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 |
Technical Documentation & Analysis: Locally Resolved Stress Measurement in Ultra-Hard Composites
Section titled “Technical Documentation & Analysis: Locally Resolved Stress Measurement in Ultra-Hard Composites”6CCVD Material Science Analysis of Research Paper: Locally resolved stress measurement in the ultra-hard composites polycrystalline diamond and polycrystalline cubic boron nitride
Executive Summary
Section titled “Executive Summary”This research validates Raman spectroscopy as the superior method for measuring residual stress in highly curved ultra-hard cutting edges (PCD and PcBN), overcoming the limitations of traditional X-ray diffraction (XRD).
- Critical Limitation Addressed: Established XRD methods fail to measure residual stress in the highly curved cutting edge area (radius rβ = 5 to 100 µm). Raman spectroscopy, with its micron-scale spot size (approx. 2 µm), provides the necessary spatial resolution.
- Key Achievement: For the first time, conversion factors (C) were accurately determined for axial load cases (bending stress) in both PCD and PcBN.
- Scientific Finding: The conversion factors for axial load differ significantly from previously published hydrostatic load factors, proving that axial factors (CBiaxial) are essential for accurate stress determination on tool surfaces.
- PCD Conversion Factor: The determined biaxial conversion factor (CDia,B) for PCD was 1.36 GPa/cm-1, showing high agreement with existing biaxial studies at high load stresses.
- PcBN Conversion Factor: Conversion factors were determined for both the LO (1.14 GPa/cm-1) and TO (1.60 GPa/cm-1) modes of cBN, providing a new standard for residual stress analysis in PcBN tools.
- Application: These new factors enable the conversion of Raman peak shifts into absolute residual stress values, crucial for optimizing tool preparation processes (grinding, EDM, laser machining) and predicting tool life.
Technical Specifications
Section titled “Technical Specifications”The following hard data points were extracted from the experimental results, focusing on material properties and measured stress states under bending load.
| Parameter | Value | Unit | Context |
|---|---|---|---|
| PCD Grain Diameter (dg) | 4 | µm | Material composition |
| PcBN Grain Diameter (dg) | 2 | µm | Material composition |
| PCD Binder Content | 10% Cobalt | % | Material composition |
| PcBN cBN Content | 85% | % | Material composition |
| PCD Residual Stress (σRS,PCD) | -1015 | MPa | Compressive stress in unloaded state (XRD) |
| PcBN Residual Stress (σRS,PCBN) | -510 | MPa | Compressive stress in unloaded state (XRD) |
| PCD Applied Load Stress (σL,PCD) | -1900 | MPa | Additional compressive stress under bending |
| PcBN Applied Load Stress (σL,PCBN) | -4515 | MPa | Additional compressive stress under bending |
| PCD Axial Conversion Factor (CDia,B) | 1.36 | GPa/cm-1 | Determined for biaxial load case |
| PcBN Axial Conversion Factor (CCBN,B,LO) | 1.14 | GPa/cm-1 | Determined for LO mode, biaxial load case |
| PcBN Axial Conversion Factor (CCBN,B,TO) | 1.60 | GPa/cm-1 | Determined for TO mode, biaxial load case |
| Raman Laser Wavelength (λ) | 532 | nm | Green laser used for spectroscopy |
| Raman Spot Diameter (dm) | approx. 2 | µm | Spatial resolution for stress measurement |
Key Methodologies
Section titled “Key Methodologies”The conversion factors were determined by correlating stress measurements obtained via XRD (sin2ψ method) with the corresponding peak shifts measured by Raman spectroscopy under controlled axial load.
- Material Preparation: PCD (0.5 mm thick layer on 1.1 mm WC substrate) and PcBN specimens (45 x 6 mm2) were prepared. PcBN surface was diamond polished for improved Raman measurability.
- Axial Load Application: A 4-point bending unit was used to apply a constant, measurable axial compressive stress (σL) to the center of the specimen surface.
- XRD Stress Measurement: Residual stress (σRS) was measured in the unloaded state and total stress (σRS + σL) in the loaded state using a Seifert XRD 3003 ETA diffractometer (Co-anode, 2 mm point collimator).
