ICMCTF2008 Session B5-1: Properties and Characterization of Hard Coatings and Surfaces

Ti_1-xAl_xN thin films can be synthesised by plasma-assisted vapour deposition in the cubic NaCl structure with AlN mole fractions (x)=0.7. At higher Al-contents the hexagonal ZnS-wurtzite modification is formed. Cubic Ti_1-xAl_xN films with highest possible Al content are preferred in industrial applications as they combine superior mechanical properties with good oxidation protection. Further improvement can be obtained by alloying reactive elements such as yttrium, which effectively retard oxide scale growth by segregation to scale grain boundaries.

Recently, we have shown that the addition of yttrium to Ti_1-xAl_xN reduces the maximum possible AlN content for single phase cubic films¹. DC magnetron sputtering of Ti_0.45Al_0.55N results in the formation of a single-phase cubic coating, whereas Ti_0.46Al_0.52Y_0.02N, having a lower Al content, contains the hexagonal ZnS-wurtzite structure phase.

Employing x-ray diffraction, scanning and transmission electron microscopy we reveal that bipolar pulsed magnetron sputtering allows the cubic stabilization of Ti_0.46Al_0.52Y_0.02N when low frequencies of 50-80 kHz but long positive pulse durations up to 5 µs are used. Consequently, as the formation of the hexagonal phase can be suppressed the hardness increases from 23.5 GPa for DC to 34.3 GPa for bipolar pulsed magnetron sputtered films. Langmuir measurements yield increased electron temperature and higher average ion fluxes for bipolar pulsed magnetron sputtering, which are related to overshoots in the negative pulse and fast MHz ringing in the positive pulse when the polarity of the voltage changes.

We can thus conclude that the different plasma conditions during bipolar pulsed compared to DC magnetron sputtering suppress the hexagonal phase formation. Consequently, we have developed a single-phase cubic Ti_0.46Al_0.52Y_0.02N film which combines excellent mechanical properties with improved oxidation protection due the incorporation of Y and the benefits of a high Al content.

¹M. Moser and P. H. Mayrhofer, Scripta Mater. 57 (2007), 357.

Titanium nitride films (TiN) are used amongst other applications as wear-protecting coatings. The residual stress in the film is a key factor determining the performance of the coating.

TiN thin films with 111 fibre texture have been deposited on 100 mm Si wafers by reactive magnetron sputtering from a Ti target in an industrial coating system. During deposition (dep. rate approx. 0.1 nm/s) at 5 kW cathode power, 125 V bias, 450°C dep. temperature and 4x10^-3 mbar, the samples performed a planetary motion in front of the target (target size 600x120 mm²). Stress analysis has been performed on TiN films of different thicknesses with X-ray diffraction by the so called sin² method and with wafer curvature measurements before and after deposition.

From wafer curvature we find high compressive stress for thinner films and less compressive stress for thicker films. Films under compressive stress were analyzed by X-ray diffraction. Using the 311 reflection we find a dependence of lattice spacing on diffraction direction in accord with plane stress. For the 111 reflection we also find this relation with the exception for the diffraction vector perpendicular to the film. Also films removed from the surface were analyzed by X-ray diffraction. Here we find that after removal from the substrate the stress is about zero and the lattice parameter is equal to the equilibrium lattice parameter of TiN.

Both from wafer curvature and X-ray diffraction analysis we conclude that in TiN compressive stress is generated at the grain boundaries.

In the past few years, the development of conventional TiN coatings was slowed down because of their too poor high temperature resistance. Fortunately, with an addition of silicon both mechanical properties and oxidation resistance were simultaneously improved, owing to the formation of a hard nanocomposite structure¹,². In this study, we intend to determine the oxidation behavior of coated steel by classical thermogravimetric measurements supported by an in situ investigation by environmental SEM (ESEM). Pure Ti, TiSi (80/20) and AlTiSi (60/30/10) targets were arc-evaporated to produce hard, single-layered coatings. Magnetron sputtered SiN_x films were also synthesized for a comparison purpose. The hardness was measured using depth sensing indentation. The nanocomposite structure was confirmed by XRD and TEM. A continuous heating (1 K/min) was applied to assess the critical temperatures of oxidation. Isothermal tests were also performed at temperatures ranging from 973 to 1223 K to calculate the oxidation activation energies. Besides, 10-cycle runs (300-1073-300 K) were carried out and compared to the results obtained in the isothermal mode to evaluate the effect of the thermal fatigue. The susceptibility of SiN_x to thermal shock is evidenced by ESEM as well as by thermogravimetric analyses. On the contrary, no cracking appears for Si-containing nanocomposite films. The beneficial role of this structure is again evidenced by the oxidation susceptibility (EaTiSiN = 260 kJ/mol vs. EaTiN = 190 kJ/mol), the oxidation rates (kinetic constants one decade lowered) and the oxidation temperatures (Tc delayed of 300 K with respect to TiN).

¹P. Steyer, D. Pilloud, J.F Pierson, J.P. Millet, M. Charnay, B. Stauder, P. Jacquot, Surf. Coat. Technol. 201 (2006) 4158.

²D. Pilloud, J.F. Pierson, P. Steyer, A. Mege, B. Stauder, P. Jacquot, Mater. Lett. 61 (2007) 2506.