THERMAL SPRAY is a "family" of particulate/droplet consolidation processes capable of forming metals, ceramics, intermetallics, composites, and polymers into coatings or free-standing structures. During the process, powders, wires, or rods are injected into combustion or arc-heated gas jets, where they are heated, melted or softened, accelerated, and directed toward the surface, or substrate, being coated. On impact at the substrate, the particles or droplets rapidly solidify, cool, contract, and incrementally build up to form a deposit. The thin "splats" undergo very high cooling rates, typically in excess of 106 K/s for metals (Ref 1).

Increased attention is being paid to thermal spray processes today because of their ability to consolidate virtually any material that has a stable molten phase, producing coatings or deposits exhibiting relatively homogeneous, fine-grained microstructures (Ref 1, 2). Thermal spray is very versatile and has also been proposed for several new applications, including rapid part prototyping (Ref 3), production of fiber-reinforced intermetallic-matrix composites (IMCs) (Ref 4), and for producing so-called "functionally gradient materials" (FGMs) (Ref 5). Thermal spray is, nevertheless, a mature process technology, having been established now for more than a century as an economic, efficient method for coating and recoating surfaces, repairing worn components and spray forming. The latter has been practiced for at least 50 years as a means of producing ceramic crucibles for the containment of molten or reactive metals, nozzles for hot gases (Ref 6), production of forging dies (Ref 7), and the structural forming of difficult-to-process particulate materials such as high-temperature alloys, refractory metals, intermetallics, and ceramics (Ref 8). Thermal spray processing has become an important powder consolidation technique, and new developments are resulting in more novel material synthesis routes and processing of unique material combinations. This article reviews thermal spray processing advances made in the last 25 years, focusing on the recent innovations that are driving its application in metallic-, ceramic-, and intermetallic-matrix composites (Ref 9), materials synthesis, and FGMs. Thermal spray processes and their characteristics are described, together with descriptions of the properties and characteristics of thermally sprayed materials. The first part describes the different classes of thermal spray processes and the materials systems that can be processed, either as coating or as spray-formed deposits. The second part focuses on thermal spray forming techniques and applications.

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