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R Hahn, S. Rosenecker, T. Wojcik, O Hunold, S. Kolozsvári, H. Riedl:
"Influence of microstructure and defects on the fracture properties of protective CrN coatings";
Talk: 2020 MRS Virtual Spring/Fall Meeting, Boston, MA, USA; 2020-11-27 - 2020-12-04.

Transition Metal Nitrides (TMN) are well known for their good mechanical stability, chemical inertness, as well as tribological properties. Hence, they successfully found application in the metal forming industry, and are widely used as protective coatings in the automotive and aerospace industry. Besides TiN, CrN is one of the most used and investigated hard coating systems, preferably applied in conditions that require a low coefficient of friction such as coatings on piston rings and bolts or punching dies. A decisive disadvantage of these hard coatings, however, is their low fracture tolerance. Premature failures due to crack initiation and propagation leads to economic disadvantages or completely excludes diverse applications. In recent years, micromechanical testing methods have made it possible to measure and hence specifically improve the fracture toughness (KIC) of thin film materials. There are various methods known for measuring KIC obtaining all advantages and drawbacks, especially concerning intrinsic material characteristics and accuracy. These methods include in-situ and ex-situ measurements such as cantilever bending, pillar splitting, and indentation fracture.
In this contribution, we perform distinct micromechanical tests on cathodic arc evaporated CrN coatings. These coatings were deposited with different bias voltages and deposition temperatures to obtain a variation in both, microstructure (specifically column size) and defect density (macroparticles as well as defects on the crystal lattice). In terms of the microstructure of the coatings, we could observe not only a variation in width but also a variation in column length. Hence, the overall picture is much more complex than considering the often-used column width for such coatings. The importance of this microstructure on fracture characteristics has recently shown by Ast et al. for Ti-Al-N deposited by diverse PVD techniques [1]. However, a clear correlation between the column sizes, defect densities, and the interface constitution of column boundaries is still missing. We found a significant influence of the residual stress state on the fracture properties of such hard coatings using the indentation fracture method. Furthermore, we used pillar splitting and cantilever bending tests to determine the intrinsic fracture characteristics of our coatings with respect to the microstructure (considering both, column width and length) and defect density. These results are discussed in-depth regarding their microstructure and were complemented by HR-TEM investigations together with x-ray diffraction studies, and further mechanical characterization techniques (nanoindentation as well as pillar compression tests).

Project Head Helmut Riedl:
CDL-SEC