Microstructure of nanometer Al₂O₃ dispersion strengthened Ni-based high-temperature protective coatings by laser cladding

The nanometer Al₂O₃ dispersion strengthened NiCoCrAlY high-temperature protective coatings by crosscurrent CO₂ laser on Ni-based superalloy GH4033 were produced. Microscopic morphologies, phase constitutions of cladding coatings and distribution of nano-Al₂O₃ particles were examined using SEM and XRD. The results show that the interface grains, after adding proper nano-Al₂O₃, grow from epitaxial to non-epitaxial shape gradually, and the columnar dendrites become thinner and denser with cellular shape. Cracks in the substrate close to the interface are eliminated. Moreover, dispersive nano-Al₂O₃ particles mainly distribute around cellular substructure and on grain-boundaries, which prevents the diffusion of alloying elements and restrains the formation of new phase. There is a critical value of nano-Al₂O₃ addition, and the most suitable content of nano-Al₂O₃ is 1% (mass fraction) in this experimental conditions. The "nanometer effect" of nano-Al₂O₃ particles plays an important role in the improvement of coating microstructure.