| Ploughing is a material deformation mechanism that is a component of all mechanical machining operations. As the demand for a greater degree of surface integrity, surface finish, and form accuracy continues to increase, there is a tendency towards smaller depths of cut in machining operations. At small depths of cut, the energy consumed by ploughing becomes the dominant component of the machining energy. It is the purpose of this research is to understand how the ploughing mechanism influences the yield stress and surface integrity of face centered cubic metals. A pyramidal indentor, made of diamond, was used to plough surfaces of gold and iridium, at depths of cut ranging from 10 nm to 1600 nm, using a modified scanned probe microscope (SPM). Since very shallow depths of cut were induced, the metallic surface had to oxide-free. The inert characteristics and vastly different mechanical properties of gold and iridium, were the reasons for choosing them as the materials to be ploughed. A rigid/perfectly plastic model of the process was developed that calculated the materials' yield stress based upon the measured normal ploughing force and the groove dimensions. These calculations revealed that the materials strain hardened. This behavior had not been previously observed when displaced material volumes were less than 1 {dollar}mu{dollar}m{dollar}sp3{dollar}. Transmission electron microscopy (TEM) was used to verify these results and to observe the details of the strain hardening zone and the material flow around the grooves. This work clearly shows that, as the depth of cut increases, there is a transition from surface dominated behavior to bulk dominated behavior within the metals. The length of the transition depends upon the stacking fault energy and the shear modulus of the metals. These investigations have provided an understanding of how surface proximity, materials properties, and indentor geometry interact to modify a material's surface during the ploughing process. |
