The Inﬂuence of Site of Co and Holes in PCD Substrate on Adhesive Strength of Diamond Coating with PCD Substrate

At a Glance

Metadata	Details
Publication Date	2023-12-19
Journal	Coatings
Authors	Cen Hao, Guoliang Liu
Institutions	Hong Kong Metropolitan University
Citations	3
Analysis	Full AI Review Included

Executive Summary

This research investigates the critical factors—residual Cobalt (Co) and resulting holes—affecting the adhesive strength of CVD diamond coatings on Polycrystalline Diamond (PCD) substrates, utilizing both Hot Filament Chemical Vapor Deposition (HFCVD) experiments and Density Functional Theory (DFT) calculations.

Core Finding: The location of residual Co atoms or holes on the PCD crystal surface significantly dictates the interfacial binding strength (W_ad) of the diamond coating.
Optimal Interface: The (110) crystal surface exhibited the highest W_ad, providing the most stable interface structure, regardless of whether Co atoms or holes were present.
Quantified Adhesion: W_ad for Co on the (110) surface was 35.17 eV/nm², representing a 31.4% increase compared to the Co on the weakest (100) surface.
Mechanism: DFT analysis showed that the (110) surface facilitates optimal charge transfer, resulting in the strongest C-C covalent bond formation between the PCD substrate and the CVD coating layer.
Engineering Recommendation: To maximize tool performance, PCD synthesis or pretreatment processes should be optimized to ensure Co atoms remain, or holes are localized, predominantly on the (110) crystal surface.
Experimental Validation: Indentation tests confirmed that both Co and holes reduce adhesion, with Co having a greater detrimental effect due to large thermal expansion mismatch.

Technical Specifications

Parameter	Value	Unit	Context
Diamond Lattice Constant (a=b=c)	3.567	A	Face-centered cubic structure model
Etching Acid Ratio (H₂SO₄:H₂O₂)	1:5	Ratio	Corrosive acid pretreatment
Etching Time	48	h	Duration of Co removal pretreatment
Filament Material	Tungsten (W)	-	HFCVD setup (1.0 mm diameter, 4 wires)
Carbon Source	Acetone	-	HFCVD deposition precursor
Filament Carbonization Temp.	2100 ± 100	°C	HFCVD process parameter
Deposition Time	4	h	Duration of CVD diamond growth
Highest Interfacial Binding Energy (Co)	35.17	eV/nm²	Co located on the (110) crystal surface
Highest Interfacial Binding Energy (Hole)	22.62	eV/nm²	Hole located on the (110) crystal surface
Lowest Interfacial Binding Energy (Hole)	0.07	eV/nm²	Hole located on the (100) crystal surface
W_ad Improvement (Hole(110) vs Hole(100))	322.1	%	Relative increase in binding energy
DFT K-point Mesh	4 x 4 x 1	-	Brillouin zone sampling (Gamma Center method)

Key Methodologies

The study combined physical experimentation (HFCVD and mechanical testing) with advanced computational modeling (DFT).

Substrate Pretreatment (Co Removal):
- PCD substrates were etched using a corrosive acid mixture (H₂SO₄:H₂O₂ = 1:5) for 48 hours at 25 °C to remove surface Co binder, creating residual holes.
HFCVD Deposition:
- Filament Carbonization: Tungsten filaments were heated to 2100 ± 100 °C for 2 hours using 4% carbon concentration (1000 mL/min gas flow).
- Diamond Growth: Deposition occurred over 4 hours, using acetone (2% carbon concentration) and hydrogen (1000 mL/min flow). Filament-substrate distance was 9 mm.
Mechanical Characterization:
- Adhesive strength was assessed using the HBRVU-187.5 Brovey hardness tester (square cone diamond indenter, 136° angle). SEM imaging was used to characterize indentation patterns and failure modes.
Geometric Modeling (DFT Input):
- PCD/CVD diamond interfacial structures were built using Device Studio (2023A) for three crystal orientations: (100), (110), and (111).
- Models included 2x2x1 supercells with a 1.3 nm vacuum layer. Co atoms were introduced by replacing C atoms, and holes were modeled by deleting atoms.
First-Principles Calculation:
- DS-PAW software (2023A) was used, employing the Projector Augmented Wave (PAW) pseudopotential method within the Generalized Gradient Approximation (GGA).
- Calculations determined the Interfacial Binding Energy (W_ad), charge density, and charge density difference to quantify C-C covalent bond formation and stability.

Commercial Applications

The findings provide direct engineering guidance for optimizing the manufacturing of superhard materials, primarily targeting tools requiring high coating integrity under extreme mechanical and thermal stress.

High-Performance PDC Cutters: Improving the adhesion of CVD diamond coatings on PDC drill bits used in oil, gas, and geothermal drilling, significantly extending tool life and operational efficiency.
Machining of Composites and Ceramics: Manufacturing diamond-coated inserts and tools for processing difficult-to-machine materials (e.g., carbon fiber reinforced polymers, silicon carbide), where coating delamination is a primary failure mode.
Tool Substrate Engineering: Developing specialized PCD synthesis or post-synthesis treatments that regulate the Co binding phase or hole distribution to preferentially expose the high-adhesion (110) crystal plane at the interface.
Wear-Resistant Components: Application in high-wear industrial components where diamond coatings are used for friction reduction and abrasion resistance, ensuring long-term coating stability.
Advanced Surface Modification: Utilizing DFT-guided surface preparation techniques to achieve targeted crystal orientation exposure prior to CVD deposition, maximizing interfacial covalent bonding strength.

View Original Abstract

Polycrystalline diamond (PCD) prepared by the high temperature and pressure method often uses Co as a binder, which had a detrimental effect on the cutting performance of PCD, thus Co needed to be removed. However, the removal of Co would cause residual holes and also make the cutting performance of PCD poorer. To address this issue, hot filament chemical vapor deposition (HFCVD) was used. During deposition, the residual holes cannot be filled fully, and Co would diffuse to the interface between CVD diamond coatings and the PCD substrate, which influenced the adhesive strength of the diamond coating with the PCD substrate. In order to investigate the influencing mechanism, both experiments and the density functional theory (DFT) calculations have been employed. The experimental results demonstrate that Co and the holes in the interface would reduce the interfacial binding strength. Further, we built interfacial structures consisting of diamond (100), (110), (111) surfaces and PCD to calculate the corresponding interfacial binding energy, charge density and charge density difference. After contrast, for Co and the holes located on the (110) surface, the corresponding interfacial binding energy was bigger than the others. This means that the corresponding C-C covalent bond was stronger, and the interfacial binding strength was higher. Based on this, conducting cobalt removal pretreatment, optimizing the PCD synthetic process and designing the site of Co can improve the performance of the PCD substrate CVD diamond coating tools.

Tech Support

Original Source

DOI: https://doi.org/10.3390/coatings14010001

References

2022 - Polishing of polycrystalline diamond using synergies between chemical and mechanical inputs: A review of mechanisms and processes [Crossref]
2020 - The manufacturing and the application of polycrystalline diamond tools-A comprehensive review [Crossref]
2020 - Application of an innovative ridge-ladder-shaped polycrystalline diamond compact cutter to reduce vibration and improve drilling speed [Crossref]
2013 - A study on PDC drill bits quality [Crossref]
2022 - Effect of transition metal carbides on mechanical properties of polycrystalline diamond by HPHT sintering [Crossref]
2018 - Study on Co-enhancement of polycrystalline diamond composite sheets
2017 - Effect of decobalting on thermal stability of polycrystalline diamond