Effect of Various Heat Treatments on the Microstructure of 316L Austenitic Stainless Steel Coatings Obtained by Cold Spray.

Industries developing cold spray aim at dense and resistant coatings for component repair. However, as-sprayed 316L coatings display non-equilibrium microstructure and brittle fracture behavior. Improving their mechanical properties requires controlling their microstructure; post-spraying heat treatment is a promising approach. The recovery and recrystallization of coatings were little studied, and heat treatments reported in literature mostly used holding for long time in furnaces, not adapted to on-site repairs. This study aimed at gaining insights into recovery and recrystallization mechanisms of 316L coatings, for a broader range of heat treatment kinetics. A study of powders and as-sprayed coatings was conducted to characterize the initial state. In situ XRD measurements provided input for heat treatment definition. Microscopy, room temperature XRD and hardness measurements allowed to better understand the microstructural evolutions and to select treatments leading to original microstructures. In this work, a variety of microstructures were produced by adapting heat treatment conditions for a given set of spraying parameters. The recrystallization path of the heterogeneous skin-core microstructure of deposited particles, as well as the interaction between grain growth and precipitation was revealed. A novel, optimized fast heat treatment led to a fully recrystallized, fine-grained coating and significantly reduced hardness.

Synthesis of few-layered graphene by ion implantation of carbon in nickel thin films.

The synthesis of few-layered graphene is performed by ion implantation of carbon species in thin nickel films, followed by high temperature annealing and quenching. Although ion implantation enables a precise control of the carbon content and of the uniformity of the in-plane carbon concentration in the Ni films before annealing, we observe thickness non-uniformities in the synthesized graphene layers after high temperature annealing. These non-uniformities are probably induced by the heterogeneous distribution/topography of the graphene nucleation sites on the Ni surface. Taken altogether, our results indicate that the number of graphene layers on top of Ni films is controlled by the nucleation process on the Ni surface rather than by the carbon content in the Ni film.