Plasma Arc Surfacing of Nickel-Based Spherical Tungsten Carbide Coatings: Friction-Wear Performance Analysis and Process Development

1. Definition and Fundamental Principles

Plasma arc surfacing (PAS) of nickel-based spherical tungsten carbide (WC) coatings is an advanced thermal spray/weld overlay technique in which a consumable composite material—consisting of a nickel-alloy binder matrix reinforced with high-hardness spherical tungsten carbide particles—is deposited onto a substrate surface using a high-temperature, high-velocity plasma arc as the heat source. The plasma arc, generated by ionizing an inert gas (typically argon or argon-hydrogen mixtures) through a constricted nozzle at temperatures exceeding 10,000–20,000 K, melts the composite wire or powder feedstock and transfers it onto the prepared substrate surface in a controlled, dilution-managed manner.

The fundamental principle governing the tribological performance of these coatings lies in the composite microstructure: the nickel-based matrix (commonly Ni-Cr-Mo or Ni-Co-W-Cr alloys) provides ductility, corrosion resistance, and metallurgical bonding to the substrate, while the spherical WC particles—typically in the 5–50 μm size range with spherical morphology—act as hard reinforcing phases that resist abrasive and adhesive wear. The spherical geometry of the WC particles, as opposed to irregular or angular particles, offers superior stress distribution during contact loading, reducing particle fracture and pull-out under sliding conditions.

The interaction between the plasma arc and the composite feedstock creates a dilution-controlled weld overlay where carbon content in the matrix is carefully managed to preserve the hardness and integrity of the WC phase. Excessive dilution leads to WC dissolution and carbide precipitation (e.g., M₇C₃, M₆C₇), which degrades coating hardness; insufficient melting results in poor bonding and delamination. The optimal dilution rate for WC-containing nickel coatings is typically maintained between 5% and 15%, depending on the specific alloy system and process parameters.

2. Category and Business Positioning

Within Cladding Technology Shanxi Co., Ltd.'s technology portfolio, the plasma arc surfacing of nickel-based spherical WC coatings falls under the advanced weld overlay and thermal processing category, complementing the company's three primary technology routes:

• TIG/MIG Weld Overlay: The plasma arc surfacing process is a specialized extension of arc-based overlay technology, employing higher energy density and more precise thermal input control than conventional TIG or MIG welding.
• Hydraulic Explosive Bonding: While hydraulic bonding produces solid-state clad structures without melting, plasma arc surfacing addresses scenarios where surface-level hardening and wear resistance modification is required on existing clad components.
• Explosion Welding: Similar to hydraulic bonding, explosion welding provides through-thickness metallurgical bonds; plasma arc surfacing serves as a surface enhancement technology for post-fabrication hardfacing applications.

This capability positions the company as a full-spectrum surface engineering provider—capable of delivering both bulk cladding solutions (via explosive/hydraulic bonding) and surface hardening treatments (via plasma arc surfacing)—thereby offering customers a comprehensive material protection strategy from design through maintenance.

3. Technical Purpose and Value

3.1 Primary Technical Objectives

• Wear Resistance Enhancement: Achieve surface hardness of 800–1200 HV (as-surfaced) or 1000–1400 HV (after heat treatment) on substrates that typically exhibit 200–400 HV, providing 3–5× improvement in abrasive and erosive wear resistance.
• Extended Component Service Life: Reduce replacement frequency of wear-critical components by 5–20× depending on service severity, directly lowering total cost of ownership (TCO).
• Friction Coefficient Reduction: In certain nickel-alloy/WC formulations, the composite coating can reduce dry friction coefficients from 0.4–0.6 (bare steel) to 0.2–0.3, improving mechanical efficiency in tribological contacts.
• Corrosion-Wear Synergy: The nickel matrix provides inherent resistance to acidic and oxidizing environments while the WC phase resists mechanical degradation, addressing combined degradation mechanisms.

3.2 Customer Value Proposition

The study on friction-wear performance of plasma arc surfaced nickel-based spherical WC coatings provides quantifiable, standards-referenced data that supports engineering design decisions, procurement specifications, and warranty commitments. For end-users in mining, power generation, cement, and oil/gas sectors, this capability translates into:

• Reduced unplanned downtime through predictable coating performance
• Capability to retrofit existing components rather than replacing entire assemblies
• Customizable coating thickness (0.5–3.0 mm per pass, multi-pass up to 10+ mm) for application-specific requirements
• Thermal input control suitable for thin-walled and pre-stressed components

4. Key Process and Implementation Points

4.1 Process Parameter Optimization

The performance of plasma arc surfaced WC coatings is critically dependent on the precise control of multiple interrelated process parameters. The following table summarizes the recommended parameter ranges and their influence on coating properties:

