Abstract
The hardness of cementite spheroidized S25C steel was measured, using an AFM ultramicro hardness tester and a special lever that has a three-sided pyramidal diamond indenter with an apical angle of 60°. A clear AFM image of the specimen surface etched in a solution of 3% nitric acid and 97% alcohol enabled us to distinguish small cementite particles with a diameter of less than 1μm. Using AFM imaging, we obtained force-penetration curves for small cementite particles as well as the ferrite portion of S25C steel. The curves show yield behavior. Below the yield force where the penetration depth was less than 30nm, the experimental curve agrees well with the theoretical one. The normalized yield stress, τY/G, is 0.46 for cementite particles and 0.24 for the ferrite portion. These values are nearly equal to those for glass and silicon, and for SNCM439 steel, respectively. It is concluded that the hardness measurement of the materials containing small particles was realized in the nanoscopic region, using the AFM ultramicro hardness tester with a special lever having a three-sided pyramidal diamond indenter with an apical angle of 60°.
