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H. Riedl, C.M. Koller, R. Rachbauer, S. Kolozsvári, F. Mendez Martin, P.H. Mayrhofer:
"Oxygen impurity related hardening mechanisms of Ti-Al-N thin films";
Poster: 14th International Conference on Plasma Surface Engineering, Garmisch Partenkirchen; 2014-09-15 - 2014-09-19; in: "Book of Abstracts", (2014), 102.

The mechanical properties, especially the hardness, of thin films are assigned to different attributes such as crystal structure, morphology, crystallite size or volume fraction of the grain boundaries. Recent investigations pointed out, that a hardness maximum can be reached at the "strongest size" of the grains which is related to the Hall-Petch effect. Impurities, such as oxygen, have a great influence on the grain size and hence the mechanical properties.
Within this study, we investigate the influence of oxygen impurities on Ti1-xAlxN coatings using a PVD magnetron sputter deposition system. We used a specially prepared powder metallurgical Ti0.5Al0.5 target to contain less than 950 ppm oxygen and base pressures of 10-5 or 10-8 mbar at the deposition temperature. The latter was varied between 500 and 800 °C to additionally investigate the grain size influence independent from the oxygen impurity variation.
The very pure Ti1-xAlxN coatings, deposited at 800 °C and base pressures of 10-8 mbar exhibit only hardnesses of 21 GPa, although being single-phase cubic structured, due to the very large grain size of about 100 nm. Their reference coatings, which have been deposited at 500°C and 10-5 mbar and otherwise identical conditions, have ~35% higher hardness of 33 GPa due to their smaller grain sizes of ~25nm. The grain boundaries and the effect of oxygen impurities were studied in detail by TEM and 3D atom probe tomography. Although starting at low as-deposited hardness, the very pure Ti1-xAlxN coatings reach 32 GPa upon vacuum annealing to 1200 °C due to the formation of small cubic Ti- and Al-rich domains. Compared to the reference coating, the age hardening peak-hardness is shifted from 1000 °C to 1200 °C, due to the increased grain size and the thereby connected reduction in grain-boundary-diffusion induced spinodal decomposition as shown earlier. Furthermore important is, that the hardness reduction - connected with the formation of the stable wurtzite structured w-AlN phase - is also shifted by 200 °C to higher temperatures.
Based on our results we can conclude that, oxygen impurities influence not only the as-deposited properties of Ti1-xAlxN but also their thermal stability.

Project Head Paul Heinz Mayrhofer:
Christian Doppler Labors für anwendungsorientierte Schichtentwicklung