Al₂O₃, Cr₂O₃, and TiO₂ are most commonly used oxide materials for thermal spray coating solutions. Each oxide shows unique properties comprising behavior in the spray process, hardness, corrosion, and wear resistance. In order to exploit the different advantages, binary compositions are often used, while ternary compositions are not studied yet. Atmospheric plasma spraying (APS) of ternary compositions in the Al₂O₃-Cr₂O₃-TiO₂ system was studied using blends of plain powders with different ratios and identical spray parameters. Coatings from the plain oxides were studied for comparison. For these powder blends, different deposition rates were observed. The microstructure, roughness, porosity, hardness, and wear resistance were investigated. The formation of the splats from particles of each oxide occurs separately, without interaction between the particles. The exception are the chromium oxide splats, which contained some amounts of titanium. The predominant oxide present in each blend has a decisive influence on the properties of the coatings. While TiO_x causes a low coating porosity, the wear resistance can be increased by adding Cr₂O₃.

Coatings prepared from chromia-rich (Al,Cr)₂O₃solid solution (ss) feedstock powders are intended to improve the properties of Cr₂O₃coatings, but are rarely studied so far. In this work, the processability of a commercial (Al,Cr)₂O₃solid solution (ss) powder containing 78 wt.% Cr₂O₃by atmospheric plasma spraying (APS), the corresponding coating microstructures and properties were investigated. Possible further improvements were expected by blending with 2, 23 and 54 wt.% TiO_xpowder. For comparison, plain Cr₂O₃and TiO_xcoatings were studied as well. The microstructures were analyzed using SEM, EDS and XRD measurements. Hardness (HV0.3) was measured, as well as the dry unidirectional sliding wear resistance and the abrasion wear resistance (ASTM G65). Moreover, the corrosion and electrical insulating properties were measured. The (Al,Cr)₂O₃ss showed only a small change of the composition, and the formation ofγ-Al₂O₃, as found for alumina-rich (Al,Cr)₂O₃ss powders, was avoided. Compared to the plain chromia coating, some improvements of the processability and coating properties for the ss (Al,Cr)₂O₃coating were found. The most balanced coating performance was achieved by blending the ss (Al,Cr)₂O₃with 2 wt.% TiO_x, as this coating showed both a high sliding and abrasion wear resistance, in combination with a high corrosion resistance.

Thermally sprayed coatings from the single oxides and binary compositions of the Al₂O₃-Cr₂O₃-TiO₂system show multifunctional properties. Ternary compositions are promising for further improvement in their performance. The stability of the composition during coating formation is an important issue for blended feedstock powders in order to obtain the desired properties. This work focuses on the compositional changes of a ternary blend of Al₂O₃, Cr₂O₃and TiO_xpowders of equal content by mass in a conventional atmospheric plasma spraying (APS) process using an Ar/H₂plasma gas mixture. By increasing the argon flow rate at constant hydrogen flow rate, the total plasma gas flow rate and the Ar/H₂ratio were varied. For the highest argon flow rate, this resulted in an average particle velocity of 140% and an average particle temperature of 90% of the initial values, respectively. Coating composition and microstructure were studied by optical microscopy, SEM, including EDS analyses, and XRD. In addition, the coating hardness and electrical impedance were also measured. Differences in the “difficulty of melting factor” (DMF) and the thermal diffusivity of the three oxides appear to be responsible for the dramatic change of the coating composition with an increasing argon flow rate. For the highest argon flow rate applied, besides TiO₂, the coating contains only 8 wt.% Al₂O₃, while the Cr₂O₃content remained almost constant. At the same time, the change of the Ar/H₂ratio resulted in the formation of stoichiometric TiO₂in the coating by oxidation of TiO_xin the feedstock powder. Moreover, a small content of titanium was found in the Cr₂O₃splats, showing that there are only limited interactions between the large oxide powder particles. Thus, the study has shown that stability of the chemical composition during spraying of ternary powder blends is strongly influenced by the process conditions.

Corrosion testing with gel electrolytes gained attention in the past decade due to the advantage of almost non-destructive and in situ electrochemical measurements of bulk materials. Regarding thermal spray coatings, gel electrolytes offered the opportunity to prevent the infiltration of the typical microstructural features such as pores and microcracks. Using the example of stainless-steel AISI 316L coatings deposited by high velocity air fuel (HVAF) spraying on mild and stainless-steel substrates, the electrochemical corrosion behavior was analyzed in 3.5% NaCl electrolytes in an aqueous and gelled state. In this context, potentiodynamic polarization tests were carried out in a three-electrode corrosion cell, which was adapted for gel electrolyte testing. Gelling was realized with a technical gelatin. The characteristic corrosion values, such as open circuit potential, corrosion potential, and corrosion current density, revealed for the gelled state that the influence of the substrate material used could be eliminated and thus, the coatings itself could be characterized. In contrast, the coating specific microstructure and substrate material significantly affected the potentiodynamic polarization curve in the 3.5% NaCl aqueous electrolyte. Optical microscopy of the coating surfaces and cross-sections proved that the corrosion attack caused by aqueous electrolytes could be mimicked with the gel electrolyte.

Austenitic steels are characterized by their outstanding corrosion resistance. They are therefore suitable for a wide range of surface protection requirements. The application potential of these stainless steels is often limited by their poor wear resistance. In the field of wrought alloys, interstitial surface hardening has become established for simultaneously acting surface stresses. This approach also offers great potential for improvement in the field of coating technology. The hardening of powder feedstock materials promises an advantage in the treatment of large components and also as a repair technology. In this work, the surface hardening of AISI 316L powder and its processing by thermal spraying is presented. A partial formation of the metastable expanded austenitic phase was observed for the powder particles by low-temperature gas nitrocarburizing. The successful deposition was demonstrated by cold gas spraying. The amount of expanded austenitic phase within the coating structure strongly depends on the processing conditions. Microstructure, corrosion and wear behavior were studied. Process diagnostic methods were used to validate the results.