# Review of Functionally Graded Thermal Sprayed Coatings
## 2 Spray Techniques and Feedstock Used to Deposit Functionally Graded Coatings
### Atmospheric Plasma Spraying (APS)  
There are many methods of coatings manufacturing on the field of TS. Among them, plasma spraying is one of the most frequently used, because of very wide range of applications and of reasonable cost of equipment [1]. The basic technique is atmospheric plasma spraying (APS). First patents, associated to the introduction of the method in the industry, were attributed in the beginning of 1960s [5,6]. The careful description of this method is made in [1,7]. The sketch of the APS method is shown in Figure 1 (inspired by [8]). The electrical arc is ignited between copper anode and thoriated tungsten cathode. Plasma gases are ionized, heated, and expanded forming the plasma jet. Then, the powder particles, being transported by a carrier gas, are injected to the hot stream of plasma jet, which are submitted to heating and to acceleration by drag force. Then, these particles hit into the substrate with relatively high kinetic energy and form splats. Finally, the splats solidify and build up the coating [1,7]. The fundamental process parameter is the composition of working gas composition. Two main groups of gases are used: (i) primary; and (ii) secondary ones. The primary gas is frequently argon (Ar) and sometimes nitrogen (N\({}_{2}\)). The primary gas should stabilize arc inside the torch's nozzle. The secondary gases is added to increase heat conductivity of plasma [1,9]. Other important APS parameters are as follows [10]:  
* Electrical power;
* Flow rate of the plasma gases;
* Feed-rate of used powder and its size distribution (most frequently, the particles sizes are in the range from 20 up to 90 microns);
* Relative speed velocity of plasma torch with regards to the substrate [10]. Nevertheless, there are many other parameters, which influence also the coatings structure and properties. Therefore, their optimization is very important [11; 12; 13]. The microstructure of sprayed coatings results from the phenomena occurring inside the powder particle in plasma jet (see Figure 2). The appropriate choice of spray parameters should lead to melting  
Figure 1: Schematic representation of the plasma spray deposition processes: atmospheric plasma spraying (APS) (up) and suspension plasma spraying (SPS) and solution precursor plasma spraying (SPPS)—(down) inspired by [8]. of injected particles before their impact on the substrate. However, the powders include frequently small and large particles. The small powder's particles may get molten in plasma and start to evaporate. On the other hand, the large ones may remain solid. There are many commercial plasma torches available. Some examples are as follows: (i) low-power one-cathode torches, such SG100 (Praxair, Indianapolis, IN, USA) or F4 (Oerlikon Metco, Pfaffikon, Switzerland), (ii) low-power three-cathodes, such TriplexPro-210 (Oerlikon Metco, Pfaffikon, Switzerland), (iii) high-power torches. The high-power torches include e.g., Plazjet (Praxair, Indianapolis, IN, USA), Axial-III (Northwest Mettech, Surrey, BC, Canada), WSP-500 (IPP-ASCR, Prague, Czech Republic), and Delta-Gun (GTV, Luckenbach, Germany). These torches operate with the electric power input up to 200 kW. The low-power torches operate with the power up to 80 kW. The powder feed-rate can reach in high-power torches 100 kg/h. Generally, the powder is injected radially, with injection angle ranging from 75\({}^{\circ}\) to 120\({}^{\circ}\) with regard to the torch axis. Only the Axial-III torch has injection angle of 0\({}^{\circ}\) i.e., axial injection. The injectors are mainly outside the plasma torch. Some torches, as e.g., SG-100 have a possibility to inject inside the torch. The flow-rate of the plasma gases vary from 40 up (low-power torch) up to 450 slpm (high-power plasma torch) [1; 10]. The powders sprayed by APS method are most frequently oxide ceramics, such as Al\({}_{2}\)O\({}_{3}\), TiO\({}_{2}\), Cr\({}_{2}\)O\({}_{3}\), ZrO\({}_{2}\), Y\({}_{2}\)O\({}_{3}\) as well as their alloys and their mixtures [15; 16; 17].