Influence of nanoparticle properties and laser settings on the surface features created by microscale pulsed laser remelting

Pulsed laser remelting is a manufacturing technique to alter the surface properties of materials. When the pulse is active, the intensive laser beam irradiates an area causing rapid melting. When the pulse is turned off, rapid solidification occurs as heat diffuses into the bulk from the relatively small melt zone. The technique can alter the microstructure, chemical distribution, and surface geometry simultaneously, and hence is an inexpensive and scalable alternative to creating patterned multi-material surface layers. In this study, S7 tool steel surfaces were coated with three different nanoparticles (SiC, Al2O3, and Ho2O3) and then pulsed laser remelted. During pulsed laser remelting, nanoparticles were found to either agglomerate on the surface forming clusters or mix with the molten substrate forming alloyed regions. The formation of the clusters or alloyed regions depends on nanoparticle properties and laser parameters. Nanoparticles with good wettability, solubility, and reactivity with the molten substrate form alloyed regions, whereas nanoparticles lacking these properties form clusters. Moreover, higher laser powers result in stronger fluid flow and assisted in mixing of nanoparticles with the substrate. The clusters exhibited improved bonding to the substrate compared to an as-coated layer. The alloyed regions exhibited considerable enhancements in hardness and wear resistance due to the strengthening effect of the nanoparticles well-dispersed in the S7 tool steel matrix.