# Review of Functionally Graded Thermal Sprayed Coatings
## 5 Perspectives of Development of Functionally Graded Coatings
### New Applications of FGC  
The number of applications of functionally graded coatings is still growing. In this section only a few examples are briefly described. The development of new materials and new deposition methods enables predicting many new implementation of FGC systems.  
Figure 14: SEM-BSE micrographs of RSW aluminum—steel joint with Ni + Al\({}_{2}\)O\({}_{3}\) interlayer (**a,b**). 1—aluminum alloy, 2—Ni + Al\({}_{2}\)O\({}_{3}\) interlayer, 3—steel substrate [353].  
Presently, the bond coats of TBC are usually deposited using expensive vacuum plasma spraying (VPS). This spray technique can be replaced by cold spray and by HVOF spraying [379; 380; 381]. The emerging technology on this field is cold spraying.  
[PERSON] and [PERSON] [380] tested cold spraying bond coats in the TBC for isothermal oxidation behavior. The authors confirmed the usefulness of cold spraying for bond coat manufacturing. [PERSON] and [PERSON] [381] found that bond coat manufactured by cold spraying has satisfactory tribological properties and high oxidation resistance in high temperatures. [PERSON] et al. [382] presented cold sprayed coatings using Cr\({}_{2}\)AlC powder for TBC bond coat.  
Another example of bond coat deposition by cold spraying was a FGC system deposited onto polymer substrate for biomedical applications [383]. The authors describe the bond coat of titanium produced on the biocompatible PEEK substrate. The nanostructured TiO\({}_{2}\) crystallized, as anatase was a top coat.  
The new idea of metal-doped HA thermally sprayed coatings was studied to improve antibacterial resistance of bone prostheses, dental implants, and macroporous scaffolds [384]. [PERSON] et al. [183] produced HA doped with Zn coatings by SPPS method. The tests carried out revealed the coatings bioactivity and their efficiency against some bacteria. The enhanced antibacterial properties of plasma sprayed HA coatings doped with Sr and Zn showed the reduction of cytotoxic effect of Zn\({}^{2+}\) ions by addition of Sr\({}^{2+}\) as observed [PERSON] et al. [385]. Copper can be a useful dopant to the HA improving bactericidal properties sprayed coatings. The characterization of Cu- and Ag-doped HA coatings obtained by APS was carried out by [PERSON] et al. [386]. The HA with Cu cermet obtained by SSPS technique was characterized by [PERSON] et al. [184; 387]. The improvement of biomedical properties of plasma sprayed HA coatings doped with Mg or with Mg and Sr were found in [388; 389]. Another popular dopant of HA coatings is silver. The Ag addition to HA coatings improves bactericidal capacities and decreases the risk of infections after surgery. This type of biomedical coatings were obtained by many thermal spray methods such as vacuum plasma spraying, flame spraying, radio frequency plasma spraying, and suspension plasma spraying [390; 391; 392; 393; 394]. The new generation of biomaterials uses magnesium alloys as a substrate on which HA coatings doped with niobium were deposited [395].  
To finish, let us have a look on an interesting application of high-pressure cold sprayed alumina composites with aluminum. The coatings were characterized by low thermal conductivity, low solar radiation absorption, comparatively high infrared emittance, and oxidation stability which rendered them useful for application in outer space [396].