Changes in the Coating Composition Due to APS Process Conditions for Al2O3-Cr2O3-TiO2 Ternary Powder Blends

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 TiO_x 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 TiO_x 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.

Keywords: ceramic coating; microstructure; plasma spraying; spaying process analysis; ternary composition.

Fig. 1

Relative particle temperature and velocity depending on the argon flow rate during spraying process

Fig. 2

Thickness and porosity of the coatings

Fig. 3

XRD patterns of the powder blend and the coatings sprayed with increasing argon flow rate

Fig. 4

Mass content of crystalline oxide components and the proportion of α-Al₂O₃

Fig. 5

SEM images of (a) powder blend and coatings produced with different argon flow rate: (b) 35 l/min, (c) 40 l/min, (d) 50 l/min, (e) 60 l/min and (f) 70 l/min

Fig. 6

Average chemical composition of coatings and powder blend determined by EDS analysis

Fig. 7

EDS point measurements: (a) different grayscales in SEM image of coating sprayed with parameter set 2, (b) chemical composition of splats with different grayscale (dark gray—Al₂O₃, medium gray areas—TiO_x/TiO₂, light gray—Cr₂O₃) in the coating sprayed with 40 l/min argon flow rate

Fig. 8

Hardness HV0.3 of the coatings (HV0.05 for 70 l/min argon flow rate)

Fig. 9

The impedance (squares) and DC resistivity (lines) of the coatings