Carburization of steel surfaces during implantation of Ti ions at high fluences

Abstract

Auger and nuclear backscattering analyses of ion-implanted steels have shown that surfaces implanted with carbide-forming ions can develop excessively high carbon concentrations well into the implanted layer. A model is presented here in which the residual gases in the vacuum chamber interact with the surface during implantation to carburize the surface. Briefly, (1) implanted Ti ions are ‘‘uncovered’’ by sputtering, (2) they react with residual gases to form surface carbide species, (3) the carbides dissociate, and (4) the carbon atoms diffuse inwards. Composition profiles of steels implanted with Ti ions (5–50×1016/cm2 at 55 and 190 keV) are analyzed and shown to support the model. Carburization occurs at a lower fluence for 55-keV implants than for 190-keV implants, consistent with a nearly energy-independent sputtering yield for Ti into Fe. Implications of the vacuum carburization effects on surface alloy chemistry and mechanical properties are also discussed.