Today, thermal spray coatings are utilized in a number of different industries. These coatings consist of wire and molten powder which are exposed to plasma or oxy-fuel combustion. The fire from the spray device will power the mixture that has been heated, and once it has been sprayed onto metal, the mixture will retain a firm coating.
Thermal spray coatings are used in a wide spectrum of many useful applications, which can include protecting airplanes, buildings, and other structures from extreme temperatures, chemicals, or environmental conditions such as humidity and rain. In this article, we will discuss what is thermal spraying and how to do it, and its application and benefits.
What is Thermal Spray Coating?
Thermal spraying techniques are coating processes in which melted (or heated) materials are sprayed onto a surface. The “feedstock” (coating precursor) is heated by electrical (plasma or arc) or chemical means (combustion flame).
Thermal spraying can provide thick coatings (approx. thickness range is 20 microns to several mm, depending on the process and feedstock), over a large area at a high deposition rate as compared to other coating processes such as electroplating, physical and chemical vapor deposition.
Coating materials available for thermal spraying include metals, alloys, ceramics, plastics, and composites. They are fed in powder or wire form, heated to a molten or semi-molten state, and accelerated towards substrates in the form of micrometer-size particles.
Combustion or electrical arc discharge is usually used as the source of energy for thermal spraying. The resulting coatings are made by the accumulation of numerous sprayed particles. The surface may not heat up significantly, allowing the coating of flammable substances.
Coating quality is usually assessed by measuring its porosity, oxide content, macro and micro-hardness, bond strength, and surface roughness. Generally, the coating quality increases with increasing particle velocities.
In addition to original equipment applications, thermal spray coatings are used to repair parts worn and damaged in service, and restore dimensions to machined parts. Thermal spray coatings are used to restore the dimensions of components that have been worn or corroded, such as printing rolls and undersized bearings.
How To do Thermal spraying?
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.
It is distinguished by its ability to deposit coatings of metals, cermet’s, ceramics, and polymers in layers of substantial thickness, typically 0.1 to 10mm, for engineering applications. Almost any material can be deposited so long as it melts or becomes plastic during the spraying operation. At the substrate surface, the particles form ‘splats’ or ‘platelets’ that interlock and build up to give the coating.
The deposit does not fuse with the substrate or have to form a solid solution to achieve a bond. This is a significant feature of thermal spraying compared to many other coating processes, particularly arc welding, brazing, and laser coating processes.
The bond between a thermally sprayed coating and the substrate is primarily mechanical, and not metallurgical or fused. Adhesion to the substrate will depend on the condition of the substrate surface, which must be clean and roughened by grit blasting or machining prior to spraying.
Thermal spraying processes have been widely used for many years throughout all the major engineering industry sectors for component protection and reclamation. Recent equipment and process developments have improved the quality and expanded the potential application range 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 (developed between 1910 and 1920) but still widely used processes such as flame spraying and wire arc spraying, the particle velocities are generally low (< 150 m/s), and raw materials must be molten to be deposited.
Plasma spraying, developed in the 1970s, uses a high-temperature plasma jet generated by arc discharge with typical temperatures >15,000 K, which makes it possible to spray refractory materials such as oxides, molybdenum, etc.
Here are we discuss the 5 most common processes to generate thermal spray coatings in detail:
1. HVOF (High-Velocity Oxy-Fuel Spraying)
HVOF is a process that makes use of a torch that allows the flame to spread whenever the nozzle is used. This creates rapid acceleration which speeds up the particles in the mixture. The end result is an exceptionally thin coating that is evenly applied. Despite being thin, this coating is strong and adheres well. Its resistance to corrosion is better than plasma coatings, but it is not well suited for high temperatures.
2. Combustion Flame Spraying
Combustion flame spraying is an excellent option for surfaces that aren’t designed to handle extreme stress. The coating which results from this process is not strongly attached to the surface since the spraying mechanism is powered by a lower flame velocity.
The flame will be generated via oxygen which has been combined with fuel, and this will melt the mixture. Combustion flame spraying is popular for low-intensity applications due to its low cost.
3. Plasma Spraying
Plasma spraying makes use of the plasma torch as the main tool for heating and spraying the coating. After the powder material has been melted down, it is then placed on the product in a manner that is similar to combustion flame spraying.
The coatings which result from plasma spraying maybe a few micrometers thick to a few millimeters thick. While the powder is the most widely used material, metals and ceramics are also used. The plasma spraying process is highly popular due to its adaptability.
4. Vacuum Plasma Spraying
Vacuum plasma spraying is done in a controlled environment but utilizes low temperatures. This maintains the vacuum while also reducing damage to the material. A variety of gas combinations can be used to get the necessary pressure for spraying.
Vacuum plasma spraying is used for items such as car bumpers, the dashboard, or housings 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
This spraying method utilizes an arc point which is created between two wires which are electrically conductive. Melting will occur at the point where the wires connect. The arc allows for heating which in turn creates deposition and melting, similar to combustion flame spraying which is used with a torch.
Compressed air will be used for spraying the coatings. This procedure is popular due to its cost-effectiveness, and will typically use aluminum or zinc as the base material.
Advantage of Thermal Sprayed Coatings
Some of the benefits of thermal spray coatings include the following:
- Reduced Cost. The cost of repairing the component is less than buying a new one. Often, the coating actually lasts longer than the original material used.
- Low Heat Input. With few exceptions, the thermal spray process leaves the component’s thermal history alone.
- Versatility. Almost any metal, ceramic or plastic can be thermal sprayed.
- Thickness Range. Depending on the material and spray system, coatings can be sprayed from 0.001 to more than 1 inch thick. The thickness typically ranges from 0.005-0.1 inch.
- Processing Speed. The spray rates range from 3-60 lb/hr depending on the material and the spray system. Typical rates for material application are 1/2 -2 lb of material per sq ft per 0.01-inch thickness.
Disadvantages of Thermal Spray Coatings
- Disguises the substrate – as thermal spray coatings are so efficient in many cases it is impossible to tell what material the substrate was made of after the coating process, unless stringent records are kept. For more info contact Green Leaf Business Solutions here.
- Cannot precisely evaluate effectiveness – once the thermal spray coating has been applied it is often difficult to tell exactly how well the coating has gone on, other than by a visual assessment.
- Costly set up – some of the methods of thermal spray coatings require very expensive apparatus, which can result in a high initial set up cost.