Thermal Spray Coating

The High Velocity Oxygen Fuel (HVOF) process is a thermal spray process developed in the 1980s that has since been commercialized for industrial use. The process involves projecting metal or ceramic particles onto a substrate at supersonic speed. To achieve this, a liquid (typically kerosene) or gas (propane, acetylene, hydrogen, etc.) fuel is mixed with oxygen and ignited in a combustion chamber. This produces a high-pressure jet of hot gas that flows through a nozzle. Powder is fed into the jet and propelled at the substrate at a velocity of more than 800 meters per second (almost 1,800 miles per hour). The particles, which have been softened but not melted, adhere to the grit-blasted surface of the target, ideally creating a hard and tough, high-strength coating with low porosity.

Advantages of HVOF Coatings

HVOF coatings provide several advantages including:

High hardness: The high velocity at which the particles are flung onto the substrate results in a dense coating, which contributes to higher levels of hardness. In addition, HVOF is an excellent option for applying carbide coatings. The short amount of time that the material is exposed to heat leads to reduced degradation of the hard carbide phases in a material.

Toughness: As compared to other thermal spray processes, the HVOF process provides coatings with high toughness. This is partly because of superior intersplat cohesion (the cohesive bonds that form between the softened particles after they are sprayed). The favorable combination of high hardness and relatively good toughness yields a coating with excellent wear resistance.

Thickness: Since the powder is often not completely melted, less residual stress in the coating occurs, which allows thicker coatings. The typical thickness of HVOF coatings is 0.005-0.010 inches (0.127-0.254 millimeters), but some coatings can be applied to a thickness up to 0.50 inch (12.7 millimeters). The thicker coating can be beneficial for wear resistance and protection of the underlying substrate from corrosion.

Corrosion resistance: HVOF coatings tend to have better corrosion resistance than most other thermal spray coatings. This depends heavily on the material applied, of course, but in general, the low porosity in HVOF coatings adds to corrosion resistance.

DISADVANTAGES

HVOF is a line-of-sight application with a spray range of about eight inches, which can restrict application on tight areas and internal surfaces of cylindrically shaped parts.

At a recommended maximum size of about 50 microns, the powder size needed for successful HVOF coating is smaller than other thermal spray applications. The distribution of particle size is also very important—it is not uncommon to see requirements for a distribution range of 20 microns (30-50 microns would be an example).

When done properly, HVOF coatings are highly effective. However, they are complex and the application requires highly trained personnel to supervise the process. Although the spraying is often performed by robots, personnel must be familiar with the process, make changes to parameters as needed for different powders and substrates, and ensure that the spraying is carried out safely.

WHEN HVOF MAKES SENSE

The HVOF process can be used to apply a wide range of metals including nickel alloys, cobalt alloys, carbides, aluminum bronze, molybdenum and some stainless steels. Also common for HVOF coatings is cermet (composite ceramic and metal) powders. In the valve industry, two of the more common cermets applied with the HVOF process are tungsten carbide particles in a cobalt matrix (WC-Co) and chromium carbide particles in a nickel-chromium matrix (CrC-NiCr). While the final coating properties depend in part on spraying parameters, these materials are favored for high wear and abrasion resistance, high bond strength and relatively good toughness.

HVOF coatings are an excellent option when an application requires high wear resistance and erosion resistance while maintaining moderate corrosion resistance. For example, in a slurry application, an HVOF coating can be a good choice on the valve ball, seat, plug or disc.

As the HVOF process has been developed over the past few decades, it has been used increasingly as an alternative to some traditional coating systems, such as plasma coating and hard chrome plating. The wear resistance offered by carbide HVOF coatings competes with that of hard chrome plating. In the best cases, the corrosion resistance, while dependent upon the environment and chosen alloy system, can match or exceed the performance of hard chrome. However, as mentioned, HVOF coating is line-of-sight, so hard chrome has an advantage when it comes to complex geometries and small inside diameters.

The cost of these two coating systems can be debated, but it depends on the geometry of the parts coated and the required thickness. HVOF is typically thought to be slightly more expensive, although when dealing with large parts with simple geometries, HVOF becomes more cost competitive and may be cheaper than hard chrome. Any polishing and grinding will contribute to cost as well. Again, cost is dependent upon the shape and complexity of the surface for both HVOF and hard chrome.

Also, while HVOF coatings are not a perfect replacement, increasing pressure to restrict hexavalent chromium for health and environmental reasons means HVOF coatings will probably grow in use in the coming years.

BENJAMIN HAGARTY is a materials engineer for Emerson. Reach him at Benjamin.Hagarty@Emerson.com.