A microstructure and mechanical property investigation on thermally sprayed nanostructured ceramic coatings before and after a sintering treatment

Coatings have been deposited by air plasma spraying of alumina powders in the form of conventional particles (C), nanostructured agglomerates (N) and sintered–nanostructured agglomerates (S). Sintering alleviated the stresses introduced in the nanopowder by the manufacturing process (high energy ball milling). The coating porosity is a direct consequence of the powder melting degree, which is related to the feedstock porosity. The mechanical performance of the coatings is also closely associated with the powder melting degree. The N coatings present the highest surface roughness due to the lowest melting degree. The slightly higher hardness values of the N and S coatings, as compared to the C coatings, are attributed to the higher percentages of α-Al2O3 and the presence of nanostructure. The S coatings exhibit superior adhesion strength, relative fracture toughness and wear resistance, due to sintering consequences (intraparticle cohesion, strain relief, tough splat boundaries), random dispersion of coherent nanozones and stress dissipation at nanograin boundaries.