Oxide Cleaning Effect of In-Flight CuNi Droplet During Atmospheric Plasma Spraying by B Addition
REN Yuan, DONG Xinyuan, SUN Hao, LUO Xiaotao()
State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an 710049, China
Cite this article:
REN Yuan, DONG Xinyuan, SUN Hao, LUO Xiaotao. Oxide Cleaning Effect of In-Flight CuNi Droplet During Atmospheric Plasma Spraying by B Addition. Acta Metall Sin, 2022, 58(2): 206-214.
A large amount of air is drawn into the high-temperature plasma jet during the atmospheric plasma spraying (APS) process because it operates in an atmospheric environment, thus oxidizing metal-spray particles. The oxide inclusion resulting from in-flight droplet oxidation inhibits the metallurgical bonding between lamellae in the coating, which limits the applications of plasma-sprayed metal coatings. In this study, a novel approach to create oxide-free molten droplets is proposed by adding B to the CuNi powder to achieve sacrificial oxidation of B in the high-temperature droplet and protect the alloy elements from oxidation. Two powders of CuNi2B and CuNi4B were prepared to deposit the coatings via APS. The effect of B content and spray distance on the microstructure, as well as the O content of CuNi coating, was studied using methods such as SEM, EDS, XRD, and inductively coupled plasma-optical emission spectrum (ICP-CES). The results show that the droplet can be heated to more than 1900oC, and the introduction of B in the powder can inhibit the oxidation of alloy elements in the high-temperature droplet during flight, thus reducing the oxygen in the CuNi coating. Moreover, the deoxidizing effect is affected by the B content of the droplet. Using 4%B CuNi alloy powder and increasing spray distance, the oxide in the coating is reduced. The oxygen in the coating is introduced via oxidation after droplet deposition, and the oxygen content of the coating prepared using the optimized spraying process is reduced to 0.43%, which is considerably lower than 3.5% of CuNiIn coating. An increase in the spray distance and a reduction in B content of CuNi powder, which contains 1.83%B, to 0.5% is insufficient to inhibit the oxidation of the alloying elements of the in-flight particles. The result yields a critical B content of approximately 0.5% for high-temperature droplet oxidation protection. The increase in the B content decreases the melting point, as well as the oxidation of the alloy, thus enhancing the metallurgical bonding between CuNi particles and improving the compactness of the coating. In addition, with the increase in the B content of the coating through the powder composition design and process parameters control from 0.26% to 3.61%, the microhardness of CuNi coating increases from 151 HV0.2 to 457 HV0.2.
Fig.1 SEM images of CuNiIn (a), CuNi2B (b), and CuNi4B (c) powders
Powder
Ni
In
B
O
Cu
CuNiIn
31.87
4.30
0
0.33
Bal.
CuNi2B
34.53
0
1.83
0.14
Bal.
CuNi4B
32.56
0
3.86
0.08
Bal.
Table 1 Chemical compositions of the original powders
Fig.2 Changes of the oxygen content of CuNiIn and CuNiB coatings with the increase of spray distance
Fig.3 XRD spectra of atmospheric plasma sprayed (APSed) CuNiIn (a), CuNi2B (b), and CuNi4B (c) coatings and corresponding powders
Fig.4 SEM images of APSed CuNiIn (a, d, g, j), CuNi2B (b, e, h, k), and CuNi4B (c, f, i, l) coatings deposited at spray distances of 80 mm (a-c), 100 mm (d-f), 120 mm (g-i), and 140 mm (j-l)
Fig.5 Cross-sectional microstructure of CuNiIn coating (a) and EDS oxygen line scanning result across the typical marked region (b)
Fig.6 Effect of spray distance on the apparent porosity of plasma-sprayed CuNiIn, CuNi2B, and CuNi4B coatings
Fig.7 SEM images of etched (a) and un-etched (b) cross-section of CuNi4B coating sprayed at distance of 120 mm
Fig.8 Evolution of CuNi4B (diameter 30-50 μm) particle temperature with spray distance (Tb—B2O3 boiling point)
Fig.9 Changes of B content in CuNi2B and CuNi4B coatings against spray distances
State
Melting point
oC
Thermal conductivity
W·m-1·oC-1
Density
kg·m-3
Specific heat
J·kg-1·oC-1
Solid
1273
23.45
8940
420
Liquid
1273
72.02
7940
890
Table 2 Physical parameters of CuNi4B alloy
Fig.10 Temperature evolution of the interface between the molten particle and the solid matrix (Tm—CuNi4B melting point, t—time)
Fig.11 Relationships between B content and hardness of CuNiB coating
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