Effect of Thermal Softening on Anisotropy and Ductile Mode Cutting of Sapphire Using Micro-Laser Assisted Machining

At a Glance

Metadata	Details
Publication Date	2016-12-08
Journal	Journal of Micro and Nano-Manufacturing
Authors	Hossein Mohammadi, John Patten
Institutions	Western Michigan University
Citations	27
Analysis	Full AI Review Included

Technical Analysis & Documentation: MPCVD Diamond for Micro-Laser Assisted Machining (µ-LAM)

Executive Summary

This research validates the critical role of high-purity, optically transparent diamond tools in enabling ductile-mode cutting (DMC) of highly brittle materials, specifically monocrystal sapphire ($\text{Al}{2}\text{O}{3}$), through Micro-Laser Assisted Machining (µ-LAM). The findings confirm that localized thermal softening via laser transmission through the diamond tool is highly effective in pushing the material’s ductile-to-brittle transition (DBT) boundary, a requirement for high-precision optics manufacturing.

Application Focus: Precision machining of highly anisotropic and hard brittle materials (Sapphire) for optics and semiconductor industries.
Core Achievement: Successful transition from brittle to ductile cutting mode by localizing heat (300-1000 °C) via a 1070 nm CW laser transmitted through the diamond cutting tool.
Anisotropy Confirmation: DBT depth and resulting cut quality vary significantly based on crystal direction ([1 1 00] vs. [11 20]).
Ductility Enhancement: Laser heating significantly increased the DBT depth, with the highest enhancement being $144%$ increase in DBT depth observed in the [1 1 00] direction at $16.8\text{ W}$ laser power.
Material Requirement: The process necessitates high-purity, optically clear, single-crystal diamond (SCD) tooling capable of transmitting high-power near-infrared (NIR) laser radiation without degradation or significant scattering.
6CCVD Value Proposition: We provide the necessary Optical Grade SCD wafers and custom tooling services (polishing, shaping, metalization) essential to replicate and advance this high-precision hybrid manufacturing technique.

Technical Specifications

The following parameters summarize the critical material properties and operational conditions used in the µ-LAM scratch tests on C-plane monocrystal sapphire.

Parameter	Value	Unit	Context
Workpiece Material	Monocrystal Sapphire ($\text{Al}{2}\text{O}{3}$)	N/A	Hexagonal-rhombohedral Structure
Sapphire Hardness (Knoop)	2200	N/A	Perpendicular to C-axis
Diamond Tool Rake Angle	-45	°	Negative rake for compressive stress
Diamond Tool Nose Radius	1	mm	Geometry used for single-point scratch test
Laser Type	Fiber Laser (CW)	N/A	Continuous Wave, 1070 nm Wavelength
Actual Laser Output Range	1.6 to 16.8	W	Measured output after transmission through diamond
Target Cutting Zone Temperature	300 - 1000	°C	Required for thermal softening (ductility enhancement)
Cutting Speed	1	µm/s	Ultra-low feed rate used on tribometer
Thrust Load Range (DBT Test)	50 - 700	mN	Used to determine critical thickness for DBT
Maximum DBT Depth Increase	$144$	$%$	Achieved using $16.8\text{ W}$ laser power in [1 1 00] direction

Key Methodologies

The experiment employed a highly controlled hybrid machining process combining mechanical scratch testing with localized thermal assistance, focusing on material anisotropy.

Diamond Tool Preparation: A transparent diamond cutting tool was mounted on a tribometer programmed for extremely low cutting speeds ($1\text{ µm/s}$). The tool featured a negative rake angle ($-45\text{°}$) designed to maximize compressive stress and suppress initial fracture.
Laser Integration ($\mu$-LAM): A $1070\text{ nm}$ Continuous Wave (CW) fiber laser was directed through the transparent diamond cutting tool, focusing the thermal energy precisely at the chip formation zone. The actual laser output powers (1.6 to $16.8\text{ W}$) were measured after transmission through the tool.
Anisotropic Testing: Scratch tests ($100\text{ µm}$ length) were performed on a C-plane sapphire wafer along four distinct crystal directions: [11 20], [1 1 00], [1120], and [1 100], to analyze the effect of anisotropy on cutting depth and mode.
Constant Load Analysis: Cuts were performed at fixed thrust loads ($100$, $200$, and $300\text{ mN}$) across the laser power range ($0\text{ W}$ to $16.8\text{ W}$) to assess depth increase and cutting mode (ductile/brittle).
Ductile-to-Brittle Transition (DBT) Tests: A separate series of tests utilized increasing thrust loads ($50\text{ mN}$ up to $700\text{ mN}$) at fixed laser power levels. The DBT depth was defined and measured as the deepest point achieved immediately before the cutting mode visibly transitioned from ductile (smooth) to brittle (fractured).
Post-Process Characterization: A white light interferometer was used to generate microscopic images and 3D profiles to quantify the depth of cut and visualize the ductile, brittle, and transition regions.

6CCVD Solutions & Capabilities

Replicating and extending this state-of-the-art research requires ultra-high-purity, optically transparent diamond material and precision engineering capabilities. 6CCVD specializes in providing the MPCVD Single Crystal Diamond (SCD) that is indispensable for µ-LAM tooling.

