Air plasma spraying

White BD, Kesler O, and Rose L. Air plasma spray processing and electrochemical characterization of SOFC composite cathodes. J. Power Sources 2008 178 334—343. 

The substrate material was TC4(Ti-6Al-4V), and the sprayed powders were 8wt%Y203 partially stabilized Zr02 and Ni-4.5Co-20Cr-4Al-lY alloy, the Zr02/NiCoCrAlY graded coating was fabricated by air plasma spraying process . ... 

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... 

At an industrial applications, the TBC coverings can be produced by thermal spraying method in the air plasma spray (APS) atmosphere, at lowered pressure low pressure plasma spray (EPPS) from APS or by the electron beam physical vapor deposition method (EB-PVD), these are all dry-route processes. By these processes, coatings have different microstructures lamellar microstructure consisting... 

Figure 6 (A) Cross-section of a TBC deposited by air plasma spraying (APS). The plate-like structure is evident in the coating shown by the inset figure, the bond coat is NiCoCrAlY fabricated by low pressure plasma spraying (LPPS) (B) Cross-section of a TBC deposited by electron beam physical vapom deposition (EBPVD) on a Pt-enriched y+y bond coat. Note the coliunnar microstructure of the coating.

Table 1 lists the wear coatings which have been under evaluation to this point in the program. Wear testing has been completed for almost all the air plasma sprayed (APS) chromium oxides, some of the high velocity oxy-fuel (HVOF) thermal sprayed materials and the slurry-sprayed silica-chromia-alumina (SCA). Microstructural characterization has been completed for all the coatings which have been wear tested. 

Extensive efforts have been made to produce ultra-hard boron carbide coatings using low pressure and inert gas shrouded air plasma spray techniques. A wide variety of test conditions were utilized (Table 2), but no deposition occurred. 

Figure 8.13 A cross-section of a thermal barrier coating fabricated by air plasma spraying (APS).

Fig. 1.6. Scanning electron micrographs showing the microstructures of plasma-spray coating of NiCrAlY on a 1020 steel substrate, (a) Air plasma spray coated layer with inter-splat cracks whose origin can be traced to the oxidation of A1 in the coated material during deposition. (6) Vacuum plasma sprayed coating of the same material without inter-splat microcracks. Reproduced with permission from Alcala et al. (2001).)...

For applications requiring higher density coatings with a strong adhesion to the substrate, low-pressure plasma spray is employed where spraying is done in an inert-gas container operating at a reduced pressure. Vacuum plasma spray is another thermal spray process which is used to improve purity of the deposited material and to reduce porosity and defect content, albeit at a higher cost than air plasma spray. 

Figure 1. Fracture surface of air plasma sprayed TBC with lamellar microstructure. The spraying splats consist of sub micron grains. Crack shaped defects exist between and within the spraying splats (inter and intra splat cracks).

Tang F, Schoenung JM. Evolution of Young s modulus of air plasma sprayed yittia-stabilized zirconia in thermally cycled thermal barrier coatings. Scripta Materialia. 2006 54 1587-1592. 

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