Ni60A-WC Composite Coating by Dual-Powder Plasma Cladding: Microstructure and Performance

Literature Overview and Technical Context

The dual-powder plasma cladding technique introduces two distinct powder streams — a metallic binder powder (Ni60A) and a ceramic reinforcement powder (WC) — into the plasma arc simultaneously. This approach allows independent control of the binder and reinforcement feed rates, enabling precise tuning of the composite microstructure. The Ni60A/WC system is widely used in the manufacture of wear-resistant components for mining, cement, and food processing industries. The study focuses on how the dual-powder feeding mechanism affects the microstructure, mechanical properties, and wear performance of the resulting composite coating compared to single-powder pre-mixed approaches.

Dual-Powder Feeding Mechanism and Process Control

The dual-powder plasma cladding process requires careful synchronization of the two powder feed systems. The Ni60A powder, which serves as the metallic binder, is typically fed at a rate of 150–300 g/min, while the WC powder, which serves as the ceramic reinforcement, is fed at 50–150 g/min. The mass ratio of Ni60A to WC is usually maintained between 3:1 and 6:1. The key challenge is ensuring that both powders enter the plasma arc at the same time and melt uniformly. If the WC powder enters the arc prematurely or in large agglomerates, it can cause arc instability, porosity, and uneven coating thickness.

The dual-powder approach offers a significant advantage over pre-mixed powder: the WC particles can be added at a higher concentration without compromising the flowability of the powder blend. Pre-mixed powders with high WC content tend to segregate during storage and feeding, leading to inconsistent coating composition. The dual-feed system eliminates this problem by maintaining separate powder streams until the moment of arc entry.

Microstructure and Phase Analysis

The microstructure of the Ni60A/WC composite coating is characterized by a dendritic Ni60A matrix with WC particles distributed throughout. At the optimal WC content (around 20–30 wt%), the WC particles are partially dissolved during the plasma arc melting process, forming a gradient transition from intact WC cores to dissolved regions with new carbide phases such as W2C and W4C3. The dissolution of WC releases tungsten into the matrix, forming (Ni,Fe)7W6 and (Ni,Fe)3W carbides that further enhance hardness. The resulting microstructure exhibits a three-phase system: retained WC cores, transition carbides, and the Ni60A matrix.

The hardness distribution shows a clear trend with WC content. At 10 wt% WC, the average hardness is approximately 650 HV. At 25 wt% WC, the hardness increases to 780–820 HV. Beyond 30 wt%, the hardness increment diminishes because the WC particles begin to agglomerate, creating weak interfaces and reducing the effective reinforcement. The wear resistance follows a similar trend, with the optimal WC content falling in the 20–28 wt% range.

Defect Analysis and Countermeasures

Several defects are commonly encountered in dual-powder plasma cladding. Porosity is the most prevalent defect, caused by incomplete powder melting, gas entrapment, or arc instability. The countermeasure is to maintain a consistent arc distance of 3–5 mm and ensure that the powder feeders are calibrated to deliver a steady, laminar flow. Cracking at the coating-base interface can occur when the thermal expansion mismatch between the Ni60A coating and the steel substrate is excessive. Preheating to 200–300°C and post-weld stress relief at 600–700°C are effective mitigations. Delamination is another concern, particularly when the WC content exceeds 30 wt%, because the high ceramic fraction creates a brittle composite with poor interfacial bonding. The study recommends limiting WC content to 25 wt% for most industrial applications.

Study Insights and Practical Implications

The dual-powder plasma cladding technique represents a practical advancement in composite coating technology. In my experience, the ability to independently control the binder and reinforcement feed rates provides a level of process flexibility that is not achievable with pre-mixed powders. This is particularly valuable when producing components with varying wear requirements across different regions. The study also highlights the importance of powder quality — the WC particles should be uniformly sized (typically 15–45 μm) and free of agglomerates to ensure consistent melting behavior. For production-scale applications, automated powder feed systems with real-time monitoring of flow rates and arc parameters are essential for maintaining coating quality. The Ni60A/WC system, when properly optimized, offers a compelling solution for components subjected to severe abrasive wear in aggressive environments.