Currently, thermal spray coatings are being used in a variety of industries. The coatings include wire or molten powder materials that have been heated either in a plasma or oxy-fuel combustion environment.
The flame from the spray apparatus will drive the heated mixture and once the heated mixture is sprayed on the metal it gives a firm coating.
Thermal spray coatings can have a range of applications and offer a number of benefits, including providing protection for airplane wings, buildings, and other structures, against temperature extremes, chemicals, or environmental conditions such as humidity and rain. This paper will define thermal spraying and describe how to perform it, its applications and benefits.
What is Thermal Spray Coating?
Thermal spraying technologies are coating processes where melted (or heated) materials are sprayed onto a surface. The ‘feedstock’ (coating precursor) is heated by electrical (plasma or arc) or chemical means (i.e., combustion flame).
Thermal spraying is capable of providing thick coatings (approx. thickness range is 20 microns to several mm depending on the process and feedstock), over a large area at a higher deposition rate compared to other types of coating processes (ex. electroplating, physical and chemical vapor deposition).
The range of coating materials available for thermal spraying is wide, and includes metals, alloys, ceramics, plastics, composites, etc. Coating materials are delivered in either powder or wire form, heated to molten or semi-molten state, and accelerated towards substrates in the form of micrometer size particles.
Thermal spraying typically uses combustion (flame) or electrical arc discharge as the energy source. Coatings arising from the particle deposition resulting from many sprayed particles. The surfaces usually do not heat up much, allowing coatings to be applied to flammable substances.
The quality of the coatings is typically rated based on its porosity, oxide content, macro and micro-hardness, bond strength, and surface roughness. Generally as the velocity of the particles increase the quality of the coating also increases.
While thermal spray coatings can be used for original equipment applications, they can also used to repair parts that have been worn and damaged in service, and restore dimensions of machined parts. Thermal spray coatings can also also be used to restore dimensions of components that have been worn, or corroded (printing rolls and undersized bearings).
How To do Thermal spraying?
Thermal spraying describes a class of coating processes in which either a solid or a powder is applied as a spray of finely divided molten or semi–molten droplets to produce a coating.
Thermal spraying is characterised as a coating process which has the ability to apply coatings of metals, cermet’s, ceramics, and polymers in substantial thickness, typically of between 0.1 to 10mm, in layers acceptable for engineering applications.
Almost any material can be deposited as long as it melts or becomes plastic at the temperatures it is sprayed.
At the substrate surface, the spraying particles deposit or impact as ‘splats’ or ‘platelets’ that interlock and accumulate to provide the coating.
The coating does not have to fuse with the substrate or form a solid solution to create a bond. This is an important aspect of thermal spraying compared to other coating processes, especially arc welding, brazing and laser coating processes.
The bond strength for thermal sprayed coating on to a substrate is predominantly mechanistic not metallurgical or fusing.
Bonding will be dependent on the substrate surface; which needs to be sufficiently clean and roughened by grit blasting or machining before thermal spraying takes place.
Thermal spraying processes have been in use for component protection and reclamation applications across all of the major engineering industry sectors for many years.
Some recent equipment and process developments have resulted in improved quality and a wider range of potential application for thermally sprayed coatings.
Types Of Thermal Spray Coating Processes
Several variations of thermal spraying are distinguished:
- Plasma spraying
- Detonation spraying
- Wire arc spraying
- Flame spraying
- High-velocity oxy-fuel coating spraying (HVOF)
- High-velocity air fuel (HVAF)
- Warm spraying
- Cold spraying
- Spray and Fuse
In classical or historical (1910 to 1920) but still in use techniques such as flame spraying and wire arc spraying, the particle velocities are typically low (< 150 m/s) and require the starting materials to be molten in order to be deposited.
In the 1970s, plasma spray was developed, which uses a high temperature plasma jet (>15,000 K) generated by arc discharge system. This allows for spraying of refractory materials such as oxides, molybdenum, etc.
In the following we will provide a more in depth discussion of the 5 most common processes for generating thermal spray coatings:
1. HVOF (High-Velocity Oxy-Fuel Spraying)
The HVOF process is similar to that of a torch that allows movement through the flame using a nozzle. So, the flame can spread and move away from the nozzle to provide quick acceleration to particles in the mixture to bring a particle size down for a spray.
The end result is a very thin coat and thinness of the coat even across a broad area. With the thickness being quite thin, the coat can actually be quite dense and have good adhesion. It has better corrosion resistance than plasma coatings, but is not as good as plasma in high temperatures.
