Ultrahigh Current Density Nitrogen Ion Implantation-- Effects on the Microstructure and High Cycle Fatigue Properties

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One of the most promising modern processes for surface modification is ion implantation. In order to achieve low cost ion implantation, a unique broad beam (10 cm.dia.) ultrahigh current density (up to 1500 Micro A/Square cm compared to conventional 6 Micro A/Square cm) ion implanter has been developed at the Colorado State University. The Principal Investigator will study the effects of high current density ion implantation. High temperatures are reached at high current densities, but the short implantation time produced no change in the bulk (beneath implanted zone) hardness or bulk microstructure in blocks of 1018 martensitic steel and many other alloys following high current density ion implantation. The PI will investigate the following aspects of high current density ion implantation: a) Permissible current densities for steels based on the time and temperature of implantation and the alloy content of steels, b) Comparison of the hard nitride phases formed due to nitrogen ion implantation in alpha-iron at low and high current densities by electron metallography, and c) Comparison of high cycle fatigue resistance of alpha-iron subjected to conventional low current density (10 Micro A/Square cm) and the CSU high current density (1500 Micro A/Square cm) nitrogen ion implantation. The PI has observed a four fold increase in the range of nitrogen ions at high current densities, resulting in a greater retardation of surface slip. Slip retardation would delay crack initiation in high cycle fatigue and increase fatigue life.