Machining Thermal Spray Coatings

Thermal spray coatings can provide wear and corrosion resistance and can quickly build up dimensions on metallic components that have been worn down or mismachined. They’re fast, affordable, and effective, and they are way off the radar of many of the engineers and machinists who might use them. The result is that, despite this technology’s flexibility and utility, there aren’t many prints out there that call for thermal spray coatings. And, when they do, manufacturers, programmers, engineers, and machinists seldom know how to work with them. Thermal spray coatings are functional and highly effective when used properly. But, to the uninitiated, they can be brittle, tough, flaky, and frustrating to work with.

This article intends to shed some light onto the nature of these materials and how to handle them when they’re being machined. Depending on the application, thermal sprayed materials are used as is, in their as-applied condition, or they are required to be finished to a specific dimension and/or surface finish. When finishing is required, grinding is often preferable because coating materials are often hard and respond well to the slow material removal rate typical of a grinding operation. Grinding is the only finishing option for very hard metallic coatings, tungsten carbide composite coatings, and ceramics.

Most metallic thermal sprayed coatings can be tooled if the proper care is taken, and this alternative may be attractive for reasons such as speed, price, and portability of the tooling process. Since tooling is a more technique-sensitive finishing method, we’ll focus on the fundamentals of tooling thermal sprayed coatings.

Preparation

It is important for the machinist to understand that thermal sprayed coatings are not wrought materials, and they won’t tool like them. First, the bonding mechanism is mechanical, not ionic or covalent. The structure is like a tightly packed and very hard sandcastle. Particles of coating lock into the particles around them using high heat and pressure, and the strength of the bond is dependent upon the intricacy of the interlocking joint and the yield strength of the sprayed material. What’s more, as the coating is built thicker, often the tensile stresses within the coating will accumulate, fighting against the mechanical forces holding the coating in place, and reinforcing the coating’s tendency to pop. In compression, even heavy, high-shrink materials might continue to perform. But, with the application of an additional mechanical stress in opposition to the bonding mechanism, failure at the bondline and within the coating is highly likely.

A further aggravation to this already sensitive system is the presence of features within the coated geometry that could lead to an uneven distribution of stresses within the coating. When a coating is wrapped over a sharp corner or peak, as it cools, more of the coating’s internal stresses will accumulate over that feature — Fig. 1. Less effort will be required to initiate a failure at that point than at another less-irregular region of the coating.

Areas where the coating integrity is compromised also produce failure-prone regions with the coating system. A common example is a coating applied to an undercut region, such as the outside diameter of a shaft, where a damaged area has been repaired with thermal spray. If the undercut is prepared with straight sides, then soot, fines, and other impurities can become trapped in the sharp relief, making the coating in that area more porous and less well bonded than the adjacent coating structure. Since this phenomenon occurs at the edge of the coating, a failure here will allow the rest of the coated area to be attacked from the side, interfering directly with the bondline and, more likely than not, leading to catastrophic coating failure.

For these reasons, it is best to ensure that thermal sprayed coatings are never applied over potential stress risers, that coating geometries do not tend to leave coated areas exposed to potential attack from impact or gouging stresses, and that, when the coating is applied within a prepared undercut pocket (often the best practice), the pocket is prepared with chamfered or radiused borders — Fig. 2.

Following these basic guidelines will ensure that the coating performs well and withstands only the abuse for which it was designed. The most common reason thermal spray coatings are tooled, rather than ground, is that the application must be done in the field, and tooling equipment is more portable than grinding. Thermal spray is well suited to repairing damage and bringing failed equipment back online in short order because the coating can be applied and finished in the field. Spun bearings and failed seals are good examples. The resulting damage is typically on a mission-critical diameter that is now undersized and possibly irregular. For the equipment to return to service, the diameter must be cleaned up, restored to size, and brought to a suitable finish. A uniform buildup on cylindrical work is easy to achieve because the consistent surface speed required for thermal spray coating can be achieved by rotating the shaft and slowly traversing the spray gun. The coating can then be tooled with a portable lathe and brought to finish with emery, sometimes without removing the piece from its service location.

Materials

Materials for repair must have performance characteristics at least equivalent to the original base material. They must also be relatively easy to machine because rework will only exacerbate downtime. Stainless steels in the 300 and 400 series are readily available in powder and wire. Whether plasma, flame, or arc sprayed, they can usually be tooled, and some respond well to being built to fairly heavy thicknesses without risk of failure.

