Are Diamond Surface Coatings Immune to Dry Running?
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2016-05-24 |
| Journal | Journal of Coating Science and Technology |
| Authors | Michael D. Seymour |
| Institutions | Quality Systems (United States) |
| Citations | 3 |
| Analysis | Full AI Review Included |
6CCVD Technical Analysis: Diamond Coatings for Extreme Tribology
Section titled â6CCVD Technical Analysis: Diamond Coatings for Extreme TribologyâExecutive Summary
Section titled âExecutive SummaryâThis technical analysis reviews the findings of the research paper comparing Polycrystalline Diamond (PCD) coatings against Plasma Assisted Chemical Vapour Deposition (PA-CVD) Diamond-Like Carbon (DLC) coatings for mechanical seal faces under dry running conditions.
- Core Finding: The study concluded that neither PCD nor PA-CVD DLC coatings are suitable for extended dry running, challenging common marketing claims of âimmunity.â
- Performance Disparity: PA-CVD DLC demonstrated significantly superior short-term dry running capability, lasting up to 18 times longer than the tested PCD coatings before reaching the critical 250°C friction-induced temperature cutoff.
- Wear Rate: The tested PCD samples (D1, D2) exhibited wear rates between 10 ”m/hour and 50 ”m/hour, whereas the PA-CVD DLC samples (C1, C2) showed much lower wear rates, ranging from 2.25 ”m/hour to 2.53 ”m/hour.
- Cost Implication: The tested PCD coated seal units were found to be approximately three times the cost of the equivalent PA-CVD DLC coated units, suggesting a poor cost-to-performance ratio for the specific PCD material tested.
- Safety Risk: High frictional temperatures (observed up to 360°C in related tests) pose a severe risk to polymer O-rings and can exceed the auto-ignition temperatures of common liquid fuels (e.g., petrol 247°C, diesel 210°C).
- 6CCVD Value Proposition: 6CCVD specializes in high-quality MPCVD PCD and SCD, offering custom thickness and superior polishing (Ra < 5 nm for PCD) to overcome the rapid wear and roughness issues observed in the lower-performing PCD samples (D1, D2).
Technical Specifications
Section titled âTechnical SpecificationsâThe following hard data points were extracted from the dry running comparison tests:
| Parameter | Value | Unit | Context |
|---|---|---|---|
| Test Termination Criterion | 250 | °C | Friction-induced temperature cutoff (to protect polymer O-rings). |
| Sliding Speed | 8.0 | m/s | Consistent seal face velocity maintained during testing. |
| PA-CVD (C1) Dry Run Time | 1:47:27 | h:m:s | Time until 250°C cutoff reached. |
| PCD (D2) Dry Run Time | 1:49 | m:s | Shortest run time recorded for PCD. |
| Performance Ratio (PA-CVD/PCD) | Up to 18 | Times | PA-CVD lasted up to 18 times longer than the lowest performing PCD sample (D2). |
| PA-CVD Contact Pressure | 234.4 | kPa | Higher face contact pressure used for PA-CVD assemblies. |
| PCD Contact Pressure | 110 | kPa | Lower face contact pressure used for PCD assemblies. |
| PA-CVD Wear Rate (C1) | 2.25 | ”m/hour | Lowest measured wear rate. |
| PCD Wear Rate (D2) | 50 | ”m/hour | Highest measured wear rate (20x higher than C1). |
| Tested Coating Thickness | 10 | ”m | Thickness of both PA-CVD and PCD samples used in the test. |
| PCD Hardness (Typical) | 10,000 | Hv | Extreme hardness required for tribological applications. |
Key Methodologies
Section titled âKey MethodologiesâThe experiment utilized a rigorous, controlled dry running test procedure to compare the tribological performance of the two coating types.
- Substrate and Materials: Silicon Carbide (SiC) faces were coated with either Polycrystalline Diamond (PCD) or Plasma Assisted - Chemical Vapour Deposition (PA-CVD) DLC.
- Test Conditions: Tests were conducted under dry running conditions at a consistent seal face velocity of 8.0 m/s (achieved via shaft speeds of 2827-2521 rpm).
- Termination: The test was terminated immediately upon the friction-induced temperature reaching 250°C, ensuring the polymer O-rings were not compromised.
- Topography Measurement: Seal face topography (form and roughness) was measured before and after testing using an Alicona Infinite Focus Microscope.
- Vertical resolution: Down to 10 nm.
- Lateral resolution: Down to 400 nm.
- Monitoring: Calibrated Type K thermocouples were mounted 180° apart on the internal diameter of the stationary face to measure temperature. Relative Humidity was also monitored within the test cell.
