Doctoral Dissertations

Date of Award

12-2007

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Materials Science and Engineering

Major Professor

Narendra B. Dahotre

Committee Members

Carl J. McHargue, Easo P. George, Dayakar Penumadu

Abstract

Alumina ceramic is an important abrasive material for grinding wheels used for rough grinding/machining of materials in manufacturing industry. Purpose of this work is to explore laser surface structuring of alumina grinding wheels for precision machining/grinding of materials by modifying surface microstructure of wheels. Major objective of this work is to study the evolution of surface microstructure and depth of modification such that microstructures/properties of modified wheels can be efficiently tailored based on fundamental understanding of physical processes taking place during laser surface structuring.

Surface structuring of alumina using a continuous wave Nd:YAG laser resulted in significant surface melting and subsequent rapid solidification. The surface modified alumina consisted of microstructure characterized by regular polygonal and faceted surface grains with well defined edges and vertices. Such multifaceted grains act as micro-cutting tools on the surface of grinding wheels facilitating micro-scale material removal during precision machining. The formation of faceted morphology is explained on the basis of evolution of crystallographic texture in laser modified alumina. Furthermore, complete crystallographic description of multifaceted morphology of surface grains is provided based on detailed analysis of surface micro-texture. Due to complexity of microstructure formed during laser surface structuring, a fractal analysisbased approach is suggested to characterize surface microstructures. Detailed analysis of the effects of laser interaction with porous alumina ceramic indicated that melt surface undergoes rapid evaporation resulting in generation of high (>105 Pa) evaporationinduced recoil pressures. These pressures drive the flow of melt through underlying porous alumina during modification extending the depth of modification. An integrative modeling approach combining thermal analysis and fluid flow analysis resulted in better agreements between predicted and experimental values of depths of melting. Finally, improvements in microindentation fracture toughness of alumina ceramic are reported with increasing laser fluence. Such improvements in the fracture toughness seem to be derived from better surface densification and coarsening of grain structure.

The understanding of the evolution of faceted morphology, depth of surface modifications and improvements in fracture properties in laser surface microstructured alumina ceramic reached in this work provides the foundation for tailoring of surface microstructures/properties of alumina grinding wheels for precision machining applications.

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