An Introduction to Thermal Spray

A question I often here is, “What is thermal spray?”

Here’s a simple answer. “Do you know how you spray paint, well do the same thing except we spray molten metals, alloys, carbides, or ceramics.”

Thermal spray is a process where a metal, alloy, or ceramic is melted, atomized, and sprayed. The molten material forms small droplets that flatten out and rapidly solidify as they hit the object being coated. The purpose is to add a layer of material to a component with a different material to change the surface properties. This layer is called the additive layer or the coating. The material that is added to the surface adheres to the base material because of mechanical bonding forces.

Thermal Spray is sometimes confused with welding and brazing. These are two very distinctively different technologies. All three of these technologies can add a layer of material to change the surface properties of parts. The methods and dynamics of how it is done are different.

Welding uses heat to melt the base material of the part being coated and it melts the material that is being added to the surface of the part. The melting of the two materials is followed by mixing of the two materials followed by rapid solidification.

With brazing, typically neither the base material or the additive layer are melted. There is a brazing alloy that is between the component and the additive layer that melts. The melting of the brazing alloy creates a chemically interaction with both materials to bond them together.

Thermal spray does not melt the base material, but does melt the additive layer. In fact, in thermal spray you want to keep the surface temperature of the part being coated below 300 degrees F. This allows you to coat parts without distorting them from significant heat being added to the base component. Additionally, it typically will not change the heat treat properties of the part being coated.

It is common to use thermal spray to coat components with dissimilar materials that are not normally compatible.

Some examples would be:

  • a metal part with a dissimilar metal or alloy
  • cast iron butterfly valves with stainless
  • plain carbon steel sculptures with bronze
  • a metal part with a carbide
  • fan blades for coal fly ash applications
  • a bearing seal area on a pump
  • a metal part with a ceramic
  • gas turbine blade with a thermal barrier ceramic
  • high definition printer (anilox) rolls with chrome oxide
  • a wood with metal
  • table tops with zinc or modern pewter
  • sculptures with bronze or copper

In the above examples all of, welding or brazing are not practical options.

All thermal spray processes have three common elements:

  1. A source of energy to melt the additive layer
  2. A source of material for the additive layer
  3. A method to atomize and spray the additive layer.

Form of Energy

You need to have a source of energy to melt the material to be used as the additive layer. The common sources of energy are combustion gases, electric arc, plasma flame, or velocity. In each case, a the energy is created in a device called a thermal spray torch or gun. The torches are deigned to be handheld or machine mounted for automation purposes. Temperatures generated from these energy sources range from 1000 degrees F to 20,000 degrees F.

The distribution of the gases and electricity for these torches are managed in a “control console”. The gun is connected to the console with hoses and cables to deliver the exact quantities of gases and electricity to the torch.

Source of Material

The only materials that can be used in standard thermal spray processes must be able to be go from a solid to a liquid and back to a solid.

The source of material for thermal spray comes in one of two forms, powder or wire. If a material can be made into a wire or powder, typically the cost of manufacturing a wire is lower. For large volume applications, the economics will point you toward selecting a thermal spray system using wire.

Some materials are only available in powder due to manufacturing or material property limits. In that case, a thermal spray system that uses powder will be the better choice.

We also have several examples where we have manufactured hollow wire with a powder. This does give us the benefit of combining the best of both worlds. There are limits as to how this technology can be used and will be covered in future articles.

The larger thermal spray operations purchase equipment to be able to do both.

Military Conformal Coating Is Effective and Long Lasting

This type of conformal coating is used in products that are exposed to outdoor elements such as moisture and heat. If the products are coated they do not get damaged when there is reduction of thermal and mechanical stress or if they are installed or handled roughly. The dielectric strength is increased between the conductors. PCBs can be designed to look small and compact as a result and the components and circuitry are protected from solvents and abrasions.

The product has a clear shiny look when coated. Sometimes the texture can be a little rubbery or hard. If it is examined under a UV light some coating display a greenish white marker that proves that the coating has been properly done on the product.

The cost of this type of military conformal coating was high and therefore used only on medical and life products. However, in the recent years, new processes and material have been developed to enable coating of all electronic consumer products. Since these electronic components and circuitries are being introduced in smaller sizes and dimensions, this type of coating will soon become a norm.

The repair and rework issues of this type of military conformal coating should be considered carefully before use on products. The requirement for performing this type of rework or repair of conformal coating arises after assemblies go through difference process and completed. Several important considerations should be placed on the requirements of component replacements and product and the selection of the coating chemistry.

The method that is not generally recommended is the thermal removal method that uses soldering irons. This is because conformal coatings require exposure for long hours and very high temperatures. This can cause residues to be left behind, discolorations and solder joints and other components/materials that are used for fabricating assemblies can be affected.

It is essential to monitor the process and ensure that de-lamination, lifting of pads or overheating of devices that are sensitive and lying around does not occur as a result of excess temperatures. Toxic vapors are emitted sometimes when conformal coating is burned which can be dangerous and therefore this type of method should be done with extreme caution.

Here is a list of the different types of coating that can be used with chemical removal methods.

Many types of solvents are available that can be selected that offer different ranges of speed. Depending on the application anyone of the following can be used:

Solvents such as alkaline activators and methanol base provide selectivity and a range in the power of dissolving. Solvents that are alkaline activators or ethylene glycol are fast and least selective.

Benefits Of Using Thermal Paper And Printers

There are many benefits that can be obtained by using these printing products. It offers impeccable and incomparable speed, print quality, efficiency, costs and many other benefits. It is considered to be one of the most economical products as it hardly requires ink or cartridges. It is also known to be one of the most reliable options. These printing machines print labels at a quicker pace compared to the ordinary machines which is considered to be an added advantage. Another basic benefit of using these machines is that it offers quality printouts. These printing devices are light in weight and portable. It can therefore be transported to different locations quite easily. Moreover, it is compact in size and for that reason it can be placed in areas where there is space constraint.

There are different types of thermal paper products made available for various purposes these days. Therefore it is important to know how to choose the right type of product from online stores. It is important to choose these products according to its size. Different machines require different types of paper which vary in length and breadth. The majority of these machines require a specific brand of paper to ensure best results. It is therefore important to use a standard size of printing paper to receive maximum benefits and reduce the amount of time required to change the rolls.

The quality of the printer and the paper is equally important to obtain good quality print-outs. Another important factor to consider while choosing thermal products is the quality of pigment used on these papers. Low quality papers fade within a short period of time so it is essential to choose good quality products. Using high quality products in the printing machines also reduces the risk of damages and frequent break-down. Therefore it is essential to utilize only superior grade materials for printing purposes as it reduces the overall maintenance costs.

There are basically two grades of paper which include the “A” grade and the “TMP” or the economy grade. The main difference between these two grades of paper is its brightness. The “A” grade products exude a professional appearance as it features high quality prints.

Most of these products can be bought online at affordable prices without the need to comprise on the quality. There are many reputed online dealers which offer remarkable products at unbelievable price rates. It is easier to compare the prices and features of different products while buying from online stores.