Evaluation of the Microstructure, Tribological Characteristics, and Crack Behavior of a Chromium Carbide Coating Fabricated on Gray Cast Iron by Pulsed-Plasma Deposition

At a Glance

Metadata	Details
Publication Date	2021-06-19
Journal	Materials
Authors	Yu. G. Chabak, V. G. Efremenko, Miroslav Džupon, Kazumichi Shimizu, V.I. Fedun
Institutions	Slovak Academy of Sciences, Muroran Institute of Technology
Citations	10
Analysis	Full AI Review Included

Evaluation of Chromium Carbide Coatings on Gray Cast Iron: A 6CCVD Analysis

This technical documentation analyzes the microstructure, tribological characteristics, and crack behavior of a high-chromium carbide coating fabricated via Pulsed-Plasma Deposition (PPD) on Gray Cast Iron (GCI). This analysis focuses on extracting key performance metrics and leveraging 6CCVD’s expertise in MPCVD diamond materials to propose superior solutions for high-wear applications.

Executive Summary

This study successfully demonstrated significant wear resistance improvements in Gray Cast Iron (GCI) through the application of a high-chromium carbide coating via Pulsed-Plasma Deposition (PPD) and subsequent heat treatment.

Coating Structure and Thickness: A composite coating (210-250 µm thick) was formed, consisting of 48 vol.% Cr-rich carbides (M⁷C³, M³C), 48 vol.% martensite, and 4 vol.% retained austenite.
Hardness Achievement: Post-heat treatment microhardness reached 980-1180 HV, a substantial increase over the GCI substrate (approx. 223 HV).
Abrasive Wear Performance: The heat-treated coating exhibited 3.0-3.2 times higher abrasive wear resistance compared to the GCI substrate.
Dry-Sliding Performance: Exceptional dry-sliding wear resistance was achieved, showing up to 1208.8 times lower volume loss when tested against the hardest counter-body (diamond cone).
Critical Failure Mechanism: The coating exhibited solidification cracking due to high tensile stress generated by the initial, low specific volume austenitic matrix. This cracking led to surface spalling under high-stress contact (diamond cone).
6CCVD Value Proposition: The study highlights the need for materials with extreme hardness and structural integrity to withstand high-stress contact. 6CCVD’s MPCVD diamond (SCD/PCD) offers an order of magnitude increase in hardness (HV > 8000) and monolithic structure, directly addressing the cracking and wear limitations observed in the Cr-carbide composite.

Technical Specifications

The following hard data points were extracted from the research paper detailing the material properties and performance metrics of the heat-treated (HT) coating.

Parameter	Value	Unit	Context
Coating Thickness	210-250	µm	Post-HT coating (Zone A)
Coating Microhardness (HT)	980 to 1180	HV	Near top to interface
Coating Microhardness (As-Dep.)	620-670	HV	As-deposited state
Carbide Volume Fraction (HT)	48.2	vol.%	Cr-rich carbides (M⁷C³, M³C)
Martensite Volume Fraction (HT)	47.6	vol.%	Heat-treated matrix
Retained Austenite (HT)	4.2	vol.%	Heat-treated matrix
Abrasive Wear Improvement	3.0-3.2	times higher	Compared to GCI (non-HT/HT)
Dry-Sliding Volume Loss (Diamond Cone)	0.49 x 10^-3	µm³	Lowest volume loss achieved
Dry-Sliding Wear Improvement (Diamond Cone)	1208.8	times lower volume loss	Compared to GCI substrate
Dry-Sliding Wear Improvement (SiC Ball)	3.9	times lower volume loss	Compared to GCI substrate
Dry-Sliding Wear Improvement (Steel Ball)	1.8	times lower volume loss	Compared to GCI substrate
Maximum Surface Temperature (Calculated)	2350	°C	During plasma collision (440 µs)
Maximum Melting Depth (Calculated)	15	µm	Transitional layer width (7-25 µm observed)

Key Methodologies

The protective coating was fabricated using an Electrothermal Axial Plasma Accelerator (EAPA) followed by specific thermal processing steps.

Substrate Preparation: Gray Cast Iron (GCI) specimens (6 mm x 12 mm x 25 mm) were used, ground to R_a = 0.638 µm.
Source Material: High-Chromium White Cast Iron rod (27.39 wt.% Cr, 2.34 wt.% C) served as the expandable cathode.
Deposition Environment: Pulsed-Plasma Deposition (PPD) was performed in the air.
Electrical Parameters: Discharge voltage stored in the capacitor was 4.0 kV, resulting in a pulsed arc discharge current of ≈18 kA and voltage of ≈4.5 kV.
Pulse Count: 10 plasma pulses were applied to the target surface.
Post-Plasma Heat Treatment (HT): Specimens were held at 950 °C for two hours.
Quenching: Oil cooling was used to achieve the martensitic transformation.
Tribological Testing:
- Three-Body Abrasion: 20 N load, 10.8 s^-1 rotating speed, Al₂O₃ abrasive particles (0.5-0.6 mm diameter).
- Dry-Sliding (Ball-on-Plate): 5 N normal load, reciprocating movement (3.5 mm stroke, 8.75 m total distance).
- Counter-Bodies: 100Cr6 steel ball, SiC ball, and a diamond cone (120° apex angle).

6CCVD Solutions & Capabilities

The research successfully demonstrated that high-hardness, carbide-rich coatings significantly enhance wear resistance. However, the inherent structural limitations of the PPD composite (cracking, porosity, complex transitional layers) resulted in failure (spalling) under the highest stress conditions (diamond cone contact).

6CCVD specializes in MPCVD diamond, the ultimate material for extreme tribological environments, offering monolithic integrity and superior hardness that eliminates the drawbacks of composite coatings.

