# Review of Functionally Graded Thermal Sprayed Coatings
## 2 Spray Techniques and Feedstock Used to Deposit Functionally Graded Coatings
### Solution Precursor Plasma Spraying  
The solution precursor plasma spraying (SPPS) allows manufacturing coatings with submicrometer and nanometer structures. The first research groups that used solutions for thermal spraying were from Stony Brook University, USA and Tampere University in Finland [46; 47; 48; 49; 50]. The evolution of SPPS method, starting from spray pyrolysis is described in [51]. The initial studies showing good quality SPPS coatings were reported in [52; 53]. Subsequently, many coatings for different applications were sprayed by SPPS. The examples are: thermal barrier coatings (TBC) [54; 55], coatings solid oxy-fuel cells [56; 57], biomaterial coatings [58; 59], photocatalytic coatings [60; 61], coatings for gas sensors [62; 63], environmental barrier coatings [64], and many others.  
Both techniques, SPS and SPPS use liquid feedstock. Nevertheless, in SPS method, there are some solid particles suspended in liquid solvent. As the solids have small dimensions, the probability of their agglomeration at spraying is very high (see Figure 6). Also, the sedimentation of feedstock during  
Figure 6: Phenomena occurring after injection of suspension droplet into plasma jet [14]. Reproduced with permission from [Wiley Books], John Wiley and Sons, 2012.  
storage can be a serious problem. These problems are negligible for SPPS method, which uses purely liquid feedstock. Such feedstock is suitable to obtain coatings with very fine microstructure [65; 66]. The solution precursors are easier to obtain with high purity, without contamination occurring frequently at suspension preparation [67].  
The composition of deposited coatings changes the initial feedstock [68]. This change is easy to carry out in SPPS technology. Consequently, it is possible to enlarge the area of thermal spray applications [69]. Another important technological advantage is the possibility of using conventional plasma spray equipment. Also, the solution feeder in SPPS can be the same as suspension feeder in SPS technique [70].  
The precursors used to spray are generally the mixtures of compounds, such as salts, acetates, or nitrates. The mixtures are dissolved in a solvent to form final solution. The solvent is frequently an organic liquid or/and water. It should be stressed up, that the mixing of the precursors occurs on the molecular level [71; 72]. More details about solution precursor preparation are described in next chapter.  
The schematic presentation of SPPS set-up is shown in Figure 1. There are many process parameters that influence the coatings microstructure and properties [73; 74].  
The phenomena occurring at SPPS process are shown in Figure 7. More details about these phenomena as well as interactions between droplets and plasma jet can be found in [51; 68; 75; 76; 77; 78; 79; 80].  
Recently, the hybrid processes of thermal spraying were studied. There are two possible hybrid processes which include solution precursor plasma spraying, namely: (i) APS and SPPS shown by [81]; and, (ii) SPS and SPPS [82]. These processes take a profit from the advantages of both methods for new applications.  
The SPPS method is a novel process that allows manufacturing a nanometer-scale coating, which results in many interesting functional properties impossible to reach in APS-deposited coatings. The advantages of this method are: pure feedstock with controlled chemical composition and different coating's microstructure adapted to application. The main drawback is the process complexity being more difficult to optimize than the conventional APS. The numerical studies may help in better understanding the phenomena occurring at SPPS method [77; 83; 84]. More details about the methods can be found in [51; 70; 84; 85; 86; 87].