An official website of the United States government
In this paper, the modified slip/fracture activation model has been used in order to understand the mechanism of ductile-brittle transition on the R-plane of sapphire during ultra-precision machining by reflecting direction of resultant force. Anisotropic characteristics of crack morphology and ductility of machining depending on cutting direction were explained in detail with modified fracture cleavage and plastic deformation parameters. Through the analysis, it was concluded that crack morphologies were mainly determined by the interaction of multiple fracture systems activated while, critical depth of cut was determined by the dominant plastic deformation parameter. In addition to this, by using proportionality relationship between magnitude of resultant force and depth of cut in the ductile region, an empirical model for critical depth of cut was developed.
more »
« less
Understanding of Residual Stress and Subsurface Damage by 2- Step Machining of Single Crystal Sapphire
Machining is in general conducted in multiple paths and thus residual stress and subsurface damage formed by previous cut may influence subsequent cutting. Ceramics materials are extremely brittle and prone to cracks. Ultra-precision machining with very small depth of cut enables ductile mode cutting. There have been various reports that critical depth of cut (CDC) for single crystal sapphire exists, where the ductile to brittle transition occurs. However, the CDC of subsequent cutting changes due to the influence of residual stress and subsurface damage by previous cut. This study investigates the indirect effect of residual stress and subsurface damage on the critical depth of cut of the second cut by analyzing the plastic deformation mechanisms activated during 2-step machining on A-plane of sapphire. It was found that the [1#100] machining orientation was most suitable since the critical depth of cut remained fairly constant due to dominant rhombohedral twinning activation during subsequent machining operations.
more »
« less
- Award ID(s):
- 2008563
- PAR ID:
- 10446549
- Editor(s):
- The Japan Society for Precision Engineering
- Date Published:
- Journal Name:
- 19th International Conference on Precision Engineering
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract In recent years, semiconductors, electronics, optics, and various other industries have seen a significant surge in the use of sapphire materials, driven by their exceptional mechanical and chemical properties. The machining of sapphire surfaces plays a crucial role in all these applications. However, due to sapphires’ exceptionally high hardness (Mohs hardness of 9, Vickers hardness of 2300) and brittleness, machining them often presents challenges such as microcracking and chipping of the workpiece, as well as significant tool wear, making sapphires difficult to cut. To enhance the machining efficiency and machined surface integrity, ultrasonic vibration-assisted (UV-A) machining of sapphire has already been studied, showing improved performance with lower cutting force, better surface finish, and extended tool life. Scribing tests using a single-diamond tool not only are an effective method to understand the material removal mechanism and deformation characteristics during such UV-A machining processes but also can be used as a potential process for separating IC chips from wafers. This paper presents a comprehensive study of the UV-A scribing process, aiming to develop an understanding of sapphire’s material removal mechanism under varying ultrasonic power levels and cutting tool geometries. In this experimental investigation, the effect of five different levels of ultrasonic power and three different cutting tool tip angles at various feeding depths on the scribe-induced features of the sapphire surface has been presented with a quantitative and qualitative comparison. The findings indicate that at feeding depths less than 6 μm, UV-A scribing with 40–80% ultrasonic power can reduce cutting force up to 50% and thus improve scribe quality. However, between feeding depths of 6 to 10 μm, this advantage of using ultrasonic vibration gradually diminishes. Additionally, UV-A scribing with a smaller tool tip angle (60°) was found to lower cutting force by 65% and improve scribe quality, effectively inhibiting residual stress formation and microcrack propagation. Furthermore, UV-A scribing also facilitated higher critical feeding depths at around 10 μm, compared to 6 μm in conventional scribing.more » « less
-
In this paper, material deformation during ultra-precision machining (UPM) on the C-, R-, and A-planes of sapphire was investigated using the slip/fracture activation model where the likelihood of activation of individual plastic deformation and fracture systems on different crystallographic planes was calculated. The stress data obtained from molecular dynamics (MD) simulations were utilized, and the slip/fracture activation model was developed by incorporating the principal stresses in calculating the plastic deformation and fracture cleavage parameters. The analysis methodology was applied to study material deformation along various cutting orientations in sapphire. The stress field at crack initiation during UPM on C-, R-, and A-planes of sapphire was calculated using molecular dynamics (MD) simulations. An equation describing the relationship between crack initiation and its triggering parameters was formulated considering the systems’ plastic deformation and cleavage fractures. The model can qualitatively predict the crack initiations for various cutting orientations. The proposed model was verified through ultra-precision orthogonal plunge cut experiments along the same cutting orientations as in the MD simulations.more » « less
-
Abstract Single crystal Ge is a semiconductor that has broad applications, especially in manipulation of infrared light. Diamond machining enables the efficient production of surfaces with tolerances required by the optical industry. During machining of anisotropic single crystals, the cutting direction with respect to the in-plane lattice orientation plays a fundamental role in the final quality of the surface and subsurface. In this study, on-axis face turning experiments were performed on an undoped (111)Ge wafer to investigate the effects of crystal anisotropy and feedrate on the surface and subsurface conditions. Atomic force microscopy and scanning white light interferometry were used to characterize the presence of brittle fracture on the machined surfaces and to evaluate the resultant surface roughness. Raman spectroscopy was performed to evaluate the residual stresses and lattice disorder induced by the tool during machining. Nanoindentation with Berkovich and cube corner indenter tips was performed to evaluate elastic modulus, hardness, and fracture toughness of the machined surfaces and to study their variations with feedrate and cutting direction. Post-indentation studies of selected indentations were also performed to characterize the corresponding quasi-plasticity mechanisms. It was found that an increase of feedrate produced a rotation of the resultant force imparted by the tool indicating a shift from indentation-dominant to cutting-dominant behavior. Fracture increased with the feedrate and showed a higher propensity when the cutting direction belonged to the <112¯> family.more » « less
-
Single-crystal calcium fluorite (CaF2) is widely used for transmissive optics in ultraviolet and vacuum ultraviolet (UV and VUV) wavelength applications because of its exceptional transmission performance. Generally, products using CaF2 are manufactured through finishing processes such as chemo-mechanical polishing (CMP), magnetorheological finishing (MRF) or ion-beam figuring (IBF) after performing precision cutting and grinding processes for profiling. However, CaF2 is known as a brittle material with high anisotropy, and subsurface damage is induced by each cutting process. But, the effects of surface integrity on the optical and functional performance of precision machined CaF2 has not been reported yet. In this research, a newly developed multiaxial adjustment system that can precisely align specimens is used in single-axis orthogonal cutting experiments with zero degree and negative rake angle diamond radius tools to prevent pre-machining and thus pre-damaging of single-crystal CaF2 specimens. Cutting forces evaluation via piezoelectric dynamometer acquisition as well as surface analysis by atomic force microscopy and white light microscopy has been performed. Finally, smooth surfaces due to ductile material removal mechanisms could be determined on all machined specimen surfaces.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Conference Paper:
- The DOI is not currently available.