Parameter	Typical Range	Influence on Coating Performance
Plasma Arc Current	80–250 A	Higher current increases dilution rate; must be balanced to preserve WC integrity
Plasma Gas Flow Rate	15–35 L/min (Ar or Ar+H₂)	Affects arc stability and energy density; H₂ addition increases arc temperature by 10–15%
Shielding Gas Flow Rate	10–25 L/min (Ar or Ar+He)	Prevents oxidation of molten pool; insufficient flow causes porosity and oxide inclusions
Travel Speed	100–400 mm/min	Higher speed reduces dilution and heat input; too high causes incomplete melting and poor bonding
Wire Feed Speed	100–500 mm/min	Controls deposition rate; must be synchronized with travel speed for uniform bead geometry
Stick-out Length	3–8 mm	Affects arc concentration; excessive stick-out causes wire burning and dilution
Preheat Temperature	100–300 °C (substrate-dependent)	Reduces thermal gradient stress; too high promotes grain growth and WC dissolution
Interpass Temperature	≤250 °C (for multi-pass)	Prevents excessive thermal cycling and residual stress accumulation

4.2 Substrate Preparation Requirements

• Surface Cleaning: Mechanical grinding to bare metal (Sa 2.5 minimum per ISO 8501-1) or carbon arc gouging for removal of prior coatings, scale, and contaminants.
• Edge Preparation: V-groove or U-groove preparation for multi-pass builds exceeding 1.5 mm total thickness, with included angles of 60°–90°.
• Base Metal Compatibility: Applicable to carbon steel, low-alloy steel (up to 0.5% C), austenitic stainless steel, and certain cast irons. Pre-weld dilution testing is mandatory for high-carbon or high-hardness substrates to prevent cracking.
• Stress Relief: Post-overlay stress relief at 400–550 °C for 2–4 hours for components subject to cyclic or impact loading.

4.3 Microstructural Control

The as-surfaced microstructure of nickel-based WC coatings typically consists of:

• Intact WC particles: Retained spherical morphology, contributing primary hardness (2800–3000 HV intrinsic WC hardness)
• WC dissolution products: M₇C₃ and M₆C₇ carbides formed from excess carbon dissolution—acceptable in controlled quantities but detrimental when dominant
• Nickel matrix: Solid solution strengthened by Cr, Mo, W additions; grain size controlled by cooling rate (typically 10–50 μm in as-surfaced condition)
• Possible intermetallics: Ni₃W, Ni₄W, Ni₇W₆ may form at high dilution—these reduce ductility and should be minimized

4.4 Friction-Wear Testing Protocol

The friction-wear characterization study employs standardized testing methodologies to generate reliable performance data:

Test Method	Standard Reference	Application Relevance
Dry sliding wear	ASTM G99 / GB/T 12444	Pin-on-disc against steel or ceramic counterfaces; evaluates adhesive and abrasive mechanisms
Abrasive wear (two-body)	ASTM G65 / GB/T 16631	Sand rubber wheel or pin-on-sand; simulates mineral processing and slurry conditions
Erosive wear	ASTM G76 / GB/T 16493	Impingement tester at controlled particle velocity and angle; relevant for pneumatic conveying
Fretting wear	ASTM G182 / ISO 18808	Reciprocating contact under normal load; applicable to bolted joints and bearing contacts
Friction coefficient	ASTM G168 / GB/T 12444	Real-time friction measurement during sliding; validates lubricity claims

5. Applicable Standards and Acceptance Criteria

5.1 Process and Material Standards

• ASTM A388 / A390 / A392: Specification for welding overlay consumables—nickel-base electrode/wire alloys applicable to plasma arc surfacing feedstock qualification.
• ASTM B546: Standard specification for tungsten carbide-cobalt cemented carbide (relevant for WC particle quality assessment).
• NB/T 47013: (Chinese National Standard for Pressure Vessel NDT)—inspection requirements for weld overlay on pressure-containing equipment.
• ASME Section IX, QW-401/QW-451: Qualification requirements for plasma arc welding processes (GTAW category with plasma torch variant).
• ISO 14732: Welding—Plasma arc welding (PAW) general specifications.
• GB/T 12467: Classification of arc welding processes (PAW classification).
• NACE SP0169: Control of corrosion on underground or submerged metal piping (relevant when overlay serves as corrosion protection).