- PCD diffraction peak: Diamond hkl 311 (2θ = 112.56°).
- PcBN diffraction peak: cBN hkl 200 (2θ = 59.30°).
- Raman Spectroscopy: Spectra were recorded using a Bruker Santerra II spectrometer (100x magnification, 532 nm green laser).
- PCD characteristic peak: 1332 cm-1.
- PcBN characteristic peaks: 1305 cm-1 (LO mode) and 1054 cm-1 (TO mode).
- Measurement settings: PCD (5 mW, 15 s accumulation), PcBN (10 mW, 20 s accumulation).
- Conversion Factor Determination: The measured stress values (σL) from XRD were plotted against the corresponding Raman peak shifts (Δω) to determine the linear conversion factor C (C = Δω / σ).
6CCVD Solutions & Capabilities
Section titled “6CCVD Solutions & Capabilities”This research highlights the critical need for high-quality, dimensionally precise PCD and PcBN materials, particularly those optimized for surface analysis and subsequent tool fabrication (brazing/soldering). 6CCVD is uniquely positioned to supply the materials and engineering services required to replicate and extend this advanced research.
Applicable Materials
Section titled “Applicable Materials”To replicate or extend this research into industrial tool applications, 6CCVD recommends the following materials:
| Material Grade | 6CCVD Specification | Application Relevance |
|---|---|---|
| Polycrystalline Diamond (PCD) | Standard or Fine Grain PCD Plates | Matches the material used (4 µm grain size). Essential for high-wear cutting applications and accurate Raman analysis. |
| Polycrystalline Cubic Boron Nitride (PcBN) | High cBN Content PcBN Plates | Required for high-fidelity Raman measurements (paper used 85% cBN content). |
| Single Crystal Diamond (SCD) | Optical Grade SCD Wafers | Ideal for fundamental studies of stress-induced peak shift in binder-free, high-purity diamond, providing a baseline for comparison with composite materials. |
| Boron-Doped Diamond (BDD) | Heavy Boron Doped PCD/SCD | Relevant for electrochemical applications where residual stress measurement is also critical, extending the methodology beyond cutting tools. |
Customization Potential
Section titled “Customization Potential”The complexity of ultra-hard tool fabrication requires precise material control, which 6CCVD provides through its advanced manufacturing capabilities:
- Custom Dimensions and Thickness: The paper used 45 x 6 mm2 specimens with a 0.5 mm PCD layer thickness. 6CCVD offers:
- PCD Plates: Custom dimensions up to 125 mm diameter.
- Thickness Control: SCD and PCD layers available from 0.1 µm up to 500 µm (0.5 mm), matching the exact layer thickness used in the study.
- Substrate Integration & Metalization: The specimens were soldered onto tungsten carbide (WC) substrates. 6CCVD offers in-house metalization services crucial for reliable brazing/soldering of diamond tools to substrates.
- Available Metal Layers: Au, Pt, Pd, Ti (critical for diamond adhesion), W, and Cu. We can apply custom Ti/Pt/Au stacks optimized for high-temperature brazing processes.
- Surface Finish: Accurate Raman spectroscopy requires a high-quality surface finish. 6CCVD provides:
- SCD Polishing: Surface roughness Ra < 1 nm.
- PCD Polishing: Surface roughness Ra < 5 nm (for inch-size plates).
Engineering Support
Section titled “Engineering Support”This research confirms that accurate residual stress measurement in ultra-hard materials is highly dependent on the load case (axial vs. hydrostatic). 6CCVD’s in-house PhD team specializes in diamond material properties and can assist researchers and engineers with:
- Material Selection: Guiding the choice between SCD, PCD, or BDD based on specific application requirements (e.g., high thermal conductivity, specific grain size, or binder content).
- Stress Analysis Projects: Providing consultation on material specifications necessary for similar Residual Stress Measurement projects, ensuring the supplied diamond material is optimized for Raman activity and subsequent tool processing.
- Global Logistics: Ensuring reliable, secure global shipping (DDU default, DDP available) for sensitive, high-value diamond materials.
For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.
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.