Microstructure evolution and wear behavior of compositionally graded Mo-enriched stellite 6 coatings with a Ni-based buffer layer via a novel laser cladding technique

Abstract

This study presents the development and characterization of compositionally graded Mo-containing Stellite 6 coatings deposited on P91 steel by a novel gravity-assisted laser cladding technique with Ni-based buffer layer. The coatings exhibited excellent interfacial integrity with no observable cracks or delamination throughout the gradient and buffer layers. A fine columnar dendritic microstructure was dominantly observed in both Stellite 6 and Mo-graded coatings. However, due to the underlying Ni and Inconel 718 buffer layers, the morphology changed to coarse columnar and cellular structures, respectively, as a result of changes in solidification conditions arising from differences in their thermophysical properties. X-ray diffraction analysis showed that the addition of Mo promoted the formation of M₆C-type complex carbides (M = Co, Cr, Mo) and solid-solution strengthening, which enhanced the hardness. The highest hardness value was obtained for the 6 wt.% Mo coating (S6M) with a maximum hardness of 577.1 HV, resulting in ~ 20% increase compared to monolithic Stellite 6. The room temperature reciprocating wear tests revealed no significant change in the coefficient of friction with Mo addition, but significant differences in wear loss were observed. The overall wear behavior was controlled by the combined effects of Mo-induced hardening and third-body abrasion by hard SiO₂ particles. S6M exhibited the highest wear resistance among all the compositions, with a decrease of the wear volume of ~ 16.7% compared to Stellite 6.

Graphical abstract