Oxide Cleaning Effect of In-Flight CuNi Droplet During Atmospheric Plasma Spraying by B Addition

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 1900 degrees C, 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.