Gas-Phase Nitridation under Kinetic Control -- A New Concept for Surface Hardening of Ti-Base Alloys

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Technical: Ti-base alloys are technologically important because of their high specific strength and resistance against corrosion in oxidizing environments. However, in many applications (aerospace engineering, biomedicine, sports, jewelry, architecture) the performance of such alloys is severely limited by their notoriously poor surface hardness and scratch resistance, very high friction coefficients and rapid wear, as well as poor corrosion resistance in reducing media. A powerful approach to counter these deficiencies is to harden the surface of Ti-alloy parts preferentially after they have been formed into their final shape. Such "case" hardening is most effectively achieved by Inward diffusion of interstitial solutes, for example nitrogen. However, titanium forms very stable compounds with all relevant interstitial solutes. Consequently, conventional methods to case-harden Ti-base alloys with interstitial solutes inevitably produce precipitates (oxides, carbides, or nitrides), which severely degrade the wear-, fatigue-, and corrosion resistance. Pioneering experiments by the PI's group have resulted in a new concept enabling inward diffusion of nitrogen into titanium base alloys from a gas phase without precipitating detrimental nitride particles. This new concept is denoted as "Nitridation under kinetic control" and has been successfully implemented in two different processing schemes, yielding precipitation-free cases with nitrogen concentrations of up to 20 at% and a tremendous increase in surface hardness. In this investigation, the PIs propose to study the surface, microstructure, and fundamental properties of these new and unusual materials. Non-Technical: The broader impacts of this work include new engineering applications of parts and components made from Ti-base alloys, new jobs related to these new applications, savings of primary energy related to the high specific strength of Ti-base alloys in some of these new applications, advances in fighting diseases by making available better materials for medicalimplants, and improvements in aircraft safety - e. g. by reducing catastrophic failureof Ti-base alloy hydraulic lines in helicopters. The results of the proposed project will also profoundly impact materials science courses at CWRU. Both PIs have a record of supporting underrepresented minorities and have had a substantial fraction of women students in their research groups. The Co-PI has a continuing effort of introducing middle school students to materials science and microscopy.