Sub-micron structuring/texturing of diamond-like carbon-coated replication masters with a femtosecond laser
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2020-02-01 |
| Journal | Applied Physics A |
| Authors | Aleksandra Michalek, Shaojun Qi, Afif Batal, Pavel Penchev, Hanshan Dong |
| Institutions | University of Birmingham, Manufacturing Technology Centre (United Kingdom) |
| Citations | 17 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive Summaryâ- Functionalization of DLC Masters: Femtosecond (fs) laser processing was successfully optimized to generate uniform Laser-Induced Periodic Surface Structures (LIPSS) on thin Diamond-Like Carbon (DLC) coatings applied to 316L stainless steel replication masters.
- Optimized Structure: Uniform LIPSS were achieved using fluences between 91-119 mJ/cm2 and 33-47 total pulses per spot (ppstotal), resulting in a periodicity of 700-800 nm.
- Tribological Preservation: The advantageous low Coefficient of Friction (CoF) of the DLC coating (mean CoF ~0.12) was retained after fs laser texturing, confirming its suitability for low-adhesion applications like micro-injection molding.
- Hardness Reduction: A significant reduction in nanohardness was measured, dropping from 22 GPa (as-received) to 4-9 GPa (laser-treated), indicating sensitivity to the fs processing parameters.
- Structural Modification: Raman spectroscopy confirmed that fs irradiation led to the formation of a thin, graphitized surface layer (indicated by an increase in I(D)/I(G) ratio to > 1.0 and a G peak shift to 1590 cm-1), while the bulk DLC structure remained largely amorphous.
- Wear Mechanism: During tribological testing, the thin graphitized layer was observed to fill the valleys of the LIPSS ripples, effectively polishing the surface but maintaining the overall low friction performance.
- Value Proposition: The technology enables the creation of functionalized replication masters that benefit from both the lubricating properties of DLC and the added functionality of sub-micron surface textures (LIPSS).
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Substrate Material | 316L Stainless Steel | - | Replication master base |
| DLC Film Thickness | 2-5 | ”m | Deposited via PACVD |
| DLC Hardness (As-received) | 22 | GPa | Equivalent to HV 2500 |
| Laser Type | fs Ytterbium-doped fiber | - | Source for structuring |
| Laser Wavelength (λ) | 1030 | nm | Near infrared |
| Pulse Duration | 310 | fs | Ultrashort pulse characteristic |
| Beam Spot Diameter (d) | 40 | ”m | At 1/e2 intensity |
| Optimized Fluence (F) Range | 91-119 | mJ/cm2 | For uniform LIPSS generation |
| LIPSS Periodicity (Î) | 700-800 | nm | Low Spatial Frequency LIPSS (LSFL) |
| LIPSS Ripple Height | 200 | nm | Average height across processed area |
| Hardness (Laser-treated) | 4-9 | GPa | Significant reduction post-processing |
| CoF (As-received DLC) | 0.12 | - | Mean value, dry condition |
| Tribology Counterpart | Alumina Ball | - | Hardness: 16 GPa (HV 1600) |
| Tribology Load | 1.47 | N | Equivalent to 150 g |
| Raman G Peak Shift | 1509 to 1590 | cm-1 | Shift observed after fs treatment |
| I(D)/I(G) Ratio (Laser-treated) | > 1.0 | - | Indicates surface graphitization |
| GAXRD Incident Angle | 3 | ° | Used to analyze crystalline structure |
Key Methodologies
Section titled âKey Methodologiesâ- DLC Coating: Thin DLC films were deposited onto 316L stainless steel substrates using Plasma-Assisted Chemical Vapour Deposition (PACVD).
- Femtosecond Laser Processing: An fs laser (1030 nm, 310 fs) was used for structuring. The beam was focused to a 40 ”m spot.
- Process Parameter Optimization: Single-spot irradiation tests were conducted to identify the fluence and pulse number thresholds required to transition from HSFL (High Spatial Frequency LIPSS) to uniform LSFL (Low Spatial Frequency LIPSS).
- Large Area Scanning Strategy: Areas were structured using multiple pulse trains with a fixed hatch distance (h) of 3 ”m, resulting in a high pulse overlap (92.5%) in the Y direction. Fluence (F) and total pulses per spot (ppstotal) were varied to establish a processing window that ensured LIPSS uniformity without damaging the underlying DLC film.
- Topographical Analysis: Surface morphology and LIPSS geometry were analyzed using Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM). Periodicity was quantified using 2D Fast Fourier Transformation (2D FFT).
- Structural Analysis (Raman): Raman spectroscopy (633 nm source) was used to analyze the short-range ordering of carbon bonds, focusing on shifts in the D and G peaks and changes in the I(D)/I(G) ratio to detect graphitization.
- Structural Analysis (GAXRD): Glancing Angle X-ray Diffraction (GAXRD) was performed at a low incident angle (3°) to check for long-range crystallization or substantial graphite formation within the DLC bulk.
- Mechanical Testing: Nanoindentation was performed using a 50 nm tip with a depth control of 400 nm (less than 10% of coating thickness) to measure the resulting nanohardness of the textured surface.
- Tribological Testing: Ball-on-plate reciprocating tests were conducted using an alumina ball (16 GPa) under a 1.47 N load to measure the Coefficient of Friction (CoF) both perpendicular and parallel to the LIPSS orientation.
Commercial Applications
Section titled âCommercial Applicationsâ- Micro-Injection Molding Tools: The primary application, where DLC acts as a solid lubricant to reduce demolding forces and increase tool life. fs-LIPSS adds functional textures (e.g., for enhanced replication quality or anti-adhesion).
- Functional Surface Replication: Manufacturing masters for replicating micro- and nano-structured surfaces onto polymers or other materials, especially for components requiring specific optical or fluidic properties.
- Anti-Wear and Protective Coatings: Utilizing the preserved low CoF and high residual hardness (still significantly harder than steel) for durable, low-friction components in mechanical systems.
- Biomedical Devices: Creating textured DLC surfaces for implants where surface topography is engineered to influence cellular response (e.g., osteoblast adhesion) while maintaining biocompatibility and wear resistance.
- Optical and Sensing Components: Employing the precise 700-800 nm periodicity of the LIPSS for diffraction gratings, anti-reflective surfaces, or other microelectronic devices.