Fe-Cr-Si Codeposition on Nb-Si based Alloys via Halide Activated Pack Cementation to Improve Oxidation Resistance

Anderson Weslei Da Cruz,Rafaela De Oliveira Nogueira,Gilbert Silva,Nabil Chaia,Raphael Felca Glória,Geovani Rodrigues

doi:10.1590/1980-5373-mr-2020-0105

Abstract

Abstract The coating formation by Fe-Cr-Si codeposition was done in a single step via Halide Activated Pack Cementation (HAPC) on Nb-Si alloy with the aim of improving the oxidation resistance. The activator salt and temperature selections for HAPC process were obtained by thermodynamic analysis. The isothermal oxidation test results for the coated alloy at 1200 °C for 62 h showed good oxidation resistance. The mass gain was 17.5 mg/cm2. Furthermore, no cracks and spallation on the oxide layers were observed for the coated alloy after the oxidation test, being that the oxidative kinetics curve is in accordance with the parabolic rate law. On the other hand, total oxidation was observed for the uncoated alloy after 12 h at 1200 °C in the isothermal oxidation test. The mass gain was 49.27 mg/cm2 and several cracks and spallation on the oxide layers led to a severe oxidation process.

Highlights

Refractory alloys of Nb-Si system have high melting points (1934 °C for αNb5Si3), low densities (7.2 g/cm[2] for Nb5Si3) and good mechanical properties at high-temperature[1,2,3]
200 °C NH4Cl activator is more unstable than CrCl3 activator and, as temperature increases, this instability becomes more evident, justifying the previous choice of NH4Cl as the Halide Activated Pack Cementation (HAPC) process activator because of its ability to decompose more and generate the species required for the coating formation reaction
Fe-Cr-Si Codeposition via HAPC process in a single step and activated by NH4Cl was performed for the formation of a coating on Nb-Si alloy to improve oxidation resistance

Summary

Introduction

Refractory alloys of Nb-Si system have high melting points (1934 °C for αNb5Si3), low densities (7.2 g/cm[2] for Nb5Si3) and good mechanical properties at high-temperature[1,2,3] Because of these characteristics, the Nb-Si based alloys have great potential for application in the segments of aeroengines/ aerospace being that they can operate between 1200-1450 °C. In order to establish a protective oxide layer, high contents of these elements are introduced in the alloy which can irreversibly alter its mechanical properties[2,7]. In this way, surface coating is an effective way to improve

Methods

Results

Conclusion