Covering all the Bases

Surface coating technologies are extremely similar, making it difficult to choose the most appropriate for specific jobs. This occurs frequently when deciding between laser cladding and thermal/plasma spray, specifically high velocity oxy-fuel coating spraying (HVOF).

05. August 2013
Bild 1: Covering all the Bases
Bild 1: Covering all the Bases

Once considered radically different, both technologies have advanced to the point that either is suitable for certain applications. HVOF coatings are growing thicker, while laser cladding coatings are becoming thinner. Additionally, HVOF has reduced porosity to levels that verge on being considered fully dense.

Still, the technologies behind HVOF and laser cladding – and the majority of applications – remain fundamentally distinct. HVOF coating involves spraying the material at a high velocity and temperature, which softens the particles and forms a mechanical bond with the roughened substrate. In contrast, laser cladding melts both the material being applied and the surface of the substrate to form a metallurgical bond.

Functional similarities

Both laser cladding and HVOF will continue to converge for the next two to five years, which will then create a shift in commercial assessment. Presently, HVOF is the only technology for thinner coatings, such as 200- to 300-microns. However, despite HVOF advances in producing thicker coatings up to a half-millimeter, laser cladding is preferred over HVOF for thicker coatings.

Another area of convergence is porosity. The fundamental principle behind thermal spray necessitates that particles are softened by heating and compacted in a solid state, leaving small spaces between them that result in a porous coating. HVOF has reduced its porosity levels to less than 1-percent, which is nearly fully dense. Still, these pockets can cause penetration of the coating when parts are exposed to high-pressure environments or long-duration tests, Despite HVOF’s decreasing porosity levels, laser cladding remains the only completely dense solution.

Both coating technologies can create residual stress on the substrate, distorting it. When a laser cladding material is heated, melted and solidified, it shrinks, resulting in temperature fluctuations that can warp a thin part. Though laser cladding has advanced to minimize distortion levels, HVOF still causes less stress and distortion risk because the material is not fully melted. The internal stresses in HVOF coatings are what limit the thickness.

Difference in application

One major application difference between these two technologies has to do with how the coatings adhere to the substrate. HVOF creates a mechanical bond between the coating and substrate surface, allowing manufacturers to use any material. Laser cladding creates an alloy in the interface zone between substrate and coating material and as a result, is limited by being able to bond only materials that are weldable. Manufacturers should ensure that the selected materials will create a successful metallurgical bond, such as a nickel coating to iron to create a nickel-iron alloy. Materials that are not compatible, such as titanium and iron, could result in a weak intermixed layer that can easily crack.

Additionally, laser cladding has a small melt pool, meaning the application process can take longer than HVOF. The thickness of the coating also adds time to the spray process. Thus, extra time for the application of laser cladding materials can offset other cost savings. HVOF still has a significantly higher deposition rate.

Surface Conditions

The part’s surface conditions play a role in determining the appropriate coating. Thermal spray’s limited bond strength is less ideal for parts that will be subjected to high stress or impact loading. The mechanical bond may cause the coating to shatter or fall off if subjected to too much stress, such as with a hammer. The stress may weaken laser cladding’s alloyed coating but will likely not cause it to disbond.

The same rule of thumb can be applied to parts that will endure many thermal cycles. Fluctuating temperatures cause different metals to expand and contract separately. This thermal shock can stress and weaken the HVOF bond line but this is not the case with laser cladding because it creates a metallurgical bond.

Corrosion

Coatings are also vulnerable to corrosion, which can be aggravated by porosity. Despite advances in HVOF, the lingering pores render the coating vulnerable to environmental pressures that deteriorate the surface. For example, a valve coated with the minimally porous HVOF would eventually succumb to harsh sea water leaking through the coating, causing it to disbond.

Such high-pressure environments often necessitate laser cladding to produce a fully dense coating but these are limited to materials that are weldable.

Manufacturing environment

Both coating technologies also have different requirements in regards to manufacturing environments. Compared to laser cladding, HVOF covers a larger spray area but is less precise. The relative velocities of the spray gun and part need to be moved quickly or the coating will accumulate, which will create excessive residual stress and bond failure.

While thermal spray can be applied both manually and via automated technology, laser cladding requires an automated factory environment for safety reasons and because of the application precision. Each weld track has to be positioned with tolerances below 1 millimeter, necessitating a robot to apply the coating. With such a small coverage area, what laser cladding gains in precision it loses in application time. Comparing the two technologies, laser coatings are applied in a narrow but relatively thick layer while HVOF uses many wider but thinner layers.

However, because HVOF is applied in fine layers to mitigate the stress and shrinkage issues, the technician must wait for the part to cool between each layer or the substrate may overheat. This start-stop process is not typically a concern with laser cladding. With some HVOF coatings requiring 50 passes, this process can decrease efficiency. Laser cladding may also require a waiting period for the part to cool as the materials are heated locally beyond the melting temperature.

