The chromia coatings were sprayed on mild steel samples using the METCO 9MB gun and 9MC spray system. At first, the steel substrates were ground and ultrasonically cleaned. Grit blasting was performed on one of the surfaces prior to the coating. This was followed by the deposition of the bond coat. Nickel-aluminum was used for this purpose. Finally, the top coat was deposited over the bond coat. Table 1 lists the spray parameters used for the preparation of the coating. The SOD and CPSP values were varied uniformly to study their effect on the coating properties.
Table 1 Spray parameters.
Figure 1 shows the SEM images of the cross section of the chromia coatings. It reveals that for a low CPSP the porosity is high and the partially melted region(PMR) is small; at a recommended value of the CPSP these defects are diminished; and at higher values of the CPSP the PMR grows while the porosity decreases.The melting point of chromia is approximately 2,435℃. At a high value of the CPSP, the plasma is aggressive in nature.A fraction of ceramic particles, chiefly the particles with the smallest diameter, was unable to penetrate the plasma and pass through the neighboring trajectory. These particles absorb a portion of the plasma heat and form a partially melted region, as shown in Fig.1. Partially melted regions appear when the particle dwell time in the plasma is insufficient for a group of ceramic particles to melt. Similarly, at smaller SODs, the porosity is low but the PMR is high, and as the SOD increases, the PMR reduces but the porosity increases. This is attributed to the coarse and wide distribution of the chromia particles, which contributes to non-uniform melting during the plasma spraying process.
Fig.1 SEM images of cross section of chromia coating at 1000x for listed parametric variations.
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