Optimisation d’un revêtement cermet Ni-Cr/Cr₃C₂ par refusion laser superficielle

Cermet Ni-Cr/Cr₃C₂ coating optimization by superficial laser remelting

¹ Laboratoire d’Automatique et de Mécanique Industrielles et Humaines, Groupe de Recherche en Génie Mécanique URA CNRS 1775, Université de Valenciennes et du Hainaut-Cambrésis, Valenciennes
² Laboratoire de Structure et Propriétés de l’Etat Solide URA CNRS 234, Université des Sciences et Technologies de Lille, Villeneuve-d’Ascq

Abstract

Plasma spraying is often used to create coatings protecting parts against physical and chemical agression or improving superficial mechanical properties. The projection of ceramic or cermet coatings involves several problems because of the high temperatures of the process : cracks, porosity, coating adhesion, residual stress.

The authors study the optimization of cermet coating Cr₃C₂/Ni-Cr (deposited on NiCrAlY covered steel) using a superficial remelting by laser. The treatment is realized under nitrogen as protection gas (pressure: 0.4 bar) with a CO₂ non focused laser beam (diameter: 0.8 mm). The variable parameters are the laser power (80-410 W), the scanning velocity (0.5-7 m/min) and the overlap rate (0-100%). The microstructural and the mechanical studies are consistent and clarify the influence of the treatment parameters.

It is shown that the cermet microstructure is refined and homogenized by the laser treatment. The cracks and porosity are reduced. There is not any segregation phenomenon.

The scanning velocity acts upon the coating hardness and leads to a hardness gradient increasing with the velocity value. The final coating thickness depends too on the velocity.

The adhesion between steel and coating is determined by interfacial indentation test; the decohesion occurs at cermet / NiCrAlY interface. A coating fracture toughness / residual stress ratio (K_1C/σ_r) is introduced. It is characteristic of the interfacial behaviour and it allows treatments grading. The residual (tensile) stress is never relaxed.

The optimum laser treatments in that study are: medium laser power (212 W), medium overlap rate (50%). The scanning velocity depends on the desired hardness or mechanical properties.