Composition Optimization and Wear Performance of Plasma Surfacing Coatings Using Orthogonal Experimental Design

Literature Overview

This paper by Lu Jianbo, Yao Shun, Lou Songnian, Du Zeyu, and Li Shaoqing, published in Mechanical Engineering Materials in 2006 (Vol. 30, No. 3, pp. 35–37), presents a systematic approach to optimizing the composition of Ni60-based plasma surfacing powder coatings. Chromium, manganese, and tungsten powders were added to the Ni60 base alloy powder, and a three-factor three-level orthogonal experimental design was employed to determine the optimal powder formulation. Plasma surfacing trials were conducted on Q235 steel plates, with hardness serving as the primary optimization criterion. The study was conducted at the Shanghai Jiao Tong University Welding Engineering Research Institute.

Core Findings and Technical Analysis

The orthogonal experimental design identified the optimal powder formulation as: ω(Cr) = 10%, ω(Mn) = 4%, ω(W) = 7%, with the remainder being Ni60 base alloy powder. Wear testing demonstrated that the optimized coating provided improved wear resistance on Q235 steel surfaces, with wear resistance increasing proportionally with coating hardness.

The Ni60 base alloy, which is a nickel-cobalt alloy with chromium and boron additions, provides an excellent foundation for plasma surfacing due to its excellent fluidity, low dilution rate, and inherent hardness. The addition of chromium enhances the formation of hard carbide and boride phases, manganese improves the austenitic matrix stability and contributes to solid solution strengthening, and tungsten introduces additional hard phase formation through the precipitation of tungsten carbides.

Interpretation of Key Technical Points

The orthogonal experimental design approach is particularly well-suited for this type of multi-factor optimization problem. With three factors and three levels, a full factorial design would require 27 experiments, while the orthogonal design reduces this to 9 experiments while maintaining the ability to evaluate main effects and identify the optimal combination. This efficiency is crucial when each experiment requires powder preparation, plasma surfacing, and property testing.

The selection of hardness as the optimization criterion reflects the well-established correlation between coating hardness and wear resistance in abrasive wear applications. However, this approach assumes that the wear mechanism is predominantly abrasive, where hardness is the dominant material property governing wear resistance. In erosive or adhesive wear scenarios, additional factors such as toughness, thermal stability, and surface roughness may be more influential.

The optimal composition reveals interesting metallurgical relationships:

• Chromium at 10%: Sufficient to form hard Cr₇C₃ and Cr₃B phases without excessive brittleness. Higher chromium levels would increase hardness but at the cost of reduced toughness and increased susceptibility to thermal cracking.
• Manganese at 4%: Provides adequate austenite stabilization in the nickel-based matrix, contributing to solid solution strengthening without promoting harmful intermetallic phases.
• Tungsten at 7%: Introduces WC and W₂C hard phases that contribute significantly to wear resistance. The relatively low level reflects the high cost of tungsten and the diminishing returns at higher additions.

Engineering Practice Implications

Plasma surfacing offers several advantages over alternative hardfacing processes for producing wear-resistant coatings:

• Low dilution rate: Typically 5–15%, resulting in coatings that closely approximate the powder composition
• High deposition efficiency: The plasma arc provides concentrated heat input with high energy density
• Precise thickness control: Suitable for thin coatings on precision components
• Versatile powder feed: Allows easy adjustment of composition for different wear requirements

For pipeline and pressure vessel applications, plasma surfacing of Ni60-based coatings is particularly relevant for:

• Valve seat hardening in high-pressure systems
• Pump impeller and casing protection
• Heat exchanger tube wear protection
• Valve stem and gland seal hardening
• Control valve trim components

The orthogonal design methodology demonstrated in this study provides a systematic approach that engineers can apply to their own coating optimization problems. The approach is particularly valuable when multiple alloying elements interact in complex ways, making intuitive optimization unreliable.

Key Questions and Reflections

Several important limitations and considerations should be noted. The study focuses exclusively on hardness as the optimization criterion, which may not capture all aspects of wear performance. For example, coatings with slightly lower hardness but higher toughness may exhibit superior performance in impact-abrasion wear scenarios. The wear testing methodology, while providing useful comparative data, should ideally include multiple wear mechanisms (abrasive, erosive, adhesive, corrosive) to provide a comprehensive performance assessment.

The plasma surfacing process parameters (current, voltage, powder feed rate, travel speed, standoff distance) were presumably held constant during the composition optimization, but their interaction with the optimized composition deserves investigation. Different compositions may require different process parameters to achieve optimal coating quality.

The cost implications of the optimized composition, particularly the 7% tungsten content, should be evaluated for commercial applications. Tungsten is a critical raw material with significant price volatility, and alternative compositions achieving comparable performance at lower cost should be considered.

Study Insights and Implications

This research demonstrates the power of systematic experimental design in optimizing complex multi-component alloy systems for plasma surfacing applications. The orthogonal design approach provides an efficient and rigorous methodology for identifying optimal compositions with minimal experimental effort. The identified Ni60-based formulation with 10% Cr, 4% Mn, and 7% W represents a practical starting point for developing wear-resistant plasma surfacing coatings on low-carbon steel substrates. For engineers in the coating and surface engineering field, this work exemplifies the importance of combining metallurgical understanding with statistical experimental design to achieve optimal results. The clear correlation between coating hardness and wear resistance, while useful as a screening criterion, should be complemented with comprehensive wear testing under representative service conditions before final coating selection for critical applications.