MATERIALS SCIENCE AND ENGINEERING

SURFACE CARBURIZATION OF TI-BASE ALLOYS

Titanium and its alloys are used for aerospace applications, in corrosive environments, and biomedical implants because of their special properties, which include a high ratio of strength to weight, high corrosion resistance, and good biocompatibility. However, Ti-base alloys usually have poor surface mechanical properties: They possess high coefficients of friction, exhibit poor wear resistance, and are susceptible to galling. Much research has been dedicated to surface hardening of these materials, e.g. by depositing surface layers by techniques like PVD and CVD (physical and chemical vapor deposition) as well as by inward diffusion of interstitial solutes (nitrogen, carbon, boron).

Both conventional approaches inevitably cause the presence of second phases and corresponding discontinuities in mechanical and electrochemical properties. Why this improves e.g. the wear resistance, it degrades other mechanical properties (e.g. fatigue resistance) as well as the corrosion resistance.

A promising technique for solving this problem is inward diffusion of interstitial solutes under conditions that do not need to precipitation of second phases (nitrides, carbides, borides), but retain the corresponding solute in homogeneous solid solution.

Recent work has demonstrated the feasibility of this approach for surface nitridation of Ti and Ti–6Al–4V by new method denoted as "gas-phase nitridation under kinetic control" (Fig. 1) [1,2]. Another, particularly potent interstitial hardener is carbon. However, the equilibrium solubility of carbon in titanium and its alloys is very small. In addition to avoiding the formation of carbides, therefore, effective surface carburization must also overcome the equilibrium solubility limit. The possibility for such a non-equilibrium supersaturation with interstitial solutes has recently been demonstrated austenitic stainless steels [3-6]. In these materials, surface properties have been successfully improved by carburizing from a gas phase at low temperatures. Choosing the processing temperatures such that carbon is highly mobile while all other atomic species are effectively immobile was found to introduce a colossal supersaturation of interstitially dissolved carbon without the formation of carbides. Correspondingly, a significant improvement in all surface mechanical properties (including fatigue resistance) was accomplished, as well as a significant (and unexpected) improvement in corrosion resistance.

The purpose of ongoing research on titanium and its alloys is to explore the possibility of courting the concept of colossal supersaturation with interstitial carbon to this class of structural alloys. The experimental approach will combine elements of nitridation under kinetic control and paraequilibrium gas-phase carburization.

This material is based upon work supported by the Defense Advanced Research Projects Agency (DARPA). Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the DARPA.

Fig. 1. Surface hardness of Ti–6Al–4V before and after surface nitridation.