Effect of Internal Interfaces on Hardness and Thermal Stability of Nanocrystalline Ti 0.5 Al 0.5 N Coatings
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A good thermal stability of hard protective coatings is a very important issue, particularly if the coatings are intended for use in machining applications at high cutting speeds or for dry cutting.[1] For such ‘‘high-temperature’’ applications, several advantages of nanocrystalline hard coatings based on the titanium aluminum nitride are used. The prominent examples of the beneficial properties of the (Ti, Al) N coatings at high temperatures are their excellent oxidation resistance (e.g., References 2 through 4) and the increase of their hardness after thermal loading up to approximately 1223 K (950 C).[5,6] The frequently reported phenomena, which negatively influence the hardness of the (Ti, Al) N coatings at high temperatures, are the relaxation of the compressive residual stresses[4,7–11] and the decomposition of the metastable supersaturated (Ti, Al) N having the fcc crystal structure into fcc-(Ti, Al) D. RAFAJA, Professor of Structure Research, C. WU¨STEFELD, Postdoctoral Student, V. KLEMM, Head of TEM Laboratory, M. DOPITA, Postdoctor, and M. MOTYLENKO, Postdoctoral Student, are with the Institute of Materials Science, TU Bergakademie Freiberg, D-09599 Freiberg, Germany. Contact e-mail: [email protected] C. BAEHTZ, Beamline Scientist, is with the Institute of Ion Beam Physics and Materials Research, Forschungszentrum DresdenRossendorf, Bautzner Landstrasse 400, D-01328 Dresden, Germany. C. MICHOTTE, Application Engineer, is with the Ceratizit Luxembourg S.a`.r.l., Route de Holzem, 101, B.P. 51, L-8201 Mamer, Luxembourg. M. KATHREIN, Head of Physical Surface Engineering, is with Ceratizit Austria GmbH, A-6600 Reutte, Austria. Manuscript submitted December 11, 2009. Article published online March 30, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
N with lower aluminum contents and into wurtzitic w-AlN.[12] Moreover, as the hardness of the nanocrystalline coatings typically increases with decreasing crystallite size[13] according to the Hall–Petch relationship[14,15] and as it reaches its maximum at the crystallite sizes of 3 to 4 nm (in the case of the fcc-(Ti, Al) N crystallites, e.g., References 16 through 18), the growth of crystallites at high temperatures can be regarded as an additional factor, which reduces the hardness of the coatings at high temperatures. In our previous publications,[17–19] we have shown that the hardness of the as-deposited, nonannealed (Ti, Al) N nanocrystalline coatings and thin film nanocomposites is controlled by an interplay of several microstructural features such as phase composition, mutual crystallographic orientation of adjacent crystallites (particularly at the phase boundaries), lattice misfit, lattice strain, and the formation of defect structures. From this point of view, the presence of the minor w-AlN separating fcc(Ti, Al) N nanocrystallites from each other was regarded as a factor increasing the hardness of the (Ti, Al) N nanocomposites. In general, the phase composition of the (Ti, Al) N nanocrystalline coatings is driven by the aluminum contents.[
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