Effect of Flaws in Coating Film on Fatigue Strength of Steel Coated with Titanium Nitride

To clarify the effect of flaws in the coating film on fatigue strength, cantilever-type rotating-bending fatigue tests were conducted in air and in saline solution (3.0% NaCl) using specimens of 0.37%C steel with flaws in the coating of titanium nitride (TiN)thin film coated by PVD and CVD methods. Flaws in the coating film on the specimen surface were introduced by the application of 1.1-1.6% static tensile strain before the test. An obvious decrease in fatigue life of the specimen with flawed coating film was observed in both environments, as compared with that of an uncoated specimen and a specimen with unflawed coating film. This behavior was marked for fatigue in air, that is, the decrease of fatigue life was 90-75% in air as opposed to 70-50% in saline solution. Although film thickness was 3-5 μm, the flaw in the film had the same effect as a notch at which cracks initiate on the substrate. Many cracks were induced in the substrate directly under a flaw and coalesced into a large crack at an early stage of fatigue in air. In corrosion fatigue, corrosion pits at which cracks initiate occur on the substrate under a flaw at an early stage of the fatigue process, and the incubation period prior to the formation of pits does not occur.