Estudo do processo de revestimento da superliga Mar-M246 utilizando o método de cementação em caixa ativado por haletos (HAPC)

Abstract

The advancement of technology has been the main reason for the production of new structural materials combining excellent mechanical properties, low manufacturing cost and applications in diverse environments. Among the most advanced materials, nickel-based superalloys are the most prominent in applications involving high temperatures. For these applications, an appropriate balance of properties is required, such as high mechanical strength, high creep resistance, high fatigue resistance, high thermal conductivity, low thermal expansion anisotropy and high oxidation resistance. Among these alloys are MarM246 and Mar-M247, which have Al and Cr in their composition, responsible for the formation of Al2O3 and Cr2O3 oxide scale, useful of increasing resistance to oxidation and corrosion at high temperatures. However, the use of these superalloys for a long period of time can cause the fragmentation of these oxide scales or the evaporation of the Cr2O3, interfering with the integrality of the material. One way to increase the useful life of these alloys is to coating them with scales that are highly resistant to oxidation without interfering with the properties of the substrate. The Halide Activated Pack Cementation (HAPC) process is a very versatile, low-cost method used to coating many materials, regardless of their geometry. In view of this scenario, the objective of this work was to study the deposition of aluminum, one of the most used in the protection against oxidation and corrosion, for the formation of coatings by the HAPC process on the Mar-M246 nickel superalloy, which until now has not been reported in the open literature. The aluminization process was carried out at four different temperatures, using NH4Cl as an activator and a powders mixture containing pure Al and alumina. A thermodynamic study, with the aid of the HSC Chemistry 6.0 software, contributed to the choice of temperatures and activator, in addition to obtaining an aluminum deposition mechanism in the formation of the coating phase for the process at 1000°C. The results showed, for all temperatures, coating without cracks, pores or adhesion failures to the substrate and a layers with Ni2Al3 and/or NiAl3 in chemical composition. The growth of the coating was evaluated by the growth kinetics in processes from 1 to 16 h, obtaining the information of a parabolic growth and activation energy of 96.55 kJ.mol-1, for the process of aluminization via HAPC, where these coatings were characterized by scanning electron microscopy (SEM). All coatings formed in a period of 9 h, at all temperatures studied, were characterized by optical microscopy (MO), SEM, dispersive energy spectroscopy (EDS) and X-ray diffractometry (XRD), showing a layer thickness between 90 to 300 μm. These coated substrates were introduced in an oxidation test at 1000°C for 240 hours, revealing an optimization in the oxidation resistance by the formation of the Al2O3 oxide layer, revealing a reduction in mass gain around 3.4 times for the layers formed in the HAPC 900 and 1000°C processes.