Evolution of surface topography in one-dimensional laser machining of structural alumina

doi:10.1016/j.jeurceramsoc.2012.06.015

Journal of the European Ceramic Society

Volume 32, Issue 16, December 2012, Pages 4205-4218

https://doi.org/10.1016/j.jeurceramsoc.2012.06.015 Get rights and content

Abstract

High energy lasers are an emerging industrial tool to fabricate complex shapes on hard and brittle structural ceramics such as alumina. The selection of laser processing parameters and the prediction of material removal rates during the laser machining are the critical issues. This paper was attempted to present the state of the art of laser machining of alumina using an integrated experimental and computational approach. A multistep computational model based on COMSOL™ Multiphysics was developed to study the influence of various single-pulse laser energy densities and associated physical phenomena (recoil pressure, Marangoni convection, and surface tension) on the temperature history, fluid velocity, crater size, and surface topography. A pulsed Nd:YAG laser was employed to machine alumina under different processing conditions. The surface topography of laser machined alumina was measured by an optical profilometer and the results were compared with the computationally predicted topographic parameters with reasonably close agreement.

Introduction

Structural ceramics, such as alumina (Al₂O₃), zirconia (ZrO₂), magnesia (MgO), silicon carbide (SiC), and silicon nitride (Si₃N₄) exhibit excellent mechanical and physical properties. These ceramics have been successfully used in the area where high hardness, superior wear resistance, low thermal and electrical conductivity, chemical stability, and high thermal resistance properties are desirable.1, 2 Also, the retention of these properties at elevated temperatures provides an exclusive solution for several industrial applications: electronic, automotive, medical and so on.³ Among them, alumina is used in making machine tool inserts, heat-resistant packing, electrical and electronic components, and attachments for the melting ducts and refractory linings.⁴ Alumina is also used as a substrate in hybrid circuits because of its excellent dielectric strength, thermal stability and conductivity.⁵ However, despite having these superior properties, alumina is hindered from several applications because of its limitations toward machining into desirable components. Since alumina is hard and brittle, it is very difficult to machine using conventional machining techniques.1, 2

In the past, substantial research on precision machining of ceramic components has been conducted for the cutting, grinding and polishing processes.1, 2 Unacceptable tool wear, insufficient accuracy, mechanical or thermal damage of the work piece are the main limiting factors that prevent the manufacturing of accurate geometries in ceramic components using conventional machining techniques.1, 2 Among them, grinding is still considered to be the most desirable technique to machine ceramic components with good dimensional accuracy as well as surface finish.⁶ However, longer machining time and higher operating costs pose a major drawback for the grinding process.7, 8 Furthermore, the finish products often demonstrate surface and subsurface cracks,9, 10, 11 some amount of plastic deformation,¹² pulverization layers,13, 14 and significant surface residual stresses.¹⁵ Hence, a cost effective ceramic machining technique is an immediate need for several industrial applications.

Laser machining is a non-contact technique that overcomes many of the drawbacks associated with conventional machining techniques. Recently, it has emerged as an innovative and potential tool for bulk material removal and fabrication of complex structures of ceramics.1, 2, 16, 17, 18, 19 A large volume of experimental and computational work has been conducted using laser-machining techniques,1, 2, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25 laser-assisted chemical etching,²⁶ and laser-assisted machining27, 28, 29, 30, 31, 32 for fabricating complex-shaped structural ceramics.

Dahotre et al. have been extensively involved in the research on laser machining of structural ceramics.1, 2, 16, 17, 23, 24 Previously, the work of Samant was focused to understand the material removal mechanisms during laser machining.1, 2, 16, 23, 24 In his work, a computational model was developed that correlated the laser machining conditions with attributes of machined cavities in various ceramics.1, 2 However, the study mainly emphasized on optimizing the laser machining parameters to improve the material removal rates during laser machining of ceramics (such as alumina, zirconia, magnesia, silicon nitride, and silicon carbide) and therefore disregarded the issues related to surface finish.

