Finish form grinding of thermally sprayed nano-structured coatings

Abstract

ABSTRACTThermally sprayed Molybdenum coatings have excellent high temperature strength and wear resistance. The as-sprayed components (surface roughness 5–15 µm) need surface finishing before operation. Because of high hardness and typical coating architectures, grinding is considered as the best surface finishing operation for thermally sprayed coatings. In the present work, nano-structured coatings of tin-bronze and molybdenum are deposited and characterised. Diamond reinforcement is added during ball milling to improve mechanical properties. Both conventional and ball-milled feedstock of bronze was deposited using a HVOF spraying facility. Molybdenum coatings were deposited using an Air-Plasma-Spraying facility. Coatings were ground in precision grinding facility using super-abrasive grinding wheels in plunge grinding mode. Both tangential and normal grinding forces increased in case of nano-structure coatings. Scanning electron microscopy reveals traces of ductile chip formation mode in grinding of conventional coatings. On the contrary, the nano-structured coatings revealed traces of micro-crack formation, brittle fracture and grain spallation upon grinding. An additional compressive residual stress was added to intrinsic stress level of the as-coated samples upon grinding. An increase in residual stress upon grinding is higher in case of nano-structured coatings.