Locally resolved stress measurement in the ultra-hard composites polycrystalline diamond and polycrystalline cubic boron nitride
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2024-03-11 |
| Journal | Forschung im Ingenieurwesen |
| Authors | Bernd Breidenstein, Nils Vogel |
| Institutions | Leibniz University Hannover |
| Citations | 1 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled “Executive Summary”This research establishes crucial conversion factors for accurately measuring residual stresses in ultra-hard polycrystalline diamond (PCD) and cubic boron nitride (PcBN) cutting tools using Raman spectroscopy.
- Problem Solved: Established X-ray diffraction (XRD) methods fail to measure residual stress on the highly curved surfaces of cutting edges (radius typically 5-100 µm). Raman spectroscopy offers the necessary spatial resolution (approx. 2 µm spot).
- Conversion Factor Necessity: Raman peak shifts (Δω) must be converted into absolute stress values (σ) using material-specific conversion factors (C). Previous hydrostatic factors were found to be unsuitable for tool applications.
- Methodology: Axial load cases were generated via 4-point bending experiments, and conversion factors were determined by correlating XRD-measured stress with Raman peak shifts.
- Key Finding (PCD): The determined axial conversion factor (CDia,B = 1.36 GPa/cm-1) for PCD differs significantly from the literature hydrostatic factor (CHydro,Dia = 2.88 GPa/cm-1).
- Key Finding (PcBN): For the first time, axial conversion factors were determined for PcBN: CCBN,B,LO = 1.14 GPa/cm-1 (LO mode) and CCBN,B,TO = 1.60 GPa/cm-1 (TO mode).
- Mechanism Insight: The strong difference between hydrostatic and axial factors is attributed to the cubic crystal structure, where axial load causes a smaller volume change, resulting in a smaller peak shift.
- Impact: The new factors enable reliable, localized residual stress mapping on cutting tools, crucial for optimizing tool preparation processes (grinding, EDM, laser machining) and predicting tool life.
Technical Specifications
Section titled “Technical Specifications”| Parameter | Value | Unit | Context |
|---|---|---|---|
| PCD Grain Diameter (dg) | 4 | µm | Cutting Material |
| PcBN cBN Content | 85 | % | Cutting Material |
| PCD Binder Content | 10 | % | Cobalt Binder |
| PCD Layer Thickness | 0.5 | mm | Soldered on tungsten carbide substrate |
| Specimen Dimensions | 45 x 6 | mm2 | 4-point bending test |
| Raman Laser Wavelength (λ) | 532 | nm | Green Laser (Bruker Santerra II) |
| Raman Spot Diameter (dm) | approx. 2 | µm | Measurement resolution |
| PCD Laser Power (PL) | 5 | mW | Raman settings |
| PcBN Laser Power (PL) | 10 | mW | Raman settings |
| PCD Unloaded Residual Stress (σRS) | -1015 | MPa | Production-induced (Compressive) |
| PcBN Unloaded Residual Stress (σRS) | -510 | MPa | Production-induced (Compressive) |
| PCD Applied Load Stress (σL) | -1900 | MPa | Bending load (Compressive) |
| PcBN Applied Load Stress (σL) | -4515 | MPa | Bending load (Compressive) |
| PCD Axial Conversion Factor (CDia,B) | 1.36 | GPa/cm-1 | Determined in this study (Biaxial/Axial) |
| PCD Hydrostatic Factor (CHydro,Dia) | 2.88 | GPa/cm-1 | Literature comparison |
| PcBN LO Axial Factor (CCBN,B,LO) | 1.14 | GPa/cm-1 | Determined in this study (Biaxial/Axial) |
| PcBN TO Axial Factor (CCBN,B,TO) | 1.60 | GPa/cm-1 | Determined in this study (Biaxial/Axial) |
| PCD Diamond Peak (Unloaded) | 1332 | cm-1 | Characteristic Raman peak |
| PcBN LO Mode Peak (Unloaded) | 1305 | cm-1 | Characteristic Raman peak |
| PcBN TO Mode Peak (Unloaded) | 1054 | cm-1 | Characteristic Raman peak |
Key Methodologies
Section titled “Key Methodologies”The conversion factors were derived by comparing absolute stress measurements (XRD) with corresponding peak shifts (Raman) under controlled axial loading.
