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Coatings prepared from chromia-rich (Al,Cr) 2 O 3 solid solution (ss) feedstock powders are intended to improve the properties of Cr 2 O 3 coatings, but are rarely studied so far. In this work, the processability of a commercial (Al,Cr) 2 O 3 solid solution (ss) powder containing 78 wt.% Cr 2 O 3 by 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 x powder. For comparison, plain Cr 2 O 3 and TiO x coatings 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) 2 O 3 ss showed only a small change of the composition, and the formation of γ -Al 2 O 3 , as found for alumina-rich (Al,Cr) 2 O 3 ss powders, was avoided. Compared to the plain chromia coating, some improvements of the processability and coating properties for the ss (Al,Cr) 2 O 3 coating were found. The most balanced coating performance was achieved by blending the ss (Al,Cr) 2 O 3 with 2 wt.% TiO x , as this coating showed both a high sliding and abrasion wear resistance, in combination with a high corrosion resistance., (© The Author(s) 2021.)

High-entropy alloys (HEAs) represent an innovative development approach for new alloy systems. These materials have been found to yield promising properties, such as high strength in combination with sufficient ductility as well as high wear and corrosion resistance. Especially for alloys with a body-centered cubic (bcc) structure, advantageous surface properties have been revealed. However, typical HEA systems contain high contents of expensive or scarce elements. Consequently, applying them as coatings where their use is limited to the surface represents an exciting pathway enabling economical exploitation of their superior properties. Nevertheless, processing conditions strongly influence the resulting microstructure and phase formation, which in turn has a considerable effect on the functional properties of HEAs. In the presented study, microstructural differences between high-velocity oxygen fuel (HVOF) and high-velocity air fuel (HVAF) sprayed coatings of the alloy AlCrFeCoNi are investigated. A metastable bcc structure is formed in both coating processes. Precipitation reactions are suppressed by the rapid solidification during atomization and by the relatively low thermal input during spraying. The coating resistance to corrosive media was investigated in detail, and an improved passivation behavior was observed in the HVAF coatings., (© The Author(s) 2021.)

Thermally sprayed coatings from the single oxides and binary compositions of the Al 2 O 3 -Cr 2 O 3 -TiO 2 system 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 Al 2 O 3 , Cr 2 O 3 and TiO x powders of equal content by mass in a conventional atmospheric plasma spraying (APS) process using an Ar/H 2 plasma gas mixture. By increasing the argon flow rate at constant hydrogen flow rate, the total plasma gas flow rate and the Ar/H 2 ratio 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 TiO 2 , the coating contains only 8 wt.% Al 2 O 3 , while the Cr 2 O 3 content remained almost constant. At the same time, the change of the Ar/H 2 ratio resulted in the formation of stoichiometric TiO 2 in the coating by oxidation of TiO x in the feedstock powder. Moreover, a small content of titanium was found in the Cr 2 O 3 splats, 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., (© The Author(s) 2020.)