Figure 1 shows the cross-section of Zn-Al alloys coatings by arc spraying.The thickness of the coating surface every samples is in a range of 500 to 1000 um.It was observed that the sample that uses less spraying distance to have smaller size of splat layer than the sample with a more spraying distance at the same current.Similar observation was reported by Djerourou et al .
Figure 1. Optical micrographs of Zn-Al coating is sprayed with different current and spraying distance at 100x magnification (a)100 A,100 mm,(b)100 A,150 mm,(c)100A,200 mm,(d)150A,100 mm,(e)150A,150mm,(f)150A,200mm,(g)200A,100mm,(h)200A,150mm,(i)200A,200mm,(j)250A,100 mm,(k)250A,150 mm, and(1)250A,200 mm.
The surface roughness of the substrate and Zn-Al coating were measured as Ra=10.47 um and Ra=8.79 um, respectively. It can be seen that the surface roughness of the coating is less than that of substrate. This is similar to Tailor et al who confirmed that the surface roughness of the substrate and Zn-Al coating were in the same range as reported in this research. The microhardness of the as-sprayed specimens were also measured. From the figure 2, it can be seen that the sample that is sprayed with current 250 A at spraying distance 100 mm has the highest hardness (45.18±1.95 HV), while the sample that is sprayed with current 100 A at spraying distance 200 mm has the lowest hardness (34.86±3.50HV). It was observed that the sample that uses higher current tends to have a higher hardness than the sample with a lower electric current at the same spraying distance. The sample that uses shorter spraying distance has a higher hardness than the sample with a longer spraying distance at the sameelectric current. Increasing spraying distance has been known to cause more porosity fraction in thecoating, which reduces the hardness. This observation is confirmed by almost all samples in thisexperiment. However, some results indicated that hardness increased with more porosity fraction (100A,100 mm) and decreased with lower porosity(150 A-200 A,200 mm). There might be other contributing factors to the hardness that needs further investigation.
Figure 2 The average hardness of various coating samples.
Figure 3 The average %porosity of various coating samples.
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