5.2 Acceptance Criteria

• Visual Inspection: No cracks, porosity exceeding 5% area coverage, undercuts, or unmelted feedstock. Surface roughness Ra ≤ 12.5 μm for as-surfaced, ≤ 6.3 μm for ground finish.
• Magnetic Particle Testing (MT):strong> Per ASTM E709 / GB/T 26952—no linear indications exceeding 3 mm in length or clustered indications exceeding 6 mm equivalent length on critical components.
• Hardness Verification: Surface hardness ≥ 800 HV0.3 minimum (as-surfaced); core hardness gradient documented; hardness transition zone ≤ 50% hardness drop within 0.5 mm from overlay surface.
• Microstructure Examination: WC retention rate ≥ 60% of original particle population (by area fraction in cross-section); no continuous intergranular carbide networks.
• Dilution Rate: ≤ 15% (by optical emission spectroscopy or XRF) for WC-containing coatings; ≤ 8% for critical applications requiring maximum hardness.
• Thickness Verification: Ultrasonic thickness measurement per ASTM E797 / GB/T 11344; minimum specified thickness tolerance ±0.2 mm.
• Impact/Bend Testing: For ductility verification of overlay welds per ASTM A563 or WPS-specified test methods.

6. Common Risks and Controls

6.1 Technical Risks

Risk	Cause	Mitigation Strategy
Cracking (hot or cold)	High dilution from high-C substrate; excessive restraint; high S/P content	Use transition layer (e.g., 309L or Ni-base); preheat 200–300 °C; limit travel speed to promote fluid weld pool; verify substrate chemistry
WC dissolution and loss of hardness	Excessive current; low travel speed; prolonged residence time	Optimize current/speed ratio; use lower-stickout configuration; employ multi-pass with cooling between passes
Delamination/poor bonding	Incomplete substrate melting; surface contamination; excessive preheat	Ensure adequate arc current for penetration; rigorous surface preparation; control preheat per WPS
Porosity	Inadequate shielding; moisture in feedstock; gas porosity from high C	Verify gas flow and nozzle alignment; use dry, certified consumables; consider vacuum-assisted or high-purity Ar
Excessive residual stress	High thermal gradient; constrained geometry; single-pass thick builds	Multi-pass with weave pattern; interpass temperature control; post-weld stress relief; back-strap technique for thick sections
Uneven coating thickness	Operator technique variation; mechanical instability; wire feed inconsistency	Use mechanized/robotic torch head; in-process thickness monitoring; WPS with fixed parameters

6.2 Quality Management Controls

• WPS/PQR System: Each coating application requires a qualified Welding Procedure Specification (WPS) backed by a Procedure Qualification Record (PQR) per ASME Section IX or NB/T 47014.
• Consumable Traceability: All nickel-base WC composite wires must carry mill test reports verifying chemical composition (Ni, Cr, Mo, W, C, Co content), WC particle size distribution, and hardness.
• Operator Certification: Plasma arc surfacing operators must hold valid certifications per ISO 9606-1 or ASME Section IX, with specific endorsement for PAW processes and overlay applications.
• In-Process Monitoring: Real-time monitoring of arc voltage, current, travel speed, and gas flow; automated data logging for traceability and deviation detection.
• Post-Weld Inspection Protocol: Sequential VT → MT → UT (thickness) → hardness survey → metallographic cross-section (for critical applications) → dimensional verification.

7. Application Scenarios Across Company Technology Routes

7.1 Integration with TIG/MIG Weld Overlay Route

Plasma arc surfacing complements conventional TIG/MIG overlay in the following ways:

• Transition Layer Sequencing: For thick overlay builds on high-dilution-sensitive substrates, the first 1–2 passes may be deposited by TIG (GTAW) using a Ni-base transition alloy (e.g., ENi-CrMo or ENi-Cl3 per AWS A5.15), followed by plasma arc surfacing of the WC-containing hardfacing layers. This hybrid approach combines the low-dilution characteristics of TIG with the high-deposition-rate capability of plasma arc.
• Repair and Maintenance: Where TIG overlay provides general corrosion-resistant cladding (e.g., 309L/316L on carbon steel), plasma arc surfacing adds localized wear protection at high-wear zones (valve seats, pump impellers, bearing journals).
• Process Selection Matrix: TIG is preferred for thin sections (<2 mm substrate thickness), precision geometry, and low-heat-input requirements; plasma arc surfacing is preferred for deposition rates >5 kg/h, thick builds (>3 mm), and high-hardness requirements.

7.2 Integration with Hydraulic Explosive Bonding Route

• Post-Bonding Surface Enhancement: Hydraulic explosively bonded clad plates (e.g., SS316L on Q345R, or Hastelloy C-276 on carbon steel) may require additional surface hardening at specific zones subject to mechanical wear. Plasma arc surfacing of Ni-WC coatings on the clad surface provides a dual-protection strategy: bulk corrosion resistance from the bonded clad layer plus surface wear resistance from the hardfacing overlay.
• Clad Component Hardening: For components fabricated from explosion-bonded clad pipe (e.g., API 5L X65 with 316L cladding), high-wear areas such as flange faces, gasket seating surfaces, and internal flow paths can be plasma arc surfaced with Ni-WC to extend service intervals.
• Material Compatibility: The plasma arc surfacing process must be qualified on the specific clad material system to ensure no detrimental effects on the bond interface. Prequalification testing involves cross-section examination of the overlay/clad/bond/base metal interface to verify bond integrity is maintained.

