Ab initio Physical Analysis of Single Dimensional Laser Machining of Silicon Nitride

Abstract

High wear resistance, low electrical conductivity, high thermal stability and hardness are some of the vital properties that make silicon nitride one of the crucial engineering ceramics used for machining purposes in the aerospace, automotive and semiconductor industries. [1][2][3] Cams, piston rings, rocker arms and bearings can be made by machining silicon nitride. Non contact processing, effective material utilization, low manufacturing costs and high productivity have led to the increased use of lasers for machining a wide range of materials including ceramics such as alumina, silicon carbide and silicon nitride. [4][5] The laser processing conditions and the material properties significantly affect the physical processes such as energy absorption, ablation, melting, material removal and evaporation taking place during laser machining. This work demonstrates single dimensional laser machining of ceramics for applications such as drilling and generates a computational model to estimate the effect of laser processing parameters on the depth of the machined region. An insight into the depth of material machined by the application of certain number of pulses can be obtained from such a comprehensive model. The physical phenomena taking place at the surface and along the depth being different, predictions of the width of the cavity at the surface are not included in this study and will be presented in future. ExperimentalDense Si 3 N 4 obtained from a commercial source (Advanced Ceramics Manufacturing, Tucson, AZ) in the form of plates (12 mm × 15 mm and 3.5 mm thick) was exposed to a JK 701 pulsed Nd:YAG laser (1064 nm wavelength) from GSI Lumonics, Rugby, England. As pulse energy of 4 J, pulse width of 0.5 ms and a repetition rate of 20 Hz produced reasonable interaction between the laser and the ceramic surface, several pulses (3, 6, 10 and 20) were randomly applied to the Si 3 N 4 surface and the resultant depth in cross-section of machined cavity was measured from the optical micrograph ( Fig. 1) The bird's eye views of machined cavities on the top surface are also shown. In order to reduce the tapering effect, the holes/cavities were drilled/machined with a lens of longer focal length and longer focal waist. Computational ApproachThe surface temperature and the corresponding thermal gradient within the material were predicted in order to estimate the depth of machined region for a given set of laser processing parameters. The simulations began with the prediction of the maximum temperature reached after the first pulse by using a heat transfer flow model in COMSOL's heat transfer mode based on Fourier's second law of heat transfer. [6] The temperature rise during the ON time and the following temperature drop during the OFF time were estimated using Equations 1 and 2 [7] which were valid as the heat transfer occurring in the direction perpendicular to the laser beam could be neglected for the short time scale used in laser processing.

Differences in physical phenomena governing laser machining of structural ceramics

“…Processing conditions and governing physical phenomena for machining of these ceramics are only briefly discussed here for the sole purpose of comparison in this section. As mentioned before, however, detailed analysis of the machining mechanisms in these ceramics has already been conducted by Samant and Dahotre and separately presented elsewhere [6][7][8][9].…”

“…3Si ðlÞ þ 2N 2ðgÞ (9) Evaporation also assists in material removal and melting, dissociation and evaporation together lead to machining in Si 3 N 4 [7].…”

“…2a) [6] and for estimating depth of machined cavity when a certain number of pulses were applied to Si 3 N 4 (Fig. 2b) [7]. The model was also applied for advance prediction of number of pulses and corresponding energy necessary for machining SiC ceramic (Fig.…”

“…The intention of this paper is just to discriminate various mechanisms such as melting, dissociation and evaporation which could be responsible for machining of the different structural ceramics. However, the details of the physical processes and computational results associated with machining of each individual ceramic are separately described elsewhere [6][7][8][9].…”

Physical Effects of Multipass Two‐Dimensional Laser Machining of Structural Ceramics

“…[18,19,21] Due to similar reasons mentioned above for alumina, absorptivity first decreases to a 1 ¼ 0.031 at 1723 K [7] to attain melting/sublimation point of Si 3 N 4 (2173 K) [22] at end of residence time and material is removed by melting, and dissociation into liquid silicon and its expulsion. The absorptivity then increases to a 2 ¼ 0.07 so that boiling point of silicon (3514 K) [23] is reached and vaporization of silicon leads to machining.…”