Thermal Spray Coatings Information

 
Thermal Spray CoatingThermal spray coatings are characterized by layers and layers of flattened liquid drops called splats, or lamallae. These overlapping lamallae and the voids between them determine the thickness and porosity of the coating. They are used to treat surfaces and enhance surface characteristics. Typically, they are supplied in either a solid wire or powdered form. They encompass a wide range of material compositions including metals, ceramics, and engineered polymers and are designed as a consumable for one of several thermal spray coating deposition techniques.
 

Specifications

One of the first considerations in selecting a thermal spray coating is the substrate to be coated. Some spray coating processes are highly versatile and can be applied to laminate surfaces, plastics, or metallic surfaces while other thermal spray coating processes are much more limited in application. 
 
The second most influential design criteria is the environmental exposure that a coating will endure or the desired surface characteristics. High temperature application may require a refractory coating or for highly corrosive applications an anodic coating may be preferred. These performance requirements along with the substrate to be coated can be considered the “meat and potatoes” of the design criteria while additional considerations include the environment in which coating will take place, service life, and cost. 
 

Techniques

The method of deposition has a profound effect on these surface characteristics and the cost and benefits of each coating method are often weighed against each other.
 

Arc Spraying

Arc spraying is considered the most productive and economical thermal spray coating technique for most applications. In arc spraying, a DC power source is used to establish an electric arc at the gun head and melt a wire feed to form a molten pool. Dry compress atomizes and propels tiny droplets of the molten material toward the treated surface. 

Arc spraying creates a semi-porous coating up to 4-mil. thick. It exhibits the highest deposition rate of all thermal spraying processes. It also requires less power, is more suitable for metallizing thermally sensitive substrates, and creates a bond strength that is superior to flame spraying. 

Disadvantages of arc spraying include material limitations as it can only be used to deposit conductive materials. A large amount of fume and dust is produced during the coating process. The coating also contains greater porosity, higher oxide levels and lower bond strengths when compared to plasma or High Velocity Oxygen Fuel (HVOF) spraying processes.

Flame Spraying

Flame spraying is another economical thermal spray process. In flame spraying, a fuel source is used and the heat of combustion melts either a wire feed or consumable powder to form a molten pool. Like arc spraying, dry compressed air atomizes and propels molten material toward the work surface.

Flame spraying is a cost-effective method for applying a thermal spray coating and is characterized by moderate spray rates. It produces less dust and fumes when compared to arc spraying. In some instances, a spray booth may not be required, like all other thermal spray methods. It is favored when coating large or complex structures and offers more material options than arc spraying, including ceramics and other non-metallic coatings.

The main disadvantage of flame spraying is coating quality. Flame spraying produces a coating with higher porosity, lower coating densities, and higher oxide levels than all other thermal spray coatings.

Plasma Spraying

Plasma spraying utilizes a high temperature plasma flame to heat and accelerate a powdered consumable toward the work surface. A DC electric arc interacts with an inert gas and the associated resistance heating from the electrical arc causes the gas to dissociate and ionize, forming a plasma jet.

Plasma spraying produces a very high quality coating from materials with exceptionally high melting points. In order to improve coating surface quality, the process may be carried out in a vacuum chamber, which is referred to as VPS.

Plasma spray is a very versatile process that accommodates both a wide range of coating materials and substrates. It is particularly well suited for the spraying of refractory materials while the disadvantages include the complex nature of the automated process, sensitivity to operating environments, and cost.

High Velocity Oxygen Fuel (HVOF) Spraying

HVOF utilizes confined combustion and an extended nozzle to heat and accelerate powdered materials. Operating up to hypersonic gas velocities, it produces a dense, well-adhered coating that is hardened, wear-resistant, and corrosion resistant.

HVOF produces a very dense coating with low porosity although the powder particle size is restricted and a narrow size distribution is required. HVOF is rarely carried out as a manual operation due to numerous process variables that affect coating consistency.

 

Materials 

Thermal spray coatings are available in a wide range of material compositions. Some of the most common materials include the following:
 

Abradables

Abradable thermal spray coatings are used on the internal walls of jet engines and gas turbines to form a seal with blade tips. Abradable thermal spray coatings consist of a softer layer with low structural integrity. The blade tip preferentially wears or abrades into the softer thermal spray coating. Porosity, graphite, plastic, bentonite, and boron nitride particle are used within a metal matrix.

Carbides

Carbides are compounds of a metal, or metalloid (B, Si), and carbon. Metal carbides are also known as hard metals such as tungsten carbide (WC), chromium carbide (Cr3C2), titanium carbide (TiC), or tantalum carbide (TaC). Metal carbides have high hardness and high hot hardness, which makes them useful for cutting tools, forming, dies and tools, and other wear applications. Metal carbides often used a cobalt, nickel, or intermetallic metal bond between grains (cemented carbides), which results in increased toughness compared to a pure carbide or ceramic.  

Ceramics

Ceramic materials are made of non-metallic minerals, such as clay, that have been permanently hardened by firing at a high temperature. Most ceramics resist heat and chemicals and are used in refractory and highly corrosive applications. 

Cermet

Cermets are a blend of a metal-oxide ceramic (cer) and metal (met) materials. Cermet thermal spray coatings are often used as an intermediate coating in high thermal cycling applications as they help control thermal expansion differences between the substrate and top-coat. 

Metals and Metal Alloys

Pure metals, ferrous metals, non-ferrous metals, and metal alloys are used in thermal spray processes. Steel, iron, nickel, molybdenum, and titanium-based alloys are often used to harden surfaces to protect against high wear and abrasion resistance. Non-ferrous metals such as zinc and aluminum are commonly used to treat steel surfaces with anodic and galvanic coatings and to improve oxidation resistance.

Related Information

Engineering360—How to Select the Right Thermal Coating

Resources 

Image credit:

Saint Gobain Coating Solutions

 

 



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