Wear Resistance of Ni-Based Cr3C2 and WC Enhanced Plasma Clad Coatings

Literature Overview

This study, published in Powder Metallurgy Materials and Engineering Science in 2015 by Yang Zaijiang, Li Yuxi, Yan Wei, Zhou Wuxi, and Yu Wei from Zigong Great Wall Hard Facing Materials Co., Ltd., investigates the tribological performance of nickel-based plasma transferred arc (PTA) cladding coatings reinforced with chromium carbide (Cr3C2) and tungsten carbide (WC) particles. The research was conducted under the National Science and Technology Support Program (2012BAE06B02), which underscores the national strategic importance of hard-facing materials in China's industrial equipment sector. The authors systematically evaluated how the combination of Cr3C2 and WC as composite reinforcements within a Ni-based matrix influences hardness, wear resistance, and microstructural integrity of the cladding layer.

Core Technical Content and Microstructural Analysis

The fundamental challenge addressed in this work is the simultaneous optimization of wear resistance and toughness in plasma-clad hard-facing layers. Pure Cr3C2 reinforcement provides excellent abrasive resistance due to its high hardness (approximately 1800–2000 HV), but tends to produce a brittle microstructure susceptible to spalling under impact loading. Conversely, WC particles offer superior hardness (2800–3000 HV) but are prone to thermal decomposition during welding, forming W2C and Fe3W3C phases that can embrittle the coating. The authors explored the synergistic effect of combining both carbide types within a Ni-Cr-B-Si base alloy system.

The microstructural evolution during PTA cladding involves several critical phenomena. The Ni-based matrix solidifies in a dendritic pattern, with carbide particles distributed along the dendrite arms and interdendritic regions. During rapid solidification, Cr3C2 remains relatively stable due to its lower melting point compared to WC, while WC particles undergo partial decomposition. The resulting microstructure typically consists of a Ni matrix with eutectic carbides (M7C3, M23C6), undecomposed Cr3C2 particles, and partially decomposed WC in the form of W2C and Fe-rich carbides.

Wear Mechanism and Performance Evaluation

The authors conducted pin-on-disk and block-on-ring wear tests under both dry sliding and abrasive conditions. The composite Cr3C2-WC reinforced coatings demonstrated superior wear resistance compared to single-reinforcement systems. The wear mechanism analysis revealed that under dry sliding conditions, the dominant mechanism transitions from adhesive wear to abrasive wear as the hardness of the coating increases. The hard carbide particles act as load-bearing phases that resist ploughing and micro-cutting by the counterface material.

A key finding was the optimal ratio of Cr3C2 to WC. At approximately 60:40 by weight, the coating achieved a balanced combination of hardness (780–820 HV) and fracture toughness. Below this ratio, excessive WC decomposition led to brittle carbide networks. Above this ratio, insufficient WC content resulted in lower overall hardness despite improved ductility. The wear rate of the optimized composite coating was reduced by approximately 40–55% compared to a conventional Ni-Cr-B-Si coating without hard carbide reinforcement.

The role of the Ni-based matrix in binding and protecting the carbide particles cannot be overstated. The ductile Ni matrix absorbs impact energy and prevents catastrophic spalling of the hard carbide phases. This matrix-reinforcement interaction is the cornerstone of the composite coating concept and represents a significant advancement over traditional single-phase hard-facing materials.

Engineering Practice Implications

From a practical standpoint, this research has direct applications in components subjected to severe abrasive and erosive wear, such as mining equipment, cement mill liners, pump impellers, and hydraulic valve bodies. The PTA process offers significant advantages over conventional arc welding overlay methods, including lower dilution rates (typically 15–25% versus 30–50% for SAW or ESW), finer microstructures, and better dimensional control.

However, several practical challenges must be addressed during production implementation. First, the powder feed system must ensure uniform mixing of Cr3C2 and WC particles with the Ni-based binder alloy. Segregation during feeding leads to heterogeneous coatings with variable properties. Second, the preheating temperature of the substrate (typically 250–400°C for steel substrates) must be carefully controlled to minimize thermal cracking without introducing excessive grain growth. Third, interpass temperature control between multiple passes is critical to prevent cracking in the heat-affected zone.

The study also highlights the importance of post-weld treatment. Stress relief annealing at 800–900°C for 2–4 hours can significantly reduce residual stresses without adversely affecting the hardness of the coating. For critical applications, a solution treatment followed by aging can further optimize the precipitation hardening response of the Ni matrix.

Key Reflections and Study Insights

This research exemplifies the power of composite reinforcement strategies in hard-facing technology. The synergy between Cr3C2 and WC is not merely additive but involves complex microstructural interactions that can be harnessed through careful process design. The authors' systematic approach to optimizing the reinforcement ratio provides a valuable methodology that can be extended to other composite coating systems.

One notable limitation is the relatively limited scope of wear testing conditions. Real-world applications often involve multi-body contact, corrosion-abrasion synergy, and cyclic loading, which may not be fully captured by standard laboratory wear tests. Future work should incorporate field trials and accelerated service simulation to validate laboratory findings under actual operating conditions.

The economic implications are also significant. While the PTA process is more expensive per unit area than conventional welding overlay, the extended service life of critical components can justify the additional cost through reduced downtime and replacement frequency. This cost-benefit analysis should be an integral part of any engineering decision regarding coating selection.