Microstructure Analysis of Plasma Surfaced Nickel-Coated SiCp Reinforced Cobalt-Based Overlay on H13 Steel

Literature Overview

Published in 2016 in Special Casting and Nonferrous Alloys, this paper by Pan Chenggang, Xiao Qin, Yang Huqun, Ma Wenchao, Chang Qingming, and Wang Huachang investigates the microstructure and properties of cobalt-based composite surfacing layers reinforced with Ni-coated SiCp particles, deposited on H13 hot work steel via plasma surfacing. Funded by the National Natural Science Foundation (51375353) and the Provincial Ministry Co-built State Key Laboratory of Refractory Materials and Metallurgy (2014QN03), this research addresses the challenge of combining high hardness with adequate toughness in surfacing applications where thermal and mechanical loads are severe.

Design Rationale and Material Selection

The selection of Ni-coated SiCp as reinforcement particles addresses two critical challenges in ceramic particle-reinforced metal matrix composites: particle degradation during surfacing and poor wettability of ceramic particles by molten metal. Silicon carbide particles, while offering exceptional hardness (2500-3000 HV) and thermal stability, are susceptible to oxidation and reaction with molten metal during plasma surfacing. The nickel coating serves as a diffusion barrier and improves interfacial bonding between the ceramic reinforcement and the cobalt-based matrix.

Material System Composition

Microstructural Characterization

Phase Identification Results

X-ray diffraction analysis reveals the following phases in the surfacing layer:

Microstructural Zones

The surfacing layer exhibits distinct microstructural zones from the substrate interface to the free surface:

1. Bonded zone (50-150 μm): Cellular dendrite structure with high dilution from H13 base metal; contains Co-Cr solid solution with dispersed carbides; hardness approximately 450-550 HV.
2. Transition zone (150-400 μm): Columnar dendrites growing perpendicular to the substrate; increasing carbide volume fraction; hardness approximately 600-700 HV.
3. Coating zone (400-600 μm): Equiaxed dendrites with fine carbide networks; highest carbide density; hardness approximately 750-800 HV.
4. Surface layer (0-50 μm from top): Contains residual SiCp particles; maximum hardness; particle-matrix interface critical for composite behavior.

Hardness Gradient Distribution

Process Optimization and Particle Retention

The improved plasma surfacing torch design is critical for maximizing SiCp particle retention. Standard plasma torches produce excessive heat input that degrades SiC particles through oxidation and reaction. The modified torch achieves:

• Reduced heat input by 15-20% through optimized nozzle geometry
• Improved powder delivery through enhanced powder carrier gas dynamics
• Better arc stability with the modified powder feed system
• Reduced particle burnout from approximately 40-60% to 10-20%

Key Process Parameters

Engineering Application Assessment

The resulting surfacing layer with surface hardness of 800 HV and gradient hardness distribution is suitable for applications requiring:

• Hot work die and mold surface protection against thermal fatigue and abrasive wear
• Hot metal contact surfaces in continuous casting equipment
• Sliding surfaces in hot metal transfer systems
• Components subjected to combined thermal cycling and abrasive contact

The gradient hardness profile is particularly advantageous because it provides maximum hardness at the surface where wear occurs while maintaining adequate toughness at the bond line to prevent spalling under thermal cycling.

Study Insights and Critical Analysis

The Ni-coating approach for SiCp particles represents an elegant solution to the fundamental challenge of ceramic particle degradation during thermal processing. The coating thickness of 2-5 μm is sufficient to prevent direct contact between SiC and the molten Co-Cr alloy while remaining thin enough to not significantly reduce the effective reinforcement volume fraction. The formation of CoSi and CrSi2 at the particle-matrix interface indicates partial reaction despite the Ni coating, but this reaction layer actually improves interfacial bonding and load transfer efficiency.

The discovery that SiCp particles survive primarily at the top of the surfacing layer, with minimal presence deeper in the deposit, suggests that particle retention is primarily a function of surface cooling conditions. This observation has implications for multi-pass surfacing strategies where particle concentration can be selectively increased in the final pass.

The hardness gradient from 850 HV at the surface to 450 HV at the bond line represents an optimal design for thermal fatigue applications, where the hard surface resists wear while the tougher bond line absorbs thermal stresses without cracking.

Reference Value and Outlook

This research demonstrates a mature approach to composite surfacing that combines metallic matrix toughness with ceramic reinforcement hardness through intelligent particle design and process optimization. The Ni-coating concept is applicable to other ceramic particle systems (Al2O3, WC, TiC) and provides a general methodology for improving particle retention in thermal spray and surfacing processes. The gradient hardness profile achieved represents a design principle that can be extended to other functional gradient surfacing applications where property transitions between substrate and coating are critical for performance and durability.