# Review of Functionally Graded Thermal Sprayed Coatings
## 2 Spray Techniques and Feedstock Used to Deposit Functionally Graded Coatings
### Feedback Used in Thermal Spray Processes  
The progress in TS technology and the establishment of new deposition methods resulted in an intensive development in feedstock used to spray. The micrometric powders are frequently used in thermal spraying. The powders have chemistry, morphology, and size distribution tailored to spray method and to the application of coating [163; 132]. Currently, the powders are made of different materials such as metals and alloys including intermetallic alloys, ceramics, polymers, and their composites. The market of powders is open and there are companies or R&D centers, where the desired powders may be produced on demand. The physico-chemical properties of powders, including e.g., melting point, crystallinity, heat conductivity, coefficient of thermal expansion or specific result from their chemical composition. However, the production route significantly influences the powder characteristic such as particle morphology, size distribution, and apparent density [164]. The most important powder manufacturing processes are [1; 165]: (i) atomization (gas or water); (ii) fusion or sintering followed by crushing; (iii) agglomeration by spray-drying; (iv) cladding; and (v) mechanical alloying. The selection of powder material suitable for a specific coating requires understanding of the application, customer requirements, and environment at coating's service. Furthermore, the feedstock should be appropriate for the specific thermal spray technology. The smallest size of powder particle sizes is generally and for all spray techniques about 10 to 15 \(\upmu\)m [164]. The greatest size depends a lot on the coating application. The value of 90 \(\upmu\)m is a typical one used for APS and the value of 30 \(\upmu\)m--for LPS. The powders used in other spray techniques have the greatest sizes between these two values [164]. Generally, it can be assumed that spherical, free of internal porosity, chemically homogeneous, and having narrow powder particle distribution are the preferred ones for all TS technologies [165]. The latest developments in micrometric powders for thermal spray technology concern:  
* New production methods, like small and flexible atomization plants where low amounts of completely new, often expensive, and high-quality powders may be produced [165; 166; 167];
* Powders with complex chemistry and controlled phase composition or even crystallinity, like amorphous metallic glasses [168; 169; 170], spinel powders [171], etc. The introduction of nanotechnologies, observed starting from late 1990s resulted in the interest in the deposition of finely grained coatings [8; 27]. The powders enabling deposition of such coatings can be prepared by [27; 48; 172]:  
* Pre-agglomeration of very fine particles (see Figure 12) into micrometric powders and then careful control of spray parameters in order to melt the powder outer surface only to preserve fine-grained structure of the particles core;
* The use of a new class of liquid feedstocks, namely suspensions and solution, to change the interaction between the hot gases and feedstock in the way to enable obtaining finely grained coatings. A suspension is a homogeneous mixture of fine solid particles, solvent, and some chemical agents [27; 75; 173]. The powders are usually sub-micrometric or nano-metric in size. They may be prepared by a _top-down_ approach such as mechanical milling of micrometric powders. Another possibility is the _bottom-up_ way consisting in a synthesis from precursors having desired chemical composition. The content of solid in suspension used starts with a few wt.% but, for industrial applications the used concentration reaches 50-70 wt.%. The solvent plays a similar role as a carrier gas in conventional TS but may also prevent powder particles against high temperature. Water, alcohol, and their mixtures are mainly used. Finally, some chemical agents are added in order to formulate a stable suspension of relatively lower viscosity. The various flocculating agents are used for this purpose.