Dynamic evolution of oxide scale on the surfaces of feed stock particles from cracking and segmenting to peel-off while cold spraying copper powder having a high oxygen content

doi:10.1016/j.jmst.2020.06.019

Abstract

Abstract:

The oxide scale present on the feedstock particles is critical for inter-particle bond formation in the cold spray (CS) coating process, therefore, oxide scale break-up is a prerequisite for clean metallic contact which greatly improves the quality of inter-particle bonding within the deposited coating. In general, a spray powder which contains a thicker oxide scale on its surface (i.e., powders having high oxygen content) requires a higher critical particle velocity for coating formation, which also lowers the deposition efficiency (DE) making the whole process a challenging task. In this work, it is reported for the first time that an artificially oxidized copper (Cu) powder containing a high oxygen content of 0.81 wt.% with a thick surface oxide scale of 0.71 μm., can help achieve an astonishing increment in DE. A transition of surficial oxide scale evolution starting with crack initiations followed by segmenting to peeling-off was observed during the high velocity particle impact of the particles, which helps in achieving an astounding increment in DE. Single-particle deposit observations revealed that the thick oxide scale peels off from most of the sprayed powder surfaces during the high-velocity impact, which leaves a clean metallic surface on the deposited particle. This makes the successive particles to bond easily and thus leads to a higher DE. Further, owning to the peeling-off of the oxide scale from the feedstock particles, very few discontinuous oxide scale segments are retained at inter-particle boundaries ensuring a high electrical conductivity within the resulting deposit. Dependency of the oxide scale threshold thickness for peeling-off during the high velocity particle impact was also investigated.

Key words: Cold spray, Deposition efficiency, Oxide scale fragmentation, Inter-particle bonding, Electrical conductivity

Xiao-Tao Luo, Yi Ge, Yingchun Xie, Yingkang Wei, Renzhong Huang, Ninshu Ma, Chidambaram Seshadri Ramachandran, Chang-Jiu Li. Dynamic evolution of oxide scale on the surfaces of feed stock particles from cracking and segmenting to peel-off while cold spraying copper powder having a high oxygen content[J]. J. Mater. Sci. Technol., 2021, 67: 105-115.

Figures/Tables 11

Table 1 Oxygen content of the gas atomized Cu feedstock powders after different pretreatments.

Powder No.	State	Oxygen content (wt.%)
1	As-received	0.09 ± 0.031
2	Ambient for 30 d in open air	0.23 ± 0.045
3	100 °C for 3.0 h in open air	0.47 ± 0.051
4	150 °C for 3.0 h in open air	0.68 ± 0.057
5	150 °C for 4.5 h in open air	0.81 ± 0.079
6	150 °C for 6.0 h in open air	0.97 ± 0.027

Fig. 1. Surface morphologies (a) and polished cross sections (b, c) of the as-received gas atomized Cu powder (No.1) and artificially oxidized gas atomized Cu powders (Nos.2-6) with increasing oxygen contents from 0.23 wt.% to 0.97 wt.%. (d) is close view of (c).

Fig. 2. XRD patterns of the as-purchased Cu powder (No.1) and artificially oxidized Cu powders (Nos. 2, 3, 4, 5, 6).

Fig. 3. Variation of the DE as a function of the oxygen content of the Cu feedstock powders with insets displaying the cross-sections the resulting deposits.

Fig. 4. Surface morphologies of the single particle deposits showing the oxide scale evolution during the high velocity impact in CS deposition; (l) is the FIB sectioned deposited single particle sprayed with powder No.6. The inset in (l) shows the trapped oxide scale segment at “south pole” of the deposited No.6 Cu particle.

Fig. 5. Cross sectional microstructures showing the oxide scale distribution in the deposits sprayed with Cu powders of different oxide scale thickness. (a-d) deposit No.1. (e-h) deposit No.2. (i, j) deposit No.3. (k, l) deposit No.4. (m, n) deposit No.5. (o) deposit No.6. The EDS line-scanning patterns in (j), (l) and (n) are taken from the across the inter-particle boundaries from as indicated by the black arrows in (i), (k) and (m), respectively. The inset in (o) is the TEM specimen made across an inter-particle boundary where evident oxide scale was not observed by SEM.

Fig. 6. The oxygen content in the deposit as a function of that in the Cu feedstock powders.

Fig. 7. A comparison of the morphologies of the deposited single particle and cross sections of the Cu coating sprayed at 470 and 607 m/s by using No.5 powder. The inset in (a) is a close view of the material jetting at the rim of the deposited particle from white frame area showing the local cracking of the oxide scale.

Fig. 8. Schematic showing the effect of oxide scale (OS) thickness on OS evolution during CS deposition.

Fig. 9. Electrical conductively of the deposits as a function of the oxygen content of the feedstock powders. The inset shows the preparation procedure of the specimen used for electrical conductivity measurement.

Fig. 10. (a, b) Microstructure schematic of the deposits with different oxide scale evolution mechanisms and (c) the simplified electrical resistance mode for one single particle along the in-plain direction. RCu is the electrical resistance of a deposited Cu particle. ROv, and ROp are electrical resistance of the oxide scale at the vertical and in-plain interfaces.

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