7.3 Integration with Explosion Welding Route

• Explosion-Welded Clad Plate Surface Treatment: Large-format explosion-welded clad plates (e.g., 304L/16Mn, 6Mo-1Ti/12Cr1MoVG) used in pressure vessel and heat exchanger fabrication may receive plasma arc surfaced wear coatings at tube sheet weld zones, nozzling areas, and support pad locations.
• Explosion-Welded Pipe Fitting Hardfacing: Explosion-welded pipe spools with corrosion-resistant cladding can have their internal surfaces plasma arc surfaced with Ni-WC coatings for slurry service, combining the metallurgical bond of explosion welding (no dilution at bond line) with the tribological protection of the hardfacing overlay.
• Value-Added Service: Offering plasma arc surfacing as a post-processing step on explosion-welded components creates differentiated value propositions—customers receive a single-source solution for both corrosion and wear protection, reducing interface management and supply chain complexity.

8. Contribution to Qualification Building, Product Delivery, and Customer Value

8.1 Qualification and Certification Advancement

The friction-wear research program on plasma arc surfaced Ni-WC coatings directly supports the company's qualification infrastructure in the following ways:

• WPS Library Development: Each parameter combination validated through wear testing generates a qualified WPS that can be applied to customer projects, reducing engineering lead time and qualification costs.
• Material Qualification: Systematic evaluation of different Ni-WC wire compositions (varying WC content from 50–70 wt%, varying binder alloy from Ni-Cr to Ni-Co-W-Cr) builds a qualified material library that supports rapid specification matching for diverse applications.
• Third-Party Certification Support: Wear test data generated per ASTM/GB standards provides objective evidence for customer qualification programs, regulatory submissions (e.g., NACE MR0175/ISO 15156 compliance for sour service components), and design authority approvals.
• Patent and IP Development: Novel coating formulations, process sequences, and performance data identified through research contribute to proprietary intellectual property that differentiates the company in competitive bids.

8.2 Product Delivery Enhancement

• Performance-Predictive Capability: Friction-wear test data enables the company to provide customers with quantified service life predictions (e.g., "5× life extension versus bare carbon steel in 60% solid slurry service at 5 m/s flow velocity"), supporting total-cost-of-ownership analysis.
• Customization Flexibility: Understanding the parameter-performance relationships allows rapid adjustment of coating properties (hardness vs. toughness, wear rate vs. corrosion resistance) to match specific customer requirements.
• Quality Consistency: Validated process windows with defined parameter ranges ensure batch-to-batch consistency, reducing rework rates and improving first-pass yield.
• Documentation Package: Each delivered component can include a complete technical dossier: WPS/PQR, material certificates, in-process records, NDT reports, hardness surveys, and wear performance predictions—providing full traceability and audit readiness.

8.3 Customer Value Realization

The plasma arc surfacing capability with validated Ni-WC coating performance represents a strategic differentiator for Cladding Technology Shanxi Co., Ltd. in addressing the combined corrosion-wear challenges faced by customers in mining, power generation, cement, pulp & paper, and oil/gas industries. By integrating this surface engineering capability with the company's core explosion bonding and weld overlay technologies, the organization offers a comprehensive material protection platform that reduces customer lifecycle costs by 40–70% compared to uncoated or conventionally protected alternatives.

Key customer value metrics supported by this capability include:

• Availability Improvement: Planned maintenance intervals extended from 3–6 months to 18–36 months for critical wear components
• Energy Efficiency: Reduced friction losses in rotating equipment translate to 2–5% energy savings in continuous-operation applications
• Environmental Compliance: Reduced material consumption and waste generation through component life extension aligns with circular economy and ESG objectives
• Retrofit Capability: Ability to extend the life of existing installed assets avoids capital expenditure on new equipment, particularly valuable for brownfield facilities

9. Conclusion

The plasma arc surfacing of nickel-based spherical tungsten carbide coatings represents a technically sophisticated, standards-compliant surface engineering capability that bridges the gap between bulk cladding technologies and specialized surface hardening requirements. Through rigorous process development, comprehensive friction-wear characterization, and systematic integration with the company's broader technology portfolio, this capability delivers measurable, quantifiable value to customers operating in demanding wear-and-corrosion environments. The investment in research, qualification, and process optimization directly translates to competitive advantage in engineering bids, customer confidence in delivered performance, and long-term technical leadership in the surface engineering market.