Fig.1 demonstrates the XRD patterns of the Ni53Nb20Ti10Zr8Co6Cu3 atomized powders and HVAF-sprayed coatings.The powders exhibit a broad halo peak, indicating a fully amor-phous structure, whereas some crystalline and oxide phases have appeared in the as-deposited coatings. The reasons for crystallization in coatings are mainly associated with two aspects.Firstly, during the HVAF spraying, the powders were exposed to a high temperature flame, resulting in the oxidation of the pow-ders during flight and subsequent deposition. Secondly, adiabatic recalescence occurred inevitably due to successive solidification of one droplet upon another. The oxygen contents(wt.%) in the Ni53Nb20Ti10Zr8Co6Cu3 alloy coatings analyzed by a nitrogen/oxygen determinator were in the range of 3.0-3.5%, which were much higher than 0.06% in the powders. This confirms the distinct oxidation of the powders occurred during spraying.
Fig. 1. XRD patterns of the atomized powders and HVAF-sprayed coatings.
SEM images of the cross-section of the HVAF-sprayed Ni53Nb20Ti10Zr8Co6Cu3 alloy coatings are shown in Fig.2.Some limited porosities are visible as very dark contrast regions but generally the coatings have a dense structure.The methodof image analysis was employed for the measurement of porosity.The SEM images with magnification of 1000 and with at least 20 view-fields from different positions and cross-sections of the samples with surface roughness 0.08um were used for the measurement by optical microscopy（MEF-4）.The porosities of the coatings are in the range of P=（1.23±0.05%）-（1.54士0.10%）for Fig.2（a）-（d），whereas Fig.2（e）exhibits a lower porosity（P=0.54±0.06%），indicating a more dense structure obtained.Due to the HVAF technique incorporating the features of both HVOF and cold spray processes，the spraying temperature was relative low(~1373 K), and hence only some of the powders were partially melted. Thus, heat transfer would be suf-ficient with increased spraying distance. The spraying distance employed in this work is increased from 150 to 200 mm, which would lead to better heat transfer of the powders. The decrease in the velocity of the particles in this range of spraying distance seemed to be not large, and thus the most dense coating mightbe formed by deposition at the spraying distance of 200mm, as shown in Fig.2(e).
Fig. 2. SEM images of typical region from cross-section of the coatings with different powder feed rate and spraying distance of (a) 10 rpm and 150 mm, (b) 15 rpm and 150 mm, (c) 20 rpm and 150 mm, (d) 20 rpm and 175 mm, and (e) 20 rpm and 200 mm.
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