Thermally activated relaxation processes in superhard nc-TiN/a-Si3N4 and nc-(Ti1-xAlx)N/a-Si3N4 nanocomposites studies by means of internal friction measurements

We investigated the processes that are responsible for the relaxation of nanostructure and/or self-hardening of superhard nc-TiN/ a-Si3N4 and nc-(Ti1-xAlx)N/a-Si3N4 nanocomposites upon annealing in nitrogen, using the internal friction measurements by means of torsion pendulum and vibrating reed method. It is shown that stable nanocomposites, which were deposited under conditions of a sufficiently high nitrogen pressure and temperature, in a plasma of intense glow discharge (power density at the surface of the growing film about 2-3 W/cm(2)), have a constant value of hardness (measured at room temperature after each annealing step) up to 1100 degrees C, and show no internal friction peak up to a temperature of 800 degrees C achievable in our internal friction measurements. In contrast, the unstable coatings that were deposited at a low temperature and/or low nitrogen pressure or low plasma density show self-hardening and a distinct internal friction peak with well defined activation energy. This peak is due to thermally activated processes within the grain boundaries of the nanostructure whose formation due to phase segregation was not completed during the deposition. Upon the annealing to >= 700 degrees C, the phase segregation is completed, the hardness increases and remains stable up to 1100 degrees C, and the internal friction peak vanishes. (c) 2004 Elsevier Ltd. All rights reserved.