Abstract:The oxidation during plasma spraying of metal coatings in the ambient atmosphere always occurs. The resultant oxide inclusion not only reduces the wettability of molten droplets to splat surface and subsequently inhibits the lamellar bonding formation, but also degrades the performance of the coatings. It is still great challenge to effectively reduce the oxide contents in atmospheric plasma sprayed metal coatings by exploring novel approaches. Accordingly, we revently proposed a novel approach to achieve oxide-free molten metal droplets in atmospheric plasma spraying by utilizing sacrificial oxidation of boron in the in-flight molten droplet following by subsequent evaporation for high performance metal coating deposition. Therefore, in this study, boron containing NiCrCu spray powders were designed to examine the effect of powder composition and spray parameters on the oxide inclusion in the NiCrCu coatings and to clarify the sacrifical oxidation effect of boron during in-flight of molten droplet. Two powders with nominal compositions of NiCrCu1.5B and NiCrCu4B were used for atmospheric plasma spraying (APS) in a particle size range of 30~50 μm. The in-flight particle temperature during plasma spraying was measured by commercial particle velocity and temperature diagnosis system DPV-2000. The microstructure of the coating was characterized by scanning electron microscopy and X-ray diffraction. The contents of oxygen and boron in the coating were measured by chemical approach using Nitrogen and Oxygen Determinator and Ion Coupled Plasma Mass Spectropy, respectively. The porosity of the coatings was estimated by image analyzing. The microVickers hardness and adhesive strength of the coatings were measured. The measurement of spraying molten particle temperature yielded the values higher than 2000℃ , which fulfilled the requirement to evaporate boron oxide. The results showed that the porosity of the coatings decreased with the increase of the arc power and the oxygen content in the coatings decreased with the increase of the torch traverse speed. However, the change of the oxygen content in the NiCrCu4B coatings plasma-sprayed with higher boron content in spray powder against the spray distance presented the trend that always decreased with the increase of the spray distance. This trend was opposite to those observed for thermal spraying of metal coatings. Taking the fact that the boron content in the coating always decreased with the increase of spray distance, the results confirmed that the oxidation of boron and subsequently the evaporation continuously occurred and the inflight oxidation of alloying elements was suppressed by the preferential sacrificial oxidation of boron in the molten droplets. Thus, the oxide inclusion in the NiCrCu4B coatings was mainly introduced by post-deposition oxidation. On the other hand, the results showed that the oxygen content in the NiCrCu1.5B coatings decreased with the increase of the spray distance up to spray distance of 110 mm and then turned to increase with the further increase of the spray distance. Taking account of boron content in the in-flight NiCrCu particle, it could be concluded that there existed a critical boron content of about 0.6 wt.% in the in-flight particle below which the boron could not provide sufficient protection of other alloying elements from oxidation. When the oxide inclusion was mainly resulted from post-deposition oxidation, the oxygen content in the coating was dependent on cooling condition of coating during deposition. It was confirmed that the oxygen content in the NiCrCu4B coating samples was significantly reduced through enhancing the cooling effect from the back of the substrate. As a result, NiCrCuB coating with an oxygen content of 0.6 wt.% was deposited. The measurement yielded the adhesive strength of about 40 MPa for NiCrCu coating which was influenced little by spray conditions. It was also found that the boron content changed with spray conditions which influenced the consume of boron during in-flight. The residual boron was present in the coating as dispersed borides by which the NiCrCu coating is strengthened. As a result, it was found that with the increase of boron content in the coating from 0.2 wt.% to 3.5 wt.% the hardness of NiCrCu coating was increased linearly from 280 HV0.3 to 700 HV0.3.