The Influence of Co binding phase on adhesive strength of diamond coating with cemented carbide substrate

At a Glance

Metadata	Details
Publication Date	2015-01-01
Journal	Acta Physica Sinica
Authors	Xiaogang Jian, Jun Chen
Citations	13
Analysis	Full AI Review Included

Technical Documentation & Analysis: Diamond Coating Adhesion on Cemented Carbide Substrates

Reference: Jian Xiao-Gang, Chen Jun. The Influence of Co binding phase on adhesive strength of diamond coating with cemented carbide substrate. Acta Physica Sinica, 64, 216701 (2015).

Executive Summary

This first-principles study provides critical atomic-level insight into the failure mechanism of diamond coatings on cemented carbide (WC-Co) substrates, directly impacting the manufacturing of high-performance cutting tools.

Core Finding: The presence of the Cobalt (Co) binding phase in the substrate severely weakens the adhesive strength of the diamond coating interface.
Quantified Reduction: Interfacial bonding energy ($W_{ad}$) dropped significantly from 6.74 J/m² (WC/Diamond, no Co) to 5.94 J/m² (WC-Co/Diamond, with Co).
Mechanism Identified: Co acts as a charge transfer agent, causing the Tungsten (W) atoms in the WC substrate and the Carbon (C) atoms in the diamond layer at the interface to acquire identical polarity.
Structural Impact: This resulting electrostatic repulsion increases the equilibrium interface distance dramatically, from 2.069 Å (WC/Diamond) to 3.649 Å (WC-Co/Diamond).
Bond Weakening: Mulliken population analysis confirms that the primary W-C chemical bond strength is reduced (0.45 to 0.39) when Co is present, leading to poor mechanical stability.
Industrial Relevance: The research validates the necessity of rigorous substrate pre-treatment (de-cobalting) prior to MPCVD diamond deposition to achieve the high adhesion required for durable cutting tools.

Technical Specifications

The following hard data points were extracted from the first-principles simulation results comparing the two interface models:

Parameter	Value	Unit	Context
Interfacial Bonding Energy ($W_{ad}$)	6.74	J/m²	WC/Diamond Model (No Co)
Interfacial Bonding Energy ($W_{ad}$)	5.94	J/m²	WC-Co/Diamond Model (With Co)
Interface Distance (WC/Diamond)	2.069	Å	Optimized stable structure
Interface Distance (WC-Co/Diamond)	3.649	Å	Optimized stable structure (Co present)
Mulliken Population (W-C Bond)	0.45	N/A	WC/Diamond Interface (Stronger bond)
Mulliken Population (W-C Bond)	0.39	N/A	WC-Co/Diamond Interface (Weaker bond)
C(Diamond) Charge Density (WC/D)	0.430	e/Å³	Interface C atom charge density (WC/D)
C(Diamond) Charge Density (WC-Co/D)	0.201	e/Å³	Interface C atom charge density (WC-Co/D)
DFT Calculation Convergence	1.0 x 10^-6	eV/atom	Calculation precision
Plane Wave Cutoff Energy	300	eV	CASTEP simulation parameter

Key Methodologies

The study utilized advanced computational methods based on quantum mechanics to analyze the interface stability:

Theoretical Framework: Density Functional Theory (DFT) was employed as the core theoretical method to study the ground state properties of the multi-particle system.
Software Implementation: Calculations were performed using the CASTEP simulation software package within the Materials Studio environment.
Functional Selection: The Generalized Gradient Approximation (GGA) framework, specifically the Perdew-Burke-Ernzerhof (PBE) functional, was used to determine the exchange and correlation potentials.
Model Construction:
- WC and Diamond single crystal cells were used to build [100] supercell models.
- WC base layer dimensions: 2.90 Å x 2.84 Å x 5.0 Å.
- Diamond coating layer dimensions: 3.56 Å x 3.56 Å x 3.56 Å.
Co Doping Simulation: The WC-Co/Diamond model was created by adding a Co atom at the center point of the periodic boundary surface of the WC/Diamond interface.
Boundary Conditions: A 12 Å vacuum layer was included in the models to ensure accurate simulation of the interface structure under periodic boundary conditions.
Convergence Criteria: Calculations were self-consistently solved (Kohn-Sham equations) with a convergence precision of 1.0 x 10^-6 eV/atom and a maximum atomic force of 0.1 eV/nm.

6CCVD Solutions & Capabilities

This research highlights the critical importance of achieving a clean, highly adherent interface for diamond coatings on cemented carbide tools. While the study focused on the substrate chemistry, 6CCVD provides the high-purity, high-quality diamond materials necessary to maximize performance once substrate preparation is optimized.

