Study of sub-surface ion-implanted hardened layers with depth-sensing indentation

Ion implantation is an effective technique for designing structural materials for different industry applications. It may be used to improve various properties of steels such as corrosion resistance, heat resistance, wear resistance, hardness, etc. Based on their origin injected ions may serve as a coating or they may form a new phases such as carbides or nitrides that drastically change the material properties. In this work ferritic martenstic steel was irradiated with Ti and N ions followed with the heat treatment process. It is expected that it would cause the formation of the titanium nitride or titanium carbide precipitates in the sub-surface region. Sample preparation procedure included radiation by Titanium and Nitrogen ions with subsequent thermal processing of the specimens. Implantation of Ti+ ions with the energy up to 4.8 MeV was carried out using Metal Vapor Vacuum Arc (MEVVA) ion source followed by N+ ions irradiation with the energy up to 1.4 MeV using duoplasmatron at the dose of approximately 3x1015 ions/cm2 for each type of ions. After irradiation procedure the part of the specimens was annealed at 350 ºC for 1 hour under high vacuum conditions. The roadmap for sample series preparation is given on Figure 1. The hardening effect of sub-surface ion-implanted layers was studied using instrumented indentation with load-partial-unload protocol combined with the mapping over the regular spatial XYZ grid. Range for Z axis (i.e. indentation depth) was selected so that it covers the predicted maximum depth of structural defects accumulated in the material due to ions concentration. The resulting volumetric map (tomograph) of hardness and elastic modulus showed distribution of mechanical properties in the sub-surface space of the material. The mechanical parameters values measured over the tomograph were correlated to structural studies by atom probe microscopy and TEM techniques. Correlation between depth of sub-layer and grain structure of the steel alloy is also investigated. The application of Nix-Gao strain gradient plasticity in terms of modeling the mechanical properties of structurally deformed ion-implanted sub-layers is discussed