Abstract
Thermally sprayed coatings from the single oxides and binary compositions of the Al2O3-Cr2O3-TiO2 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 Al2O3, Cr2O3 and TiOx powders of equal content by mass in a conventional atmospheric plasma spraying (APS) process using an Ar/H2 plasma gas mixture. 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 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 Cr2O3 content remained almost constant. At the same time, the change of the Ar/H2 ratio resulted in the formation of stoichiometric TiO2 in the coating by oxidation of TiOx in the feedstock powder. 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 stability of the chemical composition during spraying of ternary powder blends is strongly influenced by the process conditions.
Highlights
Coatings sprayed from the single oxides of the Al2O3Cr2O3-TiO2 system and some commercially available binary compositions are widely used for many technical applications
As the stepwise increase in total plasma gas flow rate and Ar/H2 ratio originated from the change of the argon flow rate, the results are discussed in dependence of the latter
The increase in the argon flow rate from 35 to 70 l/min leads to an average particle velocity of 140% and an average particle temperature of 90% of the values for the argon flow rate of 35 l/min, respectively
Summary
Coatings sprayed from the single oxides of the Al2O3Cr2O3-TiO2 system and some commercially available binary compositions are widely used for many technical applications. These coatings are used as wear and as (sealed) corrosion-resistant coatings. Atmospheric plasma spraying (APS) using fused and crushed feedstock powders with a typical particle size in the range [15-45] lm is the most common process for manufacturing these coatings (Ref [1,2,3,4]). Each of the single oxides shows a specific material behavior during spraying, which is detrimental to the processing and/or coating properties (Ref [1,2,3,4]).
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