Browsing by Author "Wellner, Patrick"
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Item Open Access Thermo-mechanical behavior of NiAl thin films(2003) Wellner, Patrick; Arzt, Eduard (Prof.)Thin films based on the intermetallic alloy NiAl are used as protective coatings for high temperature applications because of their excellent oxidation resistance. However, insufficient mechanical strength of the coatings may lead to failure as a result of thermal stresses. This work describes the influence of film thickness (0.2 to 3.1 µm) and Al content (45 to 52 at % Al) on the yield strength, the fracture toughness and the creep strength of polycrystalline NiAl films. The NiAl films were sputter-deposited on Si substrates and were thermally cycled up to 700 °C. The stress evolution during thermal cycling was measured using a laser-optical substrate-curvature technique. The fracture toughness of the NiAl films was found to depend on the chemical composition: upon thermal straining, Al-rich films exhibited crack formation while stoichiometric and Ni-rich films remained devoid of cracks. The fracture toughness of the Al-rich films was determined to be 2.2 to 2.9 MPa m1/2, independent of film thickness. However, the fracture stress increased with decreasing film thickness. These results are discussed on the basis of thin film cracking models. Plastic deformation in stoichiometric and Ni-rich films is strongly influenced by the thickness of the films. Upon cooling to room temperature, a strong increase in yield strength with decreasing film thickness was found. The yield strength increased from 420 MPa for 3.0 µm thick films to about 2000 MPa for 0.2 µm thick films. In contrast, heating above 400 °C yielded the opposite trend: thinner films showed stronger plastic deformation, which is related to a diffusional creep process. The influence of Al content and film thickness on the stress evolution of stoichiometric and Ni-rich films is discussed with respect to thin film deformation mechanisms.