ICMCTF2007 Session B5-3: Properties and Characterization of Hard Coatings and Surfaces

Mechanical behaviour and stability of thin films consisting of alternating nanolayers has been the subject of a great research activity over the past decade due to the technological impact of such nanoscale systems for protective hard coatings¹. Original elastic and mechanical properties have been reported in a large variety of multilayers due to the high interface density and small-scale dimensions associated with these systems. For example, a softening of shear elastic constants or enhancement of hardness and tensile strength as compared to the rule of mixture values have been reported in metal/metal, nitride/nitride or metal/nitride systems. The present communication will focus on the elastic and plastic properties of three nanolayered systems, namely TiN/Cu, ZrN/W and Mo/Ni, corresponding to different crystal structure and lattice mismatch combinations. They were grown epitaxially on either MgO(001) or Al₂O₃(1120) substrates by a dual ion beam sputtering techniques. Low-angle and high-angle X-ray diffraction experiments as well as Transmission Electron Microscopy observations were used to characterize the microstructure and crystalline orientation, structure of interfaces and type of growth defects. Mechanical properties were studied by nanoindentation tests using a Berkovich tip and coupled with Atomic Force Microscopy surface observations around the indents. Also, a combined FIB-TEM technique was implemented to image the deformed nanolaminates beneath the indentor. The following issues will be addressed: Stress and intermixing in Mo/Ni multilayers: influence on elastic properties² and mechanical behaviour in TiN/Cu³ and ZrN/W multilayers.

¹S.A. Barnett and A. Madan, Scripta Materialia 50, 739 (2004)

²F. Martin, C. Jaouen, J. Pacaud, G. Abadias et al., Phys. Rev. B. 71, 045422 (2005)

³G. Abadias, Y. Y. Tse, A.Michel, C. Jaouen, M. Jaouen, Thin Solid Films 433, 166 (2003).

Titanium nitride films (TiN) are used amongst other applications as wear-protecting coatings and as diffusion barrier layers. Control of stress in these films enables improvement in the performance of these coatings.

TiN thin films have been deposited on 100mm Si wafers by reactive magnetron sputtering from a Ti target in an industrial coating system. During deposition (dep. rate approx. 0.1nm/s) at 5kW power, 125V bias voltage, 450°C dep. temperature and 4x10^-3mbar pressure, the samples performed a planetary motion in front of the target (target size 600x120mm²). A thin Ti layer was deposited before for better adhesion of the TiN film.

From wafer curvature before and after deposition the stress was inferred. At all thicknesses the stress was compressive, however these films show a more compressive stress with increasing film thickness. We measured 1.4GPa compressive stress for a 30nm thick film and 1.6GPa compressive stress for a 380nm thick film, both grown under identical conditions. The same tendency of thickness dependent stress has been reported by us for Cr films and associated with evolutionairy grain growth [APL 85 3086 (2004)]. Our results are in contrast to the stress thickness relation on work steel substrates reported by Kamminga [JAP 88 6332 (2000)], who found less compressive stress in thicker TiN films. We will discuss the thickness dependent stress in terms of the microstructure of the TiN film to elucidate this different stress behaviour.

This article gathers the results of a comparative study of the oxidation behaviour and the tribomechanical properties of a series of different CrAlN coatings subjected to thermal annealing cycles in an atmosferic furnace.

CrAlN coatings with different Cr-Al stoichiometry were deposited by using of a cathodic arc PVD machine. The behaviour of these coatings has been compared with the oxidation performance of nc-CrAlN/Si₃N₄ nanocomposite coating and with the more conventional TiAlN coating deposited in rotary arc PVD equipment.

Ultramicrohardness, friction and wear resistance measures were carried out before and after thermal annealing in order to study the evolution of the tribomechanical properties. The oxidation process, at different temperatures was studied by means of GDOES analysis looking for the evolution of the composition in depth for the oxygen and other elements. These profiles and the microstructure of the coatings were also investigated by FE-SEM.

As a conclusion this paper looks for the relations between the microstructure, compostion and tribological properties of the different coatings before and after the oxidation experiments.