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.