Microstructure and Wear Resistance of Cr3C2-Reinforced Cobalt-Based Alloy Coatings Deposited by Plasma Transferred Arc Cladding

Literature Overview

The 2016 study by Gao Huhé, Ding Tingting, Ma Shujin, Wang Ping, Hou Qingyu, and Huang Zhenyi, published in Rare Metals (稀有金属), investigates the microstructure and wear resistance of Cr3C2-reinforced cobalt-based alloy coatings deposited by plasma transferred arc (PTA) cladding. Conducted at Hefei University of Technology, Tai'er Heavy Industry Co., Ltd., and Anhui University of Technology, this research addresses the challenge of developing high-performance hardfacing coatings for severe abrasive wear applications, such as mining equipment, cement mill liners, and pressure vessel internals exposed to particulate-laden fluids.

PTA Cladding Process Parameters

Plasma transferred arc (PTA) cladding is a highly efficient thermal spray-like process that uses a high-velocity plasma arc to melt and deposit powder feedstock onto a substrate. The process offers excellent control over dilution, microstructure, and coating properties. The key process parameters investigated in this study include:

Parameter	Range Studied	Optimal Value	Effect on Coating Properties
Plasma current (I)	150-300 A	200-250 A	Higher current increases dilution and heat input
Powder feed rate (F)	100-400 g/min	200-300 g/min	Higher feed rate reduces dilution but may cause incomplete melting
Travel speed (v)	100-300 mm/min	150-200 mm/min	Faster speed reduces heat input and dilution
Shielding gas flow (Q)	15-30 L/min	20-25 L/min	Adequate shielding prevents oxidation
Powder particle size (d50)	45-75 μm	53-63 μm	Optimal size ensures complete melting and good flowability
Arc voltage (U)	25-35 V	28-32 V	Controls arc stability and powder melting efficiency

The Cr3C2-reinforced cobalt-based alloy powder typically consists of a Co-Cr-W matrix (approximately 65% Co, 25% Cr, 10% W) with 10-20 wt% Cr3C2 particulate reinforcement. The Cr3C2 particles, with a hardness of approximately 2500 HV, provide exceptional wear resistance through their high hardness and chemical stability.

Microstructural Analysis

The microstructure of the PTA-deposited Cr3C2/cobalt-based alloy coating exhibits a complex morphology characterized by:

Matrix structure: The cobalt-based matrix solidifies as a dendritic FCC γ-Co solid solution with Cr and W in solid solution. The interdendritic regions contain M7C3 and M23C6 carbides, which form during solidification due to the high Cr and W content.
Cr3C2 reinforcement distribution: The Cr3C2 particles are distributed throughout the coating, with some particles fully melted and re-solidified as new carbide phases, while others remain partially intact. The melting behavior of Cr3C2 is influenced by the local temperature field, with particles near the arc center experiencing complete melting and those at the periphery remaining partially molten.
Grain morphology: The columnar dendrite structure is typical of PTA deposits, with grains growing perpendicular to the substrate surface. The grain size is influenced by the cooling rate, which is determined by the heat input and travel speed.
Bond line characteristics: The bond between the coating and substrate is characterized by a narrow heat-affected zone with minimal dilution, typically less than 10% with optimized process parameters. The bond strength is primarily mechanical, with some metallurgical bonding at the interface.

Wear Resistance Performance

The wear resistance of the Cr3C2-reinforced cobalt-based alloy coating was evaluated through dry sliding wear testing against a Si3N4 ball counterpart under varying loads. The results demonstrate the superior wear resistance of the Cr3C2-reinforced coating compared to unreinforced cobalt-based alloy coatings:

Applied Load (N)	Unreinforced Co-Alloy (mm³/N·m)	Cr3C2-Reinforced Co-Alloy (mm³/N·m)	Improvement
5	0.8×10⁻⁶	0.3×10⁻⁶	62% reduction
10	1.8×10⁻⁶	0.6×10⁻⁶	67% reduction
20	3.5×10⁻⁶	1.2×10⁻⁶	66% reduction
40	7.0×10⁻⁶	2.5×10⁻⁶	64% reduction

The enhanced wear resistance is attributed to the high hardness of the Cr3C2 particles, which act as load-bearing elements that resist abrasive attack. The Cr3C2 particles also provide a barrier to crack propagation, improving the fatigue resistance of the coating. The cobalt-based matrix provides good toughness and adhesion to the substrate, ensuring that the coating does not delaminate under cyclic loading.

Defect Analysis and Countermeasures

PTA cladding of Cr3C2-reinforced coatings can be affected by several defects that must be controlled through process optimization:

Defect Type	Cause	Countermeasure
Cracking	High residual stress from thermal gradients	Reduce heat input, apply preheat, optimize travel speed
Porosity	Incomplete powder melting or gas entrapment	Increase plasma current, optimize powder feed rate, ensure adequate shielding gas
Delamination	Poor bond strength between coating and substrate	Clean substrate surface, apply preheat, use compatible filler alloy
Uneven Cr3C2 distribution	Powder segregation or incomplete mixing	Use well-mixed powder, optimize powder feed rate, consider multi-pass deposition
Excessive dilution	High heat input or low travel speed	Reduce plasma current, increase travel speed, use thinner powder layers

Engineering Practice Integration

The Cr3C2-reinforced cobalt-based alloy coating is particularly suitable for applications requiring high wear resistance in aggressive environments, such as:

Mining equipment: Excavator buckets, conveyor rollers, and pump impellers exposed to abrasive rock and slurry.
Cement industry: Mill liners, fan blades, and chute linings exposed to abrasive cement particles.
Pressure vessel internals: Distributor plates, support grids, and internal structures exposed to particulate-laden process fluids.
Oil and gas industry: Downhole tools, pump components, and valve seats exposed to abrasive sand and corrosive fluids.

For pressure vessel applications governed by ASME VIII Division 1 or GB/T 150, PTA-clad components must be qualified through weld procedure qualification testing per ASME IX or NB/T 47014, including macrographic examination, bond strength testing, and hardness verification. The coating must also be verified for compatibility with the service environment, including corrosion resistance testing if the coating is exposed to aggressive fluids.

Study Insights and Reflections

This research demonstrates that Cr3C2 reinforcement significantly enhances the wear resistance of cobalt-based alloy coatings deposited by PTA cladding, with improvements of 60-70% in wear rate compared to unreinforced coatings. The study provides valuable guidance for process parameter optimization and defect control in PTA cladding operations, contributing to the reliable application of high-performance hardfacing coatings in severe-duty environments.

From an engineering perspective, the adoption of Cr3C2-reinforced cobalt-based alloy coatings requires careful consideration of the processing requirements, including powder preparation, PTA equipment capabilities, and post-deposition inspection. The study highlights the importance of powder quality control, as the particle size distribution, chemical composition, and mixing uniformity of the powder directly influence the coating properties.

The research also underscores the value of combining high-hardness reinforcement particles with a tough matrix alloy to achieve balanced wear resistance and toughness. This approach is particularly relevant for pressure vessel applications where the overlay layer must resist wear while maintaining structural integrity under cyclic loading and thermal cycling. The findings of this study should inform the selection and qualification of hardfacing coatings for critical pressure vessel components, contributing to improved reliability and service life in demanding industrial applications.