# Review of Functionally Graded Thermal Sprayed Coatings
## 3 Principal Properties of Thermal Spray Coatings and Methods of Their Characterizations
### Physical Properties  
The basic properties of coating are thickness and porosity. In general, TS technology may provide coatings having thickness of a few micrometers up to a few millimeters. Each spray pass introduces some thermal energy to the coatings structure, this results in correlations between thickness, residual stresses, and bond strength [243].  
The design of FGCs may include gradients of porosity, density, and of pores size. Such gradients are useful especially in bio-coatings manufacturing, for bone implants. The dense structure near to the interface between coating and implant assures high adherence. The porous structure in the outer part of FGC promoted the in-growth of bone [244, 245].  
The physical properties, which are important for FGC include thermal transport properties and coefficient of thermal expansion (CTE) among many others [246]. These properties are important for thermal barrier coatings (TBC). Thermal transport properties include thermal diffusivity and thermal conductivity. The TBC includes, generally, two layers with metallic bond coat (BC) and ceramic top coat (TC). The layers have different thermal transport properties (low conductivity of the top coat) and different thermal expansion (high CTE of bond coat) [247, 248]. The literature concerning TBCs is very rich and shows many data related to these two parameters [249, 250, 251].  
The electrical properties of FGC include electron field emission. This emission of electrons from coating's surface occurs under action of strong electric field. The important parameter characterizing the field emitters is the electric field enhancement factor. The TiO\({}_{2}\) emitters were sprayed using the SPS method [252]. The sprayed emitters had similar parameters to nano-diamond and diamond-like films described in [253]. The electron emission of TiO\({}_{2}\), TiO\({}_{2}\)/Al\({}_{2}\)O\({}_{3}\) coatings depends on their temperature [254, 255, 256].  
The dielectric properties of thermally sprayed ceramic coatings have been frequently tested. The electrical breakdown strength and dielectric constant of APS, SPS, and HVOF sprayed alumina were determined to be in the range 9-40 kV/mm and 5.3-8.3 respectively [257, 258, 259, 260, 261]. The breakdown strength of alumina was the highest for HVOF sprayed alumina coatings.  
Titanium dioxide is used for photocatalytic decomposition of impurities found in water. Various toxic and hardly degradable compounds are broken down owing to the hydroxyl radicals. The research on the field was done to optimize the catalyst and its deposition on supports. Consequently, TiO\({}_{2}\) coatings were deposited on different substrates. The stainless steel substrates coated using SPS were studied by [262, 263, 264]. The photocatalytic activity of sprayed coatings was determined by measuringthe degradation of an aqueous solution of dye methylene blue. The photon efficiency of titania deposits was the measure of the photocatalytic performance and varied from 0.022% up to 0.032%. Titania coatings were also sprayed using HVOF method. The coatings were used to degrade phenol in the solution by authors of [265]. Finally, HVOF and APS techniques were used to spray titania coatings tested by decomposition of acetaldehyde [266].