Indentation size effect in nickel and cobalt laser clad coatings

In this work it is reported a comparative study of the indentation response at different length scales in nickel and cobalt samples produced by laser cladding. The indentation tests were carried out in the load range 57 μN to 2 N. Both in Ni and Co, the indentation hardness increases as the indentation dimension decreases from the micrometer to the nanometer range. It is shown that the experimental results can be accurately described by an analytical function, whose terms account for the contributions of dislocation density and surface free energy to the material's hardness. Although the good relation between the model and the experimental points is not a fully evidence, it strongly suggests that, as the hardness scale decreases, the increase of dislocation density and the increase of the surface-to-volume ratio of the plastically deformed region are the most important causes for the observed indentation size effect.