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 mean values of the particle temperatures and velocities were normalized with respect to parameter set 1 and are shown in Fig. 1. For this parameter set, an average particle temperature of 3288 ± 104 K and an average particle velocity of 145 ± 3 m/s were measured. 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.
Fig. 1 Relative particle temperature and velocity depending on the argon flow rate during spraying process
The change in the coating thickness based on the investigation of the coating cross sections by optical microscopy is illustrated in Fig. 2. The coating thickness decreased from 296 ± 11 um when using parameter set 1 to 47 ± 9um for parameter set 5. The coating porosity changed from 5.7 ± 0.5% to 3.7 ± 1.2%, respectively. The porosity of the coating deposited with parameter set 5 shows a significantly higher standard deviation due to the small coating thickness.
Fig. 2 Thickness and porosity of the coatings
Figure 3 illustrates the XRD patterns of the powder blend and the coatings in the range 2θ= 20-80°. Compared to the powder blend, the coating patterns from parameter sets 1-4 exhibited two additional peaks at about 2θ = 5 3 . 8° And 2θ = 79.6°, which are related to the appearance of c-Al2O3. T h e peak intensities of both a-Al2O3 and c-Al2O3 decrease for the coatings with increasing argon flow rate. In the coating sprayed with parameter set 5, Al2O3 peaks were not observed. In the pattern of the powder blend, weak peaks of TiOx are present, which are weaker still in the coatings. In all coatings, additional peaks at about 2θ = 3 2 . 0° and 2θ = 6 4 . 0° are found, corre- sponding to the (110) and (211) peaks of rutile, respectively. It should be mentioned that the (110) peak is characteristic for stoichiometric rutile, while the (211) peak shows a high intensity for stoichiometric rutile but is present also as a weak peak in the patterns of Magne´li-phases. In addition to the influence of stoichiometry, the relative intensity of the peaks could be influenced by preferential crystalline orientation (Ref 17, 36). Thus, with the increase in the argon flow rate, the XRD patterns indicate an increasing amount of near-stoichiometric or stoichiometric rutile.
Fig. 3 XRD patterns of the powder blend and the coatings sprayed with increasing argon flow rate
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