Synthesis and nanoindentation study of high-velocity oxygen fuel thermal-sprayed nanocrystalline and near-nanocrystalline Ni coatings

In the present work, the nanoindentation technique was used to study the behavior of nanocrystalline Ni coatings. Two different types of Ni coatings were synthesized. One of the coatings was prepared with a commercial-grade Ni powder (as received, near-nanocrystalline), and the second coating was sprayed with the same powder, after having been mechanically milled in liquid nitrogen for 15 hours (nanocrystalline). Identical high-velocity oxygen fuel (HVOF) spray parameters were used for both types of coatings. The oxide-phase content in each coating was analyzed. The microstructure and properties of the milled powders and as-sprayed coatings were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and nanoindentation. The average grain size of the as-received powder was 140±52 nm, and that of the as milled powders was 15.7±5.1 nm. The near-nanocrystalline coating microstructure was composed of grains with an average grain size of 280±39 nm, and the nanocrystalline coating was composed of nanocrystalline grains with an average grain size of 92±41 nm. The nanoindentation technique was applied to characterize the coating hardness under different penetration depths. The indentation size effect (or ISE) has been observed and correlated to the microstructure of the coatings. The results show that the assumption of geometrically necessary dislocations was valid for this study. A critical indentation depth was identified for measuring the intrinsic properties of the constituent material of the coating (≲500 nm).