High temperature creep deformation of nanocrystalline diamond films
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2022-01-01 |
| Journal | International Journal of Materials Research (formerly Zeitschrift fuer Metallkunde) |
| Authors | Markus Mohr, H.âJ. Fecht, K. A. Padmanabhan |
| Institutions | Anna University, Chennai, UniversitÀt Ulm |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis research investigates the high-temperature plastic deformation (creep) of nanocrystalline diamond (NCD) films, demonstrating their potential for high-performance engineering applications.
- Low-Stress Deformation: NCD films exhibit time-dependent inelastic deformation (creep) at 1000 °C under applied stresses (0.3-0.8 GPa) significantly lower than the yield strength of microcrystalline diamond (7.9-10 GPa).
- Creep Mechanism Confirmed: The plastic flow is dominated by Grain Boundary Sliding (GBS), not traditional crystal plasticity (dislocation motion), evidenced by a low creep stress exponent (n â 1.7-1.8).
- Thermal Threshold: Creep was observed at 1000 °C, but samples tested at 700 °C showed only fully elastic deformation, establishing a critical temperature range for inelastic behavior.
- Transient Behavior: The initial transient creep phase follows a logarithmic character (Andrade creep), typical of general creep phenomena.
- Model Validation: The steady-state creep results are quantitatively consistent with predictions from a physics-based model specifically designed for grain/interphase boundary sliding controlled flow.
- Material Integrity: XRD analysis confirmed that the nanocrystalline structure (7.9 nm grain size) remained stable, with no evidence of grain growth after 90 minutes of annealing at 1000 °C.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Film Thickness (h) | 50 | ”m | CVD growth parameter |
| Average Grain Size (L) | 7.9 | nm | Estimated via Scherrer equation (XRD) |
| Creep Test Temperature (T) | 1000 | °C | Temperature where steady-state creep was observed |
| Elastic Limit Temperature | 700 | °C | Temperature below which deformation was purely elastic |
| Applied Load (F) Range | 0.414 to 0.666 | N | Constant loads used in three-point bending |
| Applied Stress Range | 0.3 to 0.8 | GPa | Maximum stress experienced during deformation |
| Steady State Stress Range | 0.08 to 0.13 | GPa | Equivalent steady-state stress |
| Stress Exponent (n) | 1.69 to 1.79 | - | Characteristic of Grain Boundary Sliding (GBS) |
| Hardness (H) | 36 ± 6 | GPa | Measured at room temperature (Oliver-Pharr method) |
| Elastic Modulus (E) | 403 ± 55 | GPa | Measured at room temperature |
| Mass Density | 2.77 ± 0.13 | g cm-3 | Reduced density attributed to high grain boundary volume and porosity |
| Vacuum Pressure | 10-5 to 10-6 | mbar | Required environment for high-temperature testing |
Key Methodologies
Section titled âKey MethodologiesâThe investigation relied on specialized material synthesis, structuring, and a custom high-temperature, ultra-high vacuum testing setup.
- Film Synthesis: Nanocrystalline diamond films were grown on 4-inch silicon wafers using Hot Filament Chemical Vapor Deposition (HFCVD) with a CH4/H2/NH3 gas mixture, achieving a thickness of 50 ”m.
- Substrate Pretreatment: Silicon substrates were pretreated by ultrasonic seeding to achieve a high seeding density (~1011 cm-2).
- Sample Structuring: Free-standing rectangular samples (4 mm x 13 mm) were fabricated by structuring the film using a 1064 nm diode laser, followed by chemical dissolution of the underlying silicon substrate in potassium hydroxide (KOH) solution.
- Microstructural Characterization: X-ray diffraction (XRD) using Cu-Kα radiation was used to estimate the average grain size (7.9 nm) via the Scherrer equation and confirm microstructural stability after annealing.
- Mechanical Property Measurement: Room temperature hardness (H) and elastic modulus (E) were determined using nanoindentation with a Berkovich indenter and the Oliver-Pharr method.
- Creep Testing Setup: A simple three-point bending apparatus, constructed of molybdenum, was used. The setup was placed inside an evacuated tube furnace to maintain an ultra-high vacuum (10-5 to 10-6 mbar) environment, preventing diamond oxidation.
- Displacement Monitoring: Downward displacement of the sample at the load point was measured non-contactually using optical photographs taken by a camera fixed relative to the bending setup.
- Data Modeling: Steady-state creep rates were analyzed using Nortonâs equation and interpreted via a physics-based model for grain boundary sliding controlled flow, which accounts for mesoscopic scale deformation.
Commercial Applications
Section titled âCommercial ApplicationsâThe demonstrated high-temperature stability and predictable plastic deformation mechanism (GBS) of nanocrystalline diamond films open avenues for applications requiring extreme material properties under operational stress.
- High-Wear, High-Temperature Coatings: NCD films are strong candidates for hard, wear-resistant coatings on components operating above 1000 °C, such as turbine blades, furnace linings, or high-speed cutting tools.
- Robust Sensor Devices: Utilizing diamondâs chemical inertness and high elastic modulus, NCD films can form robust sensor components (e.g., pressure or strain sensors) capable of functioning reliably in extremely harsh, high-temperature, or corrosive industrial environments.
- Micro- and Nano-Electromechanical Systems (MEMS/NEMS): NCDâs superior mechanical properties and small grain size make it ideal for fabricating durable, high-frequency components that require dimensional stability under thermal load.
- Structural Materials in Extreme Environments: The quantitative understanding of creep behavior allows engineers to design NCD components where low-stress, high-temperature deformation must be accurately predicted, such as in specialized aerospace or nuclear applications.
- Tribological Systems: NCD films, known for their smooth surfaces and high hardness, are crucial for advanced tribological applications where friction and wear must be minimized, especially where operating temperatures are high.
View Original Abstract
Abstract Diamond displays a combination of unique properties, including the highest hardness among materials, chemical inertness and high thermal conductivity. Therefore, nanocrystalline diamond films offer a huge potential for industrial applications. In fine-grained ceramics as well as metallic materials, high temperature creep deformation is dominated by grain-boundary-deformation mechanisms that become increasingly important with decreasing grain size. In this work we demonstrate that it is possible to inelastically deform nanocrystalline diamond films at elevated temperatures and stresses that are significantly lower than those reported for single-crystal diamond. The initial, isothermal, transient creep flow exhibits a logarithmic character, typical of creep in general. The isothermal steady state creep deformation, which follows transient creep, is analyzed using a physics-based model for grain boundary sliding rate controlled flow.