2009 Hosokawa Powder Technology Foundation
IntroductionModern industrial technologies call for the development of novel materials with improved properties, lower costs and environmentally suitable processes. Surface engineering that attempts to create functional layers on the surface is obviously the most economical way to provide high per formance to machiner y and equipment. Among the wide range of available methods (including varieties of atomistic and par ticulate deposition, bulk coatings wetting processes and surface modification), thermal spray coatings offer the most versatile solutions. Thermal spray processes form a continuous coating by melting the consumable material (feedstock) to form droplets and impinging these droplets on the substrate. The mechanism of bonding to the substrate in thermal sprayings (TS) is the same as plating, both mechanical interlocking and atomic interaction, with the shear strength around 7 MPa 1). The thickness of the coatings can range from 10 µm to a few millimeters 2) . Other advantages of thermal spraying include a practically unlimited assortment of powders to be sprayed, high efficiency and relatively low substrate temperature (373-583 K), thus minimizing shape distortion, oxidation and phase transformations in the near-surface layer. Demanded characteristics for thermal spray feedstock powders can be very different, depending on the spraying process, the operating conditions, the desired properties of the final coating, etc. Besides the intrinsic material properties, the technical requirements for the TS feedstock powders include good flowability and sprayability. They are greatly affected by the particle size, shape and morphology as well as particle size distribution. That is why thermal spray feedstock powder production processes must be reliable and flexible, while remaining as inexpensive as possible.Self-propagating high-temperature synthesis (SHS) or combustion synthesis, discovered by A.G. Merzhanov and colleagues in 1967, is known as a ver y promising technique for processing materials (ceramics, intermetallics, cermets, etc.) with good physical and chemical properties at relatively low costs 3,4) . The main point of the process is that after localized initiation, the reaction propagates as a narrow zonecombustion wave -along a sample driven by the exothermic reaction between components of the charge mixture without the application of external heating (furnaces, etc.). Because of the extreme conditions in the SHS wave (high temperature of up to 3500 , fast heating of up to 10 6 K/s, steep temperature gradient of up to 10 5 K/cm, rapid cooling in the after-burn zone of up to 100 K/s, and fast accomplishment of reaction, 0.5 s to 1 min), chemical interaction mechanisms during the SHS are often non-equilibrium, resulting in the formation of materials with improved structure and properties, especially in multi-component composite systems [3][4][5][6] . Moreover, a product with increased purity can be obtained due to evaporation of volatile impurities at th...