Photoelectron Spectroscopic Studies of the Electronic Structure and Bonding in TiC and TiN

Abstract

Titanium carbide (TiC) and titanium nitride (TiN) possess remarkable physical properties, such as extremely high hardness and melting point, that promote their use as antiwear materials under harsh tribological conditions. These physical properties must arise from chemical bonding phenomena that result from the inclusion of the non-metal atom within the metallic matrix, and these bonding phenomena should be apparent in measurements of the valenceband electronic structures of TiC and TiN. This paper explores the surface electronic structure and bonding in TiC(100) and TiN(110) with core and valence level photoelectron spectroscopies (PES's) using X-rays (1486.6 eV) and synchrotron radiation in the range 28-180 eV. Intensity changes in the valence-band features are followed as a function of incident photon energy; these changes are then compared to theoretical atomic photoionization cross sections to determine the atomic origins of these features. Resonant PES at the Ti 3p absorption edge is used to determine titanium 3d contributions to the valence band and to show differences in the electronic structures in TiC and TiN. A new resonance phenomenon near the Ti 3s edge in TiC was observed, and its possible assignment is discussed. The electronic structure and bonding in these materials is well described by molecular orbital theory, where the Ti and non-metal ions in their formal oxidation states (e.g., Ti4+ and C4- in TiC) undergo covalent bonding interactions. Overall, the PES results indicate greater covalent mixing for TiC as compared to TiN, consistent with the differences in the electronegativities of the atoms. Specifically, stronger covalent interactions between the C 2s, 2p and the Ti 3d, 4s, 4p levels must occur to explain the spectroscopic differences between TiC and TiN. In addition, there is no evidence for an occupied TiC valence level having predominantly Ti character (unlike TiN), precluding the existence of direct Ti-Ti bonding in TiC. Any such orbital overlap is significantly affected by the carbon atoms in the lattice. © 1994, American Chemical Society. All rights reserved.