Abstract
This paper presents the first study of the plastic behavior of NiAl thin films with thicknesses in the micron and sub-micron range. The influence of geometrical and microstructural constraints was studied for different Al contents (45 to 50 at.% Al) and film thicknesses (0.2 to 3.1 μm) in the temperature range from 20 to 700 °C. The films were deposited on Si substrates and subjected to temperature changes resulting in thermal strains of up to 0.84 %. The stress evolution in the films was measured by the substrate curvature method. The residual stress at room temperature of near-stoichiometric NiAl films significantly increased with decreasing film thickness indicating an increase of the film strength. At temperatures above 400 °C strong stress relaxation was found, which was more pronounced in thinner films suggesting the contribution of diffusional creep processes. The existence of a critical film thickness is suggested at which a transition from diffusion-controlled to dislocation-controlled plasticity occurs. The results have potential implications for the reliability of NiAl films as protective coatings in thermal engines.
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