Nondestructive Measurement of Fatigue Damage of Thermally Sprayed Al2O3/NiCr Using ESPI Method Under High Temperature

High-temperature fatigue (R = 0) damage and deformation behaviors of SUS304 steel thermally sprayed with an Al₂O₃/NiCr coating were investigated using a servopulse fatigue-testing machine, SEM, and an electronic speckle pattern interferometry (ESPI) method. The relation between crack/delamination and strain variation is discussed. Surface cracks occurred at the outer Al₂O₃ coating but stopped at the inner NiCr coating after one fatigue cycle when the tensile stress was 202 MPa at 873 K. They propagated into the NiCr coating but stopped at the substrate, and local delamination occurred along the NiCr/substrate interface after 1 × 10⁵ cycles test in condition (_max = 202 MPa, T = 873 K). Cracks and delamination largely decreased when _max = 115 MPa or T = 573 K. No influence of cycle frequencies (6.7 or 14 Hz) was detected. The strain value measured by ESPI method was confirmed to be almost the same as that obtained with strain gauges at 293 K. Strain values along cracks measured with the ESPI method were much larger than other areas as a result of crack opening under the tensile load, referred to as the strain concentration zone in this work. Positions of strain concentration zones on strain distribution figures by the ESPI method corresponded well to positions of cracks on sprayed coatings. Moreover, strain values largely decreased where local delamination occurred.