Fig. 2-10 Microscope picture of the cross section of the film made by the plasma spray method The 230 micrometer alumina film is formed on stainless steel through a nickel-chrome intermediate layer.

Fig. 2-11 Outside look of the facilities for testing shock-durability Repetitive stresses were applied to the test piece by gravity forces (weight). The stress of each shock was 640 MPa. The thickness of the alumina film became 50 mm after the shocks of 70,000 cycles and then the electrical insulation was broken.

A fusion reactor is a complex structure of metal. Furthermore, it is also a sort of electric circuit. Eddy currents are induced in the complex structures and result in parasitic effects such as error magnetic fields and excessive electromagnetic forces. Electric insulation is needed to protect the reactor from such parasitic effects. The insulator should withstand radiation doses, and high stress. It should also be as thin as possible. We have developed the technology to form a firm alumina film on stainless steel substrates. We have discovered that the alumina film never comes off, when we make a particular intermediate layer with a plasma spray method. The intermediate layer has the same components of nickel (Ni) and chrome (Cr) to those of the stainless steel, and the same crystal structure to that of the alumina film. A microscope picture of the cross section of alumina coatings is shown in Fig. 2-10. Figure 2-11 shows the test facilities for testing the shock-durability of alumina coatings. Experimental results showed that an alumina film of 230 micrometer thickness withstands the shocks of 70,000 cycles for a stress of 640 MPa. This stress corresponds to the limit in which the stainless steel has almost changed its shape. Many applications are to be expected for this technology in improving the surface characteristics while keeping the character of the substrate.

Reference M. Kanari et al., Durability of Alumina Electrical Insulation Films by Plasma Spraying Coating Method under Repeated Impact Loads, JAERI-Research 98-029 (1998).

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