Date of Award
Doctor of Philosophy
Engineering Mechanics; Aerospace Engineering
Ashraf F. Bastawros
Paired polishing process (PPP) is a variant of the chemical mechanical polishing process which facilitates defect mitigation via minimization of maximum force as well as effective planarization via profile driven determination of force gradient. The present embodiment of PPP machine employs two polishing wheels, radially spanning the wafer surface on a counter-gimbaled base. The PPP machine is deployed to experimentally investigate the role of the process parameters on the surface roughness evolution, and the effective material removal rate. Two sets of copper and aluminum blanket layers were polished under a range of applied down force, polishing wheel speed and transverse feed rate to examine the scalability of the process parameters for different material constants. The experimental measurements along with the topological details of the polishing pad have been utilized to develop a mechanistic model of the process. The model employs the soft wheel-workpiece macroscopic contact, the polishing wheel roughness and its amplification to the local contact pressure, the kinematics of abrasive grits at the local scale, and the collective contribution of these individual micro-events to induce an effective material removal rate at the macroscale. The model shows the dependence of the material removal on the ratio of wheel rotational to feed speed for the PPP process, in a form of an asymptote that is scaled by the surface hardness of each material. The PPP machine exploits this insight and utilizes an oblique grinding technique that obviates the traditional trade-off between MRR and planarization efficiency.
High speed grinding with cubic boron nitride (CBN) wheels are industrially attractive options for hard-to-machine metallic alloys, due to their low cost, reliability, reduced thermal damage and superior workpiece surface finish. However, thermal issues and transient behavior of the grinding wheel wear directly affect the workpiece surface integrity and tolerances. This thesis investigates the topological evolution of an electroplated CBN grinding wheel, characterization of its wear and life expectancy, when utilized in nickel-based alloy grinding. Depth profiling, digital microscopy and scanning electron microscopy are utilized to investigate topological evolution and mechanisms of grit failure. The results are used to elucidate the statistical evolution of the grinding wheel surface. It is found that when a grit is pulled out, load redistribution commences in its neighboring domain, with localized rapid grit wear. The unique experimental findings are used to develop a novel phenomenological model for the progressive wheel wear, including the combination of grit pullout (Stage I) and grit wear (Stage II). For Stage-I, the model employs the grinding kinematics, thermal shock to the grit-wheel interface, Paris law type fatigue approach. For Stage-II, Preston type wear approach is employed. The molding framework is utilized to infer electroplated CBN grinding wheels’ life expectancy for the high speed grinding and high efficiency deep grinding (HEDG) processes. The model provides the process design domain for different grinding process parameters, while maintaining a targeted wheel life, and averting potential damage of the workpiece.
Yu, Tianyu, "Material removal modeling and life expectancy of electroplated CBN grinding wheel and paired polishing" (2016). Graduate Theses and Dissertations. 16045.