Splat boundaries

Another mode of failure would be the transformation of the surface crack to a T-shaped crack at a relatively weak fracture plane parallel to the surface. This may be microcracks forming along the oxidized splat boundaries or the interface between the thermally grown... 

Figure 5.34 compares the microstructures of coatings deposited by the novel HVSFS process (a) and conventional APS (b). The appearance of the HVSFS coatings suggests that the reason the lamellae are almost unrecognisable across the coating cross-sections and fracture surfaces is presumably a result of splat boundary sintering as shown in the circled area of panel (a). 

Figure 5.39 (a) Cross-sectional SEM image of an 80%Ti/20%HAp composite coatings, CGD-sprayed at 35 bar, 600 °C (see also Figure 5.40a). (b) Magnified image of an etched sample to reveal the splat boundary... 

The failure mechanisms of air-plasma-sprayed TBCs are somewhat different than those described for EB-PVD coatings. Figure 10.29 shows the cross-section of a degraded APS TBC, which is about to spall. Microcracks associated with splat boundaries in the region of the TBC have grown and linked up to form a large crack... 

The case study demonstrates that even the base NiCr hybrid spray coatings outperformed the alloy 625 coatings in the presence of corrosive salts. The presence of second phase particles, especially Si02, showed improved oxidation and corrosion characteristics. Incorporation of ultrafine and nano sized oxide p>articles is expected to improve the creep properties by pinning the splat boundaries and reduce the oxidation rate. Chromia addition by itself did not help improve the properties significantly. However, it could act as a... 

In the case of the Stellite-6 coating, chromium and oxygen were found to coexist along the splat boundaries in both environments studied, whereas cobalt-rich splats remained unoxidized. 

Recently, some innovative plasma-spray techniques have been employed to produce TBCs with lower thermal conductivities and more durable performance. The solution precursor plasma spray (SPPS) has been exploited to deposit TBCs, in which an aqueous chemical precursor feedstock, rather than YSZ powders, is injected into plasma jet [25,26]. The lack of large-scale splat boundaries effectively toughens the TBCs, making SPPS TBCs highly durable relative to APS TBCs. 

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