Applicable Materials for Replication and Extension

To replicate or significantly extend the performance achieved in this research, particularly in high-stress applications like tooling, dies, and rolling equipment components, 6CCVD recommends the following materials:

Application Requirement	6CCVD Recommended Material	Rationale
Extreme Abrasive/Sliding Wear	Optical Grade Single Crystal Diamond (SCD)	SCD offers the highest purity and structural perfection (Ra < 1 nm achievable), providing unparalleled hardness (HV > 8000) and thermal stability, far surpassing the 1180 HV achieved by the Cr-carbide.
Large-Area Wear Plates/Dies	High-Purity Polycrystalline Diamond (PCD)	Available in large formats (up to 125 mm diameter), PCD provides isotropic hardness and wear resistance suitable for industrial scale-up of components like die forms and rolling equipment.
High-Stress Tooling/Counter-Bodies	Custom SCD/PCD Components	To accurately test or utilize the highest wear resistance, 6CCVD can supply custom diamond components (e.g., cones, spheres, or specialized inserts) that are structurally superior to the Cr-carbide coating tested.
Electrodes/Sensors (BDD)	Boron-Doped Diamond (BDD)	While not the primary focus, BDD offers high conductivity and electrochemical stability, ideal for integrating sensing or electrochemical functions into wear-resistant systems.

Customization Potential for Advanced Research

The paper highlighted the need for improved structural integrity and crack mitigation. 6CCVD’s capabilities directly address these engineering challenges:

Thickness Control: The paper achieved 210-250 µm thickness. 6CCVD offers precise thickness control for both SCD (0.1 µm to 500 µm) and PCD (0.1 µm to 500 µm), allowing engineers to specify the exact diamond layer required for optimal performance without the structural compromises of PPD.
Surface Finish: The Cr-carbide coating required extensive polishing (R_a = 0.076 µm). 6CCVD provides ultra-smooth polishing down to R_a < 1 nm for SCD and R_a < 5 nm for inch-size PCD, ensuring minimal friction and eliminating surface defects that initiate wear.
Metalization Services: The study mentions the need for strong metallurgical bonding. 6CCVD offers in-house, custom metalization (including Ti, W, Pt, Au, Cu) for robust bonding and integration of diamond layers onto various substrates, crucial for high-performance hybrid components.
Custom Dimensions: 6CCVD can supply PCD wafers up to 125 mm in diameter, enabling the transition of this research from small lab specimens (6 mm x 12 mm) to full-scale industrial components.

Engineering Support

The research concluded that preventing cracking requires obtaining a stable, high-specific volume martensitic matrix or a composite structure that decreases thermal contraction.

6CCVD’s in-house PhD team specializes in the material science of diamond and can assist researchers in transitioning from complex, crack-prone composite systems to monolithic diamond solutions for similar High-Wear Tooling and Protective Coating projects. Diamond’s intrinsic properties (high thermal conductivity, extreme hardness, and chemical inertness) inherently solve the tribological and structural stability issues encountered in this study.

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

View Original Abstract

The structural and tribological properties of a protective high-chromium coating synthesized on gray cast iron by air pulse-plasma treatments were investigated. The coating was fabricated in an electrothermal axial plasma accelerator equipped with an expandable cathode made of white cast iron (2.3 wt.% C-27.4 wt.% Cr-3.1 wt.% Mn). Optical microscopy, scanning electron microscopy, energy-dispersive spectroscopy, X-ray diffraction analysis, microhardness measurements, and tribological tests were conducted for coating characterizations. It was found that after ten plasma pulses (under a discharge voltage of 4 kV) and post-plasma heat treatment (two hours of holding at 950 °C and oil-quenching), a coating (thickness = 210-250 µm) consisting of 48 vol.% Cr-rich carbides (M7C3, M3C), 48 vol.% martensite, and 4 vol.% retained austenite was formed. The microhardness of the coating ranged between 980 and 1180 HV. The above processes caused a gradient in alloying elements in the coating and the substrate due to the counter diffusion of C, Cr, and Mn atoms during post-plasma heat treatments and led to the formation of a transitional layer and different structural zones in near-surface layers of cast iron. As compared to gray cast iron (non-heat-treated and heat-treated), the coating had 3.0-3.2 times higher abrasive wear resistance and 1.2-1208.8 times higher dry-sliding wear resistance (depending on the counter-body material). The coating manifested a tendency of solidification cracking caused by tensile stress due to the formation of a mostly austenitic structure with a lower specific volume. Cracks facilitated abrasive wear and promoted surface spalling under dry-sliding against the diamond cone.

Tech Support

Original Source

DOI: https://doi.org/10.3390/ma14123400

References

2006 - Damping and tribological properties of Fe-Si-C cast iron prepared using various heat treatments [Crossref]
2018 - Wear behaviour of grey cast iron with the presence of copper addition
2020 - Effects of quench-tempering and laserhardening treatment on wear resistance of gray cast iron [Crossref]
2015 - Effect of bionic coupling units׳ forms on wear resistance of grey cast iron under dry linear reciprocating sliding condition [Crossref]
2017 - Effect of alternate biomimetic coupling units on dry sliding wear resistance of grey cast iron [Crossref]
2018 - Lubricating property of cast iron combined with two-scale unit structure by biomimetic laser treatment [Crossref]
2018 - A bio-inspired concept to improve crack resistance of gray cast iron [Crossref]
2017 - Application of the Q-n-P-treatment for increasing the wear resistance of low-alloy steel with 0.75 %C [Crossref]
2014 - In-situ surface hardening of cast iron by surface layer metallurgy [Crossref]
2015 - Improved fatigue wear resistance of grey cast iron by localized laser carburizing [Crossref]