Channel cracking of β-NiAl thin films on Si substrates

Thin intermetallic films, which serve e.g. as protective coatings against oxidation, are prone to cracking due to thermal strains. In the present paper, we report on the cracking behavior of thin polycrystalline NiAl films on Si substrates as a model system. The stress-temperature evolution and the cracking behavior of thermally strained NiAl films was studied as a function of Al content (45-52 at.% Al) and film thickness (0.4-3.0 μm). Ni-rich NiAl films were found to sustain higher tensile stresses than Al-rich films. Al-rich NiAl films failed by the formation of intergranular cracks, which even extend into the Si substrates. The fracture toughness K_c of the Al-rich films was determined from the fracture stress, substrate crack depth and film thickness, revealing K_c values of 2.2-2.9 MPam^1/2 independent of film thickness. Thus, the fracture toughness is smaller than for polycrystalline bulk NiAl of stoichiometric composition. It is speculated that this is caused by the high Al content of the Al-rich NiAl films. The fracture stress was found to increase with decreasing film thickness indicating that the film thickness corresponds to a critical crack length.