a 1GPH = 3,785
L/hb 1SCFH = 0,472 L/min
Metallographic samples were prepared for the measurement of the
coating’s thickness, porosity and microhardness, as well as to the
microstructure analysis. The samples were sanded up to 2000 Mesh and
polished through 1 μm diamond paste. Nital 2 % was applied as etching
reagent. Three metallographic samples of each coating were prepared. For
each sample, 6 images were obtained at different sites of the coating,
which were treated by the Axiovision program to the porosity analysis.
The Buehler Micromet 2001 microhardness tester allowed to perform the
Knoop microhardness (HK) on the coatings. The Knoop scale was chosen to
ensure adequate microhardness measurement on the thin layers produced.
Five indentations for each sample were performed, applying load of 0.05
kgf [29]. An optical microscope (Zeiss 1500LCD) and stereomicroscope
(Zeiss Stemi 2000C) also were used to coatings analysis.
The thresholding method are used for porosity analysis, when the
amplitudes of the gray levels are sufficient to characterization
[30]. The Images were obtained through the optical microscope. In
the program used in digital imaging processing, segmentation of objects
(group of pixels) takes place. Darker shades, in this case, meaning
voids between fused particles.
The roughness (Ra) were evaluated longitudinally in 5 specimens after
spraying. The Mitutoyo Roughness SJ-410 was used at velocity of 0.2 mm/s
and at 2.5 mm median line, obtaining 10 measurements for each specimen.
The 3 points bending fatigue test was performed using the MTS Landmark
370.02, to give the SN curve in 17 specimens of each coating as well as
the substrate. The parameters applied were 30 Hz frequency, loading
ratio R = 0.1, cylinders of 5 mm diameter as supports and plunger, and
spam of 30 mm. The staircase method, according to
BS ISO 12107 standard,
was conducted to give the endurance limit. The first stress level
matching the first level of non-failure event up to 5 x
106 cycles is given. The tests proceed by decreasing
or increasing the stress level by the standard deviation of the fatigue
strength if the preceding event is failure or non-failure, respectively.
The S-N curve was achieved for each group studied.
The analysis of variance ONE WAY ANOVA (p ≤ 0.05) allowed to determine
statistical differences between groups in regard to thickness, porosity,
hardness, roughness and endurance limit.
Results and Discussions
Figure 1 shows the particles diameter as a function of the HEM time. The
microscopic powder mean diameter is 45 μm [27]. The first two hours
of milling promotes a drastic reduction of the particle’s diameter,
tending to saturation after 5 hours. The HEM promotes diameter reduction
to 13 nm after 12 hours of milling. Therefore, it can be assumed that
the nanometric powder may consist of amorphous structures and crystal
dispersions.