Optical studies of distortions of metal surfaces produced by indenting

Abstract

The definitions and types of measurement of hardness are reviewed with special reference to the static indentation test. The present state of the theory of static indentation hardness testing of metals is described, together with previous measurements of the distortions of metal surfaces caused by indenting. Multiple beam interferometry is employed to measure the surface distortions. The apparatus and techniques employed, which include the application of multilayer dielectric films to the interferometrie study of metal surfaces, are described. The specific aspects of the surface distortions studied are: (1) Flow pattern shape, with special reference to the transition from ridging to sinking impressions, and variations due to the geometric shape of the indentation. (2) Volume of the flow pattern. This is found to be smaller than the volume of the indentation for all the indentations studied. Possible explanations of this discrepancy are discussed. (3) Shallowing or elastic recovery of ball indentations. The shallowing of ball indentations in a wide range of metals is studied, and its variation with load, hardness, and indenter is analysed. (4) The shape of recovered ball indentations. Shallow ball indentations are found to be not strictly spherical, in contrast to deeper ball indentations. Measures of their deviation from sphericity are proposed and an explanation of these deviations is discussed. (5) Critical specimen size. The mode of deformation occurring when a hardness test indentation is made near to the edge of the specimen is studied. The results are compared with the mode of deformation for wedge indentations in ideal and real metals. On the basis of the results a fuller description of the processes involved in ball hardness tests is possible. The surface distortions from the onset of plasticity to the fully plastic stage are described in detail, and support Tabor's theory of ball indentation. The treatment of the hardness test as predominantly a problem in plasticity is justified.