Improvement of the fatigue resistance of materials with thin coatings developed from dynamic ion mixing

Abstract

A dynamic ion mixing (DIM) technique, which involves a physical vapor deposition (PVD) combined with a simultaneous ion implantation method, has been applied to elaborate adherent NiTi and SiC amorphous thin coatings. Two different metallic materials, a Ti—6Al—4V titanium alloy and a 316L type austenitic stainless steel were used as substrates. The characteristics of such deposits were established by performing scratch-tests and micro hardness measurements, and by measuring their Young's moduli. This process leads to a significant improvement of the fatigue life for both materials, at room temperature, in the low cycle fatigue range. Scanning electron microscopy observations revealed that thin coatings constitute a barrier at the surface which modifies the near surface deformation mode impeding the formation of extrusions—intrusions pairs and strain localization within intense slip bands. Consequently, crack initiation can be considerably delayed or even suppressed. Several factors controlling beneficial effects of DIM treatments on fatigue resistance are pointed out. These factors concern the nature and properties of the coatings and the cyclic deformation processes in the substrates.