PHOTOELECTRON SPECTROSCOPIC STUDIES OF THE ELECTRONIC-STRUCTURE AND BONDING IN TIC AND TIN

Titanium carbide (TiC) and titanium nitride (TiN) possess remarkable p hysical 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 bondin g phenomena that result from the inclusion of the non-metal atom withi n the metallic matrix, and these bonding phenomena should be apparent in measurements of the valence-band electronic structures of TiC and T iN. This paper explores the surface electronic structure and bonding i n TiC(100) and TiN(110) with core and valence level photoelectron spec troscopies (PES's) using X-rays (1486.6 eV) and synchrotron radiation in tbe range 28-180 eV. Intensity changes in the valence-band features are followed as a function of incident photon energy; these changes a re 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 structur es in TiC and TiN. A new resonance phenomenon near the Ti 3s edge in T iC was observed, and its possible assignment is discussed. The electro nic structure and bonding in these materials is well described by mole cular 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 mixin g for TiC as compared to TiN, consistent with the differences in the e lectronegativities of the atoms. Specifically, stronger covalent inter actions between the C 2s, 2p and the Ti 3d, 4s, 4p levels must occur t o explain tbe spectroscopic differences between TiC and TiN. In additi on, there is no evidence for an occupied TiC valence level having pred ominantly Ti character (unlike TiN), precluding the existence of direc t Ti-Ti bonding in TiC. Any such orbital overlap is significantly affe cted by the carbon atoms in the lattice.