Carbon atoms segregate in the surface region for polished AISI 440 Cstainless steel.After ion implantation,the surface carbon atoms exist in different forms.To elucidate their existence,surface structures and carbon chemical states of unimplanted,N+implanted,Ti+implanted and N+/Ti+co-implanted samples were analyzed by X-ray diffraction(XRD),transmission electron microscopy(TEM)and X-ray photoelectron spectroscopy(XPS).The results indicated that various phases form in the surface or subsurface region after ion implantation,while the surface topography of the samples remains intact.For polished unimplanted sample,besides some Fe3 C phase and CC phase,CrxCyphase dominates its surface region.Little change of carbon chemical states occurs after N+ion implantation.For Ti+implanted sample,besides some metal oxycarbide phases,most carbon amorphous phases form in surface region.Concerning N+/Ti+co-implantation,CrxCycompound as well as Fe3 C phase dominates the surface region while no C-C phase is found.In addition,compared with single-ion implantation,N+/Ti+co-implantation would increase the ion implantation depth significantly.The formed phases of the carbon atoms play an important role in affecting the surface properties of AISI 440 Cstainless steel. Carbon atoms segregate in the surface region for polished AISI 440 Cstainless steel. After ion implantation, the surface carbon atoms exist in different forms. To elucidate their existence, surface structures and carbon chemical states of unimplanted, N + implanted, Ti + implanted and N + / Ti + co-implanted samples were analyzed by X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The results indicate that various phases form in the surface or subsurface region after ion implantation , while the surface topography of the samples remains intact. For polished unimplanted samples, besides some Fe3 C phase and CC phase, CrxCyphase dominates its surface region. Little change of carbon chemical states occurs after N + ion implantation. For Ti + implanted sample, besides some metal oxycarbide phases, most carbon amorphous phases form in the surface region. Concerning N + / Ti + co-implantation, CrxCycompound as well as Fe3C phase dominates the surface region while no CC phase is found. In addition, compared with single-ion implantation, N + / Ti + co-implantation would increase the implantation rate of AISI 440 Cstainless steel.