Patterning and fusion of alumina particles on S7 tool steel by pulsed laser processing

The performance of many engineering instruments is directly influenced by the instrument's subparts and their surface properties due to the frequent occurrence of interacting, mating, and sliding surfaces. Due to these effects, significant work is on-going studying the unique tribological properties that multi-scale surface patterns can induce. When a micro-sized pulsed laser is used to irradiate metallic samples, fluid forces in the melt pool cause flows due to surface tension and Marangoni flow. These induced flows can be used to form multi-scale surface patterns. If the metallic melt pool contains individual, non-dissolving constituents, these flows can also cause those constituents within the melt pool to separate and multi-material surface patterns to form. In this work, the formation of multi-material surface patterns through pulsed laser remelting of S7 tool steel that has been dip-coated with alumina nanoparticles through the sol-gel process is demonstrated. Various characterization techniques are used to quantify the materials and mechanical properties of those laser-induced multi -material patterns. Within the melt zone, martensite, carbide particles, and retained austenite appear after rapid cooling, resulting in increased hardness. Additionally, the melt zone and the heat affected zone is back tempered by consecutive laser pulses, causing a nonuniform hardness across the individual laser patterns. The flows within the melt pool cause the alumina nanoparticles to concentrate at the edge of melt zone, forming a pattern higher than the surface outside of the melt zone. Those alumina patterns possess a denser structure, higher hardness, and better cohesion to the base compared to the unprocessed dip-coated layer. Therefore, pulsed laser processing is a promising method to produce consolidated patterns on the S7 tool steel surface with oxide coatings such as alumina. The method is flexible and scalable and could be used to obtain various multi-material surface patterns inexpensively.