Part I: characterization of Cr 3C 2-25% NiCr reactive plasma sprayed coatings produced at different pressures

Abstract

The present work was performed with the aim of characterizing various plasma sprayed Cr 3C 2–NiCr coatings produced by using different processing pressures between 300 and 1200 mbars, in a nitrogen controlled atmosphere CAPS system. X-Ray diffraction was carried out on all coatings by using Bragg–Brentano geometry. The phases identified in the as-supplied powder Cr 3C 2/Ni–Cr were fcc Ni–Cr and orthorhombic Cr 3C 2. In contrast to the original powder, the coatings showed evidence of Cr 3C 2, Ni–Cr and either Cr 6.2C 3.5N 0.3 or Cr 3C 1.52N 0.48 carbo-nitride phases depending on the ratio of C/N in the coating. The presence of Cr 7C 3 and CrO 2 was also identified in the coatings deposited at atmospheric pressure. The volume fraction of carbide plus nitride phase in the coating was always less than the volume fractions of the carbide phase in the original feed stock powder. The volume fraction of carbide plus nitride phase was found to depend quite markedly on the spraying parameters such as pressure, power input, spraying distance and substrate cooling. The results have been presented in terms of spraying efficiency, ξ. The highest value of ξ (86.4%) was obtained for coatings produced at 1200 mbar pressure, a spraying distance of 120 mm, without N 2 cooling and the lowest value (25.2%) was found for the coatings deposited at 300 mbar for the same spraying distance but with N 2 cooling. The presence of graphitic carbon was detected in all samples especially for specimens where the fraction of carbide plus carbo-nitride phases was the greatest. The influence of spraying parameters such as pressure, power input, spraying distance and substrate cooling on the microstructure was determined. The highest microhardness value of 2296 HV 50 was found for the carbide plus carbo-nitride phase contained in the specimens produced at 1200 mbar spraying pressure, 120-mm spraying distance and substrate heating at 600°C.