DEVELOPMENT OF CRYOGENIC HARDINESS TESTER AND FRICTION TESTER AT CRYOGENIC TEMPERATURE

点击翻译按钮获取中文摘要

In this work we developed the new Vickers-type cryogenic hardness tester, using GM-refrigerator. The old one was cooled with liquid helium, but the temperature and temperature rising rate could not be controlled because of the evaporation of liquid helium. Therefore, we introduced a GM-refrigerator to control in a vacuum chamber. The performance of the hardness tester was acceptable. The problem was not to cool a specimen at around 4 K due to the contact resistance of conduction from the second cooling stage head of the refrigerator and the specimen. The temperature was cooled down at around 30 K in this tester.We measured the hardness not only metallic specimens such as SUS316L stainless steel, but also high temperature superconductors such as YBCO, and ((Nd,Eu,Gd)BaCuO). The latter was used as the bulk magnet, but it breaks down easily with Lorentz force during magnetizing. The fracture toughness K_<IC> was evaluated from the hardness and the crack size initiated from an indented scar during hardness test. Generally, hardness was increased with decreasing temperature monotonically with rather great scatter, but the fracture toughness was slightly decreased with temperature. For example, the hardness of (Nd,Eu,Gd)BaCuO was 9,800 MPa at 40 K. The fracture toughness depends on the crystal plane because there were many inherent cracks on the (100) plane and it was easier for cracks to propagate.We also equipped the pin-on-desk type friction tester with a reciprocating sliding motion in the chamber. The temperature of the specimen was valid from 40 K to room temperature. Unfortunately, we could not obtain the frictional date due to lack of time. The performance of the frictional tester was examined at 40 K and room temperature, and we are satisfied. Later, we will obtain the temperature dependence of coefficient of friction of materials.