![]() Their frequent function is regeneration of surfaces, electrical conductivity, corrosion resistance, wear resistance, biomedical, etc. The MMCs coatings can have various applications. The papers describe the characterization of Al 2O 3 cermets with metals as Al, Cu, Zn, Sn, and Ni. Therefore, a lot of recent studies deal with deposition of metal matrix composites (MMC). On the other hand, the LPCS is the cheapest method enabling cermet coatings to be obtained. The HPCS enables better coatings properties to be achieved. The mixing of powders having particle size of −50 + 5 µm is the most popular feedstock preparation method in cold spraying. The deposition of composites as the cermets is also possible and becomes increasingly frequent. Therefore, the application of LPCS is significantly limited to deposit the easy-plastic-deformable materials such as tin, zinc, copper, aluminum, and nickel. On the other hand the sprayed particle reaches much lower velocity than that in HPCS installation. The LPCS installation is much cheaper in equipment price and processing costs than the HPCS one. The powder is injected radially to the gun’s nozzle. ![]() The gas is also heated prior to arriving to the gun. LPCS ( Figure 9) operates with nitrogen or air supplied by a compressor. ![]() The heating leads to an increase of the powder particles temperature which favors plastic deformation of particles upon impact. The working gas is heated before reaching the nozzle. The powder is injected axially into the gun’s nozzle. A stream of carrier gas passes through a powder feeder and transports powder to the gun. HPCS (see Figure 9) uses usually nitrogen or helium as the working gas. The initial pressure of working gas enables categorization of cold spraying method as: (i) high pressure cold spraying, HPCS (gas pressure > 1 MPa) and, (ii) low pressure cold spraying, LPCS (gas pressure < 1 MPa). ![]() The working gases are pushed through de Laval type nozzle of the gun. The particles are accelerated by working gas, as nitrogen, helium, or air, to reach the supersonic velocities. The high kinetic energy at impact with substrate results from very high particles velocity. The FGC may be obtained by: (i) using premixed powders or (ii) co-injection of different powders. Hardness and ductility are the HVOF coatings properties which are often tailored to avoid the brittle breakup of coating’s spallation. The FGC are particularly useful in reducing thermal stresses in thick deposits. The FGC enables gradual change in coefficient of thermal expansion (CTE) and in Young’s modulus which decreases stress level and increases bond strength. This is the main reason for applying functionally graded coatings, FGC. In spite of relatively low deposition temperature, the thermal stresses are still present in the deposited coatings. Consequently, the process is characterized by high deposition efficiency, good adhesion of coatings, which have low content of oxides and low porosity. The HVOF process enables reaching higher particles velocity and lower particle temperature than APS. HVOF (high velocity oxy-fuel) and similar processes such as HVAF (high velocity air fuel) (see Figure 8) or HVSFS (high velocity suspension flame spraying) use gas combustion as energy sources for melting and accelerating powder particles. Finally, some of the future possible fields of functional thermal sprayed coatings applications are discussed, e.g., to coat polymer substrates or to use the cheap technology of low pressure cold gas spray method instead of expensive technology of vacuum plasma spraying to obtain bond coatings. These properties are useful in present applications of functionally graded coatings as thermal barriers, the bioactive coatings in prostheses, photo-catalytic coatings in water treatment, coatings used in printing industry (anilox and corona rolls). The reviewed properties of functional coatings include: (i) mechanical (adhesion, toughness, hardness) (ii) physical (porosity, thermal conductivity and diffusivity, thermal expansion, photo-catalytic activity), and (iii) bioactivity and simulated body fluid (SBF) corrosion. Then, the solid and liquid feedstocks used to spray and their properties are shortly discussed. The examples of combined spray processes as well as some examples of post spray treatment including laser and high temperature treatments or mechanical one, are described. ![]() The paper briefly describes major thermal spray techniques used to spray functionally graded coatings such as atmospheric plasma spraying, high velocity oxy-fuel spraying, suspension and solution precursor plasma spraying, and finally low and high pressure cold gas spray method. ![]()
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