- Specimen Preparation: PCD (4 µm grain, 10% Co binder) and PcBN (85% cBN, 2 µm grain) specimens (45 x 6 mm2) were used. PcBN surfaces were polished to enhance Raman measurability.
- Axial Loading Setup: A 4-point bending unit was employed to apply a constant, monitored axial compressive load to the center segment of the specimens. Load stresses reached up to -4515 MPa (PcBN) and -1900 MPa (PCD).
- XRD Stress Measurement:
- Measurements were performed using the sin2ψ method (Seifert XRD 3003 ETA, Co-anode, 2 mm point collimator).
- PCD: Diamond diffraction peak hkl 311 (2θ = 112.56°) was evaluated.
- PcBN: Cubic boron nitride diffraction peak hkl 200 (2θ = 59.30°) was evaluated.
- Both unloaded residual stress (σRS) and total loaded stress (σRS + σL) were determined.
- Raman Spectroscopy:
- Spectra were recorded using a Bruker Santerra II spectrometer (532 nm laser, 100x objective).
- 16 measurements were taken per load case, positioned with a 10 µm offset in the center of the bending segment.
- PCD: Diamond peak (1332 cm-1) was analyzed.
- PcBN: cBN LO mode (1305 cm-1) and TO mode (1054 cm-1) were analyzed.
- Data Analysis: The program Fityk was used for baseline adjustment (spline) and peak position determination (Gaussian fit). The resulting peak shifts (Δω) were correlated with the absolute stress values (σ) determined by XRD to calculate the biaxial conversion factors (C).
Commercial Applications
Section titled “Commercial Applications”The validated conversion factors and methodology are critical for quality control and optimization in the manufacturing and application of ultra-hard cutting tools.
- Advanced Machining: Used in the production and quality control of PCD and PcBN cutting inserts for high-performance machining of difficult materials, such as hardened steels (e.g., 100Cr6) and highly eutectic aluminum alloys.
- Tool Preparation Optimization: Enables quantitative assessment of the mechanical and thermal damage induced by tool shaping processes (grinding, electrical discharge machining (EDM), laser machining) on the subsurface stress state.
- Tool Life Prediction: The ability to accurately map localized residual stress near the cutting edge allows engineers to correlate preparation-induced stresses with subsequent wear behavior and tool life, leading to optimized tool selection.
- Metrology for Curved Surfaces: Provides a reliable, non-destructive method (Raman spectroscopy) for measuring stress on highly curved or complex geometries where traditional XRD is physically impossible.
- Composite Material Development: The methodology can be extended to investigate the influence of different binder phases (e.g., cobalt, tungsten, aluminum) and cutting material grades on the resulting stress state and conversion factors.
View Original Abstract
Abstract Cutting tools made of the ultra-hard composites polycrystalline diamond and polycrystalline boron nitride are being used in more and more sectors of machining. Due to the laborious preparation processes such as grinding, brushing, electrical discharge and laser machining, the subsurface of these tools is strongly stressed mechanically and thermally. This also changes the residual stress state in the highly loaded cutting edge area. The measurement of these residual stresses is not possible by established XRD methods due to the highly curved surface of the cutting edge. The measurement method Raman spectroscopy shows high potential for this application, but conversion factors are necessary for the application. These factors enable the conversion of the stress-induced peak shift in the Raman spectrum into absolute residual stress values. Previous conversion factors are mainly based on hydrostatic load cases, which, however, cannot be transferred to the application on cutting tools. In this work, axial load cases were provided by bending and conversion factors were determined by comparing XRD stress measurements and Raman peak shifts. The conversion factors determined were then plotted against existing results from other studies and the causes for the deviations that occurred were determined. By this, for the first time, a conversion factor for an axial load case for cubic boron nitride could be determined and it could be shown that, as for diamond, it differs significantly from the hydrostatic load case.