New numerical evaluation techniques for laser processed ceramic substrates

Abstract

To date laser drilling has not been clearly defined by a standard set of measurement parameters that fully define the features they produce. This article presents criteria for solving some of these issues, which in turn assist in the scientific characterization of the laser drilling process. The measurement criteria presented here fall into four categories: laser parameter, material science, in-process, and geometry measurements. Generation of thermal damage is the Achilles heel of laser processing of ceramics. The fundamental process of laser radiation interaction with ceramic substrates generates a severe thermal gradient between the recast layer and bulk substrate. This in turn leads to the stresses, which cause cracks to form. The numerical quantification of these cracks, in terms of size and distribution, is essential for developing an understanding of how cracks form, and how they can be avoided. It was found that the width of these cracks fell into two discrete size ranges. scanning electron microscope micrographs showed that when the crack width was 7 μm or more, the cracks propagate through the recast layer and into the bulk substrate, significantly weakening the component. These cracks were termed “macrocracks.” The cracks having a width smaller than 7 μm were termed “microcracks,” which were only found in the recast layer, and did not propagate into the bulk material. The numerical evaluation techniques presented here now provide a numerical route for the comparison of strength testing data, with respect to the change in size and distribution of cracking seen in laser processed thermally sensitive materials. This article also presents results from a study in techniques for the quantification of geometry features produced by the laser drilling process. This work employed high/low power optical and electron microscopy, linked with an image processing system, to define these features. The study focused on developing numerical methods for analyzing the surface hole dimensions (diameter, area, shape), and axial hole geometry (cross-sectional area, and material removal rate).