AWS WHC-3.11:2008

Thermal Spraying and Cold Spraying
Scope : Thermal spraying and cold spraying are two closely related but fundamentally different spray-deposition processes. Thermal spraying encompasses a group of processes in which finely divided metallic or nonmetallic materials are spray-deposited in a molten or semimolten condition onto a substrate to form a coating or to build up a free-standing shape on a mandrel.1, 2 Cold spraying is a comparatively recent and emerging process that is often included in the family of thermal spray processes because it also involves spraying finely divided materials onto a substrate to form a coating or freestanding shape. However, unlike traditional thermal spray processes, cold spray particles are not heated to a molten or semi-molten state, but arrive at the target surface in solid state, often at or near room temperature. The fundamental differences in bonding mechanisms and process characteristics related to the use of molten or semi-molten spray particles in thermal spraying versus solid, relatively low-temperature spray particles in cold spraying are discussed in this chapter. Major commercial thermal spray processes include electric arc spraying, atmospheric plasma spraying (APS), vacuum plasma spraying (VPS), flame spraying (flame spray), detonation flame spraying, and highvelocity oxyfuel spraying (HVOF). The photograph on the title page of this chapter shows a rotary plasma spray gun as it is translated up and down through the cylinder bores of an aluminum automobile engine to quickly apply a wear-resistant coating to the cylinder walls. Because traditional thermal spray processes are more widely used and have greater commercial importance, the majority of this chapter is devoted to the traditional thermal spray technologies. However, cold spraying offers important advantages for some applications, and the commercial use of this technology is increasing. This highly versatile group of spray processes can rapidly deposit an exceptionally wide range of metals, ceramics, glasses, polymers, and composites on many different substrate materials. Feedstock for thermal spray processes may be in the form of powder, rod, wire, or cord. A few examples of the diverse variety of commercial applications of thermal spraying include anti-corrosion and anti-skid coatings on ships and bridges; laser-engraved ceramic coatings for analox rolls used in the printing industry; and wear-resistant coatings and thermal barriers used in industrial and aerospace turbine engine applications. Thermal spraying also is used in biomedical applications, such as titanium and hydroxy-apatite (artificial bone) coatings on human joint and dental implants. Although there are many sophisticated applications, sprayed coatings applied to enhance or extend performance in severe thermal, wear-intensive, or corrosive environments remain the mainstay of the thermal spray industry. The thickness of the deposited layer for applications typically is in the range of 0.125 millimeter (mm) to 1.0 mm (0.005 inch [in.] to 0.040 in.). Much thicker spray deposits more than 25 mm (1 in.) can be produced with some spray materials, and thermal spraying sometimes is used to make freestanding shapes. For example, injection molding dies can be produced by spraying onto removable patterns or mandrels. Cold spraying currently (~2006) is limited to depositing ductile metals and a few metal-ceramic composites. Developed in Russia in the late 1980s, the earliest commercial applications exploited some of the special advantages of cold spraying for uses such as high-therma lconductivity copper coatings for computer chip heat sink assemblies. The cold spray process often can produce much thicker coatings than thermal spray processes due to a more favorable compressive residual stress state inherent in cold-sprayed deposits. For example, copper has been successfully cold-sprayed to thicknesses of more than 10 centimeters (cm) (4 in.), with possible commercial use in the cold-spray forming of large rocket nozzles. Highly sophisticated thermal spray applications are modern, but the process is not new. The first documented thermal spray patent in the English language was recorded by Max Ulrich Schoop, a Swiss scientist, in 1911. Patent literature shows that the fundamental principles of modern thermal spray processes were well established by the early 1920s. During this period and through the 1950s, most applications were concerned with reclamation of industrial parts, such as worn shafts. Exponential advances in technology since then have dramatically expanded the number and diversity of thermal spray processes, materials, and applications. The cold spray process (originally called the cold-gas dynamic spray method) was developed in the mid- 1980s by a group of Russian scientists researching gas dynamics in Siberia. Although this process was not widely known outside Russia until the early-to-mid 1990s, cold spraying continues to be studied in research laboratories around the world and evaluated for new commercial applications. This chapter provides an overview of the thermal spray and cold spray processes and describes the equipment, materials, and techniques used for both. Sections on applications, post-treatments, quality control, economics, and safe practices are included. Sources of additional information are listed in the Supplementary Reading List at the end of the chapter.