Deposition, structure, and hardness of polycrystalline transition-metal nitride superlattice films
- PDF / 434,855 Bytes
- 8 Pages / 612 x 792 pts (letter) Page_size
- 26 Downloads / 163 Views
MATERIALS RESEARCH
Welcome
Comments
Help
Deposition, structure, and hardness of polycrystalline transition-metal nitride superlattice films X. Chu, M. S. Wong,a) W. D. Sproul,b) and S. A. Barnett Department of Materials Science and Engineering and the Advanced Coating Technology Group, Northwestern University, Evanston, Illinois 60208 (Received 8 June 1998; accepted 25 January 1999)
Polycrystalline TiNyVN, NbNyVN, and TiNyNbN superlattices with periods L between 2 and 160 nm were deposited onto steel substrates using an opposed-cathode reactive magnetron sputtering system. The nitrogen partial pressure and the substrate bias values were optimized in order to obtain dense stoichiometric films, which yielded the highest Vickers hardnesses HV . HV for TiNyVN and TiNyNbN superlattices reached maximum values of ø5000 kgfymm2 at L ø 5–10 nm, compared with ø2000 kgfymm2 for homogeneous TiN, NbN, and VN films. In contrast, HV ø 2000 kgfymm2 was obtained for VNyNbN superlattices independent of L. Model calculations in which the hardness enhancement was proportional to the difference in layer shear moduli gave good agreement with the data. The lack of hardness enhancement in VNyNbN indicates that any other hardening mechanisms, such as coherency strains and dislocation blocking by interfacial misfit dislocations, were not important.
I. INTRODUCTION
Strength and/or hardness enhancements have been reported for a wide range of metal/metal, nitride/nitride, and metal/ceramic superlattice thin films, and a number of explanations for these effects have been proposed.1,2 The most widely accepted theory, first applied to superlattices by Koehler,3 is that strengthening results from a resistance to dislocation glide caused by image forces at interfaces. In this case the strengthening is proportional to the difference between the layer shear moduli. Other proposed mechanisms include the supermodulus effect,1 coherency strain effects on dislocation motion,4 effect of interfacial misfit dislocations on dislocation glide,5 and a reduction of grain sizes by the presence of superlattice layers.6 Experimental results from singlecrystal TiNy(Vx Nb12x )N4,7 and (Vx Nb12x )NyNbN8 superlattices have shown that most of the strengthening is related to modulus differences, with coherency strains contributing a small effect. Recent detailed models have quantitatively explained strength/hardness enhancements in metal/metal and ceramic/ceramic superlattices based on the modulus difference effect.2,9,10 For the case of nitride/nitride superlattices, most of the results reported have been for single-crystal superlattices. High-hardness polycrystalline nitride superlattices are of interest since they have potential practical applications as wear-resistant protective coatings, e.g., cutting-tool coatings.11 While results reported for a)
Present address: Institute of Materials Science and Engineering, Shoufeng, Haulien, Taiwan, Republic of China. b) Present address: Sputtered Films, Inc., 320 Nopal Street, Santa Barbara, California 93103. 2500
ht
Data Loading...
