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2021
Journal Article
Titel
Changes in the coating composition due to APS process conditions for Al2O3-Cr2O3-TiO2 ternary powder blends
Abstract
The stability of the composition during coating formation is an important issue for blended feedstock powders in order to obtain the desired properties. Binary blends from the ternary Al2O3-Cr2O3-TiO2 system, such as Al2O3-TiO2, are often used for commercial coating manufacturing. For discovering the potential of ternary compositions and implementation of the expected improvements, the compositional changes of a conventional atmospheric plasma spraying (APS) using an Ar/H2 gas mixture was studied. By increasing the argon flow rate at constant hydrogen flow rate, the total plasma gas flow rate and the Ar/H2 ratio were varied. For a ternary blend of Al2O3, Cr2O3 and TiOx powders of equal content by mass, changing the plasma gas flow rate and composition 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 initial composition with an increasing argon flow rate. For the highest argon flow rate applied, the coating practically does not contain Al2O3, while the Cr2O3 content decreased moderately as well. The change of the Ar/H2 ratio also caused an increase in the amount of TiO2 in the coating. Moreover, a small content of titanium was found in the Cr2O3 splats, showing that there are only limited interactions between the large oxide powder particles. Thus, the study has shown that maintenance of the chemical composition during spraying of ternary powder blends is strongly influenced by the process conditions. A proper selection of feedstock powder properties and process parameters is necessary to reach the desired coating properties.
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