[en] Durability of dielectric breakdown (>2,000 MΩ, 1 kV) characteristics of alumina electrical insulation films (alumina films, thickness 228 μm) has been examined by applying repeated impact loads to alumina films plasma spray coated on a stainless steel substrate with a Ni-Cr intermediate layer. In the present experiment, alumina overcoated specimens having flat ended surfaces were impacted with a stress of 638 MPa. Alumina films have shown high durability of greater than 70,000 cycles under the repeated impact loads. And local fracture of the alumina films was observed at some pinholes of about 100 μm diameter at the maximum, eleven holes in total at 76,828 cycles, and the local exfoliation of the alumina film was observed between alumina film and Ni-Cr intermediate layer. To examine the evolutions of the thickness and hardness of the alumina film, cross-sectional SEM observation and hardness measurement were performed for some specimens after the predetermined cycles. As the results, the thickness of alumina films of an as-received specimen was 228 μm, and linearly decreased as the increase of the number of the impact load cycles by the rate of 2.43 nm/cycle. When the dielectric breakdown occurred in a specimen, the thickness of the alumina film was only about 20% (50% μm) of that of the as-received one and the cross-sectional view of alumina film exhibited a duplex region of Ni-Cr and alumina layers. While the thickness of Ni-Cr intermediate layers after the test was almost the same as that of the as-received one of 50 μm, even in the specimen in which dielectric breakdown occurred. Hardness of the as-received alumina film was Hv=1119, and decreased with the increase of the number of the load cycles; 76% of the as-received one at 38,000 cycles and finally 60% at 76,828 cycles, respectively. (author)