The mechanism governing cutting of Polycrystalline Cubic Boron Nitride (PCBN) tool blanks with phase transformation induced fracture

Abstract

The thesis presents a combined experimental and computational investigation of a novel thermochemical material removal mechanism for cutting of polycrystalline cubic boron nitride (PCBN) substrates through controlled crack propagation. The CO2-Laser/Waterjet machining system developed by the Iowa State University’s Laboratory for Lasers, MEMS, and Nanotechnology was utilized to achieve specimen cutting. It has been proved that the hybrid machining method is more efficiency and able to provide high quality cutting that overcomes the major drawbacks with current EDM and Nd:YAG laser machining techniques. The LWJ method implemented a high power laser heating followed by low pressure waterjet quenching that achieved fracture initiation and propagation along the cutting path. The purposes of water in LWJ machining are: (1) help the phase transformation as to induce larger tensile stresses for crack propagation and (2) provide thermal shock that reduce the fracture toughness of material in specimen. Two forms of PCBN specimens: the solid compact and the composite with tungsten carbide substrate were studied in this thesis. The mechanism governing the fracture behavior was studied through Raman spectroscopy, Scanning Electron Microscopy (SEM), and surface profile measurement with profilometer. FEA model associated with machining parameters was developed to validate the machining mechanism and provide prediction of fracture behavior. Good agreement between simulation prediction and experimental observation was achieved.