Influence of the surface microstructure on the adhesion of a CVD-diamond coating on steel with a CrN interlayer

At a Glance

Metadata	Details
Publication Date	2018-01-01
Journal	MATEC Web of Conferences
Authors	M. Prieske, Richard Börner, Andreas Schubert
Institutions	Bremen Institute for Applied Beam Technology, Chemnitz University of Technology
Citations	5
Analysis	Full AI Review Included

Technical Documentation: Adhesion Enhancement of Polycrystalline CVD Diamond on Steel Substrates via Microstructure Engineering

6CCVD Ref: MPCVD-PCD/STEEL-ADHESION-V1.0 Source Paper: MATEC Web of Conferences 190, 14008 (2018)

Executive Summary

This research investigates methods for improving the adhesion of Polycrystalline Chemical Vapor Deposition (CVD) diamond coatings on tool steel substrates (Material 1.2379), focusing primarily on the critical role of the substrate surface microstructure and thermal stress management.

Diffusion Barrier Success: Chromium Nitride (CrN, 2.4 µm thick) was confirmed as an effective diffusion barrier, successfully preventing C/Fe diffusion; however, delamination remained a challenge, occurring specifically at the CrN/Diamond interface.
Surface Microstructure Control: Utilizing Ultrasonic Vibration Assisted Milling (UVAM) and blasting allowed for the generation of defined, reproducible surface topographies (Sa values from 0.1 µm to 3.0 µm).
Adhesion Mechanism: Adhesion significantly increases with arithmetic mean height (Sa) above 0.1 µm, supporting mechanical interlocking between the coating and the CrN interlayer.
Inhomogeneous Growth Challenge: Surfaces featuring sharp peaks (high peak-to-valley height) inhibit the formation of a closed diamond layer in the valleys due to localized build-up temperature effects, resulting in inhomogeneous microcrystalline diamond formation.
Thermal Stress Mitigation: Optimized deposition temperatures (700 °C - 840 °C) were found necessary to manage the severe thermal expansion mismatch (diamond: 1.0-4.5 x 10^-6 m/(m·K); steel: 13.0 x 10^-6 m/(m·K)). Adjusting temperature relative to the steel’s Austenite-to-Ferrite transformation (Ac₁ at 840 °C) is vital for reducing residual stresses during cooling.
Process Scalability: The experiment validated a chamber-free, atmospheric Laser-Induced Plasma CVD (LaPlas CVD) process, demonstrating relevance for coating large or complex-geometry forming tools.

Technical Specifications

Extracted physical and process parameters relevant to CVD diamond deposition and substrate characteristics.

Parameter	Value	Unit	Context
Substrate Material	1.2379 (X135CrMoV12)	-	Tool Steel, soft annealed state (255 HV)
CrN Interlayer Thickness	2.4	µm	Applied via High-Power Impulse Magnetron Sputtering (HIPIMS)
Diamond Coating Thickness (High T)	8	µm	Deposited at 840 °C (40 min duration)
Diamond Coating Thickness (Low T)	2	µm	Deposited at 700 °C (60 min duration)
Recommended Roughness (Sa)	> 0.1	µm	Minimum arithmetic mean height supporting adhesion
Maximum Measured Roughness (Sa)	3.0	µm	Corundum Blasted specimen
Developed Interfacial Area Ratio (Sdr) Range	2% to 297%	%	Polished (lowest) to Corundum Blasted (highest)
Steel Phase Transformation (Ac₁)	840	°C	Austenite to Ferrite transformation start temperature
Lowest Deposition Temperature Tested	700	°C	Limited by water-cooled copper specimen holder
Highest Diamond Growth Rate	Up to 20	µm/h	Achieved via LaPlas CVD process
Steel Thermal Expansion Coeff.	13.0 x 10^-6	m/(m·K)	In range 20 °C to 400 °C
CVD Process Gases (CH₄ flow)	0.02	slm	Required for CVD synthesis
CVD Process Gases (H₂ flow)	1.98	slm	Required for CVD synthesis

Key Methodologies

A summary of the substrate preparation, interlayer deposition, and CVD recipe utilized to produce adherent polycrystalline diamond coatings on steel.

