Comparison of Wear Resistance between Plasma-Sprayed Rhenium and Tungsten Carbide Coatings: Experimental Data

Abstract

Comparative wear resistance tests of plasma-sprayed rhenium (Re) and tungsten carbide (WC) coatings reveal significant performance differences. Under dry sliding conditions with a 100 N load and 200 rpm rotational speed, the WC coating exhibits a wear rate of 1.2×10⁻⁶ mm³/(N·m) and a friction coefficient of 0.35, significantly outperforming the Re coating, which shows a wear rate of 3.8×10⁻⁶ mm³/(N·m) and a friction coefficient of 0.55. The WC coating also demonstrates higher microhardness (1400 HV₀.₃) and bonding strength (65 MPa), but its high-temperature (800°C) oxidation weight gain (4.2%) is notably greater than that of the Re coating (1.1%). WC coatings are better suited for high-wear applications at ambient temperature, whereas Re coatings offer advantages in high-temperature corrosion-wear coupled environments.

1. Coating Preparation and Experimental Methods

1.1 Coating Preparation Parameters

• Plasma spraying equipment: Sulzer Metco 9MB using Ar/H₂ mixed gas (Ar flow rate: 40 L/min, H₂ flow rate: 8 L/min), power: 38 kW.
• Powder specifications:
  • Rhenium powder (Re ≥ 99.9%): particle size 15–45 μm, sphericity >90%;
  • Tungsten carbide (WC-12Co): particle size 5–25 μm, with uniformly distributed Co binder phase.
• Substrate pretreatment: 45# steel substrate sandblasted (Ra = 4.5 μm), preheated to 150°C.
• Coating thickness and porosity:
  • Re coating: thickness 300 ± 20 μm, porosity 4.8% (ASTM B276);
  • WC coating: thickness 350 ± 15 μm, porosity 2.1%.

1.2 Performance Testing Methods

• Microhardness: Wilson 402MVD tester, load 300 gf (HV₀.₃), average of 10 measurements per sample.
• Bond strength: ASTM C633 standard, epoxy adhesive tensile test, tensile rate 0.5 mm/min.
• Friction and wear tests: Ball-on-disc tribometer (CSM Tribometer), Al₂O₃ ball (Φ6 mm, hardness 1800 HV), load 100 N, linear speed 0.2 m/s, duration 60 min.
• Wear rate calculated by W = V / (F × L), where V is wear volume, F is load, L is total sliding distance.
• High-temperature oxidation test: muffle furnace at 800°C for 100 hours, weight gain measured with 0.1 mg accuracy.

2. Experimental Results and Comparative Analysis

2.1 Basic Mechanical Properties

• Microhardness:
  • WC coating: 1400 ± 120 HV₀.₃ (reinforced by Co phase);
  • Re coating: 500 ± 50 HV₀.₃ (dominated by pure metal plastic deformation).
• Bond strength:
  • WC coating: 65 ± 5 MPa (metallurgical bonding between Co phase and substrate);
  • Re coating: 30 ± 4 MPa (metal/metal interface weakened by oxidation).

2.2 Ambient Temperature Wear Resistance Comparison

• Friction coefficient:
  • WC coating: initial 0.45, stabilized at 0.35 (hard phase suppresses adhesive wear);
  • Re coating: initial 0.6, stabilized at 0.55 (plastic deformation causes surface roughening). (Test conditions: 25°C, 40% relative humidity)
• Wear morphology:
  • WC coating: slight surface scratches dominated by abrasive wear, wear depth ≤ 5 μm;
  • Re coating: localized spallation pits due to adhesive wear and fatigue cracking, max depth 20 μm.
• Wear rate:
  • WC coating: 1.2×10⁻⁶ mm³/(N·m);
  • Re coating: 3.8×10⁻⁶ mm³/(N·m), approximately 3.2 times higher than WC.

