Improved adhesion of polycrystalline diamond films on copper/carbon composite surfaces due to in situ formation of mechanical gripping sites

At a Glance

Metadata	Details
Publication Date	2017-04-18
Journal	Surface and Coatings Technology
Authors	Clio Azina, Mengmeng Wang, Emilien Feuillet, Loïc Constantin, B. Mortaigne
Institutions	Université de Limoges, Institut de Recherche sur les Céramiques
Citations	3
Analysis	Full AI Review Included

Technical Analysis and Commercial Solutions Brief: Improved PCD Adhesion for Thermal Management

Reference: Improved Adhesion of Polycrystalline Diamond Films on Copper/Carbon Composite surfaces due to in situ formation of mechanical gripping sites (Azina et al.)

Executive Summary

This research successfully addresses the critical Coefficient of Thermal Expansion (CTE) mismatch challenge between diamond coatings and high-conductivity copper substrates, highly relevant for power electronics and heat sink applications.

Core Value Proposition: Achieved stable, highly adherent Polycrystalline Diamond (PCD) films on Cu/Carbon Fiber (CF) composite substrates by tailoring the substrate CTE.
Stress Mitigation: Optimization to 40 vol% CF in the composite substrate reduced residual compressive stresses in the diamond film by 3 to 4 times compared to bare copper.
Adhesion Mechanism: Utilized the high-temperature combustion CVD environment to selectively burn exposed CF, creating natural, fiber-shaped voids (up to 10 µm deep) that act as mechanical gripping sites.
Film Quality: Films showed dense, columnar growth characteristics (Volmer-Weber model) and excellent phase purity, reaching a Diamond Quality Factor of 95.1% at 25 µm thickness.
Application Relevance: Demonstrates a viable surface engineering pathway for manufacturing high-integrity diamond-on-metal composite heat spreaders critical for demanding thermal management applications.
Growth Performance: Achieved a consistent growth rate of approximately 0.12 µm/min using low-cost, open-air combustion flame CVD.

Technical Specifications

Extracted material properties and experimental results from the research paper.

Parameter	Value	Unit	Context
Substrate Composition (Optimal)	40	vol% CF	Minimizes residual stress
Deposition Method	Combustion Flame CVD	N/A	Open-air process, high temperature
Deposition Temperature	710 - 740	°C	Process temperature range
Gas Mixture Ratio	1:1:2	C₂H₂:C₂H₄:O₂	Precursor volume ratio
Max Film Thickness Achieved	25.41	µm	After 240 min deposition time
Average Growth Rate	0.12	µm/min	Consistent across deposition times
Max Diamond Quality Factor	95.1	%	Achieved at 240 min film thickness
Residual Stress (240 min)	-0.62	GPa	Compressive; significant reduction vs. Cu
Cu CTE at 140 °C	17	10^-6/K	Isotropic Cu powder data
CF CTE at 140 °C (x, y, z)	-1, 12, 12	10^-6/K	Orthotropic Carbon Fiber data
CF Aspect Ratio (D/L)	0.05	N/A	Ratio of 10 µm diameter to 200 µm length

Key Methodologies

The following is an ordered summary of the key steps and parameters used to synthesize and characterize the improved diamond-composite assemblies.

Substrate Fabrication (Cu/CF Composite)

Materials: Dendritic Copper powder (matrix, d₅₀ < 35 µm) and short CF reinforcements (K223HM, 200 µm average length).
Sintering: Powders mixed and compacted in a steel mould under vacuum (10^-2 mbar) to prevent copper oxidation.
Sintering Parameters: Pressed at 650 °C under 40 MPa for 30 minutes.
Final Form: Pellets produced at 40 mm diameter, 2 mm thickness, then cut into 6×6×2 mm³ pieces.
Anisotropy: Processing induced strong anisotropic structure with CF oriented perpendicular to the compaction direction.

Diamond Deposition (Combustion Flame CVD)

Setup: Open-air combustion system using a torch with a 1.5 mm diameter.
Gas Feedstock: Acetylene (C₂H₂, 99.999%), Ethylene (C₂H₄, 99.6%), and Oxygen (O₂, 99.996%).
Torch Position: Substrate maintained at 1.5 mm distance from the inner flame cone.
Temperature Control: Cooling system regulated substrate temperature via infrared pyrometer feedback (710 - 740 °C).
Interfacial Modification: Open-air, high O₂ environment consumed exposed Carbon Fibers within 5 minutes, creating fiber-shaped voids (mechanical gripping sites) up to 10 µm deep.

