[Back]

T. Glechner, P.H. Mayrhofer, R Hahn, T. Wojcik, D. Holec, S. Fritze, D. Primetzhofer, H. Bolvardi, S. Kolozsvári, H Zaid, S. Kodambaka, H. Riedl:
"Tuning structure and mechanical properties of Ta-C coatings by N-alloying and vacancy population";
Talk: EMRS Spring Meeting 2019, Nizza; 2019-05-27 - 2019-05-31.

Transition metal carbides (TMC) are known for their excellent thermo-mechanical stability, but their use in structural or thin film applications is limited by their poor fracture tolerance. Therefore, enhancing their fracture toughness, while retaining other thermo-mechanical properties is desirable for increasing the use of TMC in any application.
One conceptional approach is non-metal alloying, involving the exchange of C by N or vacancies on the non-metallic sublattice. Based on its high thermal stability as well as hardness Ta-C is used as a base system. Ta-C thin films were synthesized via non-reactive sputtering, while ternary Ta-C-N coatings have been deposited in N2/Ar gas mixtures. Based on ab initio calculations, we could experimentally verify that structural defects, especially Ta vacancies, stabilize the preferred cubic structure for high N contents. Furthermore, our DFT results predicted a softening of the films (confirmed by nanoindentation) and an increase of ductility-according to the Pugh´s criterion-with increasing N content. During uniaxial compression of superhard (43.3 GPa) 110-oriented Ta0.47C0.34N0.19 pillars, we observed yielding at 16.9 GPa followed by plastic deformation where we identified {111} <011 ̅> as the most active slip system. From micro-cantilever tests, we determined KIC values of 2.9 compared to 1.8 MPa√m for Ta0.47C0.34N0.19 and Ta0.55C0.45 respectively, indicating that Ta-C-N exhibits indeed superior fracture tolerance compared to Ta-C.