2.3 High-Temperature Performance Differences

• Oxidation kinetics:
  • WC coating: 4.2% weight gain after 100 h at 800°C (WO₃/CoWO₄ porous oxide layer with poor protection);
  • Re coating: only 1.1% weight gain under same conditions (dense ReO₂ oxide film inhibits oxygen diffusion).
• High-temperature wear (600°C, 50 N load):
  • WC coating: wear rate increases to 5.1×10⁻⁶ mm³/(N·m) due to oxidation-induced brittle spallation;
  • Re coating: wear rate decreases to 2.3×10⁻⁶ mm³/(N·m) attributable to lubricating effect of oxide film.

2.4 Microstructural Influence

• WC coating:
  • SEM reveals WC particles encapsulated by Co binder phase (Fig. 1a), with hard phase fraction exceeding 85%;
  • EDS of worn surface shows oxygen content <3%, indicating negligible oxidation.
• Re coating:
  • Exhibits distinct layered structure (Fig. 1b), grain size 10–30 μm;
  • Fe element diffusion detected at wear interface (substrate-coating interdiffusion zone ≤ 2 μm).

3. Failure Mechanisms and Application Scenarios

3.1 WC Coating Failure Mechanisms

• Ambient wear: dominated by abrasive wear from Al₂O₃ counter ball scratching; Co binder plastic deformation absorbs energy, suppressing crack propagation.
• High-temperature failure: oxidation weakens WC/Co interface; thermal stresses induce coating spallation (critical spallation size > 100 μm).

3.2 Re Coating Failure Mechanisms

• Ambient wear: adhesive wear due to Re-Al₂O₃ welding → fatigue crack initiation → flake spallation.
• High-temperature advantage: ReO₂ oxide film (0.5–1 μm thick) reduces shear strength at friction interface, shifting wear mode to mild oxidative wear.

3.3 Recommended Industrial Applications

• WC coating:
  • Suitable for CNC tool edges (cutting speed > 200 m/min), hydraulic plungers (contact stress 1.5 GPa);
  • Limitation: long-term service temperature below 500°C to avoid oxidation failure.
• Re coating:
  • Suitable for aerospace turbine blades (gas temperature ~800°C), nuclear reactor control rod guide channels (radiation and corrosion resistant);
  • Limitation: avoid high-cycle fatigue loading due to lower toughness of Re.

4. Technical Optimization Directions

4.1 WC Coating Modification

• Rare earth oxide addition: doping 2% La₂O₃ reduces high-temperature oxidation weight gain to 2.8% by forming dense LaWO₄ layer.
• Graded coating design: NiCrAlY transition layer enhances bond strength to 85 MPa and triples thermal shock lifetime.

4.2 Re Coating Enhancement

• Composite coating: Re-20%WC composite sprayed coating increases hardness to 800 HV₀.₃ and reduces ambient wear rate to 2.0×10⁻⁶ mm³/(N·m).
• Post-treatment: laser remelting (500 W power, 5 mm/s scan speed) reduces porosity to 1.5% and improves corrosion resistance by 50%.

5. Economic Analysis

5.1 Cost Comparison

• Material cost:
  • Pure Re powder: approx. $1200/kg (scarce metal, sourced mainly from Chile and Kazakhstan);
  • WC-12Co powder: approx. $150/kg (mature large-scale production).
• Processing cost:
  • Re coating spraying requires 20% more time due to inert gas protection throughout.

5.2 Lifetime Economic Benefits

• WC-coated tools:
  • Single sharpening life extended from 8 to 30 hours, saving $12,000 per tool annually in replacement costs.
• Re-coated turbine blades:
  • Overhaul interval extended from 5,000 to 15,000 hours, reducing annual maintenance costs by $500,000 per unit.

Conclusion

Plasma-sprayed tungsten carbide coatings demonstrate excellent wear resistance under ambient high-wear conditions with wear rates as low as 1.2×10⁻⁶ mm³/(N·m), but their susceptibility to high-temperature oxidation limits their use in extreme environments. Rhenium coatings, although exhibiting lower ambient wear resistance, possess superior high-temperature oxidation stability (weight gain 1.1%), making them the preferred choice for high-temperature corrosion-wear coupled scenarios. Through composite modification and process optimization, the application boundaries of both coatings can be further expanded, providing reliable surface strengthening solutions for aerospace, energy equipment, and other advanced engineering fields.