High-temperature stability of epitaxial, non-isostructural Mo/NbN superlattices
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High-temperature stability of epitaxial, non-isostructural Mo/NbN superlattices C. Engstro¨m Advanced Coating Technology Group and Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois, and Physics Department, Linko¨ping University, Sweden
A. Madan Advanced Coating Technology Group and Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois
J. Birch Physics Department, Linko¨ping University, Sweden
M. Nastasi Los Alamos National Laboratory, Los Alamos, New Mexico
L. Hultman Physics Department, Linko¨ping University, Sweden
S.A. Barnett Advanced Coating Technology Group and Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois (Received 12 February 1999; accepted 23 November 1999)
The effect of 1000 °C vacuum annealing on the structure and hardness of epitaxial Mo/NbN superlattice thin films was studied. The intensity of superlattice satellite peaks, measured by x-ray diffraction, decreased during annealing while new peaks corresponding to a MoNbN ternary phase appeared. The results are consistent with the Mo–Nb–N phase diagram, which shows no mutual solubility between Mo, NbN, and MoNbN. Even after 3-h anneals and a loss of most of the superlattice peak intensity, the room-temperature hardness was the same as for as-deposited superlattices. The retained hardness suggests that a residual nanocomposite structure is retained even after the formation of the ternary structure.
I. INTRODUCTION
There is an increasing need for coatings that retain high hardness at elevated temperature. For cutting tool applications, for example, there is industrial interest in achieving increased cutting speed and feeding rates, as well as for dry-cutting, i.e., without lubricants, all of which lead to increased cutting temperatures. Most traditional nitride cutting-tool coatings show a substantial decrease in hardness at elevated temperatures, i.e., 700–1000 °C,1 limiting their utility for high-temperature cutting. The hardness decrease is related to the increasing ease of dislocation motion at higher temperatures. One approach for achieving high hardness is the use of superlattice coatings, structures consisting of alternating layers with thicknesses of typically a few nanometers, that can exhibit room-temperature hardnesses 2 to 3 times that of the corresponding alloys or rule-ofmixtures values.2 Superlattices may maintain a higher fraction of their hardness at elevated temperatures, compared to homogeneous materials, since superlattice inter554
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J. Mater. Res., Vol. 15, No. 2, Feb 2000 Downloaded: 19 Mar 2015
faces can provide a larger resistance to dislocation motion than the Peierls force. However, it is not known whether the nanometer-thick layers are stable at elevated temperatures. For example, recent studies of TiN/NbN superlattices with periods ⌳ of 4.4 and 12.3 nm showed that the miscible nitride layers interdiffused measureably fo
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