Thermal Spray Basics
By Todd Degitz & Klaus Dobler
Reproduced with the permission of Welding Journal and the American Welding Society
A heat exchange tube bundle being sprayed with a tungsten carbide-cobalt coating. Since its inception almost a century ago, thermal spraying has evolved from a technology designed to be a cost-effective repair of worn components and mismachined parts to a process used to provide improved part performance and longer life to OEM components. As part of its growth process, thermal spray has developed from the original flame spray process to electric arc, plasma, and high-velocity oxyfuel systems. In addition, the palette of materials available for thermal spraying has expanded from metal alloys to ceramics, polymers, and carbides. One of the many industrial areas in which thermal spray has established itself is as a low-cost hardfacing alternative to weld cladding and chrome plating. The aim of this article is to introduce the characteristics of the four thermal spray processes - flame, arc, plasma, and high-velocity oxyfuel (HVOF) - and to discuss the different types of wear-resistant and/or corrosion-resistant coatings these processes can produce. An Intro to Thermal Spray
Thermal spraying, like weld cladding or chrome plating, is a coating process. In thermal spray, wire or powder is melted by a flame or electricity and sprayed onto the workpiece. During the actual process, the spray torch makes successive passes across the workpiece to produce a coating. Like all industrial processes, thermal spraying has its advantages and limitations. These have to be kept in mind in order to take proper advantage of thermal-sprayed coatings. The following are some of the benefits of thermal spray coatings.
Reduced Cost. In lieu of making the entire part out of an expensive material, a high-performance material is sprayed onto a low-cost base material. Low Heat Input. Thermal-sprayed coatings do not impact the substrates' microstructure. The coating does not penetrate the base material, i.e., there is no heat-affected zone. Versatility. Almost any metal, ceramic, or plastic can be sprayed. Thickness Range. Coatings can be sprayed from 0.001 in. to more than 1 in. thick, depending on the material and spray system. Coating thickness generally range from 0.001 to 0.100 in.
- Processing Speed. Spray rates range from 3 to 60 lb/h depending on the material and the spray system.
Some of the limitations of thermal spray include the following:
- The bond mechanism between the coating and workpiece is primarily mechanical, not metallurgical. Thermal spraying is a line-of-sight process. The coatings are considerably stronger in compression than in tension.
- The coatings have poor resistance to pinpoint loading.
The Thermal Spray Processes Flame Spraying
In the flame-spraying process, oxygen and a fuel gas, such as acetylene, propane, or propylene, are fed into a torch and ignited to create a flame. Either powder or wire is injected into the flame where it is melted and sprayed onto the workpiece.
Flame spraying requires very little equipment and can be readily performed in the factory or on site. The process is fairly inexpensive and is generally used for the application of metal alloys. With relatively low particle velocities, the flame spray process will provide the largest buildups for a given material of any of the thermal spray processes. Low particle velocities also result in coatings that are more porous and oxidized as compared to other thermal spray coatings. Porosity can be advantageous in areas where oil is used as a lubricant. A certain amount of oil is always retained within the coating and thus increases the life of the coating. The oxides increase hardness and enhance wear resistance. With regard to hardfacing, self-fluxing alloys are typically applied by flame spraying and then fused onto the component. The fusing process ensures metallurgical bonding to the substrate, high interparticle adhesive strength, and very low porosity levels.
The tungsten carbide-cobalt coating applied to this drill cone provides high wear resistance.
In the arc spray process, two wires are inserted into the torch and brought into contact with each other at the nozzle. The electrical load placed on the wires causes the tips of the wires to melt when they touch. A carrier gas such as air or nitrogen is used to strip the molten material off the wires and to transport it to the workpiece. Arc spraying is relatively inexpensive, easy to learn, portable, and fairly simple to maintain. Low particle velocities enable high maximum coating thickness for a given material. Recent advancements in nozzle and torch configurations are providing greater control over coating quality and the spray pattern. With the right equipment, it's possible to produce an elongated spray pattern or to spray components with very small internal diameters. As far as its shortcomings, arc spraying is limited to electrically conductive solid wires and cored wires.
The plasma spray process is considered to be the most versatile of all the thermal spray processes. During operation, gases such as argon, nitrogen, helium, or hydrogen are passed through a torch. An electric arc disassociates and ionizes the gases. Beyond the nozzle, the atomic components recombine, giving off a tremendous amount of heat. In fact, the plasma core temperatures are typically greater than 10,000°C, well above the melting temperature of any material. Powder is injected into this flame, melted, and accelerated to the workpiece.
Plasma spraying was initially developed to spray ceramics and is still the premier process for applying them. Metals and plastics can also be sprayed with this technique. The particle velocities for plasma are higher than for those of flame and arc spraying and result in coatings that are typically denser and have a finer as-sprayed surface roughness. The tradeoff of increased density, however, is that the maximum coating thickness for a given material is usually reduced. As both metals and ceramics can be effectively sprayed with this technique, plasma spraying lends itself to automation and to reducing process steps. For instance, ceramic coatings typically require a metallic bond coat to improve bond strength. With the plasma system, it's possible to initially apply the bond coat and then immediately follow with the ceramic material.
The HVOF process being used to apply a chromium carbide coating to this ball valve.
The high-velocity oxyfuel (HVOF) process was invented only 20 years ago, yet it has expanded the application possibilities for thermal spraying into areas that were once unattainable. In HVOF spraying, a combination of process gases such as hydrogen, oxygen, propylene, air, or kerosene are injected into the combustion chamber of the torch at high pressure and ignited. The resultant gas velocities achieve supersonic speeds. The powder is injected into the flame and also accelerated to supersonic speeds. The results are the densest thermal spray coatings available.
The HVOF process is the preferred technique for spraying wear-resistant carbides and is also suitable for applying wear- and/or corrosion-resistant alloys like Hastelloy, Triballoy, and Inconel®. Due to the high kinetic energy and low thermal energy the HVOF process imparts on the spray materials, HVOF coatings are very dense with less than 1% porosity, have very high bond strengths, fine as-sprayed surface finishes, and low oxide levels. These properties have enabled HVOF sprayed coatings to become an attractive alternative to cladding and chrome plating. Following are examples of applications using the HVOF process. Figure 1 shows a heat exchanger tube bundle sprayed with a tungsten carbide-cobalt coating. The coating is being applied with the HVOF process in lieu of cladding because the dense, erosion-resistant coating provides a low-cost alternative. In the second application, the same type of coating was applied to a drill cone - Fig. 2. The tungsten carbide-cobalt coating was specified to provide high wear resistance. In Fig. 3, the HVOF process was used to apply a chromium carbide coating to a ball valve. Chromium carbide was selected in order to provide wear and corrosion resistance. After spraying, the coating may be ground and polished to dimension or left in the as-sprayed condition. Outlook
Thermal spraying, like all processes, has inherent advantages and limitations. By understanding the variety of successful applications, a choice can be made that will save the manufacturer or processor substantial downtime and increase profits, thereby resulting in an excellent return on investment. Case histories from industries such as power, chemical, petrochemical, construction, mining, and pulp and paper show component service life increased by 50 to 75%. By reducing premature component failure, thermal sprayed parts will save thousands of dollars in forced outages.
With a variety of choices as to application methods and coating selections, thermal sprayed surfaces offer a solution for parts renewal, wear prevention, and corrosion resistance.
Todd Degitz (firstname.lastname@example.org) is Sales Manager and Klaus Dobler is Thermal Spray Engineer, St. Louis Metallizing Co., St. Louis, Mo.