Structural features of the coatings of the Ni-Al system obtained by air-plasma spraying

This article deals with the structural features of nickel aluminides coatings obtained by air-plasma spraying with the unit of annular injection of powder. The subsonic (plasma velocity is 1200 m/s) and supersonic (plasma velocity is 1900 m/s) regimes were used for coatings deposition. Ni₃Al powder (Ni is base; 13-15 wt. % Al) was applied to the pipes of low-carbon steel 20 (0.2 % C). The structure and phase composition of the coatings were investigated by optical microscopy, scanning electron microscopy, transmission electron microscopy, and X-ray phase analysis. It is established that the porosity of the coatings obtained at supersonic regimes is reduced by a factor of 2. Ni₃Al and Ni₅Al₃ are shown to be the main phases of coatings. Ni₅Al₃ phase volume fraction of the coatings obtained at supersonic regimes is revealed to be lower which reduces the microhardness of these coatings. It is shown that Ni₃Al phase has subgrain structure and Ni₅Al₃ phase has lamellar structure. (paper)