Effects of Physical Vapor Deposition on the Microstructure of Atmospheric Laminar Plasma-sprayed Mo Coatings

Relevant studies based on atmospheric plasma spraying have proven that laminar plasma jets have the characteristics of a high length, low velocity, and high energy density, which can effectively heat particles by prolonging their dwell time in the plasma jet. Previous studies have shown that increasing the particle temperature can effectively improve the interlayer bonding rate. During laminar plasma spraying, the particle and substrate temperatures can be smoothly improved by prolonging the particle dwell time and heating the substrate in situ. However, the coating still exhibits a low bonding rate and contains numerous pores. Therefore, studying the deposition mechanism of laminar plasma-sprayed coatings is critical. In this study, the deposition mechanism of a laminar plasma-sprayed Mo coating with a high melting point is analyzed, and the general deposition behaviors of metal and ceramic coatings are deduced by analogy. Further, the structures of the Mo coatings under three spraying parameters are characterized and analyzed using scanning electron microscopy. The main variables of the three spraying parameters are the spray, distance, and powder feeding rates. The tests reveal that coatings with different structures are obtained under the three spray parameters. When the spray distance is short and powder feeding rate is low, numerous protrusions arise on the surface of the coating and several pores are observed inside the coating. When the spray distance is short and powder feeding rate is high, the surface of the coating is flat and the internal bonding is good. When the spray distance is short and powder feeding rate is low, the surface of the coating is flat; however, numerous unbound interfaces appeares inside the coating. During the spraying process, the substrate temperature can reach up to 650 celcius, thus indicating that the spray distance controls the degree of in-situ heating of the substrate by the plasma jet (substrate temperature and vapor phase content in plasma jet). The presence of numerous fluffy structures on the coating surface and inside the coating indicates that the powder feeding rate controls the average heat input of the particles (temperature and evaporation of the particles). Therefore, during the spraying process, the evaporation of molybdenum oxide from the surface of the particles leads to a large composition of the gas phase in the plasma jet. Under the in situ heating effect of the plasma jet and high-temperature particles on the substrate, the molybdenum oxide vapor can adhere to and deposit on the coating surface during or after the plasma jet sweeps the substrate, thus affecting the subsequent deposition of the molybdenum particles and changing the microstructure of the coatings. The structure of the coating is related primarily to the evaporation of the molybdenum particles; a higher substrate temperature and stronger evaporation is more likely to result in the coating exhibiting a porous island structure, whereas a weaker particle evaporation is more likely to result in the coating exhibiting a layered structure. Therefore, to obtain a dense metal coating with a low oxidation by laminar plasma spraying, low powder feeding rate and short spray distance must be ensured. Our results suggest that laminar plasma spraying can yield coatings with a low oxidation content and high density by changing the spraying parameters. Thus, the study demonstrates that laminar plasma spraying has the potential to achieve atmospheric physical vapor deposition in an atmospheric environment.