Improving the toughness of thermally sprayed Cr ₃C ₂-NiCr hardmetal coatings by laser post-treatment

Thermally sprayed hardmetal coatings typically exhibit a pronounced embrittlement of the metallic binder matrix due to carbide dissolution during the deposition process. This characteristic renders the coatings prone to wear by brittle fracture, which lays at the core of many relevant wear phenomena. The present work introduces laser post-treatments as a suitable means of "curing" the microstructure of high-velocity oxygen-fuel sprayed Cr ₃C ₂-NiCr coatings from this spray-process induced deterioration. While operating well below the remelting threshold, the essential impact of the laser-generated heat flux is precipitation of secondary chromium-carbides from the supersaturated binder matrix. The concomitant transition from a solid-solution to a precipitation-hardened phase significantly increases the fracture toughness of the binder matrix and renders the coatings more resistant against mechanical wear. In the present work, the microstructural modifications of the coatings upon laser post-treatments were investigated by means of scanning electron microscopy and microhardness probing, and the corresponding impact on the abrasive wear resistance was tested under both high- and low-stress conditions. Major improvements of the high-stress abrasive wear resistance by up to a factor of three were determined and discussed in the context of the microstructure of the wear scars.