In addition to their flexibility for a variety of applications, these materials also tend to be hard and durable, a good fit for the repair of rotating equipment that has seen significant material loss. Another attractive feature of these materials is the ease with which a quality repair can be made using arc spray. Since this spray process has few utility requirements, only power and compressed air, it is often a simple matter to get the hardware in place to do a repair in the field, even under less than favorable conditions. High-bonding alloys like nickel aluminide, aluminum bronze, and molybdenum also make great repair and salvage materials, depending on the application. Like the steels, they can nearly universally be built to significant thickness without risk of failure and can be tooled, rather than ground. Though some of these materials are softer than steel, this may not be a concern, for example, in corrosive environments, where wear is less a concern than chemical attack or galling.

Applications

Following are some generic application categories where coatings are commonly applied and then machined to dimension for repair purposes. The selection is designed to illustrate commonly successful types of repairs rather than unusual or exotic applications.

As mentioned previously, journal and bearing area repair is common because of the depth and breadth of rotating equipment in service. In nearly any instance where a machine component includes a spinning shaft, that shaft will be supported by some kind of bearing, and that bearing is protected by some kind of lubrication system. When the lubrication system fails, if the machine continues to operate, the deterioration of the shaft beneath the bearing can be quick and significant. To repair an area where a bearing has seized or spun, the area must usually be excavated to restore the diameter to some uniform dimension. Again, it is important to make the undercut with chamfered sides.

Depending on the thickness of the coating required to restore dimension, either a high-building steel or a bond coat with steel top coat will likely be used. A tight fit with a close tolerance is usually required for bearing areas, and, while this usually requires grinding between centers, a field repair can be brought into approximate finish by tooling and then polishing with emery tape.

Military standard procedures for the repair and overhaul of marine equipment have been in place for decades. While becoming qualified to perform these repairs for government agencies can be time consuming and difficult, the procedures demonstrate the usefulness of corrosion-resistant thermal sprayed materials for the salvage of mission-critical equipment in hard-wearing and corrosive environments.

In typical applications, the material applied for repair or rework is similar to the base material, e.g., aluminum bronze or nickel aluminum for naval bronze castings, Monel™ or nickel copper for nickel-based alloys, and austenitic steels for carbon and low-alloy stainless steels. For pump housings and impellers, valves, hatches, and other components that come in contact with seawater, effective cleaning prior to coating is essential. Oxides remaining on the surface prior to coating can negatively impact bond strength and cause the coating to delaminate under lower stress. Depending on the geometry and the nature of the repair, either hand spray or automation can be used. Most materials used in these applications are easily tooled, so the accuracy of the spray application is less critical than for many other jobs. Hand blending, including die grinding and sanding, can be used to effect many repairs.

A large percentage of commonly applied roll coatings are used as surface enhancements. Traction coatings are often applied and used in their as-sprayed condition. Hard coatings used to extend the life of roll surfaces are typically ground, rather than tooled. The chief exception is restoration of dimension. When diameter is critical, and a roll has been ground or reground and is now under the minimum serviceable diameter, its life can be extended almost limitlessly by rebuilding the surface and then tooling it back to size. This can be especially valuable for the salvage of rolls that have suffered minor damage, such as gouges or dings — Fig. 3.

Twin-arc-sprayed 400-series stainless steel is often compatible, in terms of chemistry and coefficient of thermal expansion, with most roll shells. It can be applied quickly and built to significant thickness, and it is easily tooled to a decent finish. Gouges and other inconsistencies in the surface are often best treated by machining the entire circumference undersize in the affected zone until the damage has been eliminated and then blasting and rebuilding. Alternately, coating the entire roll face and then tooling to smooth can be used to clear up multiple small blemishes and/or restore cylindricity.

As with spun bearings, sealing or packing areas on rotating shafts often develop circumferential wear patterns that can be significantly under the original shaft diameter. Unlike bearing fits, deterioration in these zones can happen far more slowly and with less obvious consequences. Nonetheless, proper functioning of a seal or packing is essential to the proper operation of valve stems and pump shafts, and restoration of the lost dimension will eliminate leaking and restore the sealing integrity of the coupling. Materials used for the repair of sealing and packing areas must typically be hard enough to withstand the constant sliding wear of the active seal but must also be free from porosity, incomplete bonds, cracks, or other discontinuities that can lead to premature deterioration of the sealing or packing material. Nickel-chromium alloys are particularly well suited to the repair of wet-service components in sealing applications. The alloys are largely corrosion resistant and wear very well but also tend to exhibit good homogeneity; there are no hard carbide particles, choppy oxides, or excessive porosity that can tear into and abrade sealing components. They also can be tooled and polished fairly easily.

