# Review of Functionally Graded Thermal Sprayed Coatings
## 4 Major Applications of Functionally Graded Coatings
### Biomedical  
Millions of different types of prostheses (of hip for example) are produced annually in the world; different biomaterials and various methods of manufacturing in production. The major group of biomaterials include: (i) bioinert as titanium and its alloys; and (ii) bioactive as hydroxyapatite (HA) and bioglasses. The bioactive materials dissolve in human body, which accelerates the processes of prosthesis implantation in a bone [1; 306]. The production of hip's and knee's prostheses, dental implants, and repairing of the bones are the major fields of activity [307]. Initial studies dedicated to application of FGC in biomedical applications were made by group of Khor [308; 309; 310]. The investigations aimed in obtaining well adhering and bioresorbable coatings. The use of TiO\({}_{2}\) as intermediate coating enabled to reach the tensile adhesion strength of the HA-TiO\({}_{2}\) system as high as 50 MPa [311]. Moreover, the use to titania bond coat increased the coating durability and improved fatigue resistance. Moreover, TiO\({}_{2}\) was a barrier for metallic ions migrating from metallic substrate to human body [312; 313]. Another idea was APS deposition of titanium coatings with gradient of porosity [314]. The coating being in contact with substrate was dense; the middle one included micro- and macro-porosity and outer one was very porous to promote tissue growing. The authors [315] proposed the multi-coatings including HA-ZrO\({}_{2}\)-Ti coatings by APS. The coatings with internal Ti coatings had reportedly satisfactory mechanical properties. An important drawback of HA is its thermal decomposition at temperatures slightly below its melting point (depending of partial pressure of steam). This was the motivation to develop functionally graded structures, multi-coatings, hybrid organic-inorganic composites by using the oxides CaO-P\({}_{2}\)O\({}_{5}\)-TiO\({}_{2}\)-ZrO\({}_{2}\)[316; 317; 318]. An important issue in FGC technology is optimizing the process parameters. The researchers often use the _response surface methodology_. This approach was proposed to design the graded HA coatings which should be [319]: (i) stable for long time; and (ii) bioactive. The intensive studies on FGC composed with HA, Ti, and TiO\({}_{2}\) used statistical methods to improve coating properties [320; 321]. The frequently used thermal spray technique to manufacture biomedical coatings is APS. Other techniques were also tested. For example, [PERSON] et al. [101] used HVOF to spray HA/TiO\({}_{2}\)-graded coatings onto the Ti6 Al4V substrate reaching satisfactory behavior at SBF testing. The use of FGC of HA and bioglasses was favorably compared with the conventional plasma-sprayed HA [322]. The improvement of mechanical properties of coatings was achieved by the use of the composites of HA or bioglasses by nano-diamond or reduced graphene oxide [323; 324]. The SPS technology to spray FGC of HA and TiO\({}_{2}\) was used by the authors of studies [43; 325]. They compared two configurations of coating: duplex and gradient one. These oxides were SPS deposited with the use of peristaltic pumps to inject the suspension (see Figure 1) resulting in relatively thin coatings [326]. The functionally graded coatings composed of bioglass and HA were produced by SPS [41; 42]. The studies aimed at combining rapid osseointegration of bioglass with long term stability of HA. The obtained coatings were immersed in SBF and exhibited strong reactivity with the medium. The authors produced all coatings by exclusively SPS technology or, by using APS technology to spray HA and SPS one to spray bioglass. The suspension was also used as a feedstock in high velocity oxy-fuel spraying by [PERSON] et al. [102]. The authors manufactured multilayer HA/TiO\({}_{2}\) coatings and improved the mechanical properties of deposits such as adhesion and wear resistance with regard to pure HA coating. The high quality FGC biomedical coatings have an important potential in medical applications.