As mentioned previously, the differences in deposition rates, or pounds per hour of coating, are notably large with HVOF having a significantly higher rate. Though HVOF coatings can reach half-millimeter, laser cladding tends to be more efficient as often only one coat is required.

When both technologies are automated and implemented using standard industrial robots, the comparison becomes more apples-to-apples. Compared to previously requiring complicated manipulation with copper mirrors, laser cladding now uses the more simplified fiber-optic bundles to control the beams. Thermal spray can be mounted at the end of a robot or manually manipulated.

Thermal spray also requires speedy manipulators and when covering a large area, a large robot. However, this presents a paradox as a very large robot is also slower.

Safety

When incorporating such potent technologies, safety is at the forefront of consideration. Both laser cladding and HVOF have their respective safety precautions for workers and are always enclosed in a cell. Laser cladding requires compliance with general laser regulations, such as protecting eyes with special glasses, shielding the workers, and safeguarding against welding fumes and laser light wavelength.

With HVOF generating heat loads up to 1 million BTUs, thermal spray booths typically require large volumes of air exchange to keep temperatures within reasonable limits. Another requirement is a dust collector with closed-circuit air filters to vacuum the dust generated from sub-25 micron particles. Workers must also shield themselves from the extremely bright light generated by the HVOF gun. The gun generates extremely high temperatures and a piercing noise that register above safe levels, so workers need to wear appropriate protective gear if manually applying the coating.

Energy and material efficiency

Both technologies consume energy and materials at different rates. Laser cladding was previously a notorious energy consumer with an efficiency of less than 10-percent. For example, a 5-kilowatt laser would demand 50-kilowatts to power it. Fortunately, laser cladding is now 30-percent efficient, a radical enough improvement to be considered power efficient.

Because of the energy-intensive gas stream needed to heat the particles, HVOF spray is considered less efficient than laser cladding.

From a material standpoint, laser cladding is more than 90-percent efficient, outperforming thermal spray’s 40- to 60-percent efficiency. This is due to the imprecision of the HVOF spray cone, which does not emit some of the particles at a fast-enough velocity. As a result, many particles bounce off the substrate and fail to bond.

Though laser cladding and HVOF use powder-based materials at similar price points and availability, the particle sizes differ. Laser cladding particles are coarser and heavy enough that no filtration device is needed. Thermal spray’s finer particles are light enough to become airborne, necessitating the dust collector and air filters.

Application Equipment

The thermal spray gun can be easily moved – up, down and into awkward corners while the target part remains stationary. If the part has a more complicated design, the HVOF gun can be easily maneuvered to cover all contours. In addition to manual application, HVOF is versatile in that it can be also used with an automated system. Because thermal spray essentially blankets an area with a continuous coating, the automated programming is relatively simple.

On the other hand, laser cladding is much more complex. Each weld track involves a repeated start-stop approach where the laser starts and stops. As mentioned previously, each laser application is comparably precise. Thus, the programming effort is much more sophisticated and requires the precise location and dimensions of the part. Crucial to reducing this time-consuming process, offline programming tools are needed.

Access

Both technologies are line-of-sight processes with different distance requirements. While the laser cladding powder nozzle needs to be less than 1-inch from the substrate, HVOF necessitates a distance of 12 inches. When coating the small space inside of a tube, HVOF is generally preferred as the gun can be easily placed into a 100-millimeter part. However, this is another area of convergence as laser heads are continually becoming smaller and can presently fit inside a 3-inch bore.

Both guns operate at extremely high temperatures, which affect the application processes. Laser cladding creates local temperatures above the melting point of the material.

When laser cladding, the powder nozzle must be cooled because one-third of the laser light is reflected toward the processing head and not absorbed by the melting process. The closer the powder nozzle is located to the substrate, the greater the risk of harming it.

The HVOF gun itself becomes so intensely hot that it needs to be cooled to dissipate the power.

Maintenance

With typical use, the laser itself is durable enough to operate without maintenance for several months. There are neither moving parts nor sensitive optical components. Because of the robot’s movement, the fiber-optic cables eventually need replacement but that is often measured in years. Depending on the material being used, powder nozzles may need to replaced after 100-500 hours.

With HVOF, the extremely high temperatures and velocities cause the components to wear quickly, measuring nozzle life in hours. Though frequent, nozzle replacements are simple and quick. The materials used also determine the change-out frequency. Materials such as carbide are applied at a much higher velocity, making the HVOF barrels and nozzles wear out quicker than with ceramics, with which the nozzles last several days.

Conclusion

There are many factors to enter into the equation of whether to use HVOF or laser cladding. Some are obvious choices, such as if a thick and fully dense coating is needed or if ceramic material is involved. As these technologies continue to evolve, their applicability will broaden. In the meantime, it is critical that users work with material and equipment suppliers that are knowledgeable about both technologies to ensure a durable part that can be coated efficiently and cost-effectively.