On the contrary, the objective of the current integrated computational and experimental approach is to understand the laser-machining mechanism on alumina and its subsequent effect on the surface topography. A computational model is developed and experimentally validated, to understand the laser-machining mechanisms and evolution of related surface topography of alumina. The model incorporated empirical boundary conditions, temperature-dependent material properties, and phase-change kinetics.

Section snippets

Evolution of surface topography

During laser machining, when the laser beam strikes the surface of material, some part of the laser energy is lost due to the reflection and the remaining part of the energy is absorbed by the material. The absorbed laser energy, in turn, causes several phenomena, such as heating, melting, vaporization, and plume formation on the surface of material.1, 2, 16, 17, 18, 19 The effects of these phenomena predominantly depend on the laser process parameters and the material properties. The various

Sample preparation and laser machining

An alumina slab (90 × 65 × 6 × 10⁻⁹ m³) with purity of 99.6 wt.% (<0.1% SiO₂, <0.05% Fe₂O₃, and <0.1% R₂O all in wt.%) obtained from Advalue Technology Inc., Tucson, AZ, was used in this study. A slow-speed diamond saw was used to prepare the coupons of size of 25 × 20 × 6 × 10⁻⁹ m³ for laser machining. The samples were then machined using JK 701 Lumonics pulsed Nd:YAG (1.064 μm wavelength) fiber optic laser system. The laser machining parameters used in this study are presented in Table 1, and the layout of

Evaporative material removal

The temperature history extracted from boundary 6 (Fig. 3) for various single-pulse laser machining conditions is presented in Fig. 8a. The rise and drop of the temperature is due to rapid heating by the high laser intensities, self-quenching by the bulk mass material, and due to the losses by external natural convection and radiation that in turn control the different physical phenomena taking place in the material. From the heating and cooling temperature histories for various laser machining

Conclusion

A computational model using COMSOL™ Multiphysics was developed to understand the influence of single-pulse one-dimensional laser machining on the surface finish of alumina under various laser energy densities. Results indicated that the material loss due to ablation increased the crater depth, while the strong velocity gradient created by the recoil pressure increased the liquid pile-up. Both of these effects increased the overall surface roughness. In this study, less material removal with a

Acknowledgments

The authors, NBD and HDV would like to acknowledge the financial support from the National Science Foundation (NSF-CMMI 1010494). The authors would also like to appreciate Dr. Radovan Kovacevic, Southern Methodist University, for providing the research facilities for surface roughness measurement.

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Evolution of surface topography in one-dimensional laser machining of structural alumina

Abstract

Introduction

Section snippets

Evolution of surface topography

Sample preparation and laser machining

Evaporative material removal

Conclusion

Acknowledgments

J Eur Ceram Soc

J Mater Process Technol

Appl Surf Sci

J Mater Process Technol

Ann CIRP

J Mater Process Technol

Int J Mach Tools Manuf

Ceram Int

Int J Mach Tools Manuf

Int J Heat Mass Transfer

Int J Heat Mass Transfer

Int J Mach Tools Manuf

J Comput Phys

Advanced ceramic structural materials

Refract Ind Ceram

Machining of ceramic components: process-technological potentials, machining of advanced materials

NIST Spec Publ

Laser assisted machining: an overview

J Manuf Sci Eng

A study of grinding damage in magnesium oxide single crystals

J Mater Sci

Damage penetration at elongated machining grooves in hot-pressed Si3N4

J Am Ceram Soc

Crack branching as a mechanism of crushing during grinding

J Am Ceram Soc

Microfracture and material removal in scratching of alumina

J Mater Sci

Precision grinding regime of advanced ceramics

Proc Annu Meeting Am Soc Precis Eng

Laser machining of advanced materials

Damage penetration at elongated machining grooves in hot-pressed Si₃N₄