Electronic states and mechanical properties in transition metal nitrides

Thin films of hard materials are of prime importance for wear-resistant, pr otective and decorative coatings. Besides adhesion, hardness is the most of ten quoted requirement, even if doubts remain on the experimental determina tion of the hardness values of thin films, on their theoretical interpretat ion and on their significance for wear protection. Transition metal interst itial compounds are extensively used because of their broad range of functi onal properties in the fields of machining, microelectronics, decoration, e tc. This article presents a summary of recent relevant results on the struc tural, mechanical, electronic and optical properties of fee TIN, VN, CrN, N bN, W2N, hexagonal MoN, and some ternary nitrides in the form of sputtered thin films. The process parameters, e.g. the reactive gas partial pressure and the substrate bias, strongly influence the film properties. The composi tion and growth parameters influence the morphology, the stress state and o ther physical properties. The systematic investigation of the electronic density of states in valence and core states of comparable nitrides provides indications of the degree of covalency in the chemical bonding in relation to properties such as cohe sive energy and hardness. For example, in molybdenum nitride the low stabil ity of the cubic MoN phase is related to an increase in the charge transfer of Mo d electrons to nitrogen with increasing stoichiometric ratio N/Mo. Ellipsometric measurements of the dielectric function interpreted in relati on to details of the band structure measure the variation of the density of conduction electrons. Vacancies and interstitials remove or add a specific number of electrons at the Fermi level. This analysis allows one to differ entiate the types of defect at various compositions in, for example, TiNgam ma films, for which the reported hardness values spread over a wide range. (C) 1999 Elsevier Science S.A. All rights reserved.