Study of elasto-plastic properties of microheterogeneities by means of nano-indentation measurements: Application to bearing steels

The nano-indentation technique is used to investigate the mechanical properties of microheterogeneities in bearing steels. First, it is recalled how load-displacement unloading curves recorded during a nano-indentation test can be analysed to derive the Young's modulus and hardness. The conditions of testing are then optimised and validated by testing pure aluminium and fused silica, in order to avoid the occurrence of artefacts, especially those linked with microcreep mechanisms, in the load-displacement data. Then the problem of testing microheterogeneous materials is considered. A finite element calculation is performed to estimate the deviation that is induced by the matrix surrounding the microheterogeneity, from the expected behaviour of a perfect semi-infinite solid. A simple method is proposed to minimise the corresponding error made on the Young's modulus measure. Finally, tests are carried out on non-metallic inclusions and primary carbides in AISI 52100 and M50 bearing steels, respectively. Fundamental observations reported in the literature on the role of inclusions in the fatigue properties of bearing steels are discussed by considering the hardness and Young's modulus values thus obtained by means of nano-indentation. Furthermore, the nano-indentation results provide reliable data that are required in models for predicting bearing life linked with the presence of inclusions in bearing steels.