This study assesses the potential use of thermally sprayed dicalcium diiron pentaoxide (Ca 2 Fe 2 O 5 ) for thermoelectric generators. Ca 2 Fe 2 O 5 coatings up to 2 mm thick were produced by atmospheric plasma spraying and examined. Compared to the bulk material, the coatings exhibit lower thermal and electrical conductivity. The Seebeck coefficient could not be measured, and the thermoelectric performance was inadequate. The limitations derive not only from the thermal spray process, but also the material itself.

The work presented in this paper addresses some of the challenges of manufacturing thermoelectric (TE) generators by thermal spraying. One of the main obstacles is achieving good coat-on-coat bonding between different types of materials. The coatings must also be mechanically stable and optimized for their respective function. At least four types of materials are required, including electrical insulators, conductors, and thermoelectrically active p- and n-type semiconductors. Four ceramic and three metal feedstock powers were deposited by APS, HVOF, and HVAF spraying using special masking systems, substrate pretreatments, and layer thickness monitoring. After process optimization for each material, multilayer TE generator modules were successfully produced.

Laser-assisted atmospheric plasma spraying (LAAPS) is a new one-step coating process performed in air whereby the laser beam interacts with the plasma torch at the substrate or coating surface during deposition to generate a coating that is metallurgically bonded to the substrate. This hybrid process was developed in order to combine the specific advantages of APS and laser cladding. In this paper, the development of a hybrid gun for coating internal surfaces of tubes and cylinder bores by LAAPS is presented. The process was optimized for spraying AlSi30 coatings on internal surfaces of aluminum alloy cylinder bores. Single-pass coatings with thicknesses of 300-400 µm and metallurgical bonding to the substrates can be realized by applying an optimized parameter set. The dependence of coating microstructure on spray parameters was investigated by metallographic preparation and optical microscopy. Surface pretreatment must be performed to eliminate the strongly adhering oxide layer on the aluminum alloy substrate and to attain metallurgical bonding of coating to substrate.

Laser-assisted atmospheric plasma spraying (LAAPS) is a newly developed one-step coating process in which the laser beam interacts simultaneously with the plasma torch on the substrate or deposited coating surface in order to generate the coating. LAAPS combines the advantages of plasma spraying, such as high coating rates, with those of laser cladding. In addition, there is no need for a special surface preparation method, such as grit blasting. In this paper, the principle of LAAPS is described and the use of this process for the preparation of NiCrBSi and Al 2 O 3 - 3%TiO 2 coatings is demonstrated. The coatings were characterized by optical microscopy, hardness and bond strength testing, X-ray diffraction measurements, and an oscillating sliding wear test. Coating microstructures and properties were compared to those of APS coatings. The bond strengths of LAAPS coatings were higher for NiCrBSi coatings, but they were lower for Al 2 O 3 -3%TiO 2 with a NiAl bond coat due to the very complex processes occurring in the contact region between the metallic substrate and the ceramic coating material.