Spraytime

As with most things, preparation is the key to making sure you achieve a good bond between a coating and a substrate. I always say that you can have the very best coating in the world, but if it is not satisfactorily adhered to your component, then it really is of no use to anyone.

This article will review the primary preparation methods for providing a mechanically activated surface and share some insights into a robust and repeatable preparation process.

Why Do We Need to Prepare Properly?

As a general rule, most thermal spray processes create a stream of molten or semimolten particles that impact onto the surface to be coated — Fig. 1. Adhesion and buildup of the subsequent deposit relies on a combination of kinetic and thermal energy to ensure conditions are right to create a well-bonded coating. This is part of the adhesion equation, but equally important are cleanliness and surface activation.

A Clean Surface

I am not going to concentrate too much on cleaning methods in this article, but it is definitely true to say that “cleanliness is next to godliness.”

A suitable degreasing process will remove contamination on the surface. Any contamination can directly affect bonding. Proper cleaning will also prevent indirect contamination of the activation process media, which could, in turn, recontaminate the surface to be coated.

Degreasing methods employed will vary depending on the material composition and geometry of the part to be coated.

Typical methods include:

• Local swab and brush
• Vapor (reduced in use due to environmental legislation)
• High temperature burn-out
• Aqueous (detergent-based systems)

Directly after the cleaning process has been completed, there is often a masking requirement to protect areas that are not to be coated. A grease-free surface also has the added benefit of improving the adhesion of any masking tape subsequently employed.

Surface Activation

Once the part to be coated has been degreased and masked, there will be a need to activate the surface to ensure the deposit has the required level of adhesion. Coatings can be applied to several accepted types of activated surfaces, including those created by mechanical roughening, laser ablation, water-jet stripping, and blasting — Fig. 2. It is the grit-blasting process that we will be concentrating on in this article.

Grit blasting can be a manual, semiautomated, or fully automated process — Fig. 3. My experience suggests that when preparing for thermal spray, the vast majority of grit-blasting applications are manual. It’s nice to think that the more automation is involved, the more reproducible the process becomes. This is true, but automating an aggressive blasting procedure can be difficult. Quantity of parts and complexity of their shape are often also a deciding factor.

The grit-blasting technique involves abrasive particles that are fired through a nozzle using compressed air. Methods of propelling the particles can vary, but the usual method employed is via the use of suction (vacuum) or pressure blast systems. Figure 4 shows the principle of operation of a suction-type grit blasting unit. Control over the blasting process is vitally important to create a reproducible surface profile. Significant control variables include but are not limited to blast distance, air pressure, nozzle diameter, blast angle, and motion profile. I would recommend the creation and use of a properly toleranced parameter sheet for any grit-blasting procedure.

Handling the Media

Of course, choosing the right grit-blast media for the job is another very important part of the equation. The choice should be made on technical as well as commercial grounds. For example, a cheap steel grit may cut well, but steel remaining at the interface between the substrate and the coating could corrode in an aqueous environment and result in coating failure.

The media most often used in engineered thermal spray coatings is one primarily made up from fused aluminum oxide (alumina). Typically, this can be a 99% or more white alumina or alumina/3% titania (titanium oxide), so-called brown alumina. The added titania provides a little more toughness. Alumina cuts well, remains sharp as it breaks down, is chemically inert, and has high temperature capabilities. The latter two points are significant in relation to trapped grit, which I will be covering later in this article.

We also need to consider the size of the grit being used. There are certainly differing opinions within the thermal spray industry as far as the preferred size of media to be used. There are also many factors that define an acceptably bonded coating. Within reason, the controlling factor is the surface finish achieved after blasting. However, common grit sizes used range from 20 to 120 ASTM mesh (850 to 90 μm) sieve size. The choice of size used is often led by customer specification.

Thermal spraying is characterized as a metallurgically cold process. This means that heat transfer to the substrate is low; therefore, concerns over part distortion and negative effects on material properties are minimized. The coating bonds to the substrate via a mechanical adhesion process, so suitable substrate preparation via a roughening process is typically required.

It’s All About Having the Right Profile

Having established our blast parameters and media, we need to make sure we create the correct surface finish to allow our coating to adhere to the best of its ability. Ideally, we should create a profile of peaks and valleys rather than hills and vales. A good blasted surface should twinkle in the light as the part is moved — Fig. 5.

Having the right amount of twinkle is subjective, so a common method of verifying surface finish is via the use of a surface profilometer. Surface roughness requirements are often specified as a minimum Ra figure with results presented in microinches or micrometers. Ra is perhaps not the most useful of reporting methods as it provides limited information on surface topography (mountains/valleys vs. hills/vales), but it is regarded as a standard within thermal spray processing.

Figure 6 provides some guideline surface roughness requirements for different deposition processes (note that 1 μm converts to approximately 40 μin Ra). Nominally, the higher the kinetic energy of the particles in the system, the lower the level of surface roughness required to provide an adequate bond. Again, customer requirements for preparation techniques and acceptable bond strengths will often overrule general guidelines (but they do help).

I’ve already mentioned the need for visual and measurable quality control techniques to make sure we have our required mountains and valleys, but how about other things to watch out for to ensure we have a robust blasting process?

Feeling Trapped?

Grit entrapment can be a major issue when blasting. It will affect coating quality and performance in a number of ways. Figure 7 shows the trailing edge of a turbine blade after blasting. As can be seen, the blast media has become trapped in the cooling slot. This must be removed by mechanical means as a compressed air blow would not be successful. If it was left in place, it would affect coating adhesion and reduce air flow through the blade. It could also become dislodged in service, causing serious damage to the engine.

The other form of entrapped grit that can cause issues is the type that is incorporated at the interface between the coating and the substrate.

When grit blasting a surface, it is in the nature of the process for some of the grit to become embedded in the material to be coated — Fig. 8. If the coating builds up on top of the grit, the end result will be areas of weakness, which can affect deposit bond strength.

The level of entrapment can be somewhat reduced by cleaning the blasted surface with a jet of clean, dry compressed air immediately prior to coating. Perhaps more importantly, control of grit break down levels during routine blasting, in combination with a defined maintenance program for the activation equipment, will create a much more repeatable process where grit entrapment levels will be to defined expectations.

The usual method for measuring entrapped grit is on a polished cross section via a method of line intersections — Fig. 9. Each piece of grit is measured in a defined number of fields of view and an average value calculated over a sampling line length.

Again, acceptable values are often customer specific, but a typical maximum value would be 20% of the interface contaminated with blast media. Values above this tend to compromise the ability of the coating to adhere adequately to the substrate.

Conclusion

Of course, there is still more to say on the subject of preparation, and this article is just a glimpse at its importance. Close control is necessary at each stage of the cleaning and activation procedure to ensure that an adequate bond is created and that any contamination is kept to a reduced level.

STEVE BOMFORD (steve.bomford@oerlikon.com) is Customer Solutions Centre manager, Oerlikon Surface Solutions, United Kingdom.

*All images used in this article were agreed upon between the author and the content providers, taken from open access Internet sources by the author, or are the property of Oerlikon Metco.