Doctoral Dissertations

Date of Award

8-2009

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Mechanical Engineering

Major Professor

Narendra B. Dahotre,

Committee Members

Craig A. Blue, Peter K. Liaw, Rao V. Arimilli

Abstract

Outstanding mechanical and physical properties like high thermal resistance, high hardness and chemical stability have encouraged use of structural ceramics in several applications. The brittle and hard nature of these ceramics makes them difficult to machine using conventional techniques and damage caused to the surface while machining affects efficiency of components. Laser machining has recently emerged as a potential technique for attaining high material removal rates. Major focus of this work is to understand the material removal mechanisms during laser machining of structural ceramics such as alumina (Al2O3), silicon nitride (Si3N4), silicon carbide (SiC) and magnesia (MgO). A 1.06 μm wavelength pulsed Nd:YAG laser was used for machining cavities of variable dimensions in these ceramics and an ab-initio computational model was developed to correlate attributes of machined cavities with laser processing conditions.

Material removal in Al2O3, Si3N4 and SiC takes place by a combination of melting, dissociation and evaporation while dissociation followed by evaporation is responsible for material removal in MgO. Temperature measurement at high temperatures being difficult, thermocouples were used to measure temperatures in the low temperature regime (700- 1150K). A thermal model was then iterated to obtain trends in absorptivity variation below phase transition temperature for these ceramics. Following this, measured machined depths were used as a benchmark to predict absorptivity transitions at higher temperatures (> 1150K) using the developed thermal model. For temperatures below phase transition, due to intraband absorption, the absorptivity decreases with increase in temperature until the surface temperature reaches the melting point in case of Al2O3, Si3N4 and SiC and the vaporization temperature in case of MgO. The absorptivity then continues to follow increasing trend with increasing temperature due to physical entrapment of laser beam in the cavity evolved during machining of certain depth in the ceramic. Rate of machining was predicted in terms of material removed per unit time and it increased with increase in heating rate.

Such a composite study based on comput ational and experimental analysis would enable advance predictions of laser processing conditions required to machine cavities of desired dimensions and thus assist in controlling the laser machining process more proficiently.

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