Using Fused Minerals to Optimize Thermal Spray Coating Processes

During production of a wide range of critical products, manufacturers from the automotive, aerospace and oil and gas industries often use thermal spray coatings. This is because manufacturers need to ensure that engine components, gas turbines, and other similar parts can endure environments that are highly corrosive and challenging for long periods of time. When thermal spray coatings are used, the components become more durable and deliver optimum performance.

As such, there are a wide range of thermal spray processes in which molten or semi-molten materials like ceramics and metals are sprayed onto a prepared surface. Besides providing excellent resistance to wear and corrosion, thermal spray coatings that include fused materials like white aluminum oxide can help enhance surface properties like thermal insulation. A range of benefits are offered by fused ceramic powders even before the actual thermal spray process begins.

Preparing the Surface

Before applying the thermal spray coating, the component’s surface should be suitably prepared to ensure the effectiveness and efficiency of the coating. The aim behind this surface preparation is to make sure that the thermal spray coating adheres properly to the substrate. If the surface is not prepared adequately, the coating will not fix properly, leading to inconsistent results, making the entire process costly and time-consuming.

A standard technique used for surface preparation for coatings is abrasive blasting. This method needs an anchor pattern and a high level of surface cleanliness. When abrasive blasting is used for surface preparation, the surface’s roughness and the coating’s adhesion strength are improved significantly.

When blast cleaning is used, factors like mill scale and intact rust can also be removed fully, creating a controlled anchor pattern with a uniform roughness. Aluminum oxide serves as an excellent blast media. This material produces a rough surface profile, which provides superior performance and coating adhesion.

Fused white aluminum and brown aluminum oxide are blast materials that have excellent abrasion properties and are resistant to chemicals and corrosion. Aluminum oxide has low iron content, and hence does not leave any impingements that can rust the surface and give way to possible issues in the future.

This material comes in macro grit sizes that span between 12 and 240 grit. Unlike grit blasting materials like steel shot, aluminum oxide can be recycled and reclaimed back by aluminum oxide producers. The majority of large-scale blasting operations tend to have an outdoor or indoor system for collecting the spent grit.

In North America, Washington Mills is the only aluminum oxide manufacturer that has installed a fully closed-loop and waste-free spent grit recycling system. The spent grit is collected from the grit blasting operation and used in the company’s furnaces to create fused aluminum oxide, which is similar to a virgin material.

While other blast material is disposed of eventually, the recyclability factor of aluminum oxide reduces costs related to disposal, landfill, and other liabilities.

Within the Coating

In addition to surface preparation, a variety of fused ceramic powders can be effectively used for a wide range of thermal spray coatings. Examples include:

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

The addition of these fused materials, either alone or combined together, can benefit thermal spray coatings.

Fused minerals have low loss on ignition (LOI), which means that few gases, usually less than 5%, are formed when heated. The ultra-high density of the microstructure of fused minerals also allows them to withstand greater velocity and heat throughout the thermal spray process. When molten and semi-molten fused minerals hit a substrate, they solidify quickly, leaving behind a thick coating and a strong adhesive bond to the substrate.

The coating is strongly bonded and displays a very high density, resulting in a more homogeneous and solid coating. This also reduces the penetration of moisture into the metal part surfaces.

A large number of fused minerals are inert and exhibit high melting points and high hardness points, thus making them resistant to the corrosive and high-temperature environments common where thermal sprays are used. Furthermore, fused oxides are highly stable, and hence cannot be changed to other materials, irrespective of the environmental conditions. Such materials are often more lightweight than metal materials. This would largely benefit the aerospace and automotive sectors where the weight of a component is of major importance.

The Fusion Process

Under the electric arc fusion process, the minerals that are being fused are allowed to reach high temperatures so that they can attain a fully liquid state. This produces a complete reaction and ensures that the end fused material is 100% of the product.

Metal producers who are looking for an optimum material for their particular application can opt for the custom fusion services provided by Washington Mills, and thus can prevent the added necessity of buying bulk quantities of unproven and unreliable materials. The custom fusion services involve developing and testing a small set of material to establish its feasibility. Using this process, manufacturers can screen the development and testing procedures whilst avoiding excess capital and material expenditures. All these measures result in improved productivity and reduced costs.

For instance, rare earths can be used to develop coatings, but the raw materials are rather costly. To overcome this issue, a compact lab-scale electric arc furnace can be used to perform the required research and development work with rare earths, without actually spending large amounts of money.

Washington Mills has the required infrastructure for testing fusions on varied formulations of rare earths, and all these processes are performed without wasting significant amount of money on a labscale furnace or huge amounts of rare earths.

As soon as the optimal fused material has been established, Washington Mills offers particle sizing systems and testing services to fine tune the fused mineral for its intended end use application. Properties like particle size and particle size distribution can be optimized using small-scale crushing and milling equipment, whilst the physical and chemical properties of the material can be tested by the fully equipped analytical lab. These procedures are repeated during the course of the scale-up process which further increases the reliability of end products. The 300-ton batches of fused material will behave in the same way as the 10 lb test batch fused material.

A representative from a thermal spray coating manufacturer said that Washington Mills provides a flexible and reliable customer service, and its dedication towards robust and uniform products goes a long way in ensuring high-quality products. The ability to work with suppliers who understand the business and make the required adjustments to a specific material chemistry is important for a successful thermal spray process.

This information has been sourced, reviewed and adapted from materials provided by Washington Mills.

For more information on this source, please visit Washington Mills.

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