Session: Government Agency Student Posters

Paper Number: 173318

High-Resolution Characterization of Machining-Induced Subsurface Strain in Single-Crystal Sapphire 

For applications requiring extremely tight tolerances on part geometry (on the order of sub-microns) and strict control of surface roughness, ultra-precision machining (UPM) has been widely applied to a range of ceramic materials. These include optical component fabrication, mold and die manufacturing, and microfluidic device production. During the machining process, localized stress concentrations are observed near the tool–workpiece interface, which can activate slip along basal, rhombohedral, or prismatic planes, as well as deformation twinning in various orientations, as confirmed by molecular dynamics (MD) simulations. These simulations suggest that local strains can reach up to 10⁻⁵. We hypothesize that such mechanisms generate complex stress fields in the contact zone, leading to residual stress accumulation that influences subsequent deformation behavior.

There are several theoretical models that have been developed to predict deformation behavior under specific crystallographic orientations. These models typically assume the activation of a single dominant deformation or fracture mode. As a result, they offer only limited probabilistic predictions and fail to capture the full complexity of stress evolution observed during actual machining processes. In practice, multiple deformation and fracture mechanisms can be activated simultaneously and interact with one another, leading to a far more intricate stress field than current models can account for. However, measuring the depth-profile of strain remains challenging. Raman spectroscopy lacks the spatial resolution and depth sensitivity needed for subsurface stress analysis, as it only captures information from the surface but the sapphire deformation during machining often extends several micrometers below the surface. Transmission electron microscopy (TEM) can be used to investigate localized deformation zones. It is inherently limited to very small regions and often fails to provide a complete picture of the field of stress. Moreover, the sample preparation process may introduce partial stress relaxation, reducing the accuracy of the measurements. To overcome these limitations, we used the nondestructive, high spatial resolution and high strain sensitivity of X-ray nanobeam microscopy combined with nanobeam fluorescence imaging in Argonne National Lab to probe the complex strains. We propose using X-ray nanoprobe microscopy at beamline 26-ID, which offers a focused beam size of 20 nm and strain sensitivity down to 10⁻⁶. This technique enables sub-micron mapping of stress fields beneath machined surfaces, allowing for spatially resolved analysis of residual stress associated with active deformation mechanisms.

With X-ray Nanoprobe, our preliminary experiments have demonstrated that stress variations resulting from different cutting directions can be resolved with a spatial resolution of approximately 200 nm. These results include clear comparisons between unmachined areas and machined regions with different crystallographic orientations, highlighting detectable differences in strain distributions. From them, we measured 5D mapping and can get 41 by 128-pixel points fitting so that we can know how difference between the strained line and unstrained line. However, these initial studies are limited to only two crystallographic orientations for now. Further experiments across additional orientations are needed to develop a comprehensive understanding of stress evolution. Building on this, we will map strain distributions across multiple crystallographic planes to uncover the interplay between stress localization and the activation of specific deformation systems and also can get the depth-profile of strains. Ultimately, this work will lay the foundation for predictive modeling and enable improved reliability and performance of sapphire components under ultra-precision machining.

Presenting Author: Rui Liang University of Wisconsin - Madison

Presenting Author Biography: Rui Liang is a Ph.D. candidate in Mechanical Engineering at the University of Wisconsin–Madison. Her research focuses on ultra-precision machining (UPM) of hard and brittle materials. Her current work investigates the anisotropic behavior of single-crystal sapphire, with an emphasis on understanding residual stress generation and subsurface damage during UPM. Rui’s research aims to improve the fundamental understanding of deformation mechanisms in anisotropic crystals and enhance surface integrity in precision manufacturing.

Authors:

Rui Liang University of Wisconsin - Madison
Sangkee Min University of Wisconsin - Madison
Rui Liu Argonne National Laboratory