A new testing procedure to evaluate the interfacial toughness of thermal-sprayed coatings has been developed. The newly designed test specimen is a modification of the pin test with an artificially introduced weak interface, which is expected to open up easily under tensile loading and act as a circumferential precrack along the interface between a coating and the substrate. This configuration makes it possible to calculate the stress intensity factor K Int at the tip of the precrack, which can be expressed aswhere r 0 is the apparent average stress, a the crack length, R the specimen radius, and F I the geometrical correction function. Finite-element analysis was carried out to calculate the correction function F I for various values of a/R. In the experiments, the flat surface of a pin was gritblasted and a ring-shaped area from the periphery was covered with carbon using a pencil and set into a mating dice. SUS316L stainless steel was plasma-sprayed onto the flat surface of the pin and the dice. Then, tensile load was applied to the pin to break the weak interface containing the carbon and finally the unmodified coating-substrate interface. The load required to pull out the pin was measured for various specimen parameters such as a and R. The results indicate that the adhesion of the tested coatings can be represented by interface toughness of 1.9 ± 0.1 MPa m 1/2 . As a consequence, this testing procedure can be considered as a viable method to evaluate adhesion of a thermal-sprayed coating on a substrate.

SUMMARYTo evaluate an appropriate level of ac test voltage for 1000-kV transformers, it is necessary to make an accurate estimation of ac V-t characteristics. For the level of impulse test voltage, V-N characteristics are important to evaluate effects of impulse stress repetition. Since both of the characteristics are dependent on insulation structures and manufacturing processes of transformers, it is important to make accurate simulations on transformer insulation for reliable insulation data.To obtain some of the basic data for 1000-kV transformer test levels, long-time ac V-t tests and impulse V-N tests of up to 1000 times voltage application were carried out on an insulation model simulating fundamental structure of shell-form transformers. The model is composed of a parallel electrodes system with a compound structure of pressboard barrier and oil gap.The tests showed the following characteristics: ac V-t characteristics of up to several ten days could be described by two lines, each representing short-time and long-time Characteristics. V-N characteristics of lightning and switching impulse were expressed by relatively flat lines, which showed a decrease in breakdown voltage by less than 10 percent after 1000 times impulse application.

Polypropylene (PP) film is generally used as insulating material for many kinds of electrical and electronic equipments because of its excellent electrical insulation performance. We investigated the electrical conduction and the charge storage of the thin PP films by means of low energy electron beam irradiation as one new attempt.

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