Nano-Cr3C2 Particle Reinforcement in Plasma Cladded Co40 Alloy Layers

Literature Overview

This study by Si Songhua, Yu Wanping, Yao Changbin, and Zhang Lei from Anhui University of Technology investigates the effect of nano-Cr3C2 particle addition on the microstructure and mechanical properties of plasma arc cladded Co40 alloy overlay layers. Published in Materials for Mechanical Engineering in 2017, this research addresses the challenge of enhancing the wear resistance and mechanical performance of cobalt-based overlay alloys through nano-particle reinforcement, a topic of significant interest in advanced surface engineering.

Co40 Alloy System and Nano-Reinforcement Concept

Co40 alloy, also known as Stellite 6 or CoCr16W6, is one of the most widely used cobalt-based overlay alloys for severe wear and corrosion applications. Its nominal composition includes approximately 6% Cr, 6% W, 5% Mo, 5% Fe, and 5% C, with the balance being cobalt. The alloy forms a solid solution matrix with dispersed M7C3 carbides (primarily Cr7C3 and W7C3) that provide wear resistance through a combination of solid solution strengthening and carbide hardening.

The concept of nano-particle reinforcement involves dispersing sub-micron particles within the overlay matrix to create a composite structure with enhanced mechanical properties. Nano-Cr3C2 is selected as the reinforcement particle because of its exceptional hardness (HV 3000-3500), high melting point (2970°C), chemical stability, and compatibility with cobalt-based matrices. The particle size typically ranges from 50-200 nm, with an average size of approximately 100 nm.

Microstructure Evolution with Nano-Particle Addition

The addition of nano-Cr3C2 particles to the Co40 powder blend significantly modifies the solidification microstructure of the overlay layer. In the unreinforced Co40 overlay, the microstructure consists of a dendritic solid solution matrix with interdendritic M7C3 carbides. The dendrite arm spacing is typically 20-50 μm, and the M7C3 carbides form continuous networks at the dendrite boundaries.

With nano-Cr3C2 addition at levels of 5-20 wt%, several microstructural changes occur:

• Grain refinement: The nano-particles act as heterogeneous nucleation sites, reducing the dendrite arm spacing to 10-25 μm and promoting a more equiaxed grain morphology.
• Carbide morphology modification: The nano-Cr3C2 particles interact with the M7C3 carbides during solidification, promoting the formation of finer, more dispersed carbide particles rather than continuous networks.
• Particle dispersion: The nano-Cr3C2 particles remain relatively uniformly distributed throughout the overlay layer, although some agglomeration occurs at higher addition levels (above 15 wt%).
• Interface bonding: The Co-Cr3C2 interface exhibits good metallurgical bonding due to the formation of a thin Cr2O3 interfacial layer during the plasma arc process, which enhances particle-matrix adhesion.

The optimal nano-Cr3C2 addition level is found to be approximately 10-15 wt%. Below this level, the reinforcement effect is insufficient to significantly improve wear resistance. Above this level, particle agglomeration becomes pronounced, leading to stress concentration sites and reduced toughness.

Mechanical Property Characterization

The mechanical properties of the nano-reinforced overlay layers were characterized through multiple testing methods:

Hardness testing: Vickers microhardness measurements show a progressive increase with nano-Cr3C2 addition. At 5 wt% addition, the surface hardness increases to approximately 600 HV. At 10 wt%, it reaches 700 HV. At 15 wt%, it peaks at approximately 800 HV. Beyond 15 wt%, the hardness slightly decreases due to particle agglomeration and reduced matrix continuity.

Wear testing: Pin-on-disc wear testing against SiC balls at 20 N load demonstrates a significant improvement in wear resistance. The volumetric wear rate decreases from approximately 2.5 × 10⁻⁴ mm³/N·m for unreinforced Co40 to 0.7 × 10⁻⁴ mm³/N·m for 10 wt% nano-Cr3C2 reinforced Co40, representing a 3.6-fold improvement.

Impact testing: The Charpy V-notch impact energy decreases from 50 J for unreinforced Co40 to 35 J for 10 wt% reinforced Co40. This reduction in toughness is an inherent trade-off of nano-particle reinforcement and must be considered in applications subject to impact loading.

Adhesion testing: The overlay layer adhesion strength, measured by the ASTM G144 method or equivalent, remains above 100 MPa for all nano-particle addition levels, indicating that the reinforcement does not significantly compromise the bond strength between the overlay layer and the substrate.

Process Considerations and Engineering Challenges

Several process challenges must be addressed when implementing nano-particle reinforced plasma arc cladding:

1. Powder preparation: The nano-Cr3C2 particles must be uniformly mixed with the Co40 powder without agglomeration. This requires careful powder blending techniques, including high-energy ball milling or attritor milling for 2-4 hours at controlled speeds.
2. Powder feeding: The powder feed rate and stability are critical for consistent nano-particle distribution. A rotating disk or oscillating feeder is recommended to prevent particle segregation during feeding.
3. Thermal stability: Nano-Cr3C2 particles are thermally stable up to approximately 1200°C, which is well above the plasma arc temperature. However, excessive heat input can cause particle growth through Ostwald ripening, reducing the nano-reinforcement effect. Process parameters should be optimized to minimize the time particles spend in the molten state.
4. Cost considerations: Nano-Cr3C2 powder is significantly more expensive than conventional Co40 powder, typically 5-10 times the cost per kilogram. The economic viability of nano-reinforcement depends on the value of the component being protected and the expected extension of service life.

Study Insights and Practical Recommendations

The research provides several valuable insights for engineers considering nano-particle reinforced overlay applications:

• The optimal nano-Cr3C2 addition level of 10-15 wt% represents a good balance between wear resistance improvement and toughness retention. Applications subject to severe abrasion but moderate impact loading will benefit most from this reinforcement.
• The microstructural refinement achieved through nano-particle addition is comparable to that achieved by rapid solidification techniques, but without the need for specialized equipment. This makes nano-reinforced plasma arc cladding a practical approach for industrial applications.
• The adhesion strength of the overlay layer is not significantly affected by nano-particle addition, which is critical for ensuring the long-term reliability of the overlay in service. This eliminates one of the primary concerns associated with composite overlay layers.
• For applications requiring both high wear resistance and good corrosion resistance, the nano-Cr3C2 reinforced Co40 overlay provides an excellent combination, as the Cr3C2 particles contribute to both hardness and corrosion resistance through their chemical stability.

Summary

This research demonstrates that the addition of nano-Cr3C2 particles to Co40 alloy powder for plasma arc cladding is an effective strategy for significantly enhancing wear resistance while maintaining acceptable toughness and adhesion strength. The optimal addition level of 10-15 wt% provides a 3-4 fold improvement in wear resistance with a manageable reduction in impact energy. The process is technically feasible with conventional plasma arc cladding equipment, requiring only modifications to powder preparation and feeding systems. Engineers working on high-value wear components such as turbine blades, pump impellers, and valve seats should consider nano-particle reinforced overlay as a viable solution for extending service life and reducing maintenance costs.