Influence of Defects in Graphene-Like Network of Diamond-Like Carbon on Silica Scale Adhesion

At a Glance

Metadata	Details
Publication Date	2022-12-26
Journal	Tribology Letters
Authors	Yuya Nakashima, Noritsugu Umehara, Hiroyuki Kousaka, Takayuki TOKOROYAMA, Motoyuki Murashima
Institutions	Fuji Electric (Japan), Tohoku University
Citations	5
Analysis	Full AI Review Included

Technical Documentation & Analysis: Advanced Carbon Materials for Silica Scale Mitigation

Research Paper Analyzed: Influence of Defects in Graphene-Like Network of Diamond-Like Carbon on Silica Scale Adhesion (Tribology Letters, 2023)

Executive Summary

This research clarifies the mechanism by which Diamond-Like Carbon (DLC) coatings suppress silica scale adhesion, a critical issue reducing efficiency in geothermal power plants.

Problem Identification: Silica scale adhesion occurs selectively at defects (specifically, dangling bonds) within the sp² graphene-like network of the DLC coating, rather than the sp² carbon atoms themselves.
Adhesion Mechanism: Dangling bonds facilitate strong chemical adsorption of silicic acid ions, resulting in high adsorption energy (up to -1.04 eV).
Suppression Mechanism: High hydrogen content (45-47%) in a-C:H coatings effectively terminates these dangling bonds, reducing the adsorption energy to -0.69 eV and shifting the adhesion mode to weaker physical adsorption.
Material Requirement: Effective anti-scaling coatings must minimize the density of dangling bonds or ensure their complete termination (e.g., via high hydrogen content).
6CCVD Value Proposition: 6CCVD’s high-purity, highly crystalline MPCVD diamond (SCD/PCD) offers a superior alternative to amorphous DLC by providing a near-100% sp³ structure, inherently eliminating the sp² graphene-like network and its associated defects.

Technical Specifications

The following hard data points were extracted from the experimental and computational results:

Parameter	Value	Unit	Context
Adhesion Test Temperature	50	°C	Imitated geothermal brine exposure
Brine pH	8.5	-	Adjusted with HCl
Silicic Acid Ion Adsorption Energy (Defect-free Graphene)	-0.25	eV	Physical adsorption mode
Silicic Acid Ion Adsorption Energy (Dangling Bonds)	-1.04	eV	Chemical adsorption mode (Strongest adhesion)
Silicic Acid Ion Adsorption Energy (Hydrogen Terminated)	-0.69	eV	Physical adsorption mode (Suppressed adhesion)
DLC sp² Fraction Range (Tested)	0.13 to 0.40	-	Outermost 2-nm layer
Hydrogen Content (a-C:H)	45 to 47	%	Measured by Elastic Recoil Detection Analysis (ERDA)
Dangling Bond Density (Range)	1.0 x 10¹⁷ to 1.0 x 10²⁰	cm^-3	Measured by Electron Spin Resonance (ESR)
Calculated Graphene Cell Size	17.19 x 17.01 x 31.70	Å³	First-principles calculation model
Interatomic Distance (Dangling Bond C to Silicic Acid O)	1.59	Å	After chemical adsorption
Interatomic Distance (H-Terminated C to Silicic Acid O)	2.08	Å	After physical adsorption

Key Methodologies

The study employed a combination of advanced surface analysis and first-principles calculations to elucidate the adhesion mechanism.

Model Selection: Highly Oriented Pyrolytic Graphite (HOPG, representing defect-free sp² network) and CVD-synthesized monolayer graphene (representing defect-containing sp² network) were used as simplified models for DLC.
Structural Confirmation: Raman spectroscopy (633 nm excitation) confirmed the presence of defects in CVD graphene (D band at 1350 cm^-1) and the defect-free nature of HOPG.
Defect Visualization: In-lens Field-Emission Scanning Electron Microscopy (FE-SEM) was used to detect work function differences, visualizing defects (dangling bonds) as black-lined patterns on the CVD graphene.
Silica Adhesion Test: Samples were dipped in an imitated geothermal brine solution (NaSiO₃-9H₂O, NaCl, pH 8.5) for 1 hour at 50 °C to precipitate and adhere silica.
Adhesion Site Correlation: SE and in-lens SE imaging confirmed that silica particles selectively adhered to the black-lined (defective) patterns on CVD graphene.
First-Principles Calculations: Density Functional Theory (DFT) using the GGA and vdW-DF methods was applied to calculate the adsorption energy (ΔE) of silicic acid ions onto three models: defect-free, dangling bond, and hydrogen-terminated graphene.
Adsorption Mode Analysis: Bader charge analysis confirmed the shift from chemical adsorption (charge sharing) at dangling bonds to physical adsorption (no charge sharing) upon hydrogen termination.

