Optical studies on metal surfaces subjected to single and multiple (ultrasonic) impact

Abstract

The Thesis commences with a brief review of hardness measurements. The physical significance of indentation hardness is described as related to the yield stress and plasticity of metals. Previous work on the measurements of distortions of metal surfaces caused by static indentation is also reviewed. The techniques of Multiple-beam interference and multilayer dielectric films are described. Hertz's theory of elastic impact is given and the validity of some of the equations to the plastic impact is discussed. In Part A, Impact ball indentations in metals over a wide range of hardness are investigated employing multiple-beam interference methods, the following aspects being studied in detail. (1) The true diameter of an indentation is defined, and its dependence on the work-hardening capacity of the specimen is verified. (2) Shallowing or elastic recovery of the indentation is found to vary with the hardness of both the indenter and the specimen and with the velocity of impact. (3) Shallow indentations are found to be spherical only in the central region of the indentation. As the velocity of impact increases this region spreads and its radius of curvature decreases. Almost the whole of the indentation becomes spherical when the fully plastic stage is attained. (4) The volume of the surface distortion (flow pattern) is always found to be smaller than that of the indentation, but its rate of increase with the velocity of impact is larger than that of the indentation. It is found that the increase in the proportional density of the plastically deformed region surrounding the indentation is in agreement with the results of an earlier work on the density change of plastically deformed metal under high pressures. An important experimental result is that indentations produced in a given specimen at a constant velocity of impact with balls of different diameters are all geometrically similar so that all have the same value for shallowing, sphericity, and increase in the proportional density. (5) Multiple impact ball indentation increases the size of indentation but the proportion of the energy used to produce the plastic deformation in the successive impacts decreases. This is the result of work-hardening of the indentation surface. In Part B, the ultrasonic technique of drilling hard materials is first described. The distortion surrounding the drills or cuts in metals of widely differing hardness are studied by multiple-beam interference methods and light-cut microscopy. It is found that the extension of the surface distortion and its level above the surface depend on a number of factors, such as the size of the abrasive particles, the hardness of the specimen, the depth of the cut, and the static load applied during drilling. The influence of each of these factors is investigated. No work-hardening was detected in the machined surface. The results are qualitatively explained and discussed in the light of the experimental results of Part A, by considering the ultrasonic technique of drilling as repeated impacts by small indenters (the abrasive).