Amorphous Carbon Coatings for Total Knee Replacements—Part II - Tribological Behavior
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2021-06-05 |
| Journal | Polymers |
| Authors | Benedict Rothammer, Max Marian, Kevin Neusser, Marcel Bartz, Thomas Böhm |
| Institutions | Heidelberg University, Forschungszentrum Jülich |
| Citations | 41 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled “Executive Summary”This study, Part II of a two-part investigation, evaluates the tribological performance of amorphous carbon (DLC) coatings designed for Total Knee Arthroplasty (TKA) components using pin-on-disk model tests and numerical Elastohydrodynamic Lubrication (EHL) modeling.
- Core Achievement: Amorphous carbon coatings (a-C:H:W on metal, a-C:H on polymer) significantly reduced wear of the ultrahigh molecular weight polyethylene (UHMWPE) tibial inlay material.
- Wear Reduction Metrics: UHMWPE disk wear was reduced by 77% when sliding against coated Ti64 (Ti64:W) and 49% against coated CoCr (CoCr:W), compared to uncoated references.
- Durability and Failure Mode: The coatings exhibited continuous, slow wear without catastrophic failure modes like spalling, delamination, or crack initiation over 2 x 105 cycles, demonstrating high adhesion and compliance.
- Friction Trade-off: Friction coefficients increased compared to uncoated pairings. This was attributed primarily to the higher surface roughness of the as-deposited DLC coatings, which intensified mixed lubrication conditions (verified by EHL modeling).
- Material Potential: The DLC-coated Ti64 pairing (UHMWPE:H | Ti64:W) showed wear rates below the uncoated CoCr reference, suggesting Ti64:W bears the potential to supplant CoCr as a preferred TKA implant material.
- Test Validation: Numerical EHL modeling confirmed that the pin-on-disk test conditions successfully replicated the mixed lubrication regime and stress levels relevant for wear-critical instants in TKA gait cycles.
Technical Specifications
Section titled “Technical Specifications”| Parameter | Value | Unit | Context |
|---|---|---|---|
| Pin Material Substrates | Co28Cr6Mo, Ti6Al4V ELI | Alloy | Femoral Component Analogues |
| Disk Material Substrate | UHMWPE (GUR 1020) | Polymer | Tibial Inlay Analogue |
| Coating on Metal Pins | a-C:H:W (Tungsten-doped DLC) | Coating | CoCr:W, Ti64:W |
| Coating on Polymer Disks | a-C:H (Pure DLC) | Coating | UHMWPE:H |
| CoCr:W Indentation Hardness (HIT) | 16.1 | GPa | Coated Pin |
| Ti64:W Indentation Hardness (HIT) | 14.4 | GPa | Coated Pin |
| UHMWPE:H Indentation Hardness (HIT) | 1.3 | GPa | Coated Disk |
| CoCr:W Coating Thickness (tc) | 1.0 | µm | Pin |
| UHMWPE:H Coating Thickness (tc) | 1.4 | µm | Disk |
| Pin Head Radius (R) | 100 | mm | Contact Geometry |
| Normal Load (F) | 10 | N | Applied Load |
| Sliding Velocity (u) | 0.1 | m/s | Rotational Sliding |
| Total Test Duration | 2 x 105 | cycles | Equivalent to 2 x 104 m sliding distance |
| Lubricant | Diluted Bovine Calf Serum (BCS) | Fluid | Artificial Synovial Fluid (20 ± 1 g/L protein) |
| Test Temperature | 37 ± 0.2 | °C | Environmental Chamber |
| Max Total Contact Pressure (Pt,max) | 4.59 - 4.61 | MPa | EHL Simulation (All pairings) |
| Solid Asperity Load Share (Coated) | 81.3 - 81.6 | % | EHL Simulation (Mixed Lubrication) |
| DLC Raman ID/IG Ratio | 0.2 - 0.3 | Ratio | Characteristic of hydrogenated amorphous carbon |
Key Methodologies
Section titled “Key Methodologies”The tribological behavior was assessed using a pin-on-disk tribometer under environmentally controlled, fully flooded conditions, complemented by numerical EHL modeling.
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Specimen Preparation and Coating:
- Metal Pins (CoCr, Ti64): Coated with tungsten-doped hydrogen-containing amorphous carbon (a-C:H:W) via Physical Vapor Deposition (PVD). A thin Chromium (Cr) adhesive layer and a Tungsten Carbide (WC) intermediate layer were used to ensure high adhesion.
- UHMWPE Disks: Coated with pure hydrogenated amorphous carbon (a-C:H) via reactive PVD, applied directly to the polymer substrate.