Applicable Materials for Tool Coating Applications

The application (cutting tools) requires robust, thick diamond films. 6CCVD specializes in the following materials ideal for replicating or extending this research:

Polycrystalline Diamond (PCD) Plates: Recommended material for high-wear tool coatings. 6CCVD offers PCD plates with thicknesses ranging from 0.1 µm up to 500 µm, allowing researchers to test the mechanical performance of films across the entire 2-20 µm range cited in the paper, and beyond.
Optical Grade SCD (Single Crystal Diamond): For fundamental research requiring the highest purity and structural perfection, 6CCVD provides SCD plates up to 500 µm thick with surface roughness Ra < 1 nm.

Customization Potential for Advanced Research

To move from theoretical modeling to practical, high-adhesion coatings, 6CCVD offers tailored manufacturing capabilities:

Requirement from Research	6CCVD Custom Capability	Benefit to Customer
Substrate Size/Geometry	Custom Plates/Wafers up to 125mm (PCD)	Supports large-scale tool coating or production-sized research batches.
Precise Film Thickness	SCD/PCD thickness control from 0.1 µm to 500 µm	Allows precise control over the diamond layer thickness (2-20 µm range) for optimizing tool life and performance.
Post-Deposition Integration	Custom Metalization Services (Au, Pt, Pd, Ti, W, Cu)	Enables robust integration (e.g., brazing) of the diamond-coated substrate into complex tool holders, crucial for industrial applications.
Surface Quality	Polishing to Ra < 5 nm (Inch-size PCD)	Provides ultra-smooth surfaces, minimizing friction and maximizing coating integrity.

Engineering Support & Consultation

The DFT results clearly demonstrate that the Co binding phase is the primary inhibitor of strong adhesion. Successful coating requires mitigating this effect through optimized substrate pre-treatment (de-cobalting).

Expert Consultation: 6CCVD’s in-house PhD team specializes in MPCVD growth and material science. We can assist engineers and scientists in selecting the optimal diamond material properties (e.g., grain size, morphology, nitrogen doping) to maximize adhesion and overcome the challenges posed by the WC-Co substrate chemistry.
Application Focus: We provide material selection support specifically for high-performance Diamond-Coated Cutting Tool projects, ensuring the material properties complement the necessary pre-treatment protocols.

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

View Original Abstract

Diamond coating has many excellent properties as the same as those of the natural diamond, such as extreme hardness, high thermal conductivity, low thermal expansion coefficient, high chemical stability, and good abrasive resistance, which is considered as the best tool coating material applied to the high-silicon aluminum alloy cutting. We can use the hot filament chemical vapor deposition method (HFCVD) to deposit a 2-20 μm diamond coating on the cemented carbide tool to improve the cutting performance and increase the tool life significantly. Many experiments have proved that the existence of cobalt phase can weaken the adhesive strength of diamond coating. However, we still lack a perfect theory to explain why the Co element can reduce the adhesive strength of diamond coating is still lacking. What we can do now is only to improve the adhesive strength of diamond coating by doing testing many times in experiments. Compared with these traditional experiments, the first principles simulation based on quantum mechanics can describe the microstructure property and electron density of materials. It is successfully used to investigate the surface, interface, electron component, and so on etc. We can also use this method to study the interface problem at an atomic level. So the first principles based upon density functional theory (DFT) is used to investigate the influence of cobalt binding phase in cemented carbide substrate on adhesive strength of diamond coating. In this article, we uses Material Studios software to build WC/diamond and WC-Co/diamond interface models to evaluate the influence of cobalt phase on the adhesive strength of diamond coating with CASTEP program which can calculate the most stablest structure of film-substrate interface. We use PBE functional form to obtain the exchange potential and relevant potential, and to solve the self-consistent Kohn-Sham equations. We calculate the interfacial bonding energy, analyse the electron density of diamond coating and the bond Mulliken population of diamond film-substrate interface. The results show that the interfacial bonding energy of WC/diamond is 6.74 J/m2 and that of WC-Co/diamond is 5.94 J/m2, which implies that the adhesive strength of WC/diamond is better than that of WC-Co/diamond. We also find that Co element can transfer the charges near the interface of WC/diamond model when the magnetic Co element exists at the WC/diamond interface. As a result, the polarity of tungsten element in tungsten carbide and the polarity of carbon element in diamond coating near the interface turn to be identical polarity, and then the charge density of tungsten in cemented carbide changes from 0.430 e/A3 to 0.201 e/A3 and the charge density of Carbon in diamond changes from-0.045 e/A3 to 0.037 e/A3, and they exclude to each other, so the distance of interface becomes larger than that from the WC/diamond model, which changes from 2.069 Å to 3.649 Å. This can explain why the existence of Co element can weaken the adhesive strength of diamond coating. Meanwhile, Mulliken population analyses show that the bond strength of WC-Co /diamond at the interface is smaller than that of WC/diamond. So this can prove that the cobalt binding phase in cemented carbide substrate can weaken the adhesive strength of diamond coating, and then we need to do some pretreatments in order to reduce the cobalt binding phase in the cemented carbide substrate before depositing diamond coating.

Tech Support

Original Source

DOI: https://doi.org/10.7498/aps.64.216701