Plasma Arc Surfacing Ni-Based Spherical Tungsten Carbide Coating Friction and Wear Study

Literature Overview

This paper, published in Surface Technology (Volume 47, Issue 2, 2018, pages 103-110) by Li Shutao et al. from CNOOC Energy Development Equipment Technology Co., Ltd., Shanghai Institute of Technology, and Shanghai Maritime University, investigates the friction and wear behavior of nickel-based composite coatings reinforced with spherical tungsten carbide (WC) particles fabricated via plasma transfer arc (PTA) surfacing. The study is supported by the Marine Public Welfare Industry Research Special Fund Project (No. 201405013-3), reflecting its relevance to offshore marine engineering applications.

Core Technical Approach

The researchers prepared composite coatings using nickel-based mixed powders with WC mass fractions of 20%, 30%, 50%, and 60% respectively, deposited onto substrates through plasma arc surfacing. Friction and wear testing was conducted on the side surfaces of the coatings using a Bruker universal tribometer. The characterization methodology encompassed surface morphology observation, friction coefficient measurement, scratch cross-sectional area analysis, and microstructural examination of wear surfaces.

Key Experimental Parameters

WC Mass Fraction (%)	Primary Wear Mechanisms	Relative Wear Resistance
20	Adhesive wear, oxidative wear	Baseline
30	Adhesive wear, oxidative wear	Moderate improvement
50	Adhesive wear, abrasive wear	Optimal balance
60	Adhesive wear, abrasive wear	Diminishing returns, cost penalty

Technical Interpretation and Key Findings

The study reveals a clear transition in wear mechanisms as WC content increases. Below 50% WC mass fraction, the dominant wear modes are adhesive and oxidative, indicating that the matrix still plays a significant role in material removal. Above 50%, abrasive wear becomes the primary mechanism alongside adhesive wear, suggesting that the hard WC particles begin to dominate the tribological interaction.

The wear resistance of the composite coating increases monotonically with WC content, and notably, the near-heat-affected-zone (HAZ) base material also exhibits improved wear resistance. This latter observation is particularly significant from an engineering perspective, as it suggests that the thermal input from PTA surfacing creates beneficial microstructural modifications in the substrate near the fusion line, potentially through grain refinement or precipitation hardening effects.

Mechanism Analysis

The enhancement in wear resistance is attributed to two synergistic factors: (1) the inherent hardness and chemical stability of WC particles resisting material removal, and (2) the continuous generation of reinforcing phases during the thermal cycle of plasma surfacing. The spherical morphology of the WC particles is noteworthy, as it likely promotes more uniform distribution within the matrix and reduces stress concentration at particle-matrix interfaces compared to irregularly shaped particles.

Engineering Practice Implications

From a practical standpoint, the recommendation of 50% WC mass fraction represents an optimal engineering compromise. Beyond this threshold, while wear resistance continues to increase marginally, the cost of the coating system rises disproportionately, and potential issues such as increased porosity, cracking susceptibility, and reduced toughness may emerge. For offshore equipment components subjected to severe abrasion and corrosion—such as subsea pipeline components, marine valves, and pump impellers—this coating system offers a viable surface engineering solution.

Process Considerations for PTA Surfacing

Process Parameter	Recommended Range	Rationale
Arc current	200-350 A	Adequate powder melting without excessive dilution
Powder feed rate	0.8-1.5 kg/h	Balanced deposition rate and dilution ratio
Travel speed	100-200 mm/min	Uniform bead profile and controlled dilution
Shielding gas	Ar or Ar/He mix	Stable arc and oxide-free surface
Preheating temperature	150-250°C	Reduce thermal cracking in HAZ

Study Insights and Reflections

The paper demonstrates that particle shape control—specifically the use of spherical WC—is a critical but often overlooked variable in composite coating design. Spherical particles achieve better packing density and more uniform dispersion, which directly translates to improved tribological performance. The observation that the base material near the fusion zone also benefits from improved wear resistance opens up interesting possibilities for hybrid surface engineering approaches where the coating and substrate work synergistically.

One area warranting further investigation is the long-term performance of these coatings under combined loading conditions typical of marine environments, where cyclic stress, corrosion, and wear act simultaneously. The current study focuses on laboratory tribometer testing, which, while valuable for fundamental understanding, does not fully replicate the complex service conditions encountered in offshore applications.

The cost-performance analysis recommending 50% WC content is well-reasoned, but it would benefit from additional data on fatigue resistance and thermal stability at elevated temperatures, which are critical for many industrial applications. Overall, this work provides a solid foundation for the development of economically viable hardfacing solutions for the marine and offshore industries.