[Back]

S Lang:
"Thermomechanical properties of Hf-C-N thin films";
Supervisor: H. Riedl, P.H. Mayrhofer; Institute of Materials Science and Technology/E308-01, 2019; final examination: 2019-07-18.

Developments in aerospace industry pose new challenges and therefore demand a constant development of ultra-high temperature materials. A class of materials which is well known for their high phase and thermal stability are transition metal carbides. Among them HfC, which has the second highest melting temperature of all binary systems, was chosen for investigations and further improvements on thin film materials. We studied the impact of alloying nitrogen on the thermomechanical properties of Hf-C and Hf-C-N thin films, sputter-deposited from a HfC compound target in Ar as well as mixed and Ar+N2 atmospheres. For varying the growth morphology and chemistry two different power densities were compared, and it was observed that already a low nitrogen to total gas flow ratios result in high nitrogen contents within the films. All Hf-C and Hf-C-N coatings exhibit a face-centered cubic structure, with a dense columnar morphology. With increasing nitrogen content, the hardness softens from super-hard HfC1.33, 40.2 1.0 GPa, to 33.1  1.0 GPa for Hf0.32C0.20N0.48, accompanied by a decrease in Indentation modulus. In addition, the impact of the growth temperature as well as deposition rate on the mechanical properties was investigated at constant nitrogen to total gas flow ratio of 0.05. An increase in hardness could be achieved for decreased deposition temperatures accompanied by increased target currents. Annealing treatments up to 1400 °C of powdered thin films in a combined DSC/TG system showed, that with increasing nitrogen content, the samples start to lose more and more weight. This is good agreement with the relatively high over-stoichiometric nitrogen content -- occupying more than 50 % of the non-metal sublattice -- suggesting for the formation of nitrogen rich grain boundary phase due to the reactive sputter deposition process. Furthermore, vacuum annealing of thin films deposited on sapphire showed, that a higher nitrogen content leads to minor decrease in hardness with increasing annealing temperature. The results indicate promising properties of Hf-C-N thin films but raise the need for an improved sputter process which allows the formation of stoichiometric HfC1-xNx thin films.

Project Head Helmut Riedl:
CDL-SEC