| Development of the thermal spray technology is usually accompanied with improving of the heat source. Recently, as compared with atmospheric plasma spray (APS) technique, very low pressure plasma spray (VLPPS) becomes attracting much attention. Currently, in the process of VLPPS, a high power plasma gun is adopted principally. The plasma jet is expanded, and jet velocity can reach an ultrasonic regimen at the pressure of50～100Pa. Under such conditions, the unique coating could be deposited and its applications would be more expected.The technologies for the characteristics of plasma jet and how to deposite unique coatings still have a great many unbeknown problems. It’s very significant to study the characteristics of plasma jet for manufacturing perfect coatings. In this work, distribution of pressure, temperature, velocity and Knudsen number of the plasma jet are investigated as a function of distance from nozzle exit by using enthalpy probe system under reduced chamber pressure. The results show that the temperature and velocity of plasma jet near the nozzle exit can reach10950K and2024m/s, and the Knudsen number is about1.0. This indicates the heat transfer from high temperature plasma to powder particles is decreased at exit of the nozzle. However, the internal powder feed torch we designed can remedy the decreased heating ability of plasma.Meanwhile,316L stainless steel coating and FeAl intermetallic compound coating are deposited by low pressure plasma spray, and their microstructures and properties are also investigated. The results indicate that the equiaxed316L stainless steel coatings with clear grain boundary could be deposited at optimized parameters when spraying power and substrate temperature are carefully controlled. Compared with lamellar coating deposited by APS, the equiaxed coating shows less oxides and pores, and has much lower microhardness and higher corrosion resistance. Though the melting temperature of FeAl is lower than that of316L stainless steel, but FeAl coating deposited at the same parameters as equiaxed316L stainless steel coating still exhibits lamellar microstructure. Furthermore, the FeAl coating deposited by LPPS presents special mechanical properties such as higher microhardness and elastic modulus as well as lower fracture toughness, when compared with those by atmospheric plasma spray or high velocity oxy-fuel spray. |
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