Nanomechanical properties of novel intermetallic coatings developed on austenitic stainless steels by siliconisation in liquid phase

Advanced nanomechanical testing has been used to evaluate mechanical properties of Ni-free Al12(Fe,Cr)3Si2 intermetallic coatings grown on the 316 LVM steel by hot dipping in a Al-12.6 at.% Si liquid alloy for various immersion times. Despite the ultrafine-grained structure of the coating (∼200 nm), the indentation size effect is more pronounced for the intermetallic coating than for the steel, which is explained by the higher geometrical necessary dislocation (GND) density of the intermetallic coating. To determine the true hardness of the coatings, the model of Nix and Gao was used. It has been shown that the hardness of the coating decreases from 6.2 GPa for the shortest time of immersion (60 s), to 3.36 GPa for the highest immersion time (600 s), which is always much higher than that for the substrate (1.82 GPa). The decrease in both hardness and GND with increasing immersion time is related to the relaxation of residual stresses, which act as a hardening factor. The net effect is an increase of the plasticity index of the coating. Young’s modulus for the intermetallic phase (146 GPa) is lower than that for the austenitic steel 316 LVM (220 GPa), which will favour the load transfer at the bone/metal interface, weakening the so-called “stress shielding effect”. Hence, the nanomechanical properties of this novel Ni-free intermetallic coating, tightly adhered to the substrate, offer a window of opportunity for orthopaedic applications.