Model to determine the depth of a diffusion layer by normal indentations to the surface

Diffusion in steels may result of one or several elements entering or leaving the material in the vicinity of the outer surface, facilitated by the presence of a temperature gradient. As a consequence, the diffusion process produces a diffusion layer which in turn generates a gradient of the mechanical properties of such layer. Generally, the characterization of the treated material requires the preparation of cross-sections in order to obtain the hardness–depth profile by indentation and to study the evolution of the microstructure across them. This procedure usually needs long time of preparation, is often tedious, and leads to a permanent damage of the sample. In the present work, we propose a different method of analysis, i.e. performing the indentations perpendicularly to the surface of the material without the need of cross-section preparation. To construct the hardness–depth profile, we propose a model using an exponential law which is based on the hemispherical expansion of the plastic zone deformation generated during the indentation process under the residual indent. From the application of the model, we defined a criterion representing the depth of the diffusion zone. The hardness–depth profile model and the depth-diffusion criterion are both validated for a steel decarburised under 4 situations aiming to a carbon content at the surface ranging between 0.1 and 0.4% respectively from an initial content of 0.55% C in the starting material.