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PSE 2014 - 2014
Helmut Riedl (H. Riedl)
"Oxygen impurity related hardening mechanisms of Ti-Al-N thin films"
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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.The deposition temperature 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.
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

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