Part I: Characterization of Cr3C2-25% NiCr reactive plasma sprayed coatings produced at different pressures

The present work was performed with the aim of characterizing various plasma sprayed Cr3C2-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 Cr3C2/Ni-Cr were fcc Ni-Cr and orthorhombic Cr3C2. In contrast to the original powder, the coatings showed evidence of Cr3C2, Ni-Cr and either Cr6.2C3.5N0.3 or Cr3C1.52N0.48 carbo-nitride phases depending on the ratio of C/N in the coating. The presence of Cr7C3 and CrO2 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 N2 cooling and the lowest value (25.2%) was found for the coatings deposited at 300 mbar for the same spraying distance but with N2 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 HV50 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. © 2001 Elsevier Science B.V. All rights reserved

Residual stresses and mechanical behavior of nickel-boron coatings

Two nickel-based coatings obtained from electroless plating baths are studied. The first one is studied as-deposited whereas the second one was subjected to a heat-treatment. Hardness and Young's modulus of coatings have been investigated by means of indentation tests. Results show that mechanical properties are homogenous in the whole thickness of the coating. On the other hand, the heat treatment leads on a significantly increase of hardness and modulus. Residual stresses in the coatings are studied by the incremental ring core method. Values of residual stresses are small, scattered and do not seem to be heat-treatment dependent. Without any modifications of residual stresses before and after heat treatment, mechanical properties evolutions were attributed to the microstructural modification of the coating and to the precipitation of hard phases