Spraytime

When getting ready to thermal spray, it is important to address the necessary evil that is masking. Masking is employed as a protection process required to prevent damage to the part being coated both from surface roughening prior to coating and from the deposition process itself. Effectively, it is used as a method of preventing a coating from being deposited where you don’t want it (or the customer won’t allow it).

To a certain extent, it is a non-value-added process. You spend time and money applying masking tape, masking compounds, lacquers, etc., only to peel them off and drop them in the trash when you are finished (masking is not that environmentally friendly a process). However, without masking, we could not:

• Precisely define the location of where we want the coating
• Protect vulnerable parts of the component
• Prevent buildup of lower quality deposits in turbulent areas, and
• Reduce notch sensitivity at coating transition points.

So, with these points in mind, as well as a few others we will discuss within this article, we really need to consider if masking should be kept anonymous or be more widely recognized for its heroic abilities.

Choosing the Right Masking for the Job

When thermal spraying, the first thought is often to turn to masking tape. This might be a suitable choice, but even then, how do you decide? Masking can have a significant cost impact on the thermal spray process as a whole and, therefore, ideally it should be used as efficiently as possible.

Tape masking is a flexible option, but other choices are available. The decision often depends on factors including:

• Process energy levels. Thermal and kinetic energy transfer from the flame and particle impingement.
• Part geometry. Conformability to the part being coated.
• Accuracy of coating location. Masking placement and reduction of bridging effects (more later).

Let’s go with tape as a starting choice. If we pick the right tape for the job, we can mask just once to cover both the grit blasting and spraying procedures. In Fig. 1, we can see that although the tape apparently stood up to the blasting process, it was less than suitable for protecting the part during spraying.

In this test, we have come across a common problem where that elusive golden nugget — masking tape that can stand up to the aggressive energies of the high velocity oxyfuel spraying (HVOF) process — was not found. There are tapes in the marketplace purporting to work in this environment, but it is certainly open to debate how well they work in every instance. 

The Taping Process

Thermal spray masking tapes are typically constructed from a combination of silicone rubber, woven fiberglass, metal foils, and a silicone adhesive.

To ensure satisfactory adhesion, tapes made from these materials must be smoothed and pressed down strongly to create a good bond between the masking tape and the component’s surface. There is nothing worse than having spent hours setting up your job only to see the masking tape gradually peel off during spraying. At what point do you abandon hope and stop? Figure 2 shows a gas turbine component that is in the process of being masked. The level of effort employed to produce a good bond can be seen by the surface markings on the tape (please avoid using the handle of a scalpel to do this).

In Fig. 3, we can see the effects of poorly placed or detached masking. The coating has been unintentionally shadowed by detached masking tape during the deposition process. This creates an area of thin or poor-quality deposit, which often means the part will have to be reworked (this can be both a costly and technically demanding issue).

While shadowing can be an effective masking technique when used intentionally, metal shadow masks are often far more effective and durable than tape for this purpose.

Getting the Best Out of Tape Masking

We have already mentioned some instances where tape masking can go wrong. As well as rubbing down the tape with all of our might, there are other procedures that are worth considering to improve our level of success. Figure 4 shows a thermal barrier coating (TBC) on a gas turbine nozzle.

In this case, when the component was initially masked, two layers of tape were applied. Subsequently, during the spraying operation, the outer layer of tape burnt away, leaving the inner layer still in position. The application of a dual layer had, therefore, protected the part from overspray.

Tape supply companies can provide tape that is already double layered. My personal preference is to apply one layer on top of the other as the random overlaps tend to improve the overall bond of the masking system.

Another important point is the buildup of ceramic coating on the tape — Fig. 4. The masking has done its job in protecting the part, but great care must be applied when removing it to ensure that the brittle coating is not chipped or de-bonded. The common practice for removing all masking tapes is to ensure the direction of removal is away from the applied deposit.

In this particular application, consideration can also be given to applying a line of weakness at the tape/coating interface using a tool such as a scalpel (care must be taken not to cause more harm than good). This method will introduce a preferential failure zone, which can reduce coating chipping.

It’s Not All About Tape

As previously mentioned, tape is not the only solution. As applications get more challenging and the geometry of parts becomes more of an issue, application of lacquers and other liquid maskants can become a useful option. Just be aware that they will perform differently.

Advantages include a more-conforming mask that interferes less with the spray stream. Disadvantages can include a masking material that will withstand spraying but not grit blasting (there are some notable exceptions to this statement). These various liquid masking materials are chosen based on their suitability for the application at hand, so generalized rules of use are difficult to define.

Liquid maskants can be applied by dipping, brushing, and spraying. An interesting development has been the optimization of the controlled application of UV-curing resins via a robot — Fig. 5. This gives an exciting opportunity to remove some of the manual aspects of the masking procedure and improve the accuracy of masking placement. 

In Fig. 5, we see one of the exceptions to the norm, where the applied masking material stands up well to both blasting and spraying.

As positional sensing technology improves, we may soon find that some of the most arduous masking can be automated. For example, turbine blade cooling holes often require significant time and patience to mask. The holes can vary in size and location depending on the condition of the blade. One day soon, automation could handle this easily, reducing cost and improving quality.

Testing Your Metal

Heading toward the less flexible side of masking, we start to enter a different, more shadowy world.

Metal masking can be enormously useful when we need to produce very accurate delineations between masked and unmasked surfaces. Also, when using high-energy deposition techniques, such as HVOF or chamber processes (i.e., vacuum plasma spray), metallic masking is normally the only way to go.

Figure 6 shows the basic shadowing principle that is often (but not exclusively) utilized with metal masks. Here, you can see that positioning the mask above the surface of the area to be coated will produce a smooth transition between the coating and substrate. This has the advantage of reducing stress concentration and, therefore, improves local adhesion of the deposit and reduces notch sensitivity effects. Lack of direct contact between the active mask and the part being coated also reduces possible bridging of the deposit and subsequent chipping.

The cost of the manufacture of metal masking can be viewed as somewhat prohibitive, but this has to be balanced against the necessities of the application, as well as the potential gains it can provide. For example, Fig. 7 shows how this type of masking can be used to prevent the negative effects of turbulence and bounce off adjacent vertical surfaces. Effectively, the mask is removing the potentially poor coating from the equation as well as providing an accurate coating position.

If cost is your concern, then don’t forget what the humble nut and bolt can do for you. The use of a properly sourced bolt will also give the added advantage of a shadow mask so that a professional job can be delivered to the customer — Fig. 8. At the end of the day, that’s what masking is all about!

Conclusion

Masking is an integral part of the thermal spray process and should be treated as such. Care must be taken over the specification of the masking materials used, their placement on the part being sprayed, the design of the masking technique with respect to the relevant spray process, and, last but not least, its final removal (ensuring that our precious coating is not damaged near the very end of a costly process).

Spraying is a dynamic process. Although we have reviewed some of the basic masking methods, there are always new techniques to be learned and optimized to protect the component effectively.

One thing is for sure: Without masking, we would not have a satisfactorily surface-engineered product. So, I guess masking is a little bit of a superhero after all.

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.