Surface preparation The "key" to the thermal spray process

Surface activation: taking the rough with the smooth

Once the part to be coated has been degreased and masked, there will be a need to activate the surface in order to ensure the deposit has the required level of adhesion. According to various industry experts, effective surface preparation is critical to achieving optimal results in the thermal spray process. Several methods can achieve an activated surface, including mechanical roughening, water-jet stripping, laser ablation, and blasting techniques (see Figure 2). In this article, we will focus primarily on the grit-blasting process.

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Grit-blasting can be performed manually, semi-automated, or fully-automated. Based on my experience, the majority of grit-blasting applications for thermal spray are manual. While automation can improve process reproducibility, automating a complex blasting procedure presents challenges, especially when considering the variety of part numbers and geometries involved.

Typically, the grit-blasting technique uses abrasive particles propelled through a nozzle by compressed air. Various methods exist for propelling these particles, with suction (vacuum) or pressure blast systems being the most common.

Controlling the blasting process is crucial for creating a consistent surface profile. Key variables to monitor include—the blast distance, air pressure, nozzle diameter, blast angle, and motion profile. Therefore, we recommend developing a detailed parameter sheet for any grit-blasting procedure.

Dealing with the media

Choosing the right grit blast media is essential for proper surface preparation (for thermal spray, avoid walnut shells and corn cobs!). The media selection should take into account both technical and commercial considerations. For instance, while cheaper steel grit might provide good cutting action, steel residues remaining between the substrate and coating could corrode in an aqueous environment, leading to coating failure.

Most engineered thermal spray coatings utilize fused aluminum oxide (alumina) as the primary abrasive media. Commonly, this may comprise 99% white alumina or a blend of alumina with 3% titania (titanium oxide), referred to as "brown" alumina. The presence of titania enhances toughness.

Alumina performs well—it remains sharp as it degrades, is chemically inert, and withstands high temperatures. These properties are crucial concerning potential grit entrapment discussed later in this article.

Additionally, selecting the appropriate grit size is vital. The thermal spray industry presents varying opinions on optimal media size. Many factors determine an adequately bonded coating, but the finish achieved post-blasting is typically paramount. Common grit sizes range from 120 to 20 ASTM mesh (90 to 850 µm), with size often dictated by customer specifications.

Thermal spraying is recognized as a metallurgically cold process, meaning heat transfer to the substrate is minimal. Consequently, concerns regarding part distortion and adverse effects on material properties are significantly reduced. The coating adheres to the substrate through mechanical bonding, necessitating suitable substrate preparation via a roughening process.