# Review of Functionally Graded Thermal Sprayed Coatings
## 2 Spray Techniques and Feedstock Used to Deposit Functionally Graded Coatings
### High Velocity Oxy-Fuel and High Velocity Air-Fuel Spraying  
HVOF (high velocity oxy-fuel) and similar processes such as HVAF (high velocity air fuel) (see Figure 8) or HVSFS (high velocity suspension flame spraying) use gas combustion as energy  
Figure 7: Phenomena occurring in plasma jet during SPPS process [75]. Reproduced with permission  
sources for melting and accelerating powder particles. The HVOF process enables reaching higher particles velocity and lower particle temperature than APS. Consequently, the process is characterized by high deposition efficiency, good adhesion of coatings, which have low content of oxides and low porosity [88]. In spite of relatively low deposition temperature, the thermal stresses are still present in the deposited coatings. This is the main reason for applying functionally graded coatings, FGC [89]. The FGC enables gradual change in coefficient of thermal expansion (CTE) and in Young's modulus which decreases stress level and increases bond strength. The FGC are particularly useful in reducing thermal stresses in thick deposits [89]. Hardness and ductility are the HVOF coatings properties which are often tailored to avoid the brittle breakup of coating's spallation [90]. The FGC may be obtained by: (i) using premixed powders or (ii) co-injection of different powders [91; 92]. To show an example, [PERSON] et al. [93] used the blends of powder with different compositions to deposit three graded coatings including WC-Co/NiAl. However, the co-injection of different powders seems to be more beneficial because the spraying process is continuous and the effort of powder preparations is reduced. Different ways of simultaneous powder co-injection are feasible such as e.g., the use of: (i) commercial twin powder feeder; (ii) two separate commercial feeders; and, (iii) modification of commercial feeders by the users. For example, [PERSON] et al. [94] designed and manufactured gravimetric powder feeder based on two separate powder chambers and control valves. The authors used it to spray the mixture of steel with aluminum. [PERSON] et al. [95] used separate powder feeders having modified injecting system. The injectors were evenly spaced in the feeder and the powder was injected radially. In case of significant differences between the injected powders, it is possible to vary the location of powder injector with regard to the torch. For example, [PERSON] et al. [96] used two separate powder feeders with internal and external injection in order to adapt the spray process for the polymer and cemet powders having very different properties. Among the parameters influencing the deposition of FG, the most important are spray distance and working gases (fuel and oxygen) flow-rate [97]. The coatings quality can be controlled by the control of the particles in-flight temperature [98]. Moreover, in case of FGC it is suggested to control the powder with the lowest melting point in order to avoid excessive evaporation [99]. [PERSON] et al. [91] compared behavior of atmospheric plasma-sprayed Mo-Mo\({}_{2}\)C/stainless steel and HVOF-sprayed WC-Co/stainless steel functionally graded coatings. An improvement of wear resistance was observed for plasma sprayed coatings. This parameter was not improved for HVOF sprayed coating. However, these coating had better resistance against crack propagation. [PERSON] et al. [100] used HVOF to deposit the FGC of aluminum/tool-steel. The authors found the optimal number of layers to reach desired hardness and residual stresses level. [PERSON] et al. [93] provided a comprehensive study about Ni-5 wt.% Al/WC-12 wt.% Co functionally graded coating. The authors observed an increase in crack resistance in FGC in comparison to the standard coatings. Figure 8: Scheme of high velocity oxy-fuel spraying, inspired by [1]. Reproduced with permission from [Wiley Books], John Wiley and Sons, 2008. [PERSON] et al. [95] showed that gradual coatings design brings advantages also for bond coats.