match the required fill profile or to feather the coating edges.
 

o   Operational safety improved by:

o   The absence of high temperature jets and radiation.

o   The simplicity of technical implementation and operation.

o   No fuel gases employed.
 

o   Environmentally friendly

o   Eliminates the effects of high temperature oxidation, evaporation, melting, crystallization, etc. that occur with other thermal coating methods.

o   Undeposited powders can be easily collected for reuse or reclamation

o   Enables parts to be repaired for reuse.
 

Additional attributes specific to the Low Pressure Cold Spray Process:

o  Low noise level (70-85dB).

o   Portability enables repairs to be done on-site to avoid expensive and time consuming disassembly.

o   Since compressed air is used the process is economically scalable to very large sizes.

o  Powder does not go through the throat of the nozzle so there is less wear and lower chance of feedstock agglomeration.
 

Additional attributes specific to the High Pressure Cold Spray Process:

o   Deposition efficiencies may be as high as 95%.

o   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:

o   Hard brittle materials like ceramics cannot be sprayed without using ductile binders.

o   Substrates must be resilient or well supported to accept coating.

o   Low ductility substrates typically have low bond strengths.

o   Line-of-sight process making internal surfaces and complex shapes difficult to spray.

o   Deposition rate substantially lower than thermal spray.

o   Coating displays near-zero ductility in the as-sprayed condition.

o   If large quantities of Helium or Nitrogen carrier gas are required, recycling may be necessary to manage the expense.

o   Many technical standards have yet to be revised or prepared.