Manufacturers in industries ranging from aerospace and oil and gas to automotive rely on thermal spray coatings during the production of many of their key products. Parts such as gas turbines and engine components must not only have the ability to survive challenging and corrosive environments—they need to be able to do so for extended periods of time. Thermal spray coatings help protect the components to ensure their optimal performance and longevity.

While there are many different types of thermal spray processes, they generally involve spraying molten (sometimes semi-molten) materials such as metals or ceramics onto a prepared surface. In addition to wear and corrosion resistance, thermal spray coatings that incorporate fused materials like white aluminum oxide can also improve surface properties such as thermal insulation. Fused ceramic powders also offer multiple benefits even before the thermal spray process begins.

Preparing the Surface

Prior to the thermal spray coating application, the surface of the component needs to be well prepared in order to ensure the coating’s effectiveness and efficiency. The objective of surface preparation is to facilitate the adhesion of the thermal spray coating to the substrate. The coating cannot adhere properly if the surface is not well prepared, and the result is often costly and time-consuming failure.

Abrasive blasting is a common method of surface preparation for coatings that require both an anchor pattern and a high degree of surface cleanliness. Surface preparation with abrasive blasting increases the roughness of the surfaces as well as the coating’s adhesion strength. Blast cleaning can also completely remove intact rust and mill scale and produce an even roughness with a controlled anchor pattern.

Materials like aluminum oxide are a very effective blast media; they create a rough surface profile that offers excellent coating adhesion and performance. Fused brown and white aluminum oxide are blast materials that exhibit superior abrasion properties, as well as high corrosion and chemical resistance. Aluminum oxide is typically available in macro grit sizes ranging from 12 to 240 grit, and it is also low in iron so it leaves behind no impingements that can leave rust on the surface and cause potential problems in the future.

An additional benefit offered by aluminum oxide is that, unlike steel shot and other grit blasting materials, it can be reclaimed and recycled back to the aluminum oxide manufacturer. Most large-scale blasting operations have an indoor or outdoor system that collects the spent grit. Washington Mills is North America’s only manufacturer of aluminum oxide that offers a completely closed-loop, waste-free spent aluminum oxide grit recycling system.

Washington Mills will pick up the spent grit from the company’s grit blasting operation and use it in its furnaces in order to produce fused aluminum oxide that is identical to virgin material.¹ Other blast material eventually has to be disposed of, but aluminum oxide’s recyclability reduces landfill and disposal costs, as well as liabilities.

Within the Coating

Beyond surface preparation, fused ceramic powders lend themselves well for various thermal spray coatings. In addition to aluminum oxides, many different fused minerals can be used in the thermal spray process (see “Examples of Examples of some Fused Ceramics Used in Thermal Spray Coatings" below).

The low loss on ignition (LOI) exhibited by fused minerals means that few gases (typically < 5%) are formed upon heating. In addition, the very high density of fused minerals’ microstructure enables them to undergo higher heat and velocity during the thermal spray process, which creates greater bonding to the substrate. The result of these combined properties is a strongly bonded coating that exhibits exceptionally high density (i.e., low porosity); this leads to a more solid and uniform coating and reduces the tendency for moisture to penetrate to the substrate.

Fused minerals’ very high hardness and high melting point make them resistant to the high-temperature and corrosive environments where thermal sprays are frequently used. Fused oxides are also very stable; they will not convert to other materials regardless of the environment. And they tend to be more lightweight than metal materials, which offers additional benefits for industries such as automotive and aerospace where a component’s weight is always an important consideration.

The Fusion Process

The electric arc fusion process takes the minerals being fused to very high temperatures so they reach their 100-percent liquid state, which creates a completely full reaction. The resulting final fused material is therefore 100 percent of the product one intended to create.

Manufacturers seeking to find the most appropriate material for their specific application can avoid purchasing large volumes of unproven material by instead turning to Washington Mills’ custom fusion services, where a small batch of material is developed and tested to determine its viability. Custom fusion helps reduce costs and increase productivity by enabling manufacturers to go through the development and testing process without taking on excess capital and material costs.

For example, coatings can be made from rare earths, which are very expensive. In order to perform the necessary R&D with rare earths without wasting a lot of money, it is helpful to use a small lab-scale electric arc furnace. Fusions on different formulations of rare earths can be tested at Washington Mills without the manufacturer having to spend a lot of money on large quantities of rare earths—or a lab-scale furnace.

Once the optimal fused material is determined, Washington Mills also provides particle sizing equipment and testing services to further adjust the fused mineral for its specified end use. Small-scale crushing and milling equipment can be used to optimize characteristics such as particle size/particle size distribution, while the fully equipped analytical laboratory can test the material’s chemical and physical properties. These steps can be repeated throughout the scale-up process, so manufacturers can be assured that their fused material in 300-ton batches behaves the same as the 10-lb test batch.

According to a representative from a manufacturer of thermal spray coatings, Washington Mills’ dedication to robust and consistent products, along with its reliable and flexible customer service, is another benefit. “The key thing in moving products forward is the ability to work with suppliers that understand our business and realize that you need to make modifications in a certain material chemistry to be successful with thermal spray,” he says. “It’s a working relationship between organizations that’s important.”

Examples of Some Fused Ceramics Used in Thermal Spray Coatings

Thermal spray coatings can benefit from the inclusion of these fused materials, either alone or in combination with each other:

• Calcia-stabilized zirconia,
• Fused aluminum oxide,
• Fused chrome,
• Fused forsterite,
• Fused magnesia,
• Fused zirconias,
• Yttria-stabilized zirconia,
• Yttrium oxide, and
• Other fused compositions are possible.

For additional information regarding the benefits of fused minerals in thermal spray coating processes, contact Washington Mills at 716/278.6600, e-mail info@washingtonmills.com, or visit www.washingtonmills.com.

Reference

1. McLeod, Don, “Recycling Spent Aluminum Oxide,” Ceramic Industry, October 2008, pp. 29-31.