Tooling Fundamentals

Tooling thermal sprayed coatings is not overly complex, so long as it is understood that the material cannot be treated as if it is solid or wrought. Understanding that there are limits to the bond strength of the coating and that tooling forces must be controlled to remain clear of these limits is the fundamental challenge. Largely, the practice of taking light cuts with a sharp tool at lower-than-usual surface speeds will translate to success in machining sprayed coatings. Erring on the side of caution is also helpful. Some more specific practices are as follows:

Avoid attempting to machine any applied material with a nominal hardness in excess of 55 HRC. Many thermal sprayed coatings, particularly those discussed previously, fall well below this limit, but attempting to tool a harder material, or overworking a work-hardening material, will lead to excessive tool wear; poor cut quality; and probably cracking, failure of the overlay, and frustration.

Be sure to mount tools rigidly, with the tool holder as close as possible to the cutting point. Any possibility of vibration or deflection of the tool that might compromise the quality of the cut will lead to premature tool wear and the headaches that follow.

Keep tools sharp. Once a tool begins to wear, the friction imparted will begin to heat the coating, eventually leading to rubbing and blistering of the coating. At the first sign of anything other than an easy, consistent cut, replace or rotate the insert and continue the cut. Use a smaller infeed or slower traverse if an uninterrupted cut is essential. A sharp pointed tool, such as a threading insert or grooving tool, will typically wear too quickly, and some amount of radius (0.0156 to 0.125 in.) will help extend the effective tool life. In general, though, the tool radius should be as small as can be tolerated for the finish required. A larger nose radius will lead to a wider cut with more surface area in contact with the tool, which will increase the risk of overheating the coating.

Across the board, harder inserts are better. Although the material being tooled may not be hard in its typical form, thermal spraying can often lead to the development of oxides within the coating structure. These hard formations can be aggressively damaging to low-grade tools, and higher quality, harder inserts will tend to perform better longer. Tungsten carbide is a de facto choice. Cubic boron nitride and diamond tools have been used, but the additional cost is typically unnecessary.

Feeds and Speeds

Zero degrees of rake (vertical tool pitch) is a safe position for machining most sprayed coatings. Negative rake (down angle) will just lead to dragging and loading, and the coating will quickly fail. Positive rake is possible, but only to a limited degree and only on fine cuts, as there is a significant risk of gouging the coating or catching an area of incomplete bond and lifting the material away, resulting in catastrophic failure. Bear in mind that once a tool has gotten beneath a coating, the coating will lift off, like when paint is scraped.

In general, harder materials will require slower feeds and speeds to cut effectively. You’ll quickly know that you’re moving too fast when the tool either snaps or grinds away. Provided the inserts are sharp and you are taking care to proceed gently with the cutting effort, some combination of feeds and speeds will cut nearly any metallic commonly sprayed. Table 1 provides a good rule of thumb for most coatings.

The following list has some common traps both inexperienced and experienced machinists fall into when tooling coatings.

• Not keeping the inserts sharp or not replacing inserts often enough — Fig. 4. This can allow the tool to plow or burnish the coating rather than cut it. When this excess heat is imposed, the coating may blister and fail.
• Attempting to make an interrupted cut in a coated zone, such as a keyway, may lead to the coating chipping at the far side of the interruption. It is preferable not to tool an interrupted coating, but, if you must, consider filling the gap with a carbon or brass insert.
• Attempting to cut too quickly will dull the tool, leading to the aforementioned plowing and blistering.
• Failing to keep the tool cool can cause some coating materials to work harden, which can quickly make a material uncuttable.
• As with any coating, there are risks of taking too much material off and breaking through in one or more areas, which will introduce a failure-prone zone in the coating. Carefully addressing runout prior to machining should ensure that the coating thickness is consistent on all sides.
• On the other side of the same coin, the spray technician must be sure to apply enough coating so that the machinist can restore the required dimension without leaving a low spot where the tool fails to take a chip.

Working carefully and taking light cuts will help you avoid nearly all of these mistakes. Again, it is most helpful to remain aware that the material being cut is a coating on top of the original piece and not an integral part of the component. Approaching the tooling of a coated surface as if it were a thin, hard shell on a mandrel, which to some extent it is, should guide a machinist to make smarter choices about tool selection and feeds and speeds. From a practical standpoint, the much lower stock-removal rates characteristic of a grinding or lapping process might make this a better option than tooling when the option is available. Grinding also offers an improved range of surface finishes and the ability to cut very hard materials like tungsten carbide and ceramic coatings by using superabrasive grinding wheels. Grinding thermal spray coatings also has its own set of peculiarities, but this is a topic for another time.

DANIEL C. HAYDEN (daniel.hayden@haydencorp.com) is president, Hayden Corp., West Springfield, Mass., chair of the AWS C2 Committee on Thermal Spray, and technical editor of SPRAYTIME.

This article was originally published in the September 2011 Welding Journal.