Morphological evolution of in situ NbC reinforcements and wear behavior of laser-surface alloyed Fe-NbC-Cu composite coatings: A Design of experiments approach

This study investigates the formation of Fe/NbC composite coatings modified with copper (0–12 wt%) using laser surface alloying (LSA). A 3-kW diode laser (350 W, 50 mm/s, 1 mm spot diameter) was employed to remelt pre-coat powder pastes on a steel substrate. Microstructural analysis (SEM/EDS, XRD) revealed that the addition of Cu induced morphological transitions in NbC reinforcements from equiaxed faceted grains to dendritic and star-like microstructures. This was driven by varying solidification rates of undercooled melt in coatings with various Cu contents. The addition of 8 wt% Cu was found to be critical for the complete elimination of surface porosity and achieving high microstructural homogeneity. Mechanical tests showed a decrease in microhardness from 800 HV to 400–450 HV with increasing Cu content. Sliding wear tests (Ball-on-Flat, Hertz Contact Stress: 1 GPa) against steel, bronze, and WC counterparts revealed that Cu changes the wear mechanism from predominant adhesive-galling to a mixed wear mode involving adhesive, abrasive, and tribo-oxidative processes (WC counterpart). Design of experiments method (DoE) was employed to evaluate the statistical significance of Cu-content and counterpart material on mean friction coefficient. The interaction plot indicates that the 8 wt% Cu-modified coating exhibits the lowest and most stable friction values among the developed coatings.