Determination of Subsurface Hardness Gradients in Plastically Graded Materials via Surface Indentation

Graded materials with high surface hardness and ductile cores are popularly used in high performance bearing applications to resist surface wear and fatigue damage. The gradient in hardness with depth is commonly determined using micro-indentation on the cross section of the material which contains the gradation in microstructure or composition. In the current study, a novel method is proposed to predict the hardness gradient profile using solely surface indentations at a range of loads. The method does not require the graded material to be sectioned, and has practical utility in the surface treatment industry. Two case hardened steels, M-50 NiL and Pyrowear® 675, and a through-hardened M50 steel, are used as model materials to illustrate the concepts. For a material with a decreasing gradient in hardness, higher indent loads result in a lower measured hardness due to the influence of the softer subsurface layers. A power-law model is presented which relates the measured surface indentation hardness under increasing load to the subsurface gradient in hardness. It is shown that the response of the material is not influenced greatly by the absolute surface hardness value, but instead sensitive to the sharpness of the gradient in subsurface hardness beneath the indented region. The proposed approach is not specific to case hardened steels and can be used to determine the subsurface hardness gradient for any plastically graded material (PGM).