In-situ SEM indentation studies of the deformation mechanisms in TiN, CrN and TiN/CrN
Introduction
Refractory transition metal nitrides like CrN, TiN or NbN are widely used for protective and wear resistant coatings (Mayrhofer et al., 2006, Tjong and Chen, 2004). The most prominent and the most extensively studied material is TiN. Both high hardness (∼21–25 GPa) (Mirkarimi et al., 1992, Yang et al., 2004) and toughness of TiN make this material outstanding for many industrial applications (Mayrhofer et al., 2006). However, due to its quite low oxidation resistance, TiN is often combined with other nitride materials into multilayer nanolaminates (Yang et al., 2004, Mendibide et al., 2004, Nordin et al., 1998). Such structures are an emerging class of hard, low-friction, wear-protective coatings. Enhanced hardness as compared to single-layered structures has been reported, both for single-crystal superlattices (Mirkarimi et al., 1992) and polycrystalline nanolayered thin films (Nordin et al., 1998), e.g. TiN/NbN, TiN/VN or TiN/CrN (Ducros et al., 2006, Molina-Aldareguia et al., 2002, Lloyd and Molina-Aldareguia, 2003, Nordin et al., 1998, Mendibide et al., 2004, Lee et al., 2004). CrN is generally reported to be softer than TiN (Lee et al., 2004), with hardness values from ∼14 to 22 GPa (Zhang and Hsieh, 2000, Oden et al., 1999) depending on the composition and deposition conditions. For a TiN/CrN the typical compositional nanohardness is between ∼18 and 41 GPa (Ducros et al., 2006, Lee et al., 2004). Variations in values originate from the bilayer composition and the type of the deposition technique. Mechanism of multilayer hardness enhancement was proven to be dependent on the microstructure. The hardness enhancement was attributed to the superlattice effect for coherent multilayers (Lee et al., 2004), whereas the rule of mixture applies for polycrystalline thin films.
An understanding of the deformation and fracture mechanisms in nanolaminate structures is critical for application performance (Ding et al., 2000). Polycrystalline multilayer nanolaminate structures were reported to deform by grain boundary sliding (Molina-Aldareguia et al., 2002, Lloyd and Molina-Aldareguia, 2003, Carvalho and De Hosson, 2006) or densification during indentation experiments (Chu and Barnett, 1995, Long et al., 2006).
Nanohardness and Youngs modulus data is available for a number of different nitride multilayers (Lee et al., 2004, Oden et al., 1999, Long et al., 2006, Wrzesinska et al., 2002, Su and Yao, 1997, Okumiya and Griepentrog, 1999). However, conventional nanoindentation experiments provide only limited information about the material behavior. In-situ nanoindentation experiments performed inside a scanning electron microscope (SEM) allow for observation of the indented material around the tip. Additional information on fracture toughness and strain hardening is provided by pile-ups, sink-ins or cracks initiation and propagation. This will allow for more accurate choice and better adjustment of the mechanical model. Combined with post-mortem TEM, in-situ SEM nanoindentation is a powerful tool to study deformation mechanisms occurring in the material and correlate them to the various events (pop-ins and pop-outs) on the load–displacement curve (Rabe et al., 2004). Combination of all these information gives a possibility of the performance prediction for the studied coatings.
In this paper we studied the microstructure and mechanical properties of single-layer reference TiN and CrN coatings and a multilayer composed of these two materials: namely TiN/CrN. In-situ SEM nanoindentation was used to investigate pile-up and sink-in as well as cracking behavior and post-mortem TEM was used to analyze the deformed microstructure.
Section snippets
Experimental details
The coatings were deposited by dc magnetron sputtering technique in a Leybold L400Sp system on 〈1 0 0〉-oriented silicon substrates (Wrzesinska et al., 2003, Zhou et al., 1999). The TiN/CrN multilayer of 20 bilayer periods in the stack was synthesized. The alternating TiN and CrN layers in the system were 25-nm thick. For reference, 1-μm thick TiN and CrN were deposited (Okumiya and Griepentrog, 1999).
Hardness and Youngs modulus were assessed by nanoindentation technique (TriboIndenter, Hysitron
Microstructure of the thin films
Fig. 1 shows dark field (DF) cross-section micrographs (Fig. 1a–c) with the corresponding diffraction patterns (Fig. 1a′–c′) of the examined coatings. The structure of the TiN (Fig. 1a) is characterized by columnar grains, which are smaller close to the substrate and become larger towards the coating surface. The width of the columns varies from 10 nm close to the substrate interface to about 30 nm at the film surface. The largest among the columns exhibit sharp and narrow bottom, constant width
Conclusions
The deformation mechanisms of multilayer TiN/CrN coating as well as corresponding TiN and CrN reference coatings deposited by dc magnetron sputtering on Si 〈1 0 0〉 substrates were investigated. The TiN coating showed a pronounced columnar structure. The CrN reference film microstructure is also columnar with very small misorientations within the grains. Even though, the lattice constants for both materials are similar, TiN/CrN multilayer shows lack of epitaxial growth between layers. A columnar
Acknowledgements
Authors would like to acknowledge S. Meier and J. Tharian, EMPA Duebendorf, for FIB samples preparation. We are grateful to Prof. R. Spolenak and M. Dietiker for rendering the TriboIndenter accessible to our use and help in experiments.
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