Microstructural Analysis of Plasma Cladded Nickel-Coated SiCp-Reinforced Cobalt-Based Overlay

Literature Overview

This study investigates the microstructure and properties of a plasma transferred arc (PTA) cladded composite overlay consisting of nickel-coated silicon carbide (Ni-coated SiCp) particles reinforced in a cobalt-based matrix alloy. Metal matrix composites (MMCs) are increasingly used in cladding applications where superior wear resistance is required without sacrificing toughness, and the combination of SiCp reinforcement with a cobalt-based matrix offers a promising approach for extreme wear environments. The study provides detailed microstructural analysis including particle distribution, bonding characteristics, phase composition, and the effects of process parameters on microstructure evolution.

Composite Overlay Composition and Process Parameters

The composite overlay consists of a cobalt-based matrix alloy (typically Stellite 6 or similar Co-Cr-W alloy) reinforced with 10–20 vol% nickel-coated SiC particles. The nickel coating on SiC particles serves a critical function in improving the wettability and bonding between the ceramic reinforcement and the metallic matrix.

The nickel coating on SiC particles is critical for achieving good interfacial bonding. Without the coating, SiC particles exhibit poor wettability with cobalt-based melts, leading to particle debonding and void formation at the particle-matrix interface.

Microstructural Analysis

Particle Distribution and Morphology

Metallographic examination reveals that the Ni-coated SiC particles are distributed throughout the cobalt-based matrix with relatively uniform spatial distribution. The particle size distribution in the cladding layer is slightly narrower than the original powder feedstock, indicating that some particles undergo partial melting and recrystallization during the PTA process.

Key microstructural observations include:

• Particle size in cladding layer: 5–35 μm (reduced from 10–50 μm feedstock)
• Particle shape: Rounded to irregular (original angular particles become rounded due to surface melting)
• Particle volume fraction in cladding: 8–15 vol% (reduced from 10–20 vol% feedstock due to dilution)
• Particle alignment: No significant alignment observed, indicating good random distribution
• Particle-matrix bonding: Good bonding at most interfaces with Ni coating layer visible as a thin reaction zone

Interface Reaction Zone

High-magnification examination of the SiC particle-matrix interface reveals a reaction zone consisting of:

• Inner layer: Unreacted SiC core with intact crystal structure
• Intermediate layer: Ni-rich interfacial zone, 2–5 μm thick, with possible Ni₃Si formation
• Outer layer: Co-Cr-W matrix with possible Cr₂C₃ carbide formation at the interface

The reaction zone thickness is influenced by process parameters, with higher arc currents and longer residence times producing thicker reaction zones. The formation of Cr₂C₃ carbides at the interface can either improve or degrade properties depending on their distribution and morphology.

Matrix Microstructure

The cobalt-based matrix in the cladding layer exhibits a complex microstructure consisting of:

• Primary phase: Face-centered cubic (FCC) Co-based solid solution
• Carbide phase: M₇C₃ and M₂₃C₆ carbides (Cr-rich) dispersed throughout the matrix
• Eutectic structure: Co-rich liquid + M₇C₃ carbides at interdendritic regions
• Grain size: 30–80 μm, with some columnar grains near the bond line
• Dilution zone: Gradient transition from composite overlay to base metal over 0.5–1.0 mm

The carbide morphology in the matrix is influenced by the presence of SiC particles. Near SiC particles, carbide precipitation is suppressed due to the carbon scavenging effect of SiC, creating a carbide-free zone around particles that can affect local hardness distribution.

Mechanical Properties and Performance