ICMCTF2009 Session D3-2: Carbon and Nitrogen-Containing Nanostructured Composite and Nanolaminated Films

The electronic structures of epitaxially grown films of MAX-phases were investigated by soft X-ray emission spectroscopy. Specifically, the symmetry characteristics in the interior of these nanolaminate carbide and nitride compounds have been revealed¹. The bulk-sensitive soft X-ray emission technique is shown to be particularly useful for obtaining detailed electronic structure information about internal monolayers and interfaces². A weak covalent Ti-Al bond is manifested by a pronounced shoulder in the Ti L-emission of Ti3AlC2, Ti2AlC and Ti2AlN. When Al is replaced by Si or Ge, the shoulder disappears. Furthermore, the spectral shapes of Al, Si and Ge in the MAX-phases are strongly modified in comparison to the corresponding pure elements³. Measurements on V2GeC via V L2,3, C K, Ge M1 and Ge M2,3 spectra, including polarization variation in the excitation process, permits detailed decomposition of the valence electronic structure and chemical bonding in the interior of the materials. The macroscopic properties of the V2GeC nanolaminate result from the chemical bonds with the anisotropic pattern as shown in this work. The measured X-ray emission spectra are compared and interpreted with ab initio density-functional theory including core-to-valence dipole matrix elements. The calculated results are found to yield consistent spectral functions to the experimental data. By varying the constituting elements, a change of the electron population is achieved causing a change of covalent bonding between the laminated layers, which enables control of the macroscopic properties of the material.

¹Anisotropy in the electronic structure of V2GeC investigated by soft x-ray emission spectroscopy and first-principles theory, M. Magnuson, O. Wilhelmsson, M. Mattesini, S. Li, R. Ahuja, O. Eriksson, H. Högberg, L. Hultman and U. Jansson; Phys. Rev. B 78, 035117 (2008).

²Bonding mechanism in the nitrides Ti2AlN and TiN: An experimental and theoretical investigation, M. Magnuson, M. Mattesini, S. Li, C. Höglund, M. Beckers, L. Hultman and O. Eriksson; Phys. Rev. B., 76, 195127 (2007).

³Electronic structure investigation of Ti3AlC2, Ti3SiC2, and Ti3GeC2 by soft-X-ray emission spectroscopy, M. Magnuson, J. -P. Palmquist, M. Mattesini, S. Li, R. Ahuja, O. Eriksson, J. Emmerlich, O. Wilhelmsson, P. Eklund, H. Högberg, L. Hultman and U. Jansson; Phys. Rev. B 72, 245101 (2005).

The MAX phases are a class of nanolaminate materials with a unique combination of ceramic and metallic properties. MAX phases mimic ceramics in that they are stiff, resistant to oxidation, and remain strong at temperatures exceeding 1400 °C. The metal like properties of MAX phases manifest themselves in their machinability, resistance to thermal shock, high damage tolerance, and electrical and thermal conductivity. This unique combination of properties suggests them as structural materials for demanding operating environments.

M_n+1AX_n phases are composed of three elements: an early transition metal (M), a main group element (A), and either carbon or nitrogen (X). They are characterised by a nanolaminate structure in which slabs of the binary carbide/nitride (MX) are separated by single atomic layers of the main group element.

Experimentally prepared MAX phases however contain as impurities elements not in the MAX phase formula, with oxygen and hydrogen being the most common. In this work we use first principles density functional theory calculations to investigate the effects of these impurities on the MAX phases elastic properties. Elastic properties are of interest for MAX phases as they underpin macroscopic properties such as lubrication, friction, and machinability, which are important for structural applications. We calculate the elastic constants of MAX phases with an increasing impurity content, taking into account the effect of the location of the impurities in the MAX phase lattice. We also investigate the mobility of these impurities within the MAX phase structure.

The MAX phase materials are a class of nanolaminated three-element compounds that have been found to possess an unusual combination of ceramic and metallic material properties. Currently, the mechanisms involved in the growth of these materials as thin films are not well understood. An interlayer between the substrate and the MAX phase film is commonly used to promote oriented growth. However, several reports have been published of successful MAX phase growth without the use of an interlayer. The orientation of the substrate has also been found to be of critical importance, both for nucleating MAX phase growth, and in affecting its orientation.

We present a systematic investigation of the substrate influence on the growth of MAX phases in our pulsed cathodic arc deposition system. X-ray diffraction has been used to determine phases present and their orientation, and cross-sectional transmission electron microscopy has been used to give further information on the film growth modes and the resulting film microstructure. Deposition by pulsed cathodic vacuum arc has the advantage that the arc plasma is entirely ionized, allowing the energy of the deposited species at the substrate to be controlled through the manipulation of the substrate bias. We have used this capability to investigate the relationship between the energy of the incident ions and the minimum temperature at which the MAX structure nucleates.

Our previous work has shown that the Ti₂AlC MAX phase structure is capable of accommodating substantial oxygen impurities. We have used X-ray absorption near edge spectrosopy (XANES) to examine oxygen bonding in order to positively identify the impurity locations within the MAX structure, and here we present results from this investigation.

TiX(X=C or N)/TiAl multilayers of various modulation wavelengths were deposited at room temperature onto Si(100) substrates or Al₂O₃(001) either by Ion Beam Assisted Deposition or Magnetron sputtering. The microstructural modifications induced by thermal annealing under vacuum (600-900°C) as well as those induced by ion-irradiation were studied by the use of several characterization techniques (X-Ray diffraction (XRD), High Resolution Transmission Electron Microscopy (HRTEM), Energy Filtered Transmission Electron Microscopy imaging and X-Ray Photoelectron Spectroscopy (XPS) experiments).

In the case of TiN/TiAl multilayers the formation of a MAX Phase was achieved during the thermal annealing so that a (Ti,Al)N/Ti ₂AlN multilayer structure was obtained (Whereas only (Ti,Al)C was obtained for the TiC/TiAl system). XRD and HRTEM characterization of the irradiated samples revealed the very strong resistance under ion-irradiation of TiX (X=C or N) but also of the MAX Phase Ti₂AlN.