The experimental study on the optimum thickness of coating film for the fatigue strength of surface modified TiAl intermetallic alloy was carried out. The main results were obtained as follows. (1) The fatigue strength of TiAl intermetallic alloy could be improved through the optimum thickness of TiAlN hard coating film, that is 3 μm. (2) From micro-fractography, two different types of fatigue fracture mechanisms were found. For the thinner coating film, the shear mode of fatigue initiation crack was occurred. On the other hand, the thicker coating film, the TiAlN film cracking triggered off the tensile mode of the fatigue initiation crack. (3) Therefore, it could be comprehended that the optimum thickness of coating film for the fatigue strength of surface modified TiAl intermetallic alloy had been controlled through the two types of roles in the hard film. Then, the one effect is the constraint of slip deformation and the other is the Griffith crack of film cracking.