2. Combustion Flame Spraying
Combustion flame spraying is a decent alternative for surfaces that are not designed to withstand high stresses. The coating created by this process will not bond well to the surface, as the spraying process is powered by a relatively low flame velocity.
The flame is change in bound oxygen with fuel to create their service, which will melt it. Combustion flame spraying is the process of choice for low-powered applications because it is the lowest priced type of thermal spray.
3. Plasma Spraying
Plasma spraying utilizes a plasma torch as the primary method of heating and spraying the coating. The powder material is melted and deposited on the product similar to combustion flame spraying.
Coatings resulting from plasma spraying are typically a few micrometers thick to a few millimeters thick. While powder is the most common material, ceramics and metals are sometimes used. The process is widely-used because of its versatility.
4. Vacuum Plasma Spraying
Vacuum plasma spraying occurs in a controlled atmosphere, but at a low temperature. This allows for vapor vacuum, but reduced damage to the material. It can use any mixture of gases to obtain the pressure necessary to spray.
Vacuum plasma spraying is used for parts like a car bumper, the dashboard, or housing for door mirrors. This process can also be used for the pre-treatment of polyethylene moldings, which provides adhesion for epoxy adhesives that are water-based.
5. Two-Wire Electric Arc Spraying
The spraying process relies on an arc point between two conductive wires, melted at the point of connection. The arc (or hot gas) provides heating to cause deposition and melting of spraying material, similar to the combustion flame spray process using a torch.
The coatings will be applied with compressed air. This spraying method is popular due to the low-cost practice it provides and will normally use aluminum or zinc as the solid material.
Advantages of Thermal Sprayed Coatings
Some of the benefits of thermal spray coatings include the following:
- Reduced Cost. The price involved with repairing the component is lower than purchasing a new component. Been made over time, often the coating lasts longer than the original material.
- Low Heat Input. Barrington, aside from a few exceptions, the thermal spray process does not change the thermal history of the component being repaired.
- Versatility. Almost any metal, ceramic or plastic can be thermally sprayed.
- Thickness Range. Depending on the material and spray system, coatings can be sprayed anywhere from 0.001 to more than 1 inch thick. The typical thickness will be from 0.005-0.1 inch in thickness.
- Processing speed. The spray rates through the process range from 3-60 lb/hr depending on the material and the spray system. The typical rates for material application at 1/2 -2 lb of material per square ft for each 0.01 inch thickness.
Disadvantages of Thermal Spray Coatings
- Conceals the substrate: thermal spray coatings are so effective that in many circumstances, you can’t tell what the substrate was made of after coating unless precise records are kept.
- Cannot specifically assess effectiveness: once a thermal spray coating has been applied, it can often be difficult to tell exactly how effective the coating has been applied or how uniform, other than by visual inspection.
- Expensive setup: some of the types of thermal spray coatings have expensive equipment, which can cause a high initial start-up cost.
FAQs
What is the thermal spraying process?
Thermal spray is an industrial coating process that heats or melts metallic or ceramic materials and deposits them onto a surface. It is used for surface enhancement or surface remanufacture.
What materials can be thermal sprayed?
All kinds of surfaces can be coated using thermal spray: aluminum, steel, and other types of metals, but also glass, ceramics, plastics, and more.
What are the disadvantages of thermal spraying?
Finally, thermal spray coatings are typically much thicker than those applied using other methods, which can lead to problems with dimensional accuracy and material build-up. Thermal spray technology disadvantages include: High operating temperatures. Difficulty bonding with certain substrates.
What is thermal spraying in welding?
Thermal spraying is a generic category of coating processes that apply a consumable as a spray of finely divided molten or semi-molten droplets to produce a coating.
What is an example of thermal spraying?
Detonation gun spraying or D-gun spraying is a thermal spraying technology wherein a combustion mixture of oxygen, fuel, and powder is fed into the combustion chamber of the barrel, which is closed at one end. Acetylene is used as a fuel.
Where is thermal spraying used?
Applications
1. Crankshaft reconditioning or conditioning.
2. Corrosion protection.
3. Fouling protection.
4. Altering thermal conductivity or electrical conductivity.
5. Wear control: either hardfacing (wear-resistant) or abradable coating.
6. Repairing damaged surfaces.
7. Temperature/oxidation protection (thermal barrier coatings).