Substrate Preparation (Tool Steel 1.2379):
- Seven distinct surface microstructures were generated to control roughness (Sa) and isotropy (Sdr).
- Methods included: Polishing (Sa 0.1 µm), Grinding (Sa 0.5 µm), Glass-bead Blasting, Corundum Blasting (Sa 3.0 µm), and Ultrasonic Vibration Assisted Milling (UVAM).
- UVAM utilized cemented carbide end mills (D_tool = 6 mm) with a resonance frequency of approximately 19.25 kHz and amplitudes up to 6.5 µm.
Diamond Nucleation:
- Substrates were immersed for 10 minutes in an ultrasonic bath containing a dispersion of 200 ml isopropanol and 210 mg diamond powder (average crystal size 0.25 µm to 0.50 µm).
Diffusion Barrier Interlayer Deposition:
- A 2.4 µm thick Chromium Nitride (CrN) coating (BALIQ® CRONOS) was applied to all specimens in one batch via High-Power Impulse Magnetron Sputtering (HIPIMS).
CVD Diamond Deposition (LaPlas CVD):
- Process Type: CO₂ Laser-based Plasma CVD (6 kW laser, 10.6 µm wavelength) operating at atmospheric pressure (chamber-free).
- Coating Area: Approximately 1 cm².
- Gas Flows: Ar (30 slm), H₂ (1.98 slm), CH₄ (0.02 slm).
- Temperature Control: Feedback loop controlled laser power based on substrate temperature measured by an IMPAC pyrometer (IGAR 12-LO), with the emission coefficient set to 0.34.
- Deposition Recipes Tested:
  - High Temp: 840 °C, 40 min duration (8 µm thickness).
  - Low Temp: 700 °C, 60 min duration (2 µm thickness).
Characterization:
- Analysis included SEM, 3D Laser Microscopy (roughness parameters ISO 25178), EDX (elemental analysis of CrN), Raman Spectroscopy (diamond quality, graphitic content, residual stress), Vickers Hardness, and Dilatometry (measuring length change due to phase transformation).

6CCVD Solutions & Capabilities

This research highlights the critical interplay between material purity, interface engineering, and controlled surface topography—areas where 6CCVD delivers industry-leading solutions for advanced engineering applications.

Applicable Materials & Film Properties

6CCVD is positioned to provide the high-quality source material necessary to replicate or extend this research into industrial forming tool coatings.

Material Recommendation: Polycrystalline Diamond (PCD) Free-Standing Wafers and Plates.
- PCD material grown via MPCVD offers exceptional purity and uniformity, essential for controlling the resultant stress and microstructure demonstrated in this study.
Thickness Control: We provide PCD films with precision thickness control from 0.1 µm up to 500 µm, allowing researchers to explore the residual stress effects associated with the 2 µm and 8 µm films studied, as well as significantly thicker structures for industrial wear applications.

Interface Engineering and Customization Potential

The challenge of thermal mismatch requires precise interface management (the CrN interlayer). 6CCVD specializes in overcoming material interface challenges.

Research Requirement / Challenge	6CCVD Specialized Solution	Technical Advantage
Need for Custom Diffusion Barriers	Custom Metalization Services	6CCVD offers extensive in-house metalization capabilities (Au, Pt, Pd, Ti, W, Cu). We can apply tailored metal or multi-layer stacks that serve as diffusion barriers or adhesion layers, allowing exploration of alternatives to CrN (e.g., Ti/W or carbide formers) for specific steel alloys.
Substrate Surface Preparation	Ultra-Precision Polishing (Ra < 5 nm PCD)	While the research focuses on roughening (Sa > 0.1 µm), 6CCVD provides extremely smooth SCD (Ra < 1nm) and inch-size PCD (Ra < 5nm) as the starting material, ensuring highly uniform baseline topography for subsequent mechanical (e.g., UVAM) or chemical surface structuring.
Complex Tool Dimensions	Custom Dimensions and Laser Cutting	6CCVD supplies PCD wafers and plates up to 125mm. We offer precision laser cutting services to achieve complex or high-aspect-ratio shapes required for coating specific forming tool inserts or specialized components.

Engineering Support

The paper demonstrates that optimal deposition parameters (temperature, roughness, film thickness) are strongly dependent on the specific substrate (steel 1.2379) and its unique phase transformation characteristics (Ac₁).

Engineering Support: 6CCVD’s in-house PhD team provides specialized material consultation to assist customers in selecting the ideal PCD grade, interface layers (metalization or doping), and thickness needed to effectively manage residual stress for similar high-wear, metal substrate coating projects. We help define parameters that account for substrate thermal expansion coefficients and critical transformation temperatures.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly. We ship globally (DDU default, DDP available).

View Original Abstract

forming tool coating. Most of the forming tools are made of steel, so that especially the coatability of steel by a polycrystalline diamond coating would rise the range of fields of application. The polycrystalline CVD-diamond coatings are deposited by a laser induced plasma CVD process, without a vacuum chamber. Various surface microstructures were investigated regarding their influence on the residual stresses to prevent a flaking of the coating: on the one hand, deterministic structures generated by ultrasonic vibration assisted milling (UVAM) and on the other hand, stochastic structures manufactured by blasting and polishing processes. For the UVAM, a surface prediction tool was used to design the surface microstructure beforehand. All steel substrates (material no. 1.2379) were coated in one batch by high-power impulse magnetron sputtering with a chromium nitride coating with a thickness of 2.4 μm. The specimens were analysed by laser microscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy and Raman spectroscopy. None of the microstructures investigated in this study was able to prevent delamination of the coating entirely. It could be shown that a roughness higher than Sa 0.1μm supports the interlocking between coating and surface as well as that sharp peaks inhibit a homogenous diamond coating deposition.

Tech Support

Original Source

DOI: https://doi.org/10.1051/matecconf/201819014008