6CCVD Solutions & Capabilities

This research highlights that surface defects are the primary drivers of adhesion in carbon-based coatings. 6CCVD’s expertise in high-purity, highly crystalline MPCVD diamond offers a superior, engineered solution for extreme tribological and anti-fouling applications like geothermal power generation.

Applicable Materials

To replicate or extend this research using materials with superior intrinsic properties compared to amorphous DLC, 6CCVD recommends:

Optical Grade Single Crystal Diamond (SCD): SCD is intrinsically 100% sp³ carbon, eliminating the sp² graphene-like network and the associated dangling bond defects identified in this study. This material offers the ultimate baseline for defect-free, chemically inert surfaces.
High-Purity Polycrystalline Diamond (PCD): For large-area industrial components, PCD provides exceptional hardness and chemical resistance. Our advanced growth recipes minimize non-diamond carbon content, ensuring the highest possible sp³ fraction and grain boundary quality.

Customization Potential

The study emphasizes the need for precise control over surface structure and termination. 6CCVD provides the necessary engineering control:

Research Requirement	6CCVD Capability & Specification	Relevance to Anti-Adhesion
Large Area Coverage (Industrial Scale)	PCD Wafers up to 125 mm diameter.	Enables coating of large components used in geothermal turbines and heat exchangers.
Precise Thickness Control	SCD/PCD thickness from 0.1 µm to 500 µm.	Allows optimization of coating thickness for durability and cost efficiency in high-wear environments.
Minimizing Surface Defects	Ultra-Polishing: Ra < 1 nm (SCD), Ra < 5 nm (Inch-size PCD).	Near-atomic flatness drastically reduces the density of surface dangling bonds and potential adhesion sites.
Surface Functionalization (Hydrogen Termination)	Custom Surface Engineering.	We offer post-growth treatments, including precise hydrogen termination, to actively passivate any residual surface defects, mirroring the successful suppression mechanism (-0.69 eV adsorption energy) demonstrated in the paper.
Substrate Integration	Custom Substrates up to 10 mm thickness.	We can grow diamond films directly onto customer-supplied or custom-engineered substrates suitable for industrial integration.

Engineering Support

6CCVD’s in-house PhD team specializes in tailoring MPCVD diamond properties for demanding applications. We can assist engineers and scientists with:

Material Selection: Choosing the optimal diamond grade (SCD, PCD, or BDD) based on specific operating temperatures, pressures, and chemical environments (e.g., high-pH brine).
Surface Optimization: Designing custom polishing and termination protocols to achieve the lowest possible surface energy and defect density for similar silica scale mitigation projects.
Metalization: Providing internal metalization services (Au, Pt, Pd, Ti, W, Cu) for integration into complex sensor or electrode systems often required in power plant monitoring.

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

View Original Abstract

Abstract Silica scale adhesion onto geothermal power plant equipment reduces the power efficiency. In our previous study, diamond-like carbon (DLC) coatings with low sp 2 fractions and high hydrogen contents were found to suppress silica adhesion. Therefore, the present study was aimed at clarifying the mechanism of silica adhesion onto the graphene-like network of DLC. In-lens scanning electron microscopic imaging of silica adhered onto defective graphene indicated that the adhesion occurred on defects in the graphene-like network. First-principles calculations revealed that the graphene with hydrogen-terminated defects exhibited reduced adsorption energy between silica and the graphene-like network. Overall, the simulations and experiments helped establish a silica adhesion model in which defects in the graphene-like network of DLC behave as silica adhesion sites. Graphical Abstract

Tech Support

Original Source

DOI: https://doi.org/10.1007/s11249-022-01690-4