Characterization and Simulation

Microstructure: Scanning Electron Microscopy (SEM) was used for surface and cross-sectional morphology (cross-sections polished using Argon ion milling).
Phase Purity & Stress: Micro-Raman Spectroscopy (514.5 nm laser excitation) analyzed phase purity (sp²/sp³ ratio) and residual stress (peak shift from 1332 cm^-1).
CTE Simulation: COMSOL software utilized to model thermal and thermo-mechanical behavior, including planar heat spreading effects of the diamond coating (100 µm thick) on the 2 mm thick substrate.

6CCVD Solutions & Capabilities

This research validates the use of highly adherent PCD films on low-CTE composite substrates for superior thermal management (heat dissipation in power electronics). While this study utilized combustion CVD, 6CCVD’s state-of-the-art MPCVD techniques offer commercially superior material quality, precision, and scalability for replicating and advancing these thermal assembly applications.

Applicable Materials: Thermal Management Diamond

To replicate or extend this research into commercial applications requiring high-purity thermal conductivity, 6CCVD recommends the following materials:

Thermal Grade Polycrystalline Diamond (PCD): 6CCVD PCD offers high uniformity and excellent thermal conductivity (up to 2000 W/m·K). Ideal for direct deposition onto optimized Cu/CF substrates, ensuring superior phase purity and reduced sp² content compared to combustion CVD.
High-Purity Single Crystal Diamond (SCD): For ultra-demanding applications (e.g., high-power laser diodes) requiring the highest possible thermal conductivity, 6CCVD can supply SCD substrates ready for integration onto low-CTE metal matrices using advanced bonding or metalization techniques.
Custom BDD Substrates: For applications requiring an integrated electrical component, Boron-Doped Diamond (BDD) can be grown by 6CCVD to provide a semi-conducting or metallic layer with high thermal stability.

6CCVD Capability	Research Requirement Addressed	Commercial Advantage
Custom Dimensions (up to 125mm)	Scaling from 6x6 mm² lab samples	Enables inch-sized, production-ready heat spreaders and wafers.
Thickness Control (0.1µm - 500µm)	Replication of 9 µm to 25 µm films	Highly precise thickness uniformity required for reproducible thermal performance.
Precision Polishing (Ra < 5nm PCD)	Rough interfaces (mechanical gripping)	Ability to provide both rough interfaces for mechanical interlocking and mirror-polished surfaces for bonding.
Advanced Metalization Stack	Bonding of diamond to non-diamond materials	Custom internal deposition of carbide-forming interlayers (Ti, W) plus diffusion barriers (Pt, Pd) and final bonding layer (Au, Cu) for chemical adhesion control, reducing reliance on substrate burning.

Customization Potential

The research highlights the complex thermo-mechanical challenges inherent in diamond-metal systems. 6CCVD’s engineering capabilities directly address these challenges:

Substrate Preparation: We offer high-precision laser cutting services for custom substrate dimensions, ensuring optimal fit into composite heat sink designs.
Interface Engineering: While the paper used voids for mechanical gripping, 6CCVD provides custom metalization stacks (e.g., Ti/Pt/Au or W/Au) applied directly to the diamond layer, enabling reliable low-stress bonding interfaces (like active brazing) to the Cu/CF composite, offering an alternative to the high-temperature CF consumption process.

Engineering Support

6CCVD’s in-house PhD material science team specializes in thermal management and stress engineering for diamond-based assemblies. We can assist partners with:

Material Selection: Consulting on the optimal blend of PCD thickness, grain size, and substrate CTE matching for specific Power Electronic Module (PEM) and High-Power RF/Laser projects.
Process Translation: Transitioning successful research methodologies (like CTE reduction using composites) into high-yield, high-purity MPCVD production.
Interfacial Stress Modeling: Providing expertise in predicting and mitigating residual stresses in high-value diamond coatings.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

Tech Support

Original Source

DOI: https://doi.org/10.1016/j.surfcoat.2017.04.037