Different feedstock powder compositions of the alumina-chromia system were deposited on steel substrates by various methods, including conventional plasma spraying, three-anode plasma spraying, and HVOF. The powders used for plasma spraying had particle sizes of -38+10 µm and for HVOF spraying -25+5 µm and -25+10 µm. The coatings were evaluated by their microstructure, phase composition, and corrosion, wear, and electrical properties. The study shows that wear properties depend strongly on the spray process and that coatings obtained by HVOF spraying have dense structures and excellent wear behavior. Coatings produced by the three-anode plasma process, despite their higher porosity, were found to be harder than conventional plasma coatings and can be sprayed with higher feed rates. The coating properties do not appear to have a linear dependence on chromia content.

Alumina is a relatively low-cost material and easily processable by thermal spraying into wear and corrosion resistant coatings. However, thermally sprayed alumina coatings show inferior corrosion resistance versus chromia coatings, particularly in low and high pH electrolytes. Further, alumina possesses decreased mechanical properties in the as-sprayed state. In the present study, the effect of chromia addition on the properties of the plasma and HVOF sprayed alumina coatings were studied. Pure alumina powder and four different Al 2 O 3 -Cr 2 O 3 powders were prepared in two different atmospheres and produced to coatings with APS and HVOF methods. Phase composition of the powders and coatings were studied by X-ray diffraction. Electron microscopy was used for the examination of the microstructure of the powders and coatings. The mechanical properties of the coatings were studied by hardness and abrasion resistance tests. The chemical properties of the coatings were examined by corrosion exposure tests. Results indicate that with chromia addition it is possible to improve the mechanical and chemical properties of the thermally sprayed alumina coatings.

Manufacturing of MnCo 2 O 4 spinel coatings by solution precursor plasma spraying (SPPS) was studied in order to produce thin ceramic coating on a ferritic stainless steel interconnect for SOFC’s. The main purpose to use MnCo 2 O 4 coating in SOFC devices is to prevent the migration of harmful CrO 3 and Cr 2 (OH) 2 compounds to the triple phase barriers (TPB) of the cathode. In this study Mn(NO 3 ) 2 •4H 2 O and Co(NO 3 ) 2 •6H 2 O were diluted to deionized water and mixture of deionized water and ethanol at 3 M mixture rate. The solutions were sprayed on 0.5 mm thick Crofer 22 APU substrate by Sulzer Metco F4-MB plasma gun with a modified solution feeder. Microstructural characterizations for the as-sprayed coatings were done by using a field-emission scanning electron microscopy (FESEM) with SE-mode. Elemental analyses were done with energy dispersive spectroscopy (EDS) and an X-ray diffraction (XRD) was used for crystallographic studies. The coating with full equivalence of the crystallographic structure of MnCo 2 O 4 spinel was sprayed using argon-helium plasma and water based solution. Plasma gas with hydrogen as a secondary or ternary gas and ethanol based solutions caused the formation of the mixed phases of CoO and MnCo 2 O 4 . Although the microstructures of sprayed coatings were still quite porous, the influence of relevant gun and solution parameters were found in order to improve coating denseness in further studies.

In order to improve both corrosion and wear resistance of standard plasma sprayed alumina coatings chromia alloyed feedstock powders and high velocity oxygen fuel (HVOF) spraying as an alternative process were used in this work. Alumina and alumina-chromia powders with different chromia contents (27% and 50%-mass., but with different amount of (Al,Cr) 2 O 3 solid solution formed) were deposited by TopGun HVOF spraying and atmospheric plasma spraying (APS). The coatings were evaluated by optical microscopy, microhardness measurements, XRD and SEM. Abrasion wear resistance of the coatings was tested with the rubber-wheel abrasion test. Corrosion characteristics of the coatings were evaluated by exposure tests at 85 °C for 310 hours at low and high pH conditions. The amount of dissolved elements in solution, as analysed by ICP, was found to be a direct indication of the corrosion resistance of the coatings. Coatings deposited by TopGun HVOF spraying resulted in a denser microstructure, higher microhardness and significantly improved abrasion wear resistance as compared to corresponding coatings deposited by APS. For the tested conditions, corrosion resistance of both APS and HVOF-sprayed alumina coatings was low but significantly improved by chromia addition for both spray processes. However, the content of chromia addition alone does not allow to draw conclusions on the effectiveness of this measure.

Suspension plasma spraying (SPS) is regarded as a promising way to produce new coating structures with improved properties. In this study, SPS was studied as a possible manufacturing process for producing thin MnCo 2 O 4 spinel coatings for used as protective coatings in metallic interconnector plates of SOFC’s. Suspension of nanosized MnCo 2 O 4 powder and ethanol was thermally sprayed by using an F4-MB plasma gun with radial suspension feeding. The influence of spraying parameters, such as plasma gas composition, total gas flow, current and spraying distance for coating architecture was studied by using field-emission scanning electron microscopy (FESEM) and X-ray diffraction method (XRD). Spraying parameters had a strong influence on the coating structure and composition. Coating with the most homogenous structure were formed when sprayed with the low energy spraying parameters whereas high energy parameters resulted in formation of a columnar microstructure containing larger cobalt rich areas.

Manganese cobalt oxide spinel doped with Fe 2 O 3 was studied as protective coatings on alloyed metallic interconnect plates for solid oxide fuel cell applications. Chromium alloying causes problems at high operation temperatures in oxidising conditions, on the cathode side of the fuel cell. The formed chromium oxide layer tends to form a thin layer of chromium trioxide or chromium hydroxide which evaporates at certain oxygen partial pressures more easily than chromium oxide and thus poisons the cathodes active area causing the degradation of the solid oxide fuel cell. Thermal spraying is regarded as a promising way to produce dense and protective layers on top of ferritic steels to lower the degradation processes and extend the lifetime of the SOFC device. In the present work, the ceramic Co-Mn-oxide spinel coatings were produced by using the atmospheric plasma spray process. The structures and compositions of the coatings were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD) was used to study the phase structure in as-sprayed and heat treated coatings and mechanical properties were examined by using micro hardness tester. Coatings with low thickness and micro structurally dense structures were produced by using optimal deposition conditions.