Based on the output of the two-level factorial design, it was found that the stand-off distance has a strong effect on the coating properties. For this reason, this section will particularly focus on the analysis of how the response variables are affected by varying spray distances in more discrete intervals.
Consistent with the first set of experiments, all the coatings retained the c- and b-phases, and no presence of oxides and/or secondary phases was observed in the XRD patterns. However, the cross-sectional microstructure of the NiCoCrAlY coatings presents several differences that can be associated with the variation of stand-off distances (Fig. 1). Depending on the distance, the substrate temperature, the residence time, and the characteristics of the powder particles in the plume differ. This results in different velocities and temperatures of the particles when impacting the substrate. For instance, the coatings deposited at 150 mm show high porosity and oxides content (around 13% quantified by image analysis), with some visible surface cracking, scarcely present semi-molten particles. At this spraying distance, the highest substrate temperature (410 ℃) is registered. Short spray distances allow the particles to reach the substrate with high temperature and velocity conditions. Here, the microstructure of the coatings is the result of having predominantly overheated particles that may cause splashing (enlarged flattening), fragmentation or cracking due to quenching resulting in high levels of porosity and oxidation. Debonding of splats by the impact of particles at high velocity as well as splat oxidation in its surface during deposition is also possible scenarios. The latter may occur because of the high heat input from the flame. The coatings sprayed at 250 and 300 mm (Fig. 1b and c) have low porosity, a homogeneous microstructure, and few partially-molten particles. This conditions show lower substrate temperatures 270 and 272.5 ℃ at 250 and 300 mm, respectively. At these spray distances, the particles seem to reach the optimum velocity and temperature at the moment of impingement onto the surface, resulting in a balance of flattening, impact, and solidification. At longer spray distances (350 mm, Fig. 1d), the coatings exhibit a large amount of partially molten particles and pores, which are associated with re-solidified or cooled particles, and the concomitant formation of oxides from interaction with entrained air in the surroundings. Also, the substrate temperature decreased slightly at this spray distance, 205 ℃. The porosity at 350 mm does not approach the levels observed in samples coated at 150 mm, and reaches a value of 4.8%.
Fig. 1 Cross section images of as-deposited NiCoCrAlY coatings at spray distances of (a) 150 mm, (b) 250 mm, (c) 300 mm, and (d) 350 mm
In general, as the distance increases porosity and oxidation decreases given that coating formation is optimized with reduced splashing, reaching nominal particle impact velocities, and substrate temperatures (see Fig. 2a). At the largest spray distance (350 mm), some particles re-solidify before reaching the substrate resulting in a coating with a high amount of partially molten or un-molten particles, which necessarily implies a slight increment in the porosity and oxide content.
Fig. 2 (a) Porosity-oxide content and (b) deposition efficiency as a function of spray distance
This can be observed in Fig. 2(a) where the porosity and oxidation decreases as the spray distance increases, but a slight increment at longer stand-off distances (350 mm) is observed.
Splat oxidation has been reduced at long stand-off distances because heat input from the flame to the substrate is reduced. The porosity and oxidation also increases because the residence time of the particles increases which increases the contact time with oxygen and therefore the oxide content.
For this study, since coating porosity and oxidation decreases as the stand-off distance increases, from 150 to 300 mm, oxidation and porosity can be mainly attributed to splat oxidation and re-solidification and/or cooling of the particles prior to impact. Moreover, at longer distances, 350 mm, the oxidation process also arises during in-flight.
Relatively high deposition efficiencies are obtained for all spray distances tested in this work. However, a decrease, from 78 to 62%, is observed as the spray distance increases from 150 to 300 mm (Fig. 2b). This decrease is associated mainly to the re-solidification of the feedstock material.
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