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Tribological Testing (Pin-on-Disk):
- Setup: K-SST tribometer operating in rotational sliding mode (pure sliding).
- Conditions: Constant normal load (10 N) and constant sliding velocity (0.1 m/s).
- Lubrication: 25 mL of artificial Synovial Fluid (diluted BCS, 20 g/L protein) pre-cooled to 8 °C.
- Measurement: Coefficient of Friction (COF) measured in-situ via strain gauges on the cantilever. Tests were run for 2 x 105 cycles, divided into 10 intervals for temporal wear analysis.
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Numerical Elastohydrodynamic Lubrication (EHL) Modeling:
- Model Type: 3D EHL model based on full-system Finite Element Method (FEM).
- Purpose: To calculate pressure and lubricant gap distribution, assess the influence of coatings on lubrication, and confirm the representativity of the test conditions for TKAs.
- Mixed Lubrication: Accounted for using the Greenwood-Williamson model to determine solid contact pressures (Pa).
-
Wear Characterization (Ex-Situ):
- Volumetric Wear: Calculated using 3D Laser Scanning Microscopy (LSM) and Light Microscopy (LM) measurements of wear track cross-sections and pin calottes (gravimetric methods were avoided due to polymer soaking effects).
- Microstructural Analysis: Scanning Electron Microscopy (SEM) and Focused Ion Beam (FIB) milling were used to examine coating integrity, wear track morphology, and cross-sections (e.g., confirming continuous wear and substrate compaction).
- Chemical Analysis: Raman Spectroscopy (457 nm excitation) was used to analyze the chemical state of the coatings and UHMWPE in the wear track (D- and G-bands, graphitization).
- Particle Analysis: Wear particles isolated from the SF via acid digestion and filtration (0.1 µm filters) were analyzed using FEG-SEM and digital image software (ASTM F1877-16) for Equivalent Circle Diameter (ECD), Aspect Ratio (AR), and Roundness (R).
Commercial Applications
Section titled “Commercial Applications”The research focuses on enhancing the longevity and performance of orthopedic implants, specifically targeting wear reduction in large joints.
- Orthopedic Implant Manufacturing: Direct application in the production of Total Knee Arthroplasty (TKA) components, particularly femoral pins and tibial inlays.
- Aseptic Loosening Prevention: Reducing the generation of sub-micron UHMWPE wear particles, which are the primary biological trigger for osteolysis and subsequent implant failure.
- High-Performance Biomaterials: Utilizing DLC coatings (a-C:H:W) to upgrade biocompatible but tribologically inferior materials (like Ti64) to surpass the performance of traditional CoCr alloys.
- Biotribological Testing Standards: Providing validated numerical and experimental methodologies for screening novel surface modifications and material pairings under conditions relevant to human joint mechanics (mixed lubrication regime).
- Polymer Protection: Developing highly compliant and adherent DLC coatings (a-C:H) capable of protecting soft polymer substrates (UHMWPE) despite significant macro-elastic deformation and compaction.
View Original Abstract
Diamond-like carbon coatings may decrease implant wear, therefore, they are helping to reduce aseptic loosening and increase service life of total knee arthroplasties (TKAs). This two-part study addresses the development of such coatings for ultrahigh molecular weight polyethylene (UHMWPE) tibial inlays as well as cobalt-chromium-molybdenum (CoCr) and titanium (Ti64) alloy femoral components. While the deposition of a pure (a-C:H) and tungsten-doped hydrogen-containing amorphous carbon coating (a-C:H:W) as well as the detailed characterization of mechanical and adhesion properties were the subject of Part I, the tribological behavior is studied in Part II. Pin-on-disk tests are performed under artificial synovial fluid lubrication. Numerical elastohydrodynamic lubrication modeling is used to show the representability of contact conditions for TKAs and to assess the influence of coatings on lubrication conditions. The wear behavior is characterized by means of light and laser scanning microscopy, Raman spectroscopy, scanning electron microscopy and particle analyses. Although the coating leads to an increase in friction due to the considerably higher roughness, especially the UHMWPE wear is significantly reduced up to a factor of 49% (CoCr) and 77% (Ti64). Thereby, the coating shows continuous wear and no sudden failure or spallation of larger wear particles. This demonstrated the great potential of amorphous carbon coatings for knee replacements.
Tech Support
Section titled “Tech Support”Original Source
Section titled “Original Source”References
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- 2007 - Characterisation of wear particles produced by metal on metal and ceramic on metal hip prostheses under standard and microseparation simulation [Crossref]
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