# Review of Functionally Graded Thermal Sprayed Coatings
## 2 Spray Techniques and Feedstock Used to Deposit Functionally Graded Coatings
### Low Pressure Cold Spraying and High Pressure Cold Spraying  
A main advantage of cold spraying is the low processing temperature compared to other thermal spray processes [103; 104]. Instead of high thermal energy, the kinetic energy of gas is used to deposit powder particles, which deform the substrate [105; 106]. The low gas temperature in cold spraying has some advantages, such as reduction of oxidation at spraying [107; 108], avoiding of phases transformations, and reducing coating porosity [109; 110]. The compressive stresses resulting from peening effect at impinging of solid particles enable deposition of many different metals and alloys [111]. The high kinetic energy at impact with substrate results from very high particles velocity. The particles are accelerated by working gas, as nitrogen, helium, or air, to reach the supersonic velocities. The working gases are pushed through de Laval type nozzle of the gun. 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). HPCS (see Figure 9) uses usually nitrogen or helium as the working gas. A stream of carrier gas passes through a powder feeder and transports powder to the gun. The powder is injected axially into the gun's nozzle. The working gas is heated before reaching the nozzle. The heating leads to an increase of the powder particles temperature which favors plastic deformation of particles upon impact. LPCS (Figure 9) operates with nitrogen or air supplied by a compressor. The gas is also heated prior to arriving to the gun. The powder is injected radially to the gun's nozzle. The LPCS installation is much cheaper in equipment price and processing costs than the HPCS one [112]. On the other hand the sprayed particle reaches much lower velocity than that in HPCS installation. Therefore, the application of LPCS is significantly limited to deposit the easy-plastic-deformable materials such as tin, zinc, copper, aluminum, and nickel. The deposition of composites as the cernets is also possible and becomes increasingly frequent [113]. One of the current trends in cold spraying is the manufacturing of multi-coatings as FGCs in additive manufacturing [114]. The FGC obtained by cold spraying use a mixture of: (i) different metal powders; or, (ii) metal with ceramic powders. The mixtures can be prepared by (see also Figure 10): (i) mixing of powders, (ii) mechanical milling followed by agglomeration and by sintering; and, (iii) powders cladding. The mixing of powders having particle size of \(-50+5\) um is the most popular feedstock preparation method in cold spraying [115; 116]. The HPCS enables better coatings properties to be achieved. On the other hand, the LPCS is the cheapest method enabling cermet coatings to be obtained. Therefore, a lot of recent studies deal with deposition of metal matrix composites (MMC). The papers describe the characterization of Al\({}_{2}\)O\({}_{3}\) cermets with metals as Al, Cu, Zn, Sn, and Ni [117; 118; 119; 120]. The MMCs coatings can have various applications. Their frequent function is regeneration of surfaces [121], electrical conductivity [122], corrosion resistance [123], wear resistance [124], biomedical [125], etc. These coatings combine elevated wear-resistance, machinability, and reasonable thermal conductivity. A low cost of technology makes it an attractive alternative to deposition of the complex and expensive alloys. The composite powder mixtures can be used in both HPCS and LPCS. The cold sprayed coatings have useful properties. To give an example, [PERSON] et al. [126] deposited 3 mm thick Al + Al\({}_{2}\)O\({}_{3}\) coating onto steel and measured the microhardness varying from 84 HV0.3 at the interface with substrate up to 130 HV0.3 on the top of the coating (see Figure 11). Consequently, the coating is hard on its surface being ductile at its interface.