Nickel-Based Composite Powder Coating Materials by Plasma Arc Cladding

Literature Overview

This paper, published in the Welding Journal (焊接学报) in 2005 by Dong Lihong, Xu Binfu, Zhu Sheng, and Du Zeyu, investigates the microstructure and properties of nickel-based composite powder coatings produced by plasma transferred arc (PTA) cladding. The research was conducted by the National Key Laboratory of Equipment Remanufacturing Technology at the Academy of Armored Force Engineering and Tianjin University, and was supported by the National Natural Science Foundation of China (Grant Nos. 50075086, 50235030) and the National Defense Science and Technology "15th Five-Year Plan" Pre-Research Project (Grant No. 413270103).

Core Technical Content

The study addresses the development of high-performance nickel-based composite powder coatings for severe service applications in military and civilian industries. The composite powder system combines a nickel-based alloy matrix with hard ceramic particles to achieve enhanced wear resistance, corrosion resistance, and thermal stability.

Composite Powder Design

The composite powder system consists of a nickel-based alloy matrix (typically Ni-Cr-Mo or Ni-Cr-Si-B) combined with hard ceramic particles such as WC, Cr3C2, TiC, or SiC. The powder design follows the following principles:

• Matrix composition: Ni-18Cr-5Mo-3Fe or similar for high-temperature applications
• Hard phase: WC (62-65 wt% W, 35-38 wt% C) or Cr3C2 for wear resistance
• Hard phase content: 15-30 vol% for optimal properties
• Particle size: -74+44 μm for good flowability and uniform distribution
• Spherical morphology: Gas-atomized or water-atomized for good powder flowability

The composite powder is prepared by mechanical mixing of the nickel-based alloy powder and hard ceramic particles in a V-blender for 2-4 hours, followed by screening to remove agglomerates and oversized particles.

Plasma Arc Cladding Process

The PTA cladding process uses a high-velocity plasma arc to melt the substrate surface and the composite powder simultaneously. The key process parameters are:

The PTA process offers several advantages over conventional arc welding processes for cladding applications:

• Lower dilution rate (5-15% vs. 20-40% for SAW)
• Higher deposition rate (1.0-3.0 kg/h vs. 0.5-1.5 kg/h for GTAW)
• Better process control and repeatability
• Ability to clad complex geometries
• Reduced thermal distortion compared to conventional welding

Microstructural Analysis

The microstructure of the PTA cladded nickel-based composite powder coatings exhibits the following features:

• A dendritic microstructure with eutectic cells at dendrite arms
• Dispersed hard ceramic particles (WC, Cr3C2, etc.) throughout the matrix
• Some dissolution of WC particles with formation of Ni3W and Ni4W intermetallic compounds
• Fine carbide precipitates at dendrite boundaries
• A transformation zone at the coating-substrate interface

The cooling rate in PTA cladding typically ranges from 50 to 500 K/s, which is higher than in conventional welding but lower than in laser cladding. This cooling rate produces a microstructure that is finer than conventional welding but coarser than laser cladding, providing a good balance between hardness and toughness.

Mechanical and Tribological Properties

The mechanical properties of the PTA cladded nickel-based composite powder coatings are as follows:

• Hardness: 800-1000 HV (compared to 400-500 HV for pure nickel-based alloy)
• Wear resistance: 2-4 times that of the unmodified nickel-based alloy
• Bond strength: >50 MPa (satisfying most industrial standards)
• Residual stress: Predominantly compressive (-200 to -400 MPa)
• Thermal stability: Retains hardness up to 600°C

The wear mechanism transitions from abrasive wear at low loads to adhesive-abrasive mixed wear at intermediate loads, and ultimately to fatigue wear at high loads. The hard ceramic particles provide primary abrasive resistance, while the nickel-based matrix provides toughness and supports the hard phases.

Engineering Practice Integration

The PTA cladded nickel-based composite powder coatings are applicable to the following engineering scenarios:

1. Wear protection of turbine components (blades, disks, casings) in gas turbine engines
2. Corrosion and wear protection of heat exchanger tubes in chemical processing
3. Surface hardening of extrusion dies and forging dies in metal forming
4. Wear protection of pump impellers and casing components in slurry service
5. Sealing surface protection in high-temperature and high-pressure valves
6. Restoration of worn military equipment components (armor, tracks, etc.)

The PTA cladding process requires careful setup and operation to achieve optimal coating quality. The following quality control measures are recommended:

1. Pre-weld inspection of the substrate surface for cleanliness and dimensional accuracy
2. In-process monitoring of arc parameters and powder feeding rate
3. Post-weld inspection of coating thickness, porosity, and dilution rate
4. Non-destructive testing (MT or PT) to detect surface cracks or defects
5. Hardness testing at multiple locations to verify coating quality

Key Questions and Reflections

The partial dissolution of hard ceramic particles during PTA cladding is a fundamental challenge that must be managed. The dissolution rate depends on the laser power, travel speed, and particle size. For WC particles, the dissolution rate increases with increasing current and decreasing travel speed. The optimal process parameters should be selected to retain a sufficient volume fraction of intact WC particles while ensuring complete melting of the nickel-based matrix.

The dilution rate is another critical parameter that must be controlled. For nickel-based alloy coatings, the dilution rate should not exceed 15% to maintain the required corrosion resistance and mechanical properties. The dilution rate can be controlled by adjusting the arc current, travel speed, and powder feeding rate. Lower arc current, higher travel speed, and higher powder feeding rate all contribute to lower dilution rates.

The thermal cycling during multi-pass PTA cladding can cause cracking in the coating, particularly if the coating composition is susceptible to hot cracking. The addition of ductile phases or the use of a backing layer of a more ductile material can help prevent cracking. Post-weld heat treatment at 800-900°C for 1-2 hours can also relieve residual stresses and improve coating ductility.

Study Insights and Implications

This research demonstrates that the PTA cladding process is an effective method for producing high-performance nickel-based composite powder coatings with enhanced wear resistance, corrosion resistance, and thermal stability. The combination of the PTA process's low dilution capability with the reinforcing effect of hard ceramic particles creates a coating system with properties that approach those of sintered hardmetal materials while maintaining the metallurgical bond and conformability of a welded overlay.

For engineers specifying PTA cladding operations, this study highlights the importance of controlling the arc current to travel speed ratio and the powder feeding rate to maintain partial retention of the hard ceramic particles. The recommended parameters of 250-350 A, 150-250 mm/min, and 30-50 g/min provide an optimal balance between coating quality and deposition rate. The findings also underscore the value of systematic microstructural characterization and tribological testing in understanding the mechanisms of wear resistance improvement and optimizing the coating design for specific applications.