- Wear Analysis: Wear depth and location were determined by comparing pre- and post-test surface profiles, allowing for the calculation of wear rates in ”m/hour.
6CCVD Solutions & Capabilities
Section titled â6CCVD Solutions & CapabilitiesâThe research highlights the critical need for high-quality, thick, and well-polished diamond coatings for demanding tribological applications, even if only for short-term dry running protection. 6CCVDâs expertise in high-purity MPCVD diamond growth directly addresses the limitations observed in the tested PCD samples (D1, D2).
Applicable Materials
Section titled âApplicable MaterialsâTo replicate or extend this research, particularly focusing on maximizing wear resistance and thermal stability, 6CCVD recommends the following materials:
- High-Purity Polycrystalline Diamond (PCD): Ideal for large-scale mechanical seal faces requiring extreme hardness (10,000 Hv) and high thermal conductivity. Our MPCVD process ensures a dense, consistent grain structure superior to potentially lower-quality CVD methods used for the tested D1/D2 samples.
- Single Crystal Diamond (SCD): For ultra-high precision or smaller seal components where Ra < 1 nm surface finish is mandatory, SCD offers unmatched purity and thermal performance.
Customization Potential
Section titled âCustomization PotentialâThe paper noted that thicker coatings (up to 40 ”m) are desirable, and surface roughness (as seen in D2) severely impacts performance. 6CCVD provides the necessary customization to optimize performance:
| Requirement from Paper | 6CCVD Custom Capability | Benefit to Engineer |
|---|---|---|
| Thickness Limitation (Tested PCD was 10 ”m) | Custom Thickness: PCD plates/wafers available from 0.1 ”m up to 500 ”m. | Allows engineers to specify the exact thickness required for extended lifetime and wear budget, far exceeding standard commercial offerings. |
| Large Dimensions (Tested shaft diameter 47.6mm) | Large Area Growth: PCD plates/wafers available up to 125 mm diameter. | Suitable for large industrial seals and turbo-machinery applications. |
| Surface Finish (Roughness caused rapid failure in D2) | Precision Polishing: Standard PCD polishing to Ra < 5 nm (inch-size). SCD polishing to Ra < 1 nm. | Ensures the required flatness and smoothness for optimal tribological contact, minimizing friction and preventing the rapid wear observed in rougher samples. |
| Adhesion/Bonding (Need for robust substrate integration) | Custom Metalization: In-house deposition of adhesion layers (Ti, W) and contact layers (Pt, Au, Pd, Cu). | Critical for ensuring robust, high-temperature bonding between the diamond layer and the SiC or WC substrate, preventing delamination. |
Engineering Support
Section titled âEngineering SupportâThe significant performance variability between the tested PCD samples (D1 vs. D2) and the PA-CVD DLC highlights that material selection and processing parameters are paramount.
- Tribology Consultation: 6CCVDâs in-house PhD team specializes in diamond material science and can assist engineers in selecting the optimal MPCVD diamond grade (PCD grain size, SCD orientation) for high-load, high-speed, or high-temperature mechanical seal projects.
- Process Optimization: We provide consultation on achieving the necessary surface finish and integrating custom metalization schemes to ensure maximum coating integrity and wear resistance, addressing the adhesion and roughness issues identified in the research.
For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.
View Original Abstract
It is well understood that mechanical seal performance is dependent upon the tribology of the seal face materials. Published material is available claiming the advantages of Polycrystalline Diamond (PCD) and its suitability for extended running under dry conditions; indeed, one such claim suggests that PCD coated faces are immune to dry running. In order to investigate this claim, the author undertook a series of rigorously controlled tests to evaluate the performance of mechanical seal faces coated with a Polycrystalline Diamond coating (PCD) against others with Diamond - Like Carbon (DLC) coatings. Seal surface topography was accurately measured before and after testing and was used to evaluate the wear behaviour. From the test results obtained it is concluded that neither coatings are suitable for extended dry running use. However, it was evident that seal faces coated with a new form of DLC identified as, Plasma Assisted - Chemical Vapour Deposition (PA-CVD) performed 18 times longer than the PCD coated ones before reaching a predetermined friction induced temperature. Publications can be cited as evidence that PCD coated seal faces are capable of producing very high frictional temperatures that could, in a dry running situation, allow certain liquid fuels such as flashing hydrocarbons to reach their auto-ignition temperatures. In addition, it was revealed that the PCD coated seal units are being sold at a higher cost than the equivalent DLC coated ones by a factor of three.