Applicable Materials and Tooling

Component Requirement	6CCVD Solution	Material Specification	Value Proposition
IR-Transparent Tooling	Optical Grade SCD Wafers	Single Crystal Diamond (SCD) offers the superior optical transparency necessary to transmit $1070\text{ nm}$ NIR laser energy directly to the cutting zone, enabling effective thermal softening.	SCD ensures maximum laser coupling efficiency and prevents tool degradation due to absorption.
Tool Wear Resistance	High Mechanical Strength SCD	Diamond (Mohs 10) is the only suitable material for efficient, long-track machining of Sapphire (Mohs 9).	6CCVD SCD minimizes tool wear observed during ultra-low speed testing, improving process reliability.
Custom Tool Geometry	Precision Laser Shaping Service	Customized fabrication of diamond plates/wafers up to $125\text{mm}$. We offer internal laser cutting to meet specific tool geometries (e.g., $1\text{ mm}$ nose radius, specific rake/relief angles).	Enables rapid prototyping and scaling of customized µ-LAM tools.
Surface Quality	Ultra-Fine Polishing	SCD wafers are polished to an exceptional surface roughness of $\text{Ra} < 1\text{nm}$.	Minimizes laser scattering and enhances metrology precision during critical depth measurements.
System Integration	Custom Metalization Layer	Internal capability to apply standard metal layers (e.g., $\text{Ti}$, $\text{Pt}$, $\text{Au}$, $\text{W}$) required for secure brazing or integration of the SCD tool into the tribometer/cutting head assembly.	Streamlines assembly and ensures reliable thermal management in hybrid systems.

Customization Potential

The research highlights that optimization of the µ-LAM process—specifically critical chip thickness and DBT control—is highly dependent on the tool’s geometry, which must be flawless.

Custom Dimensions: 6CCVD delivers SCD plates and wafers in custom sizes and thicknesses (from $0.1\text{µm}$ to $500\text{µm}$) required for next-generation µ-LAM apparatus.
Orientation Control: For research involving anisotropic materials like sapphire, 6CCVD offers SCD materials with verified crystal orientations, crucial for modeling and validating results across different cutting directions, as demonstrated in this study.
Global Delivery: We offer reliable global shipping (DDU default, DDP available) to research institutions and production facilities worldwide.

Engineering Support

The successful execution of high-precision hybrid machining hinges on selecting the exact right diamond properties (e.g., thermal management, optical clarity, crystallographic orientation). 6CCVD’s in-house PhD engineering team possesses deep expertise in MPCVD diamond properties and can provide consultation on:

Material Selection: Guiding researchers toward the optimal Optical Grade SCD specifications needed for high-power $1070\text{ nm}$ laser transparency.
Machining Optimization: Assisting with material choice and tool design for similar $\text{µ}$-LAM projects involving other hard, brittle semiconductors or ceramics (e.g., $\text{SiC}$, Spinel) where precise Ductile Mode Control is essential.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

Ceramics and semiconductors have many applications in optics, micro-electro-mechanical systems, and electronic industries due to their desirable properties. In most of these applications, these materials should have a smooth surface without any surface and subsurface damages. Avoiding these damages yet achieving high material removal rate in the machining of them is very challenging as they are extremely hard and brittle. Materials such as single crystal silicon and sapphire have a crystal orientation or anisotropy effect. Because of this characteristic, their mechanical properties vary significantly by orientation that makes their machining even more difficult. In previous works, it has been shown that it is possible to machine brittle materials in ductile mode. In the present study, scratch tests were accomplished on the monocrystal sapphire in four different perpendicular directions. A laser is transmitted to a diamond cutting tool to heat and soften the material to either enhance the ductility, resulting in a deeper cut, or reducing brittleness leading to decreased fracture damage. Results such as depth of cut and also nature of cut (ductile or brittle) for different directions, laser powers, and cutting loads are compared. Also, influence of thermal softening on ductile response and its correlation to the anisotropy properties of sapphire is investigated. The effect of thermal softening on cuts is studied by analyzing the image of cuts and verifying the depth of cuts which were made by using varying thrust load and laser power. Macroscopic plastic deformation (chips and surface) occurring under high contract pressures and high temperatures is presented.

Tech Support

Original Source

DOI: https://doi.org/10.1115/1.4035397

References

2013 - Experimental Study on Brittle-Ductile Transition in Elliptical Ultrasonic Assisted Grinding (EUAG) of Monocrystal Sapphire Using Single Diamond Abrasive Grain [Crossref]
2015 - Fractal Analysis of Surface Topography in Ground Monocrystal Sapphire [Crossref]
2010 - Femtosecond Laser Ablation of Sapphire on Different Crystallographic Facet Planes by Single and Multiple Laser Pulses Irradiation [Crossref]
2016 - Surface Characterization of Diamond Film Tool Grinding on The Monocrystal Sapphire Under Different Liquid Environments [Crossref]
2016 - Laser Processing of Sub-Wavelength Structures on Sapphire and Alumina for Millimeter Wavelength Broadband Anti-Reflection Coatings [Crossref]
2015 - Atmospheric Pressure Plasma Enabled Polishing of Single Crystal Sapphire [Crossref]
2016 - Study on Planarization Machining of Sapphire Wafer With Soft-Hard Mixed Abrasive Through Mechanical Chemical Polishing [Crossref]
2017 - Grinding Forces in Micro Slot-Grinding (MSG) of Single Crystal Sapphire [Crossref]
2015 - Anisotropy of Deformation and Fracture Processes in Sapphire Surface [Crossref]