Thermal-Spray Processing of Materials
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Processing of Materials
S. Sampath and R. McCune, Guest Editors The enhancement of engineering materials by surface modification has extended the operational envelope for many structures in terms of resistance to corrosion, wear, fatigue, and other forms of surface degradation and has made feasible the introduction of novel materials into high-performance applications. Surface engineering, using thermal-spray coatings, represents a pragmatic and highly costeffective means of satisfying stringent design criteria (e.g., in aerospace applications), operating under extreme environments (e.g., high temperatures, wear, and corrosion), and introducing a multiplicity of functions (e.g., thermal barriers, biomedical implants, and electronic multilayers). Thermal-spray technology as a family of processes is distinguished by its ability to deposit overlays of metals, ceramics, polymers, and composites of these materials in layers of substantial thickness (e.g., 25 m) for engineering applications, often with equipment that can operate in the atmosphere and can be portable for use in the field. While traditional applications of thermal-spray coatings have addressed issues of surface protection, there is a growing activity in the use of the basic technology concepts for producing engineered functional surfaces and devices that offer the materials engineer a new scale of construction between thin films and macroscopic structures. The roots of thermal spray date to the introduction of powder metallurgy and to the manufacture of paint. These disparate fields both involve the incorporation of particles. The early methods of particle production employed pulverization, which by its nature is expensive and very material-dependent (i.e., ductile metals versus refractory materials). The concept of less-expensive melt spraying was introduced in the late 19th century and yielded far greater rates of production. Both gas
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and mechanical atomization were subsequently developed. One can only guess when it was realized that the atomized molten-particle stream, upon impingement on a substrate, could form a deposit. Traditionally, Schoop1 of Switzerland is given the credit for early recognition that mechanical or gas atomization of molten materials could be used to form a coating on a clean, well-prepared substrate. The early work led to a family of technologies involving the spraying of molten materials as protective coatings and for rebuilding machine elements. It was quickly realized by early workers that thermal spray, whether using powder or wire feedstock, was both cost-effective and versatile. The early combustion heat sources were eventually supplanted by electric arcs—to melt two electrically conductive wires or to create high-energy plasmas—allowing melt-spraying of refractory metals and ceramics. Further developments have focused on high-velocity combustion and noncombustion (cold) processes that produce material structures of very high density and strength. Thermal-spray coatings have become crucial to the economical and effic
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