Figure 8 shows the Bode plot of the 140 MXC
coating after heat treatment. A signi#cant increase
in the impedance modulus was observed in the
samples under heat treatment when compared
to the untreated coating. In turn, an increase in
the impedance of the system was observed after
isothermal oxidation, which can be related to
thermal degradation after the onset of oxides.
After a 4-h exposure at 800ºC, the results describe
some variations; for example, overlapping the
curve at 1000ºC during 24 h at low frequencies.
Nonetheless, it was expected that under hightemperature
treatment over long periods of time,
an increase in the concentration of oxides would
be observed. However, this tendency was not
observed. "e XRD spectra in Figure 9 shows
that at 800°C, there was the highest concentration
of oxides, such as chromium oxide (Cr2O3) with
rhombohedral crystal structure, manganese oxide
(MnO) with a face-centered cubic structure, and
molybdenum oxide (MoO3) with a monoclinic
structure, which may act as a thermal barrier.
Samples subjected to treatment at 1000°C for 4
and 24 h presented ferrous oxides, which are not