Yasuhiro Yoshino

The development of a Vickers-type hardness tester for cryogenic temperatures down to 4.2 K

Hardness is one of the most important parameters for evaluating the tribological properties of structural materials for magnets at cryogenic temperatures. We have developed a Vickers-type hardness tester for cyrogenic temperatures. This paper outlines the hardness tester and presents the results of hardness measurements. The temperature of specimens can be varied from 4.2 K to room temperature. A controlled d.c. servo motor and a ball-screw device drive the indentation system and control the velocity of the indenter, the applied load, and the loading rate. Another d.c. motor rotates the specimen which is placed at the bottom of a hollow cylindrical holder. The hardnesses of JN1, JN2, SUS304, SUS316L, Ti alloy, and copper were measured. Hardness increased with decreasing temperature to 77 K for all metals. The hardness of steels at 77 K is more than 1.8 times higher than at 293 K, and JN2 shows the greatest increase with a factor of 3.3. However, the hardness at 4.2 K does not always increase from 77 K. SUS304 shows the greatest increase (1.4 times). On the other hand, JN2, SUS316L, Cu, and CuNi soften between 77 and 4.2 K. Comparisons of hardness with yield stress and tensile strength are also made.

An evaluation of mechanical properties of YBaCuO and (Sm,Gd)BaCuO bulk superconductors using Vickers hardness test at cryogenic temperatures

Vickers hardness properties of bulk superconductors have been investigated to estimate the mechanical properties of bulk materials. However, the test at cryogenic temperatures had not been conducted at all. Then we conducted Vickers hardness test on (001) plane of YBaCuO and (Sm,Gd)BaCuO bulk superconductors from 40 to 293 K. The hardness of YBaCuO and (Sm,Gd)BaCuO increased with decreasing temperature up to 19 000 MPa and 10 000 MPa, respectively. Furthermore, the cracks caused by the indentation were observed with SEM, and the fracture toughness KIC at cryogenic temperatures was calculated. The KIC of YBaCuO and (Sm,Gd)BaCuO decreased with decreasing temperature to 0.6 MPa√m and 0.8 MPa√m, respectively.

Fracture toughness of Sm123 bulk superconductors evaluated by tensile and bending tests

The effects of test temperature and Sm211 (Sm/sub 2/BaCuO/sub 5/) secondary phase particles on the fracture toughness of the Sm123 (SmBa/sub 2/Cu/sub 3/O/sub x/) single grain bulk superconductors were investigated by tensile and bending tests using the V-notched specimens. The correlation between the obtained fracture toughness by tensile and bending test was not necessarily good. The fracture toughness obtained by the indentation method was lower than them. The tensile fracture toughness for the crack propagation in the c-axis, 1.2-1.3 MPa m/sup 1/2/, was higher than that in the direction perpendicular to the c-axis, 1.0-1.1 MPa m/sup 1/2/. Although the former increased with increase in the Sm211 molar fraction, the latter did not increase. There was no appreciable difference between the fracture toughness obtained by the tensile tests at LNT and that at RT, while the fracture toughness from the indentation method at LNT was slightly lower than that at RT.

Frictional properties of ceramics, MoS2 coated films and polyethylene fibre reinforced plastics at 4.2 K in liquid helium

Abstract Frictional problems in superconducting magnets are related to the stability and reliability of the magnets, because frictional heating causes quench of the magnet and the frictional resistance determines the limit of the shear force at the contacting surfaces. To find those materials possessing low friction, the frictional properties of ceramics of ZrO2 and Si3N4, three different kinds of MoS2 coated films and polyethylene fibre reinforced plastics (DFRPs) were examined at 4 K in liquid helium under reciprocating sliding conditions. The two ceramics showed similar tendencies in terms of friction. The organically bonded MoS2 film exhibited the lowest coefficient of friction of 0.06, while the coefficient of friction of DFRP was lower than that of glass fibre reinforced epoxy (GFRP).

Fracture toughness of SmBaCuO bulk superconductors at cryogenic temperatures

Fracture toughness of Sm123 bulk superconductors with 16.7 and 28.6 mol% of Sm211 particles have been investigated by Vickers hardness test at cryogenic temperatures. The hardness test was carried out on [001] plane for two types of SmBCO bulk materials during 40-293 K. However, the hardness of Sm29 increased to 7,380 MPa with decreasing temperatures. In addition, fracture toughness of Sm17 and Sm29 are discussed calculating from Vickers hardness, median cracks length, Youngs modulus and applied load. The fracture toughness of Sm17 became constant to 0.8 MPa/spl radic/m following the hardness property. The fracture toughness of Sm29 decreased from 0.73 Mpa/spl radic/m at 293 K to 0.67 Mpa/spl radic/m at 40 K.

Frictional properties of structural steel JN2 at cryogenic temperatures in a vacuum

Abstract To assess the frictional properties at the wedge of a toroidal field coil of a tokamak fusion reactor, friction tests were conducted using structural steel, JN2, against various materials in a vacuum at low temperatures under a normal load of 10 N and a peak-to-peak sliding amplitude of 100 μm. The temperature was mainly ≈ 5 K and the ambient pressure ≈ 10 −3 Pa. The relationship between the frictional characteristics and the number of cycles depends on both the material combination and temperature. Two typical patterns of behaviour were observed at 5 K depending on the combination; fairly constant friction and very high and fluctuating friction. The high friction was caused by severe adhesion between surfaces. Temperature dependence of friction was also observed for JN2-copper.

Property of Mechanical Heat Generation Inside the Superconducting Coil Installed in MAGLEV Inner Vessel

Superconducting magnets for MAGLEV train are vibrated around 300 Hz when the train passes through the ground coils. This mechanical vibration will make the frictional heat generation. The coefficient of friction and the liquid helium evaporation for the combination of copper versus polyimide films on substrate was measured at 300 Hz in this study. The coefficient of friction increased around 2.7 with increasing the film thickness. The heat generation increased around 0.95 W with increasing the film thickness. However, the heat generation with single thin films did not increase