# Review of Functionally Graded Thermal Sprayed Coatings
## 2 Spray Techniques and Feedstock Used to Deposit Functionally Graded Coatings
### Suspension Plasma Spraying (SPS)  
One of the serious limitations of conventional, powder-based, thermal spray technologies is the processing of fine powders. Small particles of low density materials are difficult to inject into high-energy jet or flame [24; 25]. This means that obtaining submicro/nano-grained coatings is also limited. In order to overcome such problem, the innovative idea of using suspensions instead of dry powders, was patented by [PERSON] et al. in Sherbrook University, Canada, in 1997 [26]. The patent describes that fine solids may be mixed with appropriate solvent (usually water or/and ethanol) and shows the chemical agents necessary to formulate a stable liquid feedstock.  
The main advantages of the suspension plasma spraying (SPS) technique, shown in Figure 1, are as follows [8]:  
* Feedstock transport from feeder to torch or gun is easy and may be realized under action of compressed gas pushing the liquid;
* Injection may be realized by atomized or continuous liquid stream (see Figure 4), creating the possibility to influence the coating's microstructure;
* Fine powders may be more easily introduced to flame or jet in form of droplets;
* Solvent may provide some protection for fine particles against a direct contact with high temperature of flame or jet.  
The use of SPS is advantageous for deposition of functionally graded coatings. The process offers the possibility to design the microstructure (see Figure 5) for example:  
Figure 4: Injection of liquid using continuous stream, (**a**) and atomized stream, (**b**), after [8]. Reproduced with permission from [Surface and Coatings Technology], Elsevier, 2008.  
Figure 5: Various microstructures of YSZ coatings produced by SPS, after [27]. Reproduced with permission from [Advances in Materials Science Research], Nova, 2016.  
* Coatings may be extremely porous (even more than 50 vol.%) or very dense (1-2 vol.%) [28; 29; 30], depending on the phenomena occurring in the plasma jet (see Figure 6);
* Pores may be of nanometer, submicrometer, or even micrometer size, having various shapes and forms connected or non-connected networks [28; 29; 30; 31; 32; 33; 34; 35];
* Morphology may vary from dense and homogeneous one, through vertically cracked up to fully columnar [28; 29; 30];
* Deposits may be formed by fully molten splats, sintered particles only, or by both as so-called _two-zones structures_[36; 37];
* Roughness of coating's surface may vary in a very broad range (as reported in [35], R\({}_{\text{a}}\) values may be between 1.6 and 14.1 \(\upmu\)m);
* Thickness may be as low as a few micrometers and reach hundreds of micrometers [38; 39].  
Over the past years, many promising examples of using SPS technology in manufacturing functionally graded coatings were presented. One of the most studied topic in that field concerns bioactive coatings [40]. [PERSON] et al. studied in the Universities of Limoges and of Modena bioactive glass/hydroxyapatite (HA) functional coatings with various designs, namely: composite, duplex, and graded coatings [41; 42]. [PERSON] et al. [43] studied SPS titania-hydroxyapatite functionally graded coatings. In this design, the TiO\({}_{2}\) layer was used as a bond one on Ti alloy substrate.  
Thermal barrier coating (TBC) is another example of functionally graded coating system that can be sprayed by SPS. [PERSON] et al. [44] described the possibility of using hybrid plasma spraying concept to avoid sharp interface between oxidation/corrosion-resistant NiCoCrAJY bond coating and thermally insulating 8 YSZ top coating. The hybrid spraying consisted of the manufacturing of bond coating by APS and the top one by SPS. [PERSON] et al. [45] proposed the concept of graded double ceramic insulation layer in which 8 YSZ and La\({}_{2}\)Zr\({}_{2}\)O\({}_{7}\) coatings were deposited by SPS.