Deformation and strain distribution in plasma sprayed nickel-aluminum coating loaded by a spherical indenter

The deformation underneath a spherical indent was investigated on samples prepared by the bonded-interface technique from a plasma sprayed nickel-5%aluminum coating. Prior to the spherical indentation a set of Vickers micro-indents was produced as fiduciaries on one of the cross-sections. This allowed the calculation of plastic strain components from the relative displacements of micro-indents during subsequent surface indentation. Using a modified procedure based on a method proposed by Tabor [1], stress-strain curve was evaluated by measuring the contact radii of spherical indentations produced at several different loads. The derived stress-strain relationship was used in 2D and 3D finite element models to estimate the plastic deformation field under the indenter. The comparison between the measured and calculated strain profiles revealed notable differences. The normal plastic strain in the direction along the vertical axis of indentation displayed higher values directly under the indenter compared to the finite element analysis which predicted lower values under the indenter and less rapid decrease of the plastic strain with depth. The differences were attributed to the compaction of the lamellar structure in the plasma sprayed coatings. The compaction of the lamellae was directly observed on polished coating cross-sections.