Study of Cold-Sprayed Coatings on IN-738 Superalloy

IN-738 nickel-base superalloy has been widely utilized for fabricating hot-section components, e.g., turbine blades, in the industrial gas turbines. The purpose of this study was to investigate the cold spray process and the coating properties of IN-738 powder on an IN-738 substrate. During recent years, the laser cladding process has been used in power plants to restore damaged turbine blades and has proven to be cost effective. However, the drawback of such a process is that the turbine blades need to be heated to high temperatures; in addition, the process is complicated and time consuming. Satisfactory coatings can be obtained on a workpiece by using the cold spray process, which is a fast process to produce low-porosity coatings.
Experimental results revealed that IN-738 powder could be sprayed on an IN-738 substrate using nitrogen carrier gas at 780~830℃. Metallographic analysis revealed that the coatings had low coating porosity (<5%) and good adhesion to the substrate. In addition, SEM examinations revealed the formation of an adiabatic shear strain zone at the outer layer of the powder particles. The adiabatic shear strain zone is the main mechanism that governs the bonding of the powder particles as well as the coatings to the substrate. TEM observations confirmed that the coatings had a nanograin zone and a coarser grain zone. The diffraction patterns revealed that the nanograin zone occurred at the outer layer of the powder particles and that the coarser grain zone occurred at the inner portion. These results implied that the difference in plastic deformation between the outer and inner portions of the powder particles was substantial. After a heat treatment of 1205℃/10 min (similar to a brazing process), the formation of irregular shaped γ΄ precipitates (~200 nm) the absence of nanograins in the coatings were resulted. Moreover, the coarsening of γ΄ at inter-particle regions was obvious due to fast surface diffusion. When the coatings subjected to a two-step treatment (1180°C/2 h + 850℃/16 h), γ΄ precipitates of either cubical (200 nm edge length) or spherical particles (50 nm diameter) were observed. Such microstructral features were similar to those of the IN-738 substrate after the same heat treatment. The inter-particle regions, which had higher contents of Ta and Nb, were likely to be the nucleation sites for carbides. Additionally, continuous γ΄ precipitates were formed along the inter-particle regions. X-ray diffraction (XRD) analysis was also performed on the raw powder as well as the as-sprayed coatings. The results indicated that the γ peaks obtained for the as-sprayed coatings shifted to lower angles and also broaden relative to the peaks obtained for the raw powder. This phenomenon could be attributed to the presence of fine grains, high dislocation density and increased oxygen content in the coatings. After heat treatment of the coatings or the raw powder, the formation of γ΄ lowered the lattice parameter of γ owing to reduced Al and Ti concentrations in the matrix, i.e., the peaks moving toward the higher angles. A comparison of the coatings after different heat treatments revealed that the diffraction peaks were nearly unchanged. This could be due to the fact that the rate of γ΄ precipitation was fast and less dependent on the heat treatment. By using the cold spray process, coatings with thicknesses of 0.5 mm can be obtained. This process also has the advantage over the brazing process (using IN-738 + DF4B powder mixtures brazed at 1205℃/10 min) to eliminate the formation of brittle chromium borides in the coatings. It is clear that the cold spray process can have many new applications and can possibly replace the brazing process for restoring turbine blades, in particular, refurbishing worn surfaces of the turbine blade.