Cold Spray Technology and How It Compares to Thermal Spray

In the world of surface coating, there are many options available to extend the life of parts and most of them are very similar in one way or another. While thermal spray has been around for a long time, cold spray is relatively new on the scene having been developed sometime in the 1980’s and 90’s. While cold spray is technically a subset of thermal spray, its application does not require a combustion process like most thermal spray applications. Cold spray and thermal spray differ in three main ways; process parameters, coating materials and the resulting coating properties.

Process Parameter Differences

The first thing you notice is the term cold versus thermal. It paints a clear picture of the most distinguishing way these processes differ. Cold spray relies on kinetic energy via high velocities to deform the material particles and adhere them to the surface of the part. This means that while the process gas is heated anywhere from 90 to 2,000° F, the particle temperature usually remains below 400° F. The gases in cold spray are heated, not to warm up the powder particles, but instead to increase the velocity of the gas and therefore the material particles. This is also aided by the use of a nozzle inside the spray gun to achieve supersonic velocities. Other thermal spray processes, such as HVAF Spray and Plasma Spray can range from 3,500° to 35,500 ° F. HVAF and HVOF can also reach supersonic velocities using similar nozzle technology.

 

These processes both propel material through a spray gun but rely on different raw material forms to do so. Thermal spray uses many different process feedstocks depending on which process you are using. Arc spray uses electricity, where the more premium process, HVAF, uses hydrogen, nitrogen and propane. Cold spray relies on nitrogen or helium gas, or sometimes just air, to create the velocity needed to deform the material particles and apply it to a substrate.

Cold spray coatings are applied by either a low pressure or a high pressure spray system. High pressure operates at around 300-1000 psi while low pressure operates below 300 psi. They also differ in that low pressure injects the powder material after the nozzle throat while high pressure injects it prior to the nozzle throat. High pressure is used for applying high strength metals and alloys and low pressure is used for spraying softer metals and mixtures of metals and ceramic powders.

Coating Material Differences

Both Cold Spray and Thermal Spray apply similar coating materials. The big difference is the particle size of the powder that can be sprayed. Cold spray can spray much finer particles, including nanocrystalline powders. Cold spray powders typically have a grain size of 1-50 µm whereas thermal spray powders vary from 10-100 µm depending on the specific thermal spray process. Some thermal spray processes, such as Flame and Arc, can spray materials in wire form.

While cold spray can spray many materials, it has yet to be reliable in spraying economically available tungsten carbide or ceramics. These cold spray applications are yet to be fully developed but some nanocrystalline combinations are available. Cold spray excels with more ductile materials such as bronze, stainless steel, zinc, and aluminum. Technology is still being developed to spray harder more brittle materials which tend to rely on high heat for thermal spray coating.

Coating Property Differences

Cold spray and thermal spray coatings have mostly the same properties since these are heavily dependent upon the feedstock material being applied. Where cold spray excels is material oxidation. Oxidation is a chemical interaction that can occur during spraying where the metallic particles oxidize over their surface. Since cold spray utilizes plastic deformation of the material particles via high velocities as opposed to melting with heat, the particles are in a solid state when they contact the part and therefore experience no oxidation. With thermal spray a small amount of oxidation does occur over the particle surface.

 

Figure 2 - The nature of thermal spray coatings. Source [1]

 

While oxidation can be detrimental to corrosion, strength, and machinability; it can also increase the hardness and wear resistance of a coating. Oxidation can be mitigated in thermal spray processes by controlling process parameters such as standoff distance, reducing flame parameters, using an inert environment, or using compressed air cooling.

Although material choice largely dictates coating properties, these processes do differ in the residual stress they create in the coating after spraying. Cold spray results in compressive residual stress whereas thermal spray has tensile residual stress. While cold sprays bonding mechanism is not thoroughly researched, at least from readily available sources, it is believed to be largely mechanical interlocking and some metallurgical bonding between particle interfaces. This metallurgical bond between particles combined with the compressive residual stress means, in general, less potential for coating cracks. As with most coating properties in thermal spray, potential cracking can be mitigated with properly engineered spray parameters.

How to Choose Between Cold Spray and Thermal Spray

Cold spray is an emerging coating technology engineered for low spray temperature applications. It is well suited for the aviation industry and some specialty applications since it allows for minimal part distortion. It is a great low heat solution. While both it and thermal spray offer similar coating materials, thermal spray may be a more economical solution in some circumstances and is offered by a wider variety of providers. As with any thermal spray coating, cold spray included, material and spray process selection come down to your priorities.

If you have a part that is prone to distortion but need a premium corrosion solution, then cold spray is a coating solution worth looking into. If you’re more concerned with coating hardness and wear resistance, then some other thermal spray options, such as HVOF and HVAF may be better options. Being a relatively new technology, cold spray has its limitations, as does any thermal spray process. It will be interesting to see where this newer technology leads the thermal spray industry in the future.