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.