February AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | F E B R U A R Y / M A R C H 2 0 1 9 3 8 iTSSe TSS iTSSe TSS C oating technologies that fall under the umbrella of ther- mal spray are highly versatile and can be found in use in several industries and applications. The wide variety of materials that can be processed into coatings is impressive. Virtually any feedstock material that melts without decom- posing can be processed by a thermal spray device. This ar- ticle highlights some examples of how these technologies are used to enhance surface properties for a few applications and industries. WHAT IS THERMAL SPRAY? Thermal spray is a generic term for a group of coating processes used to apply metals, alloys, ceramics, plastics, and composites. These processes are grouped into three major categories: plasma arc spray, flame spray includingHVOF (high velocity oxy fuel), and electric arc spray. These energy sources are used to heat the coating material (in powder, wire, or rod form) to a molten or semi-molten state. The resultant heated particles are accelerated and propelled toward a prepared surface by either process gases or atomization jets. Upon im- pact, a bond forms with the surface with subsequent particles causing thickness buildup. The technology can produce thick coatings at high deposition rates compared to other coating technologies such as electroplating and physical or chemical THERMAL SPRAY IMPACTS MULTIPLE INDUSTRIES The use of coatings to enhance the performance of components is one of the most powerful tools in materials engineering. William Lenling, FASM* Thermal Spray Technologies Inc., Sun Prairie, Wisconsin 4 FEATURE vapor deposition. Coating thicknesses from approximately 20 microns to several mm can be realized. Figure 1 shows simpli- fied graphics of the processes that make up thermal spray. The technology of cold spray also falls under the umbrel- la of thermal spray. This process is unique when compared to the other thermal spray processes in that cold spray solely re- lies on kinetic energy to create a coating as opposed to other processes that require thermal energy. Because it is a kinetic process, the powder particle feedstock remains in the solid state and does not oxidize or change phases while spraying (Fig. 2). AUTOMOTIVE APPLICATIONS The use of thermal spray coatings in the production of automotive components exhibits that the technology can be adapted for high volume production. For example, piston rings have been coated for many years with molybdenum-based materials to reduce friction. Aluminum engine block cylin- ders are now being coated to replace cylinder liners. Several automotive manufacturers have adopted the technology, and the practice is spreading. In the case where a lot of power is desired in a small space, such as outboard boat motors, the technology is also finding a home. Yamaha is now using coat- ed cylinders in their V Max SHO outboardmotors. According to Fig. 1 — Schematic diagrams of thermal spray processes: (a) Flame spray, (b) electric arc spray, (c) high velocity oxy fuel (HVOF), and (d) plasma spray. *Member of ASM International (a) (c) (b) (d)