Impact of Peak Material Volume of Polycrystalline CVD Diamond Coatings on Dry Friction Against Aluminum
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2020-05-07 |
| Journal | JOM |
| Authors | M. Prieske |
| Institutions | Bremen Institute for Applied Beam Technology |
| Citations | 3 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis research establishes a critical surface topology parameter for optimizing Chemical Vapor Deposited (CVD) diamond coatings used in dry friction applications against aluminum.
- Core Finding: The Peak Material Volume (Vmp), which quantifies the volume of the top 10% of the surface, is the most reliable predictor for both the coefficient of friction and the wear rate of the aluminum counter body.
- Performance Requirement: To minimize the abrasion of the aluminum alloy (EN AW-5083), the Vmp of the diamond coating must be strictly maintained at less than 0.04 ml/m2.
- Friction Correlation: The coefficient of friction increases near-to-linearly with increasing Vmp, saturating at approximately 0.5 for Vmp values greater than 0.1 ml/m2.
- Topology Hypothesis Confirmed: The hypothesis that a decrease in a specific topology size (Vmp) predicts a decrease in wear rate and friction coefficient for both polished and non-polished coatings was confirmed.
- Recommended Solution: For dry forming processes involving aluminum, microcrystalline CVD diamond layers combined with a subsequent surface post-treatment (polishing or rubbing) are recommended to achieve the necessary low Vmp.
- Insufficient Parameters: Traditional roughness parameters like Arithmetical Mean Height (Ra or Sa) or Root Mean Square Deviation (Rq) were found to be unsuitable for establishing a clear correlation with friction and wear, especially when comparing treated and untreated coatings.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Critical Topology Parameter | Peak Material Volume (Vmp) | ml/m2 | Predictor for friction and wear |
| Target Vmp for Low Wear | < 0.04 | ml/m2 | Required to minimize aluminum abrasion |
| Counter Body Material | EN AW-5083 | N/A | Aluminum alloy (AlMg4.5Mn0.7) |
| Hertzian Contact Stress | 759 | MPa | Used in oscillating ball-on-plate test |
| Total Test Cycles | 99,900 | cycles | Total sliding distance of 1 km |
| Lowest Achieved Wear Rate | 4.5 x 10-9 | mm3/Nm | Achieved by polished CVDD 10.25p coating |
| Lowest Friction Coefficient | 0.12 | N/A | Achieved by polished CVDD 10.25p coating |
| CVD Deposition Temperature Range | 750 to 1050 | °C | Range used to produce nine different coatings |
| CH4/H2 Ratio Range | 1 to 5 | % | Range used for plasma CVD process |
| Diamond Raman Peak | 1332 | cm-1 | Used to confirm sp3 bonds (diamond quality) |
| Graphite G-Band Peak | 1560 | cm-1 | Used to confirm sp2 bonds (coating impurity) |
| Coating Delamination Vmp | 0.047 and 0.066 | ml/m2 | Observed for coatings CVDD 0.48 and CVDD 0.94 |
Key Methodologies
Section titled âKey MethodologiesâThe study utilized a laser-based plasma CVD process to deposit polycrystalline diamond coatings, followed by standardized tribological testing and advanced surface metrology.
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Substrate Preparation:
- K10 hard metal discs (94% WC, 6% Co) were etched using Murakami reagent and Caroâs reagent.
- Diamond nucleation was performed by immersing substrates in a diamond powder dispersion (0.25 ”m to 0.50 ”m) within an ultrasonic bath.
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CVD Deposition:
- Coatings were deposited using a laser-based plasma CVD process at atmospheric pressure.
- Nine different surface topographies were created by varying temperature (750 °C to 1050 °C), deposition duration, and the methane/hydrogen (CH4/H2) ratio (1% to 5%).
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Surface Post-Treatment:
- One coating (CVDD 10.25p) was mechanically polished to a mirror finish.
- Two coatings (CVDD 8.89r and CVDD 1.91r) were mechanically polished by rubbing two diamond layers against each other.
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Tribological Testing:
- Dry oscillating ball-on-plate tests were conducted using a CETR Universal Mechanical Tester UMT-3MT.
- The counter body was a hemisphere of aluminum alloy EN AW-5083.
- Tests ran for 99,900 cycles (1 km sliding distance) under a constant force of 10 N.
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Metrology and Analysis:
- Surface Roughness: 3D laser microscopy (Keyence VK 9710) was used to measure surface roughness parameters (Sa, Spk, Sxp, Svk, Vmp, Vmc) according to DIN EN ISO 25178.
- Wear Rate: Calculated for the aluminum counter body pins after testing.
- Coating Quality: Micro-Raman spectroscopy was used to calculate the diamond quality factor (Q), estimating the concentration of sp3 bonds relative to sp2 bonds (graphite).
Commercial Applications
Section titled âCommercial ApplicationsâThe findings directly support the development of advanced tool coatings for manufacturing processes focused on efficiency and environmental sustainability.
- Dry Forming and Shaping: Enabling the transition from oiled forming processes to dry forming, reducing environmental pollution, shortening process chains, and lowering costs in metal shaping operations.
- Automotive Manufacturing: Specifically relevant for forming components made from high-strength aluminum alloys like EN AW-5083, commonly used in the automotive industry.
- High-Performance Tooling: Application of post-treated microcrystalline CVD diamond coatings for tools (e.g., dies, punches) requiring extreme hardness, wear resistance, and chemical inertness against non-ferrous metals.
- Surface Engineering Quality Control: Utilizing Vmp as a robust quality control metric for diamond coating suppliers, ensuring that tool surfaces meet the strict topological requirements necessary for low-friction, low-wear tribological contact.
View Original Abstract
Abstract For economic and environmental reasons, dry forming is of increasing interest due to the shortening of process chains, cost savings and reduction of environmental pollution. The aim of these investigations is to examine to what extent chemical vapor deposited (CVD) diamond coatings are suitable for dry forming of aluminum and to identify the surface topology requirements for a low friction coefficient and low wear. Nine different surface topologies of CVD diamond coatings were tested in an oscillating ball-on-plate tribometer test against aluminum balls with a Hertzian contact stress of 759 MPa and 99,900 cycles. It could be concluded that the peak material volume (Vmp) of the diamond coating is the most important factor for achieving a low abrasion of aluminum as well as a low friction coefficient against aluminum. The Vmp should be smaller than 0.04 ml/m 2 . Microcrystalline CVD diamond with a post-treated surface